CHEMICAL TECHNOLOGY I

Water for the chemical process industry and its treatment: Boiler feed-water, Cooling tower water, Process
Plant water, Treatment of water: lime-soda process, Flocculation, aeration, deaeration, ion-exchange

Acid industries: Sulfuric, hydrochloric and nitric.

Chlor-Alkali industries: Caustic soda, Sodium carbonate, Chlorine, Bleaching powder.

Pulp and paper

Paints and Varnishes.

Cement.

Fertilizers: Nitrogen fertilizers - synthetic ammonia, urea, ammonium chloride, CAN, ammonium sulphate.
Phosphorous fertilizers - phosphate rock, phosphoric acid, superphosphate and triple super phosphate,
MAP, DAP. Potassium fertilizers – potassium chloride and potassium sulphate.

Books
Shreve's Chemical Process Industries by George T. Austin
Dryden's Outlines of Chemical Technology for the 21st Century 3rd Edition
Water
The Water Cycle
The major chemical and physical properties of water are:
•Water is a good solvent and is often referred to as the universal solvent. Substances that dissolve in water, e.g., salts,
sugars, acids, alkalis, and some gases – especially oxygen, carbon dioxide (carbonation) are known as hydrophilic
(water-loving) substances, while those that do not mix well with water (e.g., fats and oils), are known as hydrophobic
(water-fearing) substances.

•Water has the second highest specific heat capacity of any known substance, after ammonia, as well as a high heat of
vaporization (40.65 kJ·mol−1), both of which are a result of the extensive hydrogen bonding between its molecules.
These two unusual properties allow water to moderate Earth's climate by buffering large fluctuations in temperature.

Groundwater and surface water are useful or potentially useful to humans as water resources.

Liquid water is found in bodies of water, such as an ocean, sea, lake, river, stream, canal, pond, or puddle. The majority
of water on Earth is sea water. Water is also present in the atmosphere in solid, liquid, and vapor states. It also exists as
groundwater in aquifers.

Water and steam are used as heat transfer fluids in diverse heat exchange systems, due to its availability and high heat
capacity, both as a coolant and for heating. Cool water may even be naturally available from a lake or the sea.
Condensing steam is a particularly efficient heating fluid because of the large heat of vaporization. A disadvantage is
that water and steam are somewhat corrosive. In almost all electric power stations, water is the coolant, which vaporizes
and drives steam turbines to drive generators.

In the nuclear industry, water can also be used as a neutron moderator. In a pressurized water reactor, water is both a
coolant and a moderator. This provides a passive safety measure, as removing the water from the reactor also slows the
nuclear reaction down.

Water has a high heat of vaporization and is relatively inert, which makes it a good fire extinguishing fluid. The
evaporation of water carries heat away from the fire. However, water cannot be used to fight fires of electric equipment,
because impure water is electrically conductive, or of oils and organic solvents, because they float on water and the
explosive boiling of water tends to spread the burning liquid.
Use of water in fire fighting should also take into account the hazards of a steam explosion, which may occur when water
is used on very hot fires in confined spaces, and of a hydrogen explosion, when substances which react with water, such
as certain metals or hot graphite, decompose the water, producing hydrogen gas.

Industrial applications
Water is used in power generation. Hydroelectricity is electricity obtained from hydropower. Hydroelectric power comes
from water driving a water turbine connected to a generator. Hydroelectricity is a low-cost, non-polluting, renewable
energy source. The energy is supplied by the sun. Heat from the sun evaporates water, which condenses as rain in higher
altitudes, from where it flows down.

Pressurized water is used in water blasting and water jet cutters. Also, very high pressure water guns are used for precise
cutting. It works very well, is relatively safe, and is not harmful to the environment. It is also used in the cooling of
machinery to prevent over-heating, or prevent saw blades from over-heating.

Water is also used in many industrial processes and machines, such as the steam turbine and heat exchanger, in addition
to its use as a chemical solvent. Discharge of untreated water from industrial uses is pollution. Pollution includes
discharged solutes (chemical pollution) and discharged coolant water (thermal pollution). Industry requires pure water
for many applications and utilizes a variety of purification techniques both in water supply and discharge.

The high heat capacity makes water a good heat storage medium (coolant) and heat shield.
Why Use Water for Cooling?
Sources of water
Two Types of Water
Sources of water
Groundwater: The water emerging from some deep ground water may have fallen as rain many decades, hundreds,
thousands or in some cases millions of years ago. Soil and rock layers naturally filter the ground water to a high degree
of clarity before it is pumped to the treatment plant. Such water may emerge as springs, artesian springs, or may be
extracted from boreholes or wells. Deep ground water is generally rich in dissolved solids, especially carbonates and
sulfates of calcium and magnesium. Depending on the strata through which the water has flowed, other ions may also be
present including chloride, and bicarbonate. There may be a requirement to reduce the iron or manganese content of this
water to make it fit for use. Disinfection may also be required. Where groundwater recharge is practised; a process in
which river water is injected into an aquifer to store the water in times of plenty so that it is available in times of drought;
it is equivalent to lowland surface waters for treatment purposes.

Upland lakes and reservoirs: Many upland sources have low pH which requires adjustment.

Rivers, canals and low land reservoirs: Low land surface waters may contain suspended solids and a variety of
dissolved constituents.

Pre-treatment
Pumping and containment - The majority of water must be pumped from its source or directed into pipes or holding
tanks. To avoid adding contaminants to the water, this physical infrastructure must be made from appropriate materials
and constructed so that accidental contamination does not occur.

Screening - The first step in purifying surface water is to remove large debris such as sticks, leaves, trash and other large
particles which may interfere with subsequent purification steps. Most deep groundwater does not need screening before
other purification steps.

Storage - Water from rivers may also be stored in bank-side reservoirs for periods between a few days and many months
to allow natural biological purification to take place. This is especially important if treatment is by slow sand filters.
Storage reservoirs also provide a buffer against short periods of drought or to allow water supply to be maintained during
transitory pollution incidents in the source river.
Pre-conditioning - Many waters rich in hardness salts are treated with soda-ash (sodium carbonate) to precipitate
calcium carbonate out utilising the common-ion effect.

Widely varied techniques are available to remove the fine solids, micro-organisms and some dissolved inorganic and
organic materials. The choice of method will depend on the quality of the water being treated, the cost of the treatment
process and the quality standards expected of the processed water.

pH adjustment
Distilled water has a pH of 7 (neither alkaline nor acidic) and sea water has an average pH of 8.3 (slightly alkaline). If
the water is acidic (lower than 7), lime, soda ash, or sodium hydroxide is added to raise the pH. For somewhat acidic
waters (lower than 6.5), forced draft gasifies are the cheapest way to raise the pH, as the process raises the pH by
stripping dissolved carbon dioxide (carbonic acid) from the water. Lime is commonly used for pH adjustment at the start
of a treatment plant for process water, as it is cheap, but it also increases the ionic load by raising the water hardness.
Making the water slightly alkaline ensures that coagulation and flocculation processes work effectively.
Water Treatment Plant
What Problems Does Scale Cause?
General corrosion Localized corrosion BIOLOGICAL
(pitting) FOULING

Calcium carbonate Microbial contamination on

scale a tube sheet
Ways to Prevent Scale
Effects of Corrosion
Natural Draft Cooling Tower
Cooling Tower Process
 Heat Transfer Principle

OPEN RECIRCULATING SYSTEM

ONCE-THROUGH SYSTEM

CLOSED RECIRCULATING SYSTEM

Cooling tower water
A cooling tower is an installation that retreats heat from water by evaporation. The industries use cooling water in
various processes. There are various types of cooling towers. When water is reused, it is pumped through the installation
into the cooling tower. After the water is cooled, it is reintroduced into the production process. Water that needs to be
cooled usually has a temperature of between 40 and 60°C.

The water is pumped to the top of the cooling tower and will than flow down through plastic or wooden shells. This
causes drop formation. While flowing down, the water emits heat which mixes with the above air flow, causing it to cool
down 10 to 20 ˚C. Part of the water evaporates, causing it to emit more heat. Water vapor can sometimes be observed
over the cooling tower.

To create an upward airflow, some cooling towers contain blades in the top, which are similar to ventilator blades. These
blades cause an upward air flow inside the cooling tower. The water falls down into a basin and will be brought back into
the production process from there.

A cooling tower is a heat rejection device, which extracts waste heat to the atmosphere through the cooling of a water
stream to a lower temperature. The type of heat rejection in a cooling tower is termed "evaporative" in that it allows a
small portion of the water being cooled to evaporate into a moving air stream to provide significant cooling to the rest of
that water stream. The heat from the water stream transferred to the air stream raises the temperature of the air and its
relative humidity to 100%, and this air is discharged to the atmosphere. Evaporative heat rejection devices such as
cooling towers are commonly used to provide significantly lower water temperatures than achievable with "air cooled"
or "dry" heat rejection devices, like the radiator in a car, thereby achieving more cost-effective and energy efficient
operation of systems in need of cooling.

Common applications for cooling towers are providing cooled water for air-conditioning, manufacturing and electric
power generation. The smallest cooling towers are designed to handle water streams of only a few gallons of water per
minute supplied in small pipes like those might see in a residence, while the largest cool hundreds of thousands of
gallons per minute supplied in pipes as much as 15 feet (about 5 meters) in diameter on a large power plant.
A direct, or open circuit cooling tower is an enclosed structure with internal means to distribute the warm water fed to it
over a labyrinth-like packing or "fill." The fill provides a vastly expanded air-water interface for heating of the air and
evaporation to take place. The water is cooled as it descends through the fill by gravity while in direct contact with air
that passes over it. The cooled water is then collected in a cold water basin below the fill from which it is pumped back
through the process to absorb more heat. The heated and moisture laden air leaving the fill is discharged to the
atmosphere at a point remote enough from the air inlets to prevent its being drawn back into the cooling tower.

The fill may consist of multiple, mainly vertical, wetted surfaces upon which a thin film of water spreads (film fill), or
several levels of horizontal splash elements which create a cascade of many small droplets that have a large combined
surface area (splash fill).

An indirect or closed circuit cooling tower involves no direct contact of the air and the fluid, usually water or a glycol
mixture, being cooled. Unlike the open cooling tower, the indirect cooling tower has two separate fluid circuits. One is an
external circuit in which water is recirculated on the outside of the second circuit, which is tube bundles (closed coils)
which are connected to the process for the hot fluid being cooled and returned in a closed circuit. Air is drawn through
the recirculating water cascading over the outside of the hot tubes, providing evaporative cooling similar to an open
cooling tower. In operation the heat flows from the internal fluid circuit, through the tube walls of the coils, to the
external circuit and then by heating of the air and evaporation of some of the water, to the atmosphere. Operation of the
indirect cooling towers is therefore very similar to the open cooling tower with one exception. The process fluid being
cooled is contained in a "closed" circuit and is not directly exposed to the atmosphere or the recirculated external water.

In a counter-flow cooling tower air travels upward through the fill or tube bundles, opposite to the downward motion of
the water. In a cross-flow cooling tower air moves horizontally through the fill as the water moves downward.

Cooling towers are also characterized by the means by which air is moved. Mechanical-draft cooling towers rely on
power-driven fans to draw or force the air through the tower. Natural-draft cooling towers use the buoyancy of the
exhaust air rising in a tall chimney to provide the draft. A fan-assisted natural-draft cooling tower employs mechanical
draft to augment the buoyancy effect. Many early cooling towers relied only on prevailing wind to generate the draft of
air.
If cooled water is returned from the cooling tower to be reused, some water must be added to replace, or make-up, the
portion of the flow that evaporates. Because evaporation consists of pure water, the concentration of dissolved minerals
and other solids in circulating water will tend to increase unless some means of dissolved-solids control, such as blow-
down, is provided. Some water is also lost by droplets being carried out with the exhaust air (drift), but this is typically
reduced to a very small amount by installing baffle-like devices, called drift eliminators, to collect the droplets. The
make-up amount must equal the total of the evaporation, blow-down, drift, and other water losses such as wind blowout
and leakage, to maintain a steady water level.
Some useful terms, commonly used in the cooling tower industry:
Drift - Water droplets that are carried out of the cooling tower with the exhaust air. Drift droplets have the same
concentration of impurities as the water entering the tower. The drift rate is typically reduced by employing baffle-like
devices, called drift eliminators, through which the air must travel after leaving the fill and spray zones of the tower.
Blow-out - Water droplets blown out of the cooling tower by wind, generally at the air inlet openings. Water may also
be lost, in the absence of wind, through splashing or misting. Devices such as wind screens, louvers, splash deflectors
and water diverters are used to limit these losses.
Plume - The stream of saturated exhaust air leaving the cooling tower. The plume is visible when water vapor it
contains condenses in contact with cooler ambient air, like the saturated air in one's breath fogs on a cold day. Under
certain conditions, a cooling tower plume may present fogging or icing hazards to its surroundings. Note that the water
evaporated in the cooling process is "pure" water, in contrast to the very small percentage of drift droplets or water
blown out of the air inlets.
Blow-down - The portion of the circulating water flow that is removed in order to maintain the amount of dissolved
solids and other impurities at an acceptable level.
Leaching - The loss of wood preservative chemicals by the washing action of the water flowing through a wood
structure cooling tower.
Noise - Sound energy emitted by a cooling tower and heard (recorded) at a given distance and direction. The sound is
generated by the impact of falling water, by the movement of air by fans, the fan blades moving in the structure, and the
motors, gearboxes or drive belts.
Cooling towers are evaporative coolers used for cooling water or other working medium to near the
ambient wet-bulb air temperature. Cooling towers use evaporation of water to reject heat from processes
such as cooling the circulating water used in oil refineries, chemical plants, power plants and building
cooling, for example. The towers vary in size from small roof-top units to very large that can be up to
200 metres tall and 100 metres in diameter, or rectangular structures that can be over 40 metres tall and
80 metres long. Smaller towers are normally factory-built, while larger ones are constructed on site.

Air flow generation methods

With respect to drawing air through the tower, there are three types of cooling towers:

Natural Draft, which utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the
density differential to the dry, cooler outside air. Warm moist air is less dense than drier air at the same
temperature and pressure. This moist air buoyancy produces a current of air through the tower.

Mechanical draft, which uses power driven fan motors to force or draw air through the tower.

Induced draft: A mechanical draft tower with a fan at the discharge which pulls air through tower.
The fan induces hot moist air out the discharge. This produces low entering and high exiting air
velocities, reducing the possibility of recirculation in which discharged air flows back into the air
intake. This fan/fill arrangement is also known as draw-through.
 Forced draft: A mechanical draft tower with a blower type fan at the intake. The fan forces air into
the tower, creating high entering and low exiting air velocities. The low exiting velocity is much
more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to
complications due to freezing conditions. Another disadvantage is that a forced draft design
typically requires more motor horsepower than an equivalent induced draft design. The forced draft
benefit is its ability to work with high static pressure. They can be installed in more confined spaces
and even in some indoor situations. This fan/fill geometry is also known as blow-through.

Crossflow

Crossflow is a design in which the air flow is directed perpendicular to the water flow. Air flow enters
one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to
the air) through the fill by gravity. The air continues through the fill and thus past the water flow into an
open plenum area. A distribution or hot water basin consisting of a deep pan with holes or nozzles in the
bottom is utilized in a crossflow tower. Gravity distributes the water through the nozzles uniformly
across the fill material.
Most industrial cooling towers use river water or well water as their source of fresh cooling
water. The large mechanical induced-draft or forced-draft cooling towers continuously
circulate cooling water through heat exchangers and other equipment where the water
absorbs heat. That heat is then rejected to the atmosphere by the partial evaporation of the
water in cooling towers where upflowing air is contacted with the circulating downflow of
water. The loss of evaporated water into the air exhausted to the atmosphere is replaced by
"make-up" fresh river water or fresh cooling water. Since the evaporation of pure water is
replaced by make-up water containing carbonates and other dissolved salts, a portion of the
circulating water is also continuously discarded as "blowdown" water to prevent the
excessive build-up of salts in the circulating water. The cooling water is often treated with a
biocide to prevent fouling in heat exchangers. High grade industrial water (produced by
reverse osmosis) and potable water is sometimes used in industrial plants requiring high-
purity cooling water.
Forced Draft Cooling Tower
Cross-Flow Cooling Tower
Induced Draft Cooling Tower
•Cooling towers reduce water temperature through evaporation, a high-energy phase change
•Only water leaves the system as a vapor, mineral content remains
•Minerals dissolve until the saturation point is reached, then scale begins to form
•Conductivity is the measure of dissolved solids
•pH is the measure of the relative acidity or basicity of the water
•These two parameters are convenient methods to monitor and control scale formation and corrosion
•Scale inhibitors- increase solubility
•Acid reacts with carbonates and releases CO2, increases saturation point
•Softeners remove Ca and Mg (scaling cations)
•Corrosion inhibitors- coat or react specifically with certain metals

Methods of Scale Prevention

•Feed Soft water
•Remove all the calcium and magnesium before it is fed it to the towers
•Increase the blowdown rate lowering the conductivity so the solubility limit of calcium carbonate is not exceeded
•Reduce the pH by dosing acid:
•Removes the carbonate (other half of scale) and increases Calcium solubility but balance against corrosion is a must.
•Adjust pH down using controlled dosage of sulfuric acid to maintain the pH at 7.3-8.1
•Can risk corrosion as the pH is adjusted down
•Add chemicals to prevent scale from forming and keep particles suspended and flushed out with the blowdown water

Scale inhibitors
These are usually phosphonate-based scale inhibitors - only work up to about 400 ppm of Calcium with good flow. Low
molecular weight polyacrylate is usually is added to keep particles suspended and flushed out.
Fouling
Organic and inorganic materials, other than scale, that coat heat transfer
surfaces and block flow through piping.
There are two types of foulants: Microbiological and Other.
Examples of Foulants
Algae Bloom in Cooling Tower Basin
A cooling tower is an installation that retreats heat from water by evaporation. When water is reused, it is pumped
through the installation into the cooling tower. After the water is cooled, it is reintroduced into the production process.
Water that needs to be cooled usually has a temperature between 40 to 60oC.

The water is pumped to the top of the cooling tower and will then flow down through plastic or wooden shells. This
causes drop formation. While flowing down, the water emits heat which mixes with the above air flow, causing it to cool
down 10 to 20˚C. Part of the water evaporates, causing it to emit more heat. Water vapor can sometimes be observed
over the cooling tower.

Microbes can thrive in untreated cooling water, which is warm and sometimes full of organic nutrients, as wet cooling
towers are very efficient air scrubbers. Dusts, flies, grass, fungal spores and so on collect in the water and create a sort of
"microbial soup" if not treated with biocides. The water carries bacterial growth in it when it flows through the cooling
tower. If this water is not treated as soon as possible, it can give rise to problems like scale formation, corrosion, and
biological fouling.

Problems with Scale Formation

Scale is what is formed when wet solids get clogged in the pipes. This scale is made from heat and cold water that
contains a very high mineral content. These deposits will continue to build up over time. If these scale deposits start to
form on a heat exchange surface, they will eventually clog the passages and slow the system down. The cooling tower
can also be affected by the scale deposits. If the deposits block the flow of the basin or fill in the cooling tower, problems
arise.
Examples Microbiological Growth
Inside Corrosion
When metal starts to dissolve, the phenomenon is known as corrosion. Oxidation effects break the metal down
significantly. The breakdown will cause the system to degrade at a faster rate than normal. The major point being that
when the metal starts to break down, the strength of the metal and the thickness off the metal are reduced. The structure
of the metal can no longer stand up to the pressure that it was designed for and pits and craters can form in the metal.

Problems with Biological Fouling

Problems with biological fouling occur when the water has not been used and left for long periods of time. When the
water is left unattended, it has the potential to form bacteria, fungus, algae, and protozoa. The microorganisms will
eventually get to large proportions in the water and will cause a biological film to form on the surface of the water. This
film is very hard to get rid of. This is what is known as biological fouling.

The biological fouling problems are commonly known to be the worst problem to encounter with the cooling tower
system. The problems that can occur with biological fouling are low heat transfer, the fill can stop working properly, the
water flow can be restricted or blocked, or corrosion can occur from the microorganisms present. It can also result in
health problems for the humans using it.

The water of the cooling towers can be treated very effectively with ozone for the control of algae, slime and
microorganisms in the water and for the control of bacteria in the dispersion plume of the tower in the air. Incidental
benefits of ozone treatment might be substantial conservation of water, reduced scaling and no toxic materials in the
wastewater discharge.
Cooling water treatment
A cooling tower functions to cool a circulating stream of water. The tower acts as a heat exchanger by driving ambient air
through falling water, causing some of the warmed water to evaporate (evaporation gives off heat--providing cooling),
and then circulating cooler water back through whatever equipment needs cooling.

In a cooling tower some of the cooling water is evaporated, this evaporation cools down the remaining cooling water,
using the effect of the evaporation heat. Very simply explained, the water needs a certain amount of energy to change
from the liquid phase (water) to the gas phase (vapor), this energy is taken from the environment (as well the water)
and so water is getting colder, but the combination of heat, expansive moist areas, and recirculating water creates
an almost ideal petri dish for bacteria and mold. Cooling tower water is continuously exposed to airborne organic
materials, and the buildup of bacteria, algae, fungi, and viruses presents hazards to the tower system and to the health of
humans encountering the water. In addition to creating occupant health risks, this biofouling decreases the system’s
heat-extracting efficiency by accumulating as a biofilm on surfaces—causing microbe-induced corrosion and
providing a substrate for scale formation.

Scale forms when dissolved minerals (generally calcium carbonate) and other solids in the water crystallize on surfaces.
This further reduces the system’s efficiency by clogging water paths and insulating heat-transfer areas: a 1/32" (0.8 mm)
layer of scale can increase energy use by close to 10%. Evaporation exacerbates scale formation, and adding make-up
water introduces more minerals to the process. This is partially controlled by “blow-down,” a process of replacing some
of the solids-laden recirculating water with make-up water.

As only water is evaporated, the concentration of solids or solved parts such as salts or minerals in the remaining water is
increasing all the time during this process. This leads over time as more and more water evaporates, to a higher
concentration of solids (TDS) in the water. Higher the TDS in the cooling water, more likely there will be technical
problems in the system, such as fouling and scaling.

Water is lost from a cooling tower in three ways: drift, evaporation, and blow down. Drift occurs when the water droplets
become entrained in the discharge air stream and can be controlled through cooling tower design. Evaporation is from air
passing through the cooling water and absorbing heat and mass. Blow down is intentional bleed-off (replaced by make-
up water) to reduce the concentration of contaminants.
In order to avoid this technical problem, the common method to counter these problems is to replace some of the cooling
water with fresh water. This is called blow down or bleeding. In most systems, this happens automatically. There is set a
threshold for TDS or conductivity, once this limit is reached a blow down valve or a bleeding valve is opened and a
certain amount of water is drained, which is Then replaced by the feed water.
As an example, if the blow down happens at a concentration of 2.500 TDS and the feed water holds only 100 TDS, the
overall concentration of the cooling water will be lowered.

Another method is chemical treatment of cooling water. Typically, chemicals such as chlorine and chelating agents are
added to cooling tower water to control biological growth (called "biofilm") and inhibit mineral build-up (called
"scale"). The control of biofilm and scale is essential in maintaining cooling tower heat transfer efficiency. As the water
volume in the tower is reduced through evaporation and drift, the concentration of these chemicals and their byproducts
increases. Cooling towers also pick up contaminants from the ambient air. To maintain chemical and contaminant
concentrations at a prudent level, water is periodically removed from the system through a process called "blowdown" or
"bleed off". The blowdown water and the water lost through evaporation and drift are replaced with fresh "make-up"
water (which will also contain minerals and other impurities).

Blow down water must subsequently be discharged to a local wastewater treatment facility. The blow down water
typically contains little organic material.

There are the costs of the chemicals itself, the dosing equipment with its sensors and control units, as well there are
the costs of logistic to get supplied with the chemicals, to store them and bring them to all the dosing points in the
system.
Ozone Treatment for Cooling Towers
Ozone is a molecule consisting of three oxygen atoms and is commonly denoted O3. Under ambient conditions, ozone is
very unstable and as a result has a relatively short half-life of usually less than 10 minutes. Ozone is a powerful biocide
and virus deactivant and will oxidize many organic and inorganic substances. These properties have made ozone an
effective chemical for water treatment for nearly a century. Ozone treatment can also reduce the need for chemical
additives added to the cooling tower water. In a properly installed and operating system, bacterial counts are reduced,
with a subsequent minimization of the buildup of biofilm on heat exchanger surfaces.

Ozone may be a corrosion stimulant rather than an inhibitor, and this can be a factor in some circumstances.
Nevertheless, it is easier to combat corrosion in a clean system than in one that is biologically and mineralogically
fouled.

Using ozone to treat cooling tower water is a relatively new practice; however, its market share is growing as a result of
water and energy savings and environmental benefits relative to traditional chemical treatment processes.

Discharge of the blow down water to the environment onsite is coming under increasing regulation due to stricter
regulation of the contaminants typically found in blow down water. Ozone will dissipate quickly and not be found in the
blow down water. This reduces the overall chemical load found in the discharged water, making it easier to comply with
regulations.

Most cooling tower ozone treatment systems include the following components: an air dryer, air compressor, water and
oil coalescing filters, particle filter, ozone injectors, an ozone generator, and a monitoring/control system. Ambient air is
compressed, dried, and then ionized in the generator to produce ozone. Ozone is typically applied to cooling water
through a side stream of the circulating tower water .
If water and sewer services are purchased from a municipal or public utility, reducing blow down and make-up water
requirements will trigger a series of resource and cost savings for those municipal utilities. If the site operates its own
water treatment and wastewater treatment facilities, reducing blow down and make-up water requirements will allow the
facility to realize these benefits directly as follows:

• reduced pumping power to extract water from source wells or reservoir and pump to water treatment facility

• reduced chemical, filtration, and maintenance costs associated with treating and purifying at the water
treatment facility

• reduced pumping power for distributing the water from the water treatment facility to the end-user

• reduced pumping power and associated costs to transport wastewater (blowdown) to the sewage treatment plant

• reduced chemical and maintenance costs, and reduced pumping power associated with sewage treatment at the
plant
Dielectric Process for Ozone Generation
Ozone generation is accomplished by passing a high-voltage alternating current (6-20kV) across a dielectric discharge
gap through which air is injected (see Figure 3). As air is exposed to the electricity, oxygen molecules disassociate and
form single oxygen atoms, some of which combine with other oxygen molecules to form ozone. Different manufacturers
have their own variations of components for ozone generators. Two different dielectric configurations exist--flat plates
and concentric tubes. Most generators are installed with the tube configuration. Cylindrical configurations offer the
easiest maintenance.

Mass transfer of the ozone gas stream to the cooling tower water is usually accomplished through a venturi in a
recirculation line connected to the sump of the cooling tower where the temperature of the water is the lowest. Since
the solubility of ozone is very temperature-dependent, the point of lowest temperature provides for the maximum
amount of ozone to be introduced in solution to the tower. Mass transfer equipment can take other forms: column
bubble diffusers, positive pressure injection (U-tube), turbine mixer tank, and packed tower. The counter-current column-
bubble contactor is the most efficient and cost-effective but is not always useful in a cooling tower setting because of
space constraints. Hence, a setup like a venturi followed by an in-line static mixer is common in the installation of an
ozone system.

Once ozone is in the liquid phase, it will last only a short period of time; thus, maintaining an ozone residual for more
than approximately 10 minutes can be difficult. This limits the application of ozonation in large cooling towers. In large
towers with 100,000 or more gallons, multiple injection points may be necessary.

Water temperature is critical to the success or failure of a system. Above 110°F (43.3°C) the solubility of ozone is
effectively zero for all concentrations of ozone in the feed gas. Even at 104°F (40°C) the solubility is very small (<3
mg/L).
Non-Chemical Water Treatment for Cooling Towers
Treating cooling-tower water to prevent biological fouling, scale, and corrosion is a complex, highly monitored process.
Most of the dirty work in the half-million cooling towers in the U.S. is accomplished with biocidal, conditioning,
dispersant, and scale-inhibiting chemicals, including chlorine, various brominated compounds, phosphates,
molybdenates, acids (including sulfuric acid), and zinc compounds (which are now banned for cooling-tower use in
about half of U.S. states). While chemicals do get the job done, there are considerations beyond the tower. Regulations
are increasingly stringent for chemical storage, handling, and disposal; in many jurisdictions, chemically treated cooling-
tower blow-down water itself is regulated. There are also consequences to worker, public, and environmental health due
to accidental spills, chronic chemical exposure (even at low levels), and bioaccumulation of persistent chemicals in the
food chain. Reducing or eliminating chemical usage in the treatment of cooling-tower water is an appealing thought.

Various alternatives are being used, including ozonation, ionization, and UV light treatment, all of which kill bacteria.
Another option is exposition recirculating cooling-tower water to electromagnetic fields.

A “pulsed-power” pipe assembly is wound with coils that “impart pulsed, high-frequency, electromagnetic energy” into
the water. Pulsed-power fields activate colloidal nucleation sites in the bulk solution and induces the coagulation of
colloids—very small, electrically charged particles—which creates nucleation sites that encourage calcium carbonate to
precipitate as a powder in the bulk solution rather than a scale on surfaces. The powder can settle in the tank and is
removed via blowdown and/or side-stream filtration. This coagulation-precipitation activity also reduces bacteria counts
by trapping bacteria within the crystal formation, an effect noted in water-treatment plants, which typically use lime or
alum to induce coagulation. The primary source of microorganism control is also the pulsed-power electric field.
Type of Boilers

Fire Tube Boiler

• Relatively small steam

capacities (12,000
kg/hour)
• Low to medium steam
pressures (18 kg/cm2)
• Operates with oil, gas
or solid fuels

(Light Rail Transit Association)

59
60
Type of Boilers

Water Tube Boiler

• Used for high steam
demand and pressure
requirements
• Capacity range of 4,500
– 120,000 kg/hour
• Combustion efficiency
enhanced by induced
draft provisions
• Lower tolerance for
water quality and needs
water treatment plant
Type of Boilers

Packaged Boiler • Comes in complete

package
To • Features
• High heat transfer
Chimney

• Faster evaporation
• Good convective
heat transfer
• Good combustion
Oil
efficiency
Burner
• High thermal
efficiency
• Classified based on
number of passes
 Type of Boilers

Fluidized Bed Combustion (FBC)

Boiler
• Particles (e.g. sand) are
suspended in high velocity air
stream: bubbling fluidized bed
• Combustion at 840° – 950° C
• Fuels: coal, washery rejects,
rice husk, bagasse and
agricultural wastes
• Benefits: compactness, fuel
flexibility, higher combustion
efficiency, reduced SOx & NOx
Type of Boilers

Pulverized Fuel Boiler

• Pulverized coal powder blown with combustion
air into boiler through burner nozzles
• Combustion
temperature at 1300 -
1700 °C
• Benefits: varying coal
quality coal, quick
response to load
changes and high pre-
heat air temperatures Tangential firing
Fire-tube boiler
Water-tube boiler
 STEAM TO
EXHAUST GAS VENT
PROCESS

STACK DEAERATOR

PUMPS

ECO-
NOMI-
ZER

VENT
BOILER
BURNER
WATER
SOURCE
BLOW DOWN
SEPARATOR FUEL

BRINE

CHEMICAL FEED
SOFTENERS

Figure: Schematic overview of a boiler room

Assessment of a Boiler

Heat Balance
Balancing total energy entering a boiler against
the energy that leaves the boiler in different forms
12.7 %
Heat loss due to dry flue gas

8.1 % Heat loss due to steam in fuel gas

1.7 %
100.0 % Heat loss due to moisture in fuel
BOILER 0.3 %
Fuel Heat loss due to moisture in air

2.4 % Heat loss due to unburnts in residue

1.0 %
Heat loss due to radiation & other
unaccounted loss
73.8 %
Heat in Steam
The tubes is dark in color at one section
and another is light in color at another
section. Hence, this indicate that the tubes
are at overheating.
This shows the tube is overheating
Boiler feedwater
A boiler is a device for generating steam, which consists of two principal parts: the furnace, which provides heat, usually
by burning a fuel, and the boiler proper, a device in which the heat changes water into steam. The steam or hot fluid is
then recirculated out of the boiler for use in various processes in heating applications. At atmospheric pressure water
volume increases 1,600 times.

Boiler feedwater is water used to supply ("feed") a boiler to generate steam or hot water. At thermal power stations the
feedwater is usually stored, pre-heated and conditioned in a feedwater tank and forwarded into the boiler by a boiler
feedwater pump.
Conditioning
The water required for boiler feed purposes i.e for steam generation should be of very high quality and thus requires a lot
of treatment. Untreated waters, containing impurities may lead to the following problems in boilers:
►Scale and sludge formation
►Boiler Corrosion
►Caustic Embrittlement
►Priming and foaming
The feedwater has to be specially conditioned to avoid problems in the boiler and downstream components.

The water circuit of a water boiler can be summarized by the following picture:
The boiler receives the feed water, which consists of varying proportion of recovered condensed water (return water) and
fresh water, which has been purified in varying degrees (make up water). The make-up water is usually natural water
treated by some process before use. Feed-water composition therefore depends on the quality of the make-up water and
the amount of condensate returned to the boiler. The steam, which escapes from the boiler, frequently contains liquid
droplets and gases. The water remaining in liquid form at the bottom of the boiler picks up all the foreign matter from the
water that was converted to steam. The impurities must be blown down by the discharge of some of the water from the
boiler to the drains. The permissible percentage of blown down at a plant is strictly limited by running costs and initial
outlay. The tendency is to reduce this percentage to a very small figure.

Proper treatment of boiler feed water is an important part of operating and maintaining a boiler system. As steam is
produced, dissolved solids become concentrated and form deposits inside the boiler. This leads to poor heat transfer and
reduces the efficiency of the boiler. Dissolved gases such as oxygen and carbon dioxide will react with the metals in the
boiler system and lead to boiler corrosion. In order to protect the boiler from these contaminants, they should be controlled
or removed, through external or internal treatment.
In the steam boiler industry, high purity feed water is required to ensure proper operation of steam generation systems.
High purity feed water reduces the use of boiler chemicals due to less frequent blowdown requirements. Lower
blowdown frequency also results in lower fuel costs. Scale buildup is reduced due to a smaller concentration of
impurities in the boiler feed water to foul heat transfer surfaces. The lower level of impurities also reduces corrosion
rates in the boiler. When boiler is used to run a steam turbine, turbine blade erosion is reduced due to higher purity
steam generated.

Silicate and colloidal deposits decrease boiler efficiency and also result in premature failure of turbines. The
reduction in particulate matter, suspended solids and total organic carbon also enhance turbine and boiler efficiency.

To meet the requirements of both ecology and economy, the filtration of boiler feed water streams allows an industrial
boiler or power plant to meet the stringent requirements for more efficient boiler performance. The treatment of the
boiler feed water is required for preventing excessive heat transfer equipment fouling and the erosion of turbine blades.

Importance of Good Boiler Feedwater Treatment

Maintaining good feedwater is an important and fundamental aspect of any steam turbine power plant. A plant that
maintains good feedwater achieves the following three benefits:

1.Help to ensure maximum life out of its boilers, steam turbines, condensers, and pumps.

2. Reduce maintenance expenses.

3. Maintain optimal thermal performance

Results of Poor Water Treatment
In the ideal situation, water would be feed to a boiler free of any impurities. Unfortunately, this is not the case. Water
clean up is always required. The following items are the most problematic to boilers and steam turbines:

Calcium (Ca) scale– Calcium is present in water in the forms of compounds like calcium sulfate, calcium bicarbonate,
calcium carbonate, calcium chloride, and calcium nitrate. During evaporation, these chemicals adhere to boiler tube walls
forming scale. Its formation increases with the rate of evaporation so these deposits will be heaviest where the gas
temperatures are highest. Scale is a nonconductor of heat which leads to a decreased heat transfer of the boiler
tubes, and can result in tube failure due to higher tube metal temperatures. Buildup of scale also clogs piping
systems and can cause control valves and safety valves to stick.

Magnesium (Mg) scale – Same issues as with calcium.

Silica (SiO2) – Silica can form scale at pressures below 600 psig. Above 600 psig, silica starts to volatize, passing over
with steam to potentially form deposits on the steam turbine diaphragms and blades. These deposits change the
steam path components’ profiles resulting in energy losses. The degree of loss depends upon the amount of the deposits,
their thickness and their degree of roughness.

Sodium (Na) – Sodium can combine with hydroxide ions creating sodium hydroxide (caustic). Highly stressed areas of
boiler piping and steam turbines can be attacked by sodium hydroxide and cause stress-corrosion cracks to occur. This
was a problem in older boiler with riveted drums because of stresses and crevices in the areas of rivets and seams. While
less prevalent today, rolled tube ends are still vulnerable areas of attack as well as welded connections.

Chloride (Cl) – Chlorides of calcium, magnesium, and sodium, and other metals are normally found in natural water
supplies. All of these chlorides are very soluble in water and therefore, can carry over with steam to the steam turbine.
Chlorides are frequently found in turbine deposits and will cause corrosion of austenitic (300 series) stainless steel
and pitting of 12 Cr steel. Corrosion resistant materials protect themselves by forming a protective oxide layer on
their surface. These oxides are better known by their generic name “ceramic.” All ceramics will pit if exposed to
chlorides. If the metal piece is under tensile stress either because of operation or residual stress left during
manufacturing, the pits formed by chlorides attacking the passivated layer will deepen even more. Since the piece is
under tensile stress, cracking will occur in the stressed portions.
Usually there will be more than one crack present causing the pattern to resemble a spider’s web. The most common
source of chloride contamination is from condenser leakage.
Iron (Fe) – High iron is not found in raw water but high concentrations can come from rusted piping and exfoliation of
boiler tubes. Iron is found in condensate return in a particle form as it does not dissolve in water. The detrimental
aspect of iron is called steam turbine solid particle erosion, which causes significant erosion of steam turbine steam
path components.
Oil contamination, problem & remedies
Main problems caused by oil in the boiler water are:
1.Oil can coat metal surfaces, cut down heat transfer, and produce metal overheating and tube damage
2.Oil can cause sludge to become sticky and adhere to heat transfer surfaces
3.Oil can produce foaming and boiler water carryover
Oil contamination should be completely eliminated whenever possible. Organic chemicals help counteract the effects of
small amounts of oil contamination, but not of gross contamination. When sudden boiler water oil contamination is
experienced, normal procedure is to blow down heavily to remove oil and to check for the source of contamination. In
case of severe contamination, the boiler needs to be taken off the line and cleaned out to remove the oil from the boiler
surfaces. When oil contamination is continuous and unavoidable, some of the methods used are:
1.Free oil can be reduced by passing the water through absorbent cartridge filters
2.Special filters are used with aids like diatomaceous earth
Oxygen (O2) – Oxygen is found in feedwater and its partial pressure is relatively high so it will requires a near saturation
temperature to disassociate itself from water. Oxygen in combination with water will attack iron and cause corrosion.
The reaction occurs in two steps:

The ferric hydroxide is highly insoluble and precipitates on heated surfaces. The precipitate is called magnetite or
rust. The closer the water is to the saturation temperature, the more corrosion will occur.
Carbon Dioxide (CO2) – Carbon dioxide can react with water to form carbonic acid (H2CO3). Carbonic acid will cause
corrosion in steam and return lines. Carbon dioxide can originate from condenser air leakage or bicarbonate
(HCO3) alkalinity in the feedwater.

pH – The pH value of water is a measure of its alkalinity or acidity and has a direct bearing on the corrosive properties.
All water contains alkaline (hydroxyl, OH) ions and hydrogen (H) ions. Low pH in local areas is the second most
common cause of corrosion in mild steel boilers. Above roughly 400°, mild steel corrosion results in the formation of
magnetite, a tight adherent that acts as a barrier between boiler water and steel. The corrosion reaction stops after a
uniform magnetite layer is formed.

Rapid general corrosion can ensue if this protective film is disrupted, so water chemistry must be carefully controlled to
maintain the film. An acidic condition can destroy the magnetite film; therefore, boiler water is maintained in the alkaline
range of a pH of 9.0 to 10.5.

Foaming – Foaming is the formation of bubbles or froth on the water surface. It is caused by a high amount of total and
suspended solids. Foam will fill the free surface area of a separating device increasing local velocities and promoting a
serious carryover of boiler water.

Priming – Priming is a violent and spasmodic discharge of water with steam into the steam space. Slugs of water are
thrown over with the steam causing damage to the steam turbine.

Carryover – When boiler water solids are carried over into the moisture mixed with steam even though there is no
indication of foaming or priming, this is considered as carryover. Carryover can be the result of high steam flow which
overloads the dryers (separators). The dryers work by sudden changes in steam velocity so that foreign particles are
thrown out by centrifugal force.
Raw Water Cleaning
Raw water can come from a variety of sources, lakes, rivers, and wells for example. Each source of water will have its
own constituents and therefore, its own requirements for cleanup. These cleanup requirements should be specified by
experts in this field of work.
Raw water from reservoirs, lakes, rivers, and wells can have varying characteristics as provided below:
In general, raw water is cleaned via four processes. They are summarized below:

1. Aeration
This process removes undesirable gases such as carbon dioxide and hydrogen sulfide by mixing water with air. The
mixing adds oxygen to the water while removing the carbon dioxide and hydrogen sulfide. Increasing the temperature,
the aeration time and the surface area of water improves the removal of gases.

2. Coagulation
This is the process of adding chemicals to reduce coarse suspended solids, silt, turbidity, and colloids in a clarifier. The
impurities will settle out of solution with the added chemicals. Some chemicals used for coagulation are filter alum,
sodium aluminate, copperas, ferrisul, activated silica, and various organic compounds.

3. Filtration
Filters remove coarse suspended matter and removes floc or sludge from coagulation or process softening systems. Beds
of gravel or coarse anthracite are common materials used for the filter beds. Specially made precoated filters can be used
to remove oil and reduce color.

4. Softening
There are several methods to remove calcium, magnesium, silica, and silt (softening). They are summarized as follows:

Lime soda softening – Calcium hydroxide (lime) is added to feedwater to precipitate the calcium bicarbonate to calcium
carbonate and magnesium salts to magnesium hydroxide. Sodium carbonate (soda ash) is added to react with calcium
chloride and calcium sulfate to form calcium carbonate. This process is more efficient at hot temperatures. After the
lime-soda process, the residual hardness will be approximately 17 to 25 ppm.
Hot-process phosphate softening – Phosphate (tri-sodium phosphate or sodium hexameta phosphate) is added to
remove calcium and magnesium. This process results in precipitated tricalcium phosphate and magnesium hydroxide.
The chemical reactions occur above 212° F and will reduce hardness to nearly zero.

Zeolite softening – Zeolites are any in a group of crystalline mineral compounds whose framework of atoms forms
microscopic tunnels and “rooms.” The internal structure of zeolites makes them useful as filters and catalysts. In water
softening processes, zeolites are used to exchange calcium and magnesium ions with sodium in the zeolite.
The calcium and magnesium are passed to waste and the zeolite is regenerated by passing a sodium chloride (salt)
solution through the softener.

Demineralization – Demineralizers are used to remove ionized mineral salts. Calcium, magnesium, and sodium cations
are removed in a hydrogen cation exchanger by sulfonic, carboxylic, and phenolic hydroxyl compounds.
Anions of bicarbonates, sulfates, chloride, and soluble silica are removed by amino or quaternary nitrogen.

Reverse Osmosis - Reverse osmosis is defined as the passage of water from a more concentrated solution to a less
concentrated solution through a semi-permeable membrane under pressure. The pressure is required to reverse the natural
process of osmosis by overcoming the osmotic pressure. Simply, osmotic pressure can be looked at as being directly
related to concentration so the higher the salt concentration in the feed, the higher the osmotic pressure.

Seawater, which contains approximately 3.5%, or 35,000 mg/l salt, the majority of which is sodium chloride (NaCl), has
an osmotic pressure of 410 psi (28.3 bars). Before desalting can occur, an RO system must be pressurized to a minimum
of 410 psi (28.3 bars) to overcome the osmotic pressure inherent in the solution. Two-stage RO is defined as running two
RO systems in series with the product (permeate) of the first acting as the feed to the second RO. Staged or series
operation is typically done when a single-stage RO system does not produce the required quality of product water.
Another justification for two-stage RO is where the additional expenses of operating the second RO system is lower than
alternative forms of polishing the first-stage RO permeate to reach a higher quality of final product water.
REVERSE OSMOSIS
Boiler Water Control
Items to control in the boiler water are oxygen and dissolved solids. Each of these is controlled in the following manner:

Oxygen – Oxygen will be found in steam condenser condensate as well as in makeup water. It is most commonly
removed via one of two system arrangements. A common method is the use of a vacuum condenser. A vacuum is created
in the condenser by steam jet air ejectors pumps and this prevents the water from absorbing oxygen because the vacuum
is lower that the oxygen’s partial pressure in water. The other method is to use an integral deaerator arrangement. For
this arrangement a deaerator is located atop the low pressure drum and water from the HRSG feedwater heater is sent to
the integral deaerator where it mixes with saturated steam from the low pressure drum thereby removing the oxygen.

Dissolved Solids – During the evaporation process, most solids stay in the water section of the drum while steam is sent
to the superheater. As the solids increase in water, they are removed by sending a small portion (typically 1 to 2% of the
feedwater flow rate) though a drum blowdown pipe to the blowdown tank. This water is most often released to a drain.
The separation ratio of solids in steam vs. drum water depends upon pressure.
Caustic embrittlement is the phenomenon in which the material of a boiler becomes brittle due to the accumulation of
caustic substances.

As water evaporates in the boiler, the concentration of sodium carbonate increases in the boiler. Sodium carbonate is
used in softening of water by lime soda process, due to this some sodium carbonate maybe left behind in the water. As
the concentration of sodium carbonate increases, it undergoes hydrolysis to form sodium hydroxide.

Na2CO3 + H2O → 2NaOH + CO2

The presence of sodium hydroxide makes the water alkaline. This alkaline water enters minute cracks present in the
inner walls of the boiler by capillary action. Inside the cracks, the water evaporates and amount of hydroxide keeps
on increasing progressively. This sodium hydroxide attacks the surrounding material and the dissolves the iron of the
boiler as sodium ferrate. This causes embrittlement of boiler parts like rivets, bends and joints, which are under stress.

This can be prevented by using sodium phosphate instead of sodium carbonate as softening reagents. Adding tannin
or lignin to boiler water, which block the hair-line cracks and prevent infiltration of NaOH into these areas. Adding
Na2SO4 to boiler water, which also blocks hair-line cracks.

This indicates that there are leaks in the boiler tubes.

White deposit appeared at the bottom end of the tubes.
It is a salt( SODIUM CARBONATE). CAUSTIC CORROSION
BOILER FEED-WATER TREATMENT

The importance of correct feed-water treatment for economic operation and for extending life of boiler and equipment cannot be over emphasized.
Feed-water treatment is essential in boilers, feed-systems, etc., more particularly in modern boilers of a high evaporative rate. (The faster a steam
boiler or generator will convert water to steam, the more rapidly will the solids in the water concentrate up.) So, large and small water-tube boilers,
the typical fire-tube packaged boiler, and steam generators are all examples of this in varying degrees. As all untreated waters carry natural salts,
they have to be treated to prevent scale forming.

The three main reasons for water treatment are :

Prevention of Corrosion in feed boiler, steam and condensate systems.

Elimination of Scale.

Economic boiler operation without carryover.

Corrosion will reduce metal thickness of tubes or shell. Result : pressure must be reduced and finally boiler condemned.

Scale reduces the heat flow from fire side to water. Result : high fire temperatures are needed to be maintained.

Basic Chemistry of the Effect of Impurities in the Boiler. If we could use water completely free from all impurities, there would be no need
for water treatment.

IMPURITY EFFECT ON A BOILER

1. Dissolved gases Corrosion

These salts are the 'hardness in the
2. Calcium salts and magnesium
boiler.
salts
Some salts can also cause corrosion
3. Silica Can form a very hard scale.
Contribute to, or cause, carryover
4. Suspended solids and dissolved
entrainment of a relatively small quantity of
solids
boiler water solids with the steam
 IMPURITY RESULTING IN GOT RID OF BY COMMENTS
Soluble Gases
Water smells like rotten Found mainly in
Hydrogen Sulphide (H2S) eggs: Tastes bad, and is Aeration groundwater, and polluted
corrosive to most metals. streams.
Filming, neutralizing
Corrosive, forms carbonic Deaeration, neutralization
Carbon dioxide (CO2) amines used to prevent
acid in condensate. with alkalis.
condensate line corrosion.
Pitting of boiler tubes, and
Deaeration & chemical
Corrosion and pitting of turbine blades, failure of
Oxygen(O2) treatment with (Sodium
boiler tubes. steam lines, and fittings
Sulphite or Hydrazine)
etc.
Suspended solids
Tolerance of approx.
Sludge and scale Clarification and 5ppm max. for most
Sediment & Turbidity
carryover. filtration. applications, 10ppm for
potable water.
 Found mostly in surface waters, caused by
rotting vegetation, and farm run offs.
Organics break down to form organic acids.
Results in lowering of boiler feed-water
pH, which then attacks boiler tubes.
Includes diatoms, molds, bacterial slimes,
iron/manganese bacteria. Suspended
Carryover, foaming, particles collect on the surface of the water
Clarification;
deposits can clog in the boiler and render difficulty in the
Organic Matter filtration, and
piping, and cause liberation of steam bubbles rising to that
chemical treatment
corrosion. surface.. Foaming can also be attributed to
waters containing carbonates in solution in
which a light flocculent precipitate will be
formed on the surface of the water. It is
usually traced to an excess of sodium
carbonate used in treatment for some other
difficulty where animal or vegetable oil
finds its way into the boiler.
Dissolved Colloidal
Solids
Foaming, deposits in
Oil & Grease Coagulation & filtration Enters boiler with condensate
boiler
Scale deposits in boiler, Forms are bicarbonates, sulphates,
inhibits heat transfer, chlorides, and nitrates, in that
Hardness,
and thermal efficiency. Softening, plus internal order. Some calcium salts are
Calcium(Ca), and
In severe cases can lead treatment in boiler. reversibly soluble. Magnesium
Magnesium (Mg)
to boiler tube burn reacts with carbonates to form
through, and failure. compounds of low solubility.
Foaming, carbonates
Deaeration of make-up
form carbonic acid in
water and condensate Sodium salts are found in most
Sodium, alkalinity, steam, causes
return. Ion exchange; waters. They are very soluble, and
NaOH, NaHCO3, condensate return line,
deionization, acid cannot be removed by chemical
Na2CO3 and steam trap
treatment of make-up precipitation.
corrosion, can cause
water.
embrittlement.
Hard scale if calcium is Tolerance limits are about 100-
Sulphates (SO4) Deionization
present 300ppm as CaCO3
 Priming, or the passage of
steam from a boiler in
Priming, i.e. uneven delivery
"belches", is caused by the
of steam from the boiler
concentration sodium
(belching), carryover of water
carbonate, sodium sulphate,
in steam lowering steam
Chlorides, (Cl) Deionization or sodium chloride in
efficiency, can deposit as salts
solution. Sodium sulphate is
on superheaters and turbine
found in many waters in the
blades. Foaming if present in
USA, and in waters where
large amounts.
calcium or magnesium is
precipitated with soda ash.

Deposits in boiler, in large

Iron (Fe) and Aeration, filtration, ion Most common form is ferrous
amounts can inhibit heat
Manganese (Mn) exchange. bicarbonate.
transfer.
Silica combines with many
elements to produce silicates.
Silicates form very tenacious
Hard scale in boilers and Deionization; lime soda deposits in boiler tubing. Very
Silica (Si) cooling systems: turbine process, hot-lime-zeolite difficult to remove, often only
blade deposits. treatment. by flourodic acids. Most
critical consideration is
volatile carryover to turbine
components.
 Conventional Surface Water
Treatment
Raw water
Screening Filtration
sludge sludge
Alum
Coagulation Cl2 Disinfection
Polymers

Flocculation Storage

Sedimentation Distribution
sludge
Floc floating at the surface of a basin
Mechanical system to push floc out of the water basin
Screening
Screening removes large solids, logs, branches, rags,
fish etc. Screening may incorporate a mechanized trash
removal system. Screening protects pumps and pipes in
WTP.

Chemical Coagulation-Flocculation
Removes suspended particulate and colloidal
substances from water, including microorganisms.
Coagulation is colloidal destabilization. Typically, alum
(aluminum sulfate) or ferric chloride or sulfate is added
to the water with rapid mixing and controlled pH
conditions. Insoluble aluminum or ferric hydroxide and
aluminum or iron hydroxo complexes form. These
complexes entrap and adsorb suspended particulate and
colloidal material.

Flocculation
Slow mixing (flocculation) that provides for a period of
time to promote the aggregation and growth of the
insoluble particles (flocs). The particles collide, stick
together and grow larger. The resulting large floc
particles are subsequently removed by gravity
sedimentation (or direct filtration).

Sedimentation is the oldest form of water treatment, it

uses gravity to separate particles from water often
follows coagulation and flocculation.

Smaller floc particles are too small to settle and are

removed by filtration.
Typical Surface Water Treatment Plant
Flocculation
Sludge formed by flocculation
Clean water from a clarifier
Suspended matter in raw water supplies is removed by various methods to provide water suitable for domestic purposes
and most industrial requirements. The suspended matter can consist of large solids, settleable by gravity alone
without any external aids, and non-settleable material, often colloidal in nature. Removal is generally accomplished
by coagulation, flocculation, and sedimentation. The combination of these three processes is referred to as
conventional clarification.

Coagulation is the process of destabilization by charge neutralization. Once neutralized, particles no longer repel each
other and can be brought together. Coagulation is necessary for the removal of the colloidal-sized suspended matter.

Flocculation is the process of bringing together the destabilized, or "coagulated," particles to form a larger
agglomeration, or "floc.“

Sedimentation refers to the physical removal from suspension, or settling, that occurs once the particles have been
coagulated and flocculated. Sedimentation or subsidence alone, without prior coagulation, results in the removal of only
relatively coarse suspended solids.

Steps of Clarification
Finely divided particles suspended in surface water repel each other because most of the surfaces are negatively charged.
The following steps in clarification are necessary for particle agglomeration:
Coagulation. Coagulation can be accomplished through the addition of inorganic salts of aluminum or iron. These
inorganic salts neutralize the charge on the particles causing raw water turbidity, and also hydrolyze to form insoluble
precipitates, which entrap particles. Coagulation can also be effected by the addition of water-soluble organic polymers
with numerous ionized sites for particle charge neutralization.
Flocculation. Flocculation, the agglomeration of destabilized particles into large particles, can be enhanced by the
addition of high-molecular-weight, water-soluble organic polymers. These polymers increase floc size by charged site
binding and by molecular bridging.
Therefore, coagulation involves neutralizing charged particles to destabilize suspended solids. In most clarification
processes, a flocculation step then follows. Flocculation starts when neutralized or entrapped particles begin to collide
and fuse to form larger particles. This process can occur naturally or can be enhanced by the addition of polymeric
flocculant aids.
Inorganic Coagulants
Typical iron and aluminum coagulants are acid salts that lower the pH of the treated water by hydrolysis. Depending on
initial raw water alkalinity and pH, an alkali such as lime or caustic must be added to counteract the pH depression of the
primary coagulant. Iron and aluminum hydrolysis products play a significant role in the coagulation process, especially
in cases of low-turbidity influent waters.
Common inorganic coagulants
Typical Forms
Typical Typical Used in Water
Name Formula Strength Treatment Density Typical Uses
Aluminum Al2(SO4)3 17% Al2O3 lump, granular, 60-70 lb/ft3 primary coagulant
sulfate · or powder
14 to 18
H2O
Alum 8.25% Al2O3 liquid 11.1 lb/gal
Aluminum AlCl3 · 35% AlCl3 liquid 12.5 lb/gal primary coagulant
chloride 6H2O
Ferric Fe2(SO4)3 · 68% granular 70-72 lb/ft3 primary coagulant
sulfate 9H2O Fe2(SO4)3
Ferric-floc Fe2(SO4)3 · 41% solution 12.3 lb/gal primary coagulant
9H2O Fe2(SO4)3
Ferric FeCl3 60% FeCl3, crystal, solution 60-64 lb/ft3 primary coagulant
chloride 35-45% 11.2-12.4
FeCl3 lb/gal
Sodium Na2Al2O4 38-46% liquid 12.3-12.9 primary coagulant;
aluminate Na2Al2O4 lb/gal cold/hot
precipitation
softening
Polyelectrolytes
The term polyelectrolytes refers to all water-soluble organic polymers used for clarification, whether they function as
coagulants or flocculants.

Water-soluble polymers may be classified as follows:

•anionic-ionize in water solution to form negatively charged sites along the polymer chain
•cationic-ionize in water solution to form positively charged sites along the polymer chain
•nonionic-ionize in water solution to form very slight negatively charged sites along the polymer chain

Polymeric primary coagulants are cationic materials with relatively low molecular weights (under 500,000). The
cationic charge density (available positively charged sites) is very high. Polymeric flocculants or coagulant aids may be
anionic, cationic, or nonionic. Their molecular weights may be as high as 50,000,000.

Because suspensions are normally non-uniform, specific testing is necessary to find the coagulants and flocculants with
the broadest range of performance.

Primary Coagulant Polyelectrolytes

The cationic polyelectrolytes commonly used as primary coagulants are polyamines. They exhibit strong cationic
ionization and typically have molecular weights of less than 500,000. When used as primary coagulants, they adsorb on
particle surfaces, reducing the repelling negative charges. These polymers may also bridge, to some extent, from one
particle to another but are not particularly effective flocculants. The use of polyelectrolytes permits water clarification
without the precipitation of additional hydroxide solids formed by inorganic coagulants. The pH of the treated water is
unaffected.

The efficiency of primary coagulant poly-electrolytes depends greatly on the nature of the turbidity particles to be
coagulated, the amount of turbidity present, and the mixing or reaction energies available during coagulation. With
lower influent turbidities, more turbulence or mixing is required to achieve maximum charge neutralization.
In low-turbidity waters where it is desirable to avoid using an inorganic coagulant, artificial turbidity can be added to
build floc. Bentonite clay is used to increase surface area for adsorption and entrapment of finely divided turbidity. A
polymeric coagulant is then added to complete the coagulation process.

The use of organic polymers offers several advantages over the use of inorganic coagulants:

The amount of sludge produced during clarification can be reduced by 50-90%. The approximate dry weight of solids
removed per pound of dry alum and ferric sulfate are approximately 0.25 and 0.5 lb, respectively.

The resulting sludge contains less chemically bound water and can be more easily dewatered.

Polymeric coagulants do not affect pH. Therefore, the need for supplemental alkalinity, such as lime, caustic, or soda
ash, is reduced or eliminated.

Polymeric coagulants do not add to the total dissolved solids concentration. For example, 1 ppm of alum adds 0.45 ppm
of sulfate ion (expressed as CaCO3). The reduction in sulfate can significantly extend the capacity of anion exchange
systems.

Soluble iron or aluminum carryover in the clarifier effluent may result from inorganic coagulant use. Therefore,
elimination of the inorganic coagulant can minimize the deposition of these metals in filters, ion exchange units, and
cooling systems.
Variation in pH affects particle surface charge and floc precipitation during coagulation. Iron and aluminum hydroxide
flocs are best precipitated at pH levels that minimize the coagulant solubility. However, the best clarification
performance may not always coincide with the optimum pH for hydroxide floc formation. Also, the iron and aluminum
hydroxide flocs increase volume requirements for the disposal of settled sludge.

With aluminum sulfate, optimum coagulation efficiency and minimum floc solubility normally occur at pH 6.0 to 7.0.
Iron coagulants can be used successfully over the much broader pH range of 5.0 to 11.0. If ferrous compounds are used,
oxidation to ferric iron is needed for complete precipitation. This may require either chlorine addition or pH adjustment.
The chemical reactions between the water's alkalinity (natural or supplemented) and aluminum or iron result in the
formation of the hydroxide coagulant as in the following:

Al2(SO4)3 + 6NaHCO3 = 2Al(OH)3 + 3Na2SO4 + 6CO2

aluminum sodium aluminum sodium carbon
sulfate bicarbonate hydroxide sulfate dioxide

Fe2(SO4)3 + 6NaHCO3 = 2Fe(OH)3 + 3Na2SO4 + 6CO2

sodium sodium carbon
ferric sulfate ferric hydroxide
bicarbonate sulfate dioxide

Na2Al2O4 + 4H2O = 2Al(OH)3 + 2NaOH

sodium aluminum sodium
water
aluminate hydroxide hydroxide
Coagulant Aids (Flocculants)
In certain instances, an excess of primary coagulant (whether inorganic, polymeric, or a combination of both) may be fed
to promote large floc size and to increase settling rate. However, in some waters, even high doses of primary coagulant
will not produce the desired effluent clarity. A polymeric coagulant aid added after the primary coagulant may, by
developing a larger floc at low treatment levels, reduce the amount of primary coagulant required.

Generally, very high-molecular-weight, anionic polyacrylamides are the most effective coagulant aids. Nonionic or
cationic types have proven successful in some clarifier systems. Essentially, the polymer bridges the small floc particles
and causes them to agglomerate rapidly into larger, more cohesive flocs that settle quickly. The higher-molecular-weight
polymers bridge suspended solids most effectively.
Coagulant aids have proven quite successful in precipitation softening and clarification to achieve improved settling rates
of precipitates and finished water clarity.

Color Reduction
Frequently, the objective of clarification is the reduction of color. Swamps and wetlands introduce color into surface
waters, particularly after heavy rain-falls. Color-causing materials can cause various problems, such as objectionable
taste, increased microbiological content, fouling of anion exchange resins, and interference with coagulation and
stabilization of silt, soluble iron, and manganese.

Most organic color in surface waters is colloidal and negatively charged. Colour can be removed by chlorination and
coagulation with aluminum or iron salts or organic polyelectrolytes. Chlorine oxidizes color compounds, while the
inorganic coagulants can physically remove many types of organic color by neutralization of surface charges. The use of
chlorine to oxidize organic color bodies may be limited due to the production of chlorinated organic by-products, such as
trihalomethanes. Additional color removal is achieved by chemical interaction with aluminum or iron hydrolysis
products. Highly charged cationic organic polyelectrolytes can also be used to coagulate some types of color particles.

Coagulation for color reduction is normally carried out at pH 4.5 to 5.5. Optimum pH for turbidity removal is usually
much higher than that for color reduction. The presence of sulfate ions can interfere with coagulation for color reduction,
whereas calcium and magnesium ions can improve the process and broaden the pH range in which color may be reduced
effectively.
Conventional Clarification Equipment
The coagulation/flocculation and sedimentation process requires three distinct unit processes:
•high shear, rapid mix for coagulation
•low shear, high retention time, moderate mixing for flocculation
•liquid and solids separation

Horizontal Flow Clarifiers

Originally, conventional clarification units consisted of large, rectangular, concrete basins divided into two or three
sections. Each stage of the clarification process occurred in a single section of the basin. Water movement was horizontal
with plug flow through these systems.
Because the design is suited to large-capacity basins, horizontal flow units are still used in some large industrial plants
and for clarifying municipal water. The retention time is normally long (up to 4-6 hr), and is chiefly devoted to settling.
Rapid mix is typically designed for 3-5 min and slow mix for 15-30 min. This design affords great flexibility in
establishing proper chemical addition points. Also, such units are relatively insensitive to sudden changes in water
throughput.
The long retention also allows sufficient reaction time to make necessary adjustments in chemical and polymer feed if
raw water conditions suddenly change. However, for all but very large treated water demands, horizontal units require
high construction costs and more land space per unit of water capacity.

Upflow Clarifiers
Compact and relatively economical, upflow clarifiers provide coagulation, flocculation, and sedimentation in a single
(usually circular) steel or concrete tank. These clarifiers are termed "upflow" because the water flows up toward the
effluent launders as the suspended solids settle. They are characterized by increased solids contact through internal
sludge recirculation. This is a key feature in maintaining a high-clarity effluent and a major difference from horizontal
clarifiers.
Because retention time in an upflow unit is approximately 1-2 hr, upflow basins can be much smaller in size than
horizontal basins of equal throughput capacity. A rise rate of 0.70-1.25 gpm/ft² of surface area is normal for clarification.
Combination softening-clarification units may operate at up to 1.5 gpm/ft² of surface area due to particle size and
densities of precipitated hardness.
Coagulation, Flocculation and Sedimentation

Raw water enters the treatment plants from the terminal reservoirs and first goes through the clarifiers where sediment
and other particulate matter are removed by processes known as coagulation, flocculation and sedimentation.
Coagulation occurs with the help of chemicals known as coagulants that use their positive charges to attract negatively
charged particles like sediment and organic matter in raw water. Flocculation then occurs as these particles clump
together and form flocs, or clumps of sediment and particulate matter. These flocs are then allowed to settle to the bottom
of the clarifier by sedimentation where rake-like structures slowly scoop the solid matter out. Clear, sediment-free water
leaves the clarifier from the top and flows onto the next step in the treatment process. Currently, both treatment plants use
a chemical called aluminum sulfate or "alum" as a coagulant. The flocculation process coagulates (joins together)
particles with alum and metal salts so that they settle out of the water as sediment. Sedimentation is simply a gravity
process that removes flocculated particles from the water.

Flocculation
Flocculation is a process which clarifies the water. Clarifying means removing any turbidity or colour so that the water is
clear and colourless. Clarification is done by causing a precipitate to form in the water which can be removed using
simple physical methods. Initially the precipitate forms as very small particles but as the water is gently stirred, these
particles stick together to form bigger particles - this process is sometimes called flocculation. Many of the small
particles that were originally present in the raw water absorb onto the surface of these small precipitate particles and so
get incorporated into the larger particles that coagulation produces. In this way the coagulated precipitate takes most of
the suspended matter out of the water and is then filtered off, generally by passing the mixture through a coarse sand
filter or sometimes through a mixture of sand and granulated anthracite (high carbon and low volatiles coal). Coagulants
/ flocculating agents that may be used include:

Iron (III) hydroxide. This is formed by adding a solution of an iron (III) compound such as iron(III) chloride to pre-
treated water with a pH of 7 or greater. Iron (III) hydroxide is extremely insoluble and forms even at a pH as low as 7.

Aluminium hydroxide is also widely used as the flocculating precipitate although there have been concerns about
possible health impacts.
Flocculants
Particles finer than 0.1 µm (10-7m) in water remain continuously in motion due to electrostatic charge (often negative)
which causes them to repel each other. Once their electrostatic charge is neutralized by the use of coagulant chemical, the
finer particles start to collide and agglomerate (combine together) under the influence of Van der Waals’ s forces. These
larger and heavier particles are called flocs.

Flocculants, or flocculating agents, are chemicals that promote flocculation by causing colloids and other suspended
particles in liquids to aggregate, forming a floc. Flocculants are used in water treatment processes to improve the
sedimentation or filterability of small particles.

Many flocculants are multivalent cations such as aluminium, iron, calcium or magnesium. These positively charged
molecules interact with negatively charged particles and molecules to reduce the barriers to aggregation. In addition,
many of these chemicals, under appropriate pH and other conditions such as temperature and salinity, react with water to
form insoluble hydroxides which, upon precipitating, link together to form long chains or meshes, physically trapping
small particles into the larger floc.

Long-chain polymer flocculants, such as modified polyacrylamides, are manufactured and sold by the flocculant
producing business. These can be supplied in dry or liquid form for use in the flocculation process. Some commonly
used flocculants include alum, aluminium chlorohydrate, aluminium sulfate, calcium oxide, calcium hydroxide, iron(II)
sulfate, iron (III) chloride, polyacrylamide, sodium aluminate, sodium silicate etc.
1. Dissolved Gases :
The two gases which cause corrosion are oxygen and carbon dioxide. The carbon dioxide does so simply by dissolving in the water and
forming a weak carbonic acid which attacks the metal in feed systems, boiler or condensate system. Oxygen is present in all waters, so that
red iron oxide forms on a mild steel surface immersed in water. This rusting or, as we call it, corrosion triunes until the metal is corroded
away. If the amount of oxygen in the water is restricted, the oxide film does not form so readily; but instead, the surface of the steel
tarnishes. This tarnish is usually the development of a thin film of iron oxide on the metal surface which is not so fully oxidized as the red
iron oxide, and is more dense, thus tending to resist further corrosive attack. In water of increasing alkalinity, the oxide film becomes more
stable and gives more protection to the steel, but until a definite alkalinity is reached, it still tends to break down in selective areas, where
pits will develop.

2. Calcium and magnesium salts :

There are two forms of hardness; temporary and permanent. Temporary hardness is due bicarbonates of calcium and magnesium which
break down to carbonates when the water is boiled. In the boiler the following chemical reaction takes place :

Calcium Bicarbonate + heat → Calcium Carbonate + carbon dioxide + water

Calcium and magnesium bicarbonate are soluble in water but the carbonates are insoluble and therefore precipitate as a fine white
powder. This precipitate will bake unto the heating surface of a boiler and form a scale. Permanent hardness is due to calcium and
magnesium sulphates, chlorides and nitrates, and these salts cannot be removed by boiling. However, under boiler conditions (resulting in
successive concentrations of these hardness salts) the solubility of these salts is soon exceeded and they deposit on the hottest part of the
heating surface. The salts of magnesium that form permanent hardness sometimes tend to cause corrosion instead of hard scale formation,
e.g. magnesium chloride in an untreated boiler hydrolyses to form corrosive hydrochloric acid.

3. Silica :
Silica forms scale in a similar way to the permanent hardness salts. When the scale formed is a mixture of silica, calcium and magnesium
salts, it is very hard and therefore presents a difficult problem at inspection time.

4. The suspended and dissolved solids :

The suspended and dissolved solids cause foaming by becoming absorbed unto the walls of individual bubbles so that small bubbles,
instead of coalescing to form large ones and bursting early, repel one another and build up a large volume of small bubbles. If these
bubbles burst near the steam outlet, the spray is taken over with the steam. If the bubbles do not burst high in the steam space, the foam
can be drawn over with the steam.
These "hardness ions" cause two major kinds of problems. First, the metal cations react with soaps, causing them to
form an unsightly precipitate. More seriously, the calcium and magnesium carbonates tend to precipitate out as
adherent solids on the surfaces of pipes and especially on the hot heat exchanger surfaces of boilers. The resulting
scale buildup can impede water flow in pipes. In boilers, the deposits act as thermal insulation that impedes the flow of
heat into the water; this not only reduces heating efficiency, but allows the metal to overheat, which in pressurized
systems can lead to catastrophic failure.

Types of water hardness

Temporary hardness
This refers to hardness whose effects can be removed by boiling the water in an open container. Such waters have
usually percolated though limestone formations and contain bicarbonate HCO3– along with small amounts of
carbonate CO32– as the principal negative ions. Boiling the water promotes the reaction
2 HCO3– → CO32– + CO2
by driving off the carbon dioxide gas. The CO32– reacts with Ca2+ or Mg2+ ions, to form insoluble calcium and
magnesium carbonates which precipitate out. By tying up the metal ions in this way, the amounts available to form
soap scum are greatly reduced.
Permanent hardness
Waters than contain other anions such as chloride or sulfate cannot be remediated by boiling, and are said to be
"permanently" hard.
LIME-SODA ASH WATER TREATMENT METHOD

Lime-soda ash treatment for the reduction of hardness involves the addition of slaked lime [Ca(OH)2] to a hard water
supply to remove the carbonate hardness by precipitation with the precipitation being removed by filtration. Non-
carbonate hardness is in turn reduced by the addition of soda ash (Na2CO3) to form insoluble precipitate which is also
removed by filtration.

This particular method of removing hardness a sometimes used by municipal water plants to reduce the amount of
calcium and magnesium in a water supply. While it is quite effective in reducing hardness, it is not a complete removal
treatment.

Lime-soda ash treatment is especially effective if a water contains bicarbonate (temporary) hardness. Where calcium and
magnesium are primarily in chloride or sulfate compounds, this treatment is noticeably less effective. Sodium ions get
into the water.

Slaked lime is used to remove calcium bicarbonate from water. In the water to be treated, the slaked lime ions react with
the calcium bicarbonate to form the very slightly soluble calcium carbonate. This precipitated material is usually removed
by first settling and then filtering.
Ca(OH) 2+ Ca(HCO3) 2 → 2 CaCO3 ↓ + 2 H20
Calcium hydroxide plus calcium bicarbonate reacts to form calcium carbonate plus water

NOTE: The arrow pointing down (↓) indicates the formation of an insoluble compound.

To remove the magnesium, additional lime is used. The reaction for this process is:
Ca(OH) 2 + Mg ++ → Mg(OH)2 ↓+ Ca++
Calcium hydroxide plus magnesium ions react to form magnesium hydroxide plus calcium ions
This step has simply replaced the magnesium with calcium. If soda ash is then fed into the water, the calcium will
precipitate as calcium carbonate:
Ca++ + Na 2CO3 → CaCO3 ↓ + Na+
Calcium ions plus sodium carbonate react to form calcium carbonate plus sodium ions.
FUNDAMENTALS OF WATER SOFTENING BY ION EXCHANGE

Water that contains calcium and magnesium ions is called "hard water" because calcium and magnesium can combine
with other ions and compounds to leave a hard scale on the surfaces they touch. An ion exchange water softener can
reduce or eliminate hardness problems.

A typical water softener has a pressure tank partially filled with ion exchange resin which consists of highly porous,
amber colored, plastic beads loaded with "exchange sites" that preferentially remove hardness ions and replace them
with sodium, a "soft" ion. A softener system also includes a brine tank to provide a source of sodium for regenerating
the resin and hydraulic controls to direct the flow of water through the softener during service and regeneration.

At the beginning of the softening cycle, sodium ions occupy the resin's exchange sites. As water passes through it, the
resin's stronger attraction for the hardness ions causes it to take on the hardness ions and give up its sodium ions. Iron
and manganese are considered hardness and they are removed also, provided they are in solution. Ion exchange
cannot remove suspended matter.
As water flows downward through the resin bed, the resin at the top of the bed gives up its sodium first. The exchange
process is not instantaneous, so exchange occurs in a band called a "reaction zone". The reaction zone's depth depends
on incoming water hardness and TDS, flow rate, water temperature and resin particle size. When the reaction zone's
leading edge reaches the bottom of the resin bed and hardness passed into the service line, the resin has become
"exhausted" and it must be regenerated before it can remove hardness again.

The regeneration cycle starts with backwash, an upward flow that loosens the resin bed and flushes out suspended
particles. Backwash usually lasts about 10 minutes.

Regeneration occurs when a solution of sodium chloride (salt) brine is passed through the resin in a downward
direction. An eductor draws concentrated brine from a storage tank and dilutes it to the right concentration. Brine
draws lasts from 10 to 30 minutes depending on salt dosage (weight of salt per volume of resin). A large excess of
sodium ions causes the resin to release its hold on hardness ions picked up during the preceding service cycle and
returns the resin to its sodium state.

The brining step is followed by a slow downflow rinse to displace spent brine from the resin. It also carries the
hardness removed from the resin to drain. The rinse rate is regulated to ensure correct contact time between the salt
and the resin. Slow rinse usually lasts about 30 minutes.

A final fast downflow rinse, or purge, flushes all remaining brine from the tank. It lasts about 5 minutes.
Ion Exchange Reactions
Ion exchange is a reversible chemical reaction wherein an ion (an atom or molecule that has lost or gained an electron
and thus acquired an electrical charge) from solution is exchanged for a similarly charged ion attached to an immobile
solid particle. These solid ion exchange particles are either naturally occurring inorganic zeolites or synthetically
produced organic resins. The synthetic organic resins are the predominant type used today because their characteristics
can be tailored to specific applications.

An organic ion exchange resin is composed of high-molecular-weight polyelectrolytes that can exchange their mobile
ions for ions of similar charge from the surrounding medium. Each resin has a distinct number of mobile ion sites that set
the maximum quantity of exchanges per unit of resin.

In a water deionization process, the resins exchange hydrogen ions (H+) for the positively charged ions (such as nickel.
copper, and sodium). and hydroxyl ions (OH-) for negatively charged sulfates, chromates and chlorides. Because the
quantity of H+ and OH ions is balanced, the result of the ion exchange treatment is relatively pure, neutral water.

Ion exchange reactions are stoichiometric and reversible, and in that way they are similar to other solution phase
reactions. For example:
NiSO4 +Ca(OH)2 = Ni(OH)2 + CaSO4

In this reaction, the nickel ions of the nickel sulfate (NiSO4) are exchanged for the calcium ions of the calcium
hydroxide [Ca(OH)2] molecule. Similarly, a resin with hydrogen ions available for exchange will exchange those ions
for nickel ions from solution. The reaction can be written as follows:

2(R-SO3H)+ NiSO4 = (R-SO3)2Ni+ H2SO4

R indicates the organic portion of the resin and SO3 is the immobile portion of the ion active group. Two resin sites are
needed for nickel ions with a plus 2 valence (Ni +2). Trivalent ferric ions would require three resin sites.

As shown, the ion exchange reaction is reversible. The degree the reaction proceeds to the right will depend on the
resins preference or selectivity, for nickel ions compared with its preference for hydrogen ions.
The resin can be converted back to the hydrogen form by contact with a concentrated solution of sulfuric acid (H 2SO4) :

(R—SO3)2Ni + H2SO4 → 2(R-SO3H) + NiSO4

This step is known as regeneration. In general terms, the higher the preference a resin exhibits for a particular ion, the
greater the exchange efficiency in terms of resin capacity for removal of that ion from solution. Greater preference for a
particular ion, however, will result in increased consumption of chemicals for regeneration.

Resin Types
Ion exchange resins are classified as cation exchangers, which have positively charged mobile ions available for exchange,
and anion exchangers, whose exchangeable ions are negatively charged. Both anion and cation resins are produced from
the same basic organic polymers. They differ in the ionizable group attached to the hydrocarbon network. It is this
functional group that determines the chemical behavior of the resin. Resins can be broadly classified as strong or weak acid
cation exchangers or strong or weak base anion exchangers.

Strong Acid Cation Resins. Strong acid resins are so named because their chemical behavior is similar to that of a strong
acid. The resins are highly ionized in both the acid (R-SO3H) and salt (R-SO3Na) form. They can convert a metal salt to the
corresponding acid by the reaction:

2(R-SO3H)+ NiCl2 → (R-SO3)2Ni+ 2HCl

The hydrogen and sodium forms of strong acid resins are highly dissociated and the exchangeable Na + and H+ are readily
available for exchange over the entire pH range. Consequently, the exchange capacity of strong acid resins is independent
of solution pH. These resins would be used in the hydrogen form for complete deionization; they are used in the sodium
form for water softening (calcium and magnesium removal). After exhaustion, the resin is converted back to the hydrogen
form (regenerated) by contact with a strong acid solution, or the resin can be convened to the sodium form with a sodium
chloride solution. For Equation 5. hydrochloric acid (HCl) regeneration would result in a concentrated nickel chloride
(NiCl2) solution.
Weak Acid Cation Resins.
In a weak acid resin. the ionizable group is a carboxylic acid (COOH) as opposed to the sulfonic acid group (SO 3H) used in
strong acid resins. These resins behave similarly to weak organic acids that are weakly dissociated.

Weak acid resins exhibit a much higher affinity for hydrogen ions than do strong acid resins. This characteristic allows for
regeneration to the hydrogen form with significantly less acid than is required for strong acid resins. Almost complete
regeneration can be accomplished with stoichiometric amounts of acid. The degree of dissociation of a weak acid resin is
strongly influenced by the solution pH. Consequently, resin capacity depends in part on solution pH. A typical weak acid
resin has limited capacity below a pH of 6.0 making it unsuitable for deionizing acidic metal finishing wastewater.
Strong Base Anion Resins.
Like strong acid resins. strong base resins are highly ionized and can be used over the entire pH range. These resins are
used in the hydroxide (OH) form for water deionization. They will react with anions in solution and can convert an acid
solution to pure water:
R--NH3OH+ HCl → R-NH3Cl + HOH

Regeneration with concentrated sodium hydroxide (NaOH) converts the exhausted resin to the hydroxide form.

Weak Base Anion Resins.

Weak base resins are like weak acid resins. in that the degree of ionization is strongly influenced by pH. Consequently,
weak base resins exhibit minimum exchange capacity above a pH of 7.0. These resins merely sorb strong acids: they
cannot split salts.

In an ion exchange water deionization unit. the water would pass first through a bed of strong acid resin. Replacement of
the metal cations (Ni+2. Cu+2) With hydrogen ions would lower the solution pH. The anions (SO4-2. Cl-) can then be
removed with a weak base resin because the entering water will normally be acidic and weak base resins sorb acids. Weak
base resins are preferred over strong base resins because they require less regenerant chemical. A reaction between the
resin in the free base form and HCl would proceed as follows:
R-NH2 + HCl → R-NH3Cl
The weak base resin does not have a hydroxide ion form as does the strong base resin. Consequently. regeneration needs
only to neutralize the absorbed acid: it need not provide hydroxide ions. Less expensive weakly basic reagents such as
ammonia (NH3) or sodium carbonate can be employed.
Batch and Column Exchange Systems

Ion exchange processing can be accomplished by

either a batch method or a column method. In the first
method, the resin and solution are mixed in a batch
tank, the exchange is allowed to come to equilibrium,
and then the resin is separated from solution. The
degree to which the exchange takes place is limited
by the preference the resin exhibits for the ion in
solution. Consequently, the use of the resins exchange
capacity will be limited unless the selectivity for the
ion in solution is far greater than for the exchangeable
ion attached to the resin. Because batch regeneration
of the resin is chemically inefficient, batch processing
by ion exchange has limited potential for application.

Passing a solution through a column containing a bed

of exchange resin is analogous to treating the solution
in an infinite series of batch tanks.
Ion Exchange Process Equipment and Operation
Most industrial applications of ion exchange use fixed-bed column systems, the basic component of which is the resin
column. The column design must:
•Contain and support the ion exchange resin
•Uniformly distribute the service and regeneration flow through the resin bed
•Provide space to fluidize the resin during backwash
•Include the piping, valves, and instruments needed to regulate flow of feed, regenerant. and backwash solutions

Regeneration Procedure. After the feed solution is processed to the extent that the resin becomes exhausted and cannot
accomplish any further ion exchange, the resin must be regenerated. In normal column operation, for a cation system
being converted first to the hydrogen then to the sodium form, regeneration employs the following basic steps:

1. The column is backwashed to remove suspended solids collected by the bed during the service cycle and to eliminate
channels that may have formed during this cycle. The back- wash flow fluidizes the bed. releases trapped particles.
and reorients the resin particles according to size.
During backwash the larger, denser particles will accumulate at the base and the particle size will decrease moving up
the column. This distribution yields a good hydraulic flow pattern and resistance to fouling by suspended solids.

2. The resin bed is brought in contact with the regenerant solution. In the case of the cation resin. acid elutes the collected
ions and converts the bed to the hydrogen form. A slow water rinse then removes any residual acid.

3. The bed is brought in contact with a sodium hydroxide solution to convert the resin to the sodium form. Again, a slow
water rinse is used to remove residual caustic. The slow rinse pushes the last of the regenerant through the column.

4. The resin bed is subjected to a fast rinse that removes the last traces of the regenerant solution and ensures good flow
characteristics.

5. The column is returned to service.

Aeration Basins
Aeration is a unit process in which air and water are brought into intimate contact. Turbulence increases the aeration of
flowing streams In industrial processes, water flow is usually directed countercurrent to atmospheric or forced-draft air
flow. The contact time and the ratio of air to water must be sufficient for effective removal of the unwanted gas.
Aeration as a water treatment practice is used for the following operations:
•carbon dioxide reduction (decarbonation)
•oxidation of iron and manganese found in many well waters (oxidation tower)
•ammonia and hydrogen sulfide reduction (stripping)
Aeration is also an effective method of bacteria control.

Methods of aeration
Two general methods may be used for the aeration of water. The most common in industrial use is the water-fall aerator.
Through the use of spray nozzles, the water is broken up into small droplets or a thin film to enhance countercurrent air
contact.
In the air diffusion method of aeration, air is diffused into a receiving vessel containing counter-current flowing water,
creating very small air bubbles. This ensures good air-water contact for "scrubbing" of undesirable gases from the water.
Water-Fall Aerators
Many variations of the water-fall principle are used for this type of aeration. The simplest configuration employs a
vertical riser that discharges water by free fall into a basin. The riser usually operates on the available head of water. The
efficiency of aeration is improved as the fall distance is increased. Also, steps or shelves may be added to break up the
fall and spread the water into thin sheets or films, which increases contact time and aeration efficiency.
Coke tray aerators are widely used in iron and manganese oxidation because a catalytic effect is secured by contact of the
iron/manganese-bearing water with fresh precipitates. These units consist of a series of coke-filled trays through which
the water percolates, with additional aeration obtained during the free fall from one tray to the next.
Wood or plastic slat tray aerators are similar to small atmospheric cooling towers. The tray slats are staggered to break up
the free fall of the water and create thin films before the water finally drops into the basin.
Forced draft water-fall aerators are used for many industrial water conditioning purposes. Horizontal wood or plastic slat
trays, or towers filled with packing of various shapes and materials, are designed to maximize disruption of the falling
water into small streams for greater air-water contact. Air is forced through the unit by a blower which produces uniform
air distribution across the entire cross section, cross current or countercurrent to the fall of the water. Because of these
features, forced draft aerators are more efficient for gas removal and require less space for a given capacity.
Air Diffusion Aerators
Air diffusion systems aerate by pumping air into water through perforated pipes, strainers, porous plates, or tubes.
Aeration by diffusion is theoretically superior to water-fall aeration because a fine bubble of air rising through water is
continually exposed to fresh liquid surfaces, providing maximum water surface per unit volume of air. Also, the velocity
of bubbles ascending through the water is much lower than the velocity of free-falling drops of water, providing a longer
contact time. Greatest efficiency is achieved when water flow is countercurrent to the rising air bubbles.

Applications
In industrial water conditioning, one of the major objectives of aeration is to remove carbon dioxide. Aeration is also
used to oxidize soluble iron and manganese (found in many well waters) to insoluble precipitates. Aeration is often used
to reduce the carbon dioxide liberated by a treatment process. For example, acid may be fed to the effluent of sodium
zeolite softeners for boiler alkalinity control. Carbon dioxide is produced as a result of the acid treatment, and aeration is
employed to rid the water of this corrosive gas. Similarly, when the effluents of hydrogen and sodium zeolite units are
blended, the carbon dioxide formed is removed by aeration.

In the case of cold lime softening, carbon dioxide may be removed from the water before the water enters the equipment.
When carbon dioxide removal is the only objective, economics usually favor removal of high concentrations of carbon
dioxide by aeration rather than by chemical precipitation with lime.

Air stripping may be used to reduce concentrations of volatile organics, such as chloroform, as well as dissolved gases,
such as hydrogen sulfide and ammonia. Air pollution standards must be considered when air stripping is used to reduce
volatile organic compounds.
Iron and Manganese Removal
Iron and manganese in well waters occur as soluble ferrous and manganous bicarbonates. In the aeration process, the
water is saturated with oxygen to promote the following reactions:
4Fe(HCO3)2 + O2 + 2H2O = 4Fe(OH)3 + 8CO2
ferrous bicarbonate oxygen water ferric carbon
hydroxide dioxide
2Mn(HCO3)2 + O2 = 2MnO2 + 4CO2 + 2H2O
manganese oxygen manganese dioxide carbon water
bicarbonate dioxide
The oxidation products, ferric hydroxide and manganese dioxide, are insoluble. After aeration, they are removed by
clarification or filtration.
Occasionally, strong chemical oxidants such as chlorine (Cl2) or potassium permanganate (KMnO4) may be used
following aeration to ensure complete oxidation.
Dissolved Gas Reduction
Gases dissolved in water follow the principle that the solubility of a gas in a liquid (water) is directly proportional to the
partial pressure of the gas above the liquid at equilibrium. This is known as Henry's Law.
However, the gases frequently encountered in water treatment (with the exception of oxygen) do not behave in
accordance with Henry's Law because they ionize when dissolved in water. For example:

H2O + CO2 ↔ H+ + HCO3-

water carbon dioxide hydrogen ion bicarbonate ion

H2S ↔ H+ + HS-
hydrogen sulfide hydrogen ion hydrosulfide ion

H2O + NH3 ↔ NH4+ + OH-

water ammonia ammonium hydroxide ion
ion
Carbon dioxide, hydrogen sulfide, and ammonia are soluble in water under certain conditions to the extent of 1700, 3900,
and 531,000 ppm, respectively. In a normal atmosphere, the partial pressure of each of these gases is practically zero.
Consequently, the establishment of a state of equilibrium between water and air by means of aeration results in saturation
of the water with nitrogen and oxygen and nearly complete removal of other gases.

Gas removal by aeration is achieved as the level of gas in the water approaches equilibrium with the level of the gas in
the surrounding atmosphere. The process is improved by an increase in temperature, aeration time, the volume of air in
contact with the water, and the surface area of water exposed to the air. As previously indicated, pH is an important
consideration. The efficiency of aeration is greater where the concentration of the gas to be removed is high in the water
and low in the atmosphere.

LIMITATIONS
Temperature significantly affects the efficiency of air stripping processes. Therefore, these processes may not be suitable
for use in colder climates. Theoretically, at 68°F the carbon dioxide content of the water can be reduced to 0.5 ppm by
aeration to equilibrium conditions. This is not always practical from an economic standpoint, and reduction of carbon
dioxide to 10 ppm is normally considered satisfactory.

Although removal of free carbon dioxide increases the pH of the water and renders it less corrosive from this standpoint,
aeration also results in the saturation of water with dissolved oxygen. This does not generally present a problem when
original oxygen content is already high. However, in the case of a well water supply that is high in carbon dioxide but
devoid of oxygen, aeration simply exchanges one corrosive gas for another.

The efficiency of aeration increases as the initial concentration of the gas to be removed increases above its equilibrium
value. Therefore, with waters containing only a small amount of carbon dioxide, neutralization by alkali addition is
usually more cost-effective.

Nonvolatile organic compounds cannot be removed by air stripping. For example, phenols and creosols are unaffected by
the aeration process alone.
Aeration and degassing
During aeration, oxygen is brought into the water which converts the dissolved ferrous and manganous compounds into
insoluble ferric and manganic hydroxides. These can be removed by sedimentation or filtration.

Aeration and degassing, or gas exchange, of water is in general the first treatment step for the preparation of drinking
water out of groundwater and bank filtration. The purpose of the gas exchange is the introduction of oxygen (O2) and the
removal of carbon dioxide (CO2), methane (CH4), sulphide (H2S), nitrogen (N2) and volatile organic compounds.

Cascade aeration

The oldest aeration system is the cascade. In a cascade the water falls over an overflow edge in a flow splitting
device in a lower situated cascade tank. During the fall of the water bubbles arise, because air is being dragged in.
The gas exchange takes place between the air in these bubbles and the water. The cascade is often applied, because
of its simplicity, indifference to pollution and water load and its visual attractiveness.
The tower aerator, also called aeration and degassing tower, is used for the removal of very hard to remove gases like
volatile hydrocarbons, and for the removal of carbon dioxide and methane.

A tower aerator consists of a steel or synthetic cylinder filled with a packing, mostly consistent of synthetic shapes.
The water is divided over the packages in the top of the tower from where it flows down over the surface area. With
the use of a ventilator air is transported upwards in counter current through the tower.
At a plate aerator the water flows horizontally over a perforated plate where a ventilator blows
so much air upward that a bubble bed arises on the plate. This way a intensive contact
between water and air arises.

Plate aerators are mainly used for the removal of methane. Because of the small building
height and small hydraulic head needed for plate aerators they are easily fitted in an existing
treatment station. Sometimes it is even possible to put them in the filter compartment just
above the pre filters.
Sprinklers are very often employed as a gas exchange system, because they are easily fitted
in. They can be hung in the top of the filter compartment and spray the water so a contact
area arises between air and water for the gas exchange. The air in the filter compartment is
refreshed by means of a ventilator or through natural draught through intakes caused by the
falling water. The sprinklers divide the water evenly over the filter area.
Empty aeration tank for iron precipitation
Deaerator

A Deaerator is a device for air removal and is used to remove dissolved gases (an alternate would be the use of water
treatment chemicals) from boiler feedwater to make it non-corrosive. A deaerator typically includes a vertical domed
deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater tank.

Necessity for deaeration

A steam generating boiler requires that the circulating steam, condensate, and feed water should be devoid of dissolved
gases, particularly corrosive ones, and dissolved or suspended solids. The gases will give rise to corrosion of the metal.
The solids will deposit on the heating surfaces giving rise to localized heating and tube ruptures due to overheating.
Under some conditions it may give rise to stress corrosion cracking.
The removal of dissolved gases from boiler feedwater is an essential process in a steam system. The presence of
dissolved oxygen in feedwater causes rapid localized corrosion in boiler tubes. Carbon dioxide will dissolve in water,
resulting in low pH levels and the production of corrosive carbonic acid. Low pH levels in feedwater causes severe acid
attack throughout the boiler system. While dissolved gases and low pH levels in the feedwater can be controlled or
removed by the addition of chemicals, it is more economical and thermally efficient to remove these gases mechanically.
This mechanical process is known as deaeration and will increase the life of a steam system dramatically.

Deaeration is based on two scientific principles. The first principle can be described by Henry's Law. Henry's Law asserts
that gas solubility in a solution decreases as the gas partial pressure above the solution decreases. The second scientific
principle that governs deaeration is the relationship between gas solubility and temperature. Easily explained, gas
solubility in a solution decreases as the temperature of the solution rises and approaches saturation temperature. A
deaerator utilizes both of these natural processes to remove dissolved oxygen, carbon dioxide, and other non-condensable
gases from boiler feedwater. The feedwater is sprayed in thin films into a steam atmosphere allowing it to become
quickly heated to saturation. Spraying feedwater in thin films increases the surface area of the liquid in contact with the
steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations. This process reduces the
solubility of all dissolved gases and removes it from the feedwater. The liberated gases are then vented from the
deaerator.
Schematic diagram of a typical tray-type deaerator
Schematic diagram of a typical spray-type deaerator
Horizontal spray-tray type deaerating
heater with storage tank