Sewage Characteristics

Municipal wastewater is mainly comprised of water (99.9%) together with a variety of

suspended and dissolved organic and inorganic solids. Among the substances present in sewage
are fats, soaps, synthetic detergents various natural and synthetic organic chemicals from the
process industries and potentially toxic elements such as arsenic, cadmium, chromium, copper,
lead, mercury, zinc, etc
Table below shows the levels of the major constituents of strong, medium and weak domestic
wastewaters. The concentrations of most materials in wastewater are expressed in milligrams per
litre (mg/I) and denote the strength of the wastewater. The higher the concentration, or mg/I, the
higher the strength.

Composition of sewage may be broken into three general areas. These areas are the physical,
chemical, and biological characteristics of wastewater

Physical characteristics
The most important physical characteristics of water is its total solids content, which is
composed of floating, suspension, colloidal and solution matter.

Other physical characteristics include odor, temperature and color.

Total solids
Wastewater is normally 99.9 percent water and 0.1 percent solids. If a wastewater sample is
evaporated, the solids remaining are called total solids.

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The amount of solids in the drinking water system has a significant effect on the total solids
concentration in the raw sewage. Industrial and domestic discharges also add solids to the plant
influent. There are many different ways to classify solids. The most common types are dissolved,
suspended, settleable, floatable, colloidal, organic, and inorganic solids.

Part of the total solids is dissolved in wastewater. Dissolved solids pass through a fine mesh
filter. Normal wastewater processes using settling or flotation are designed to remove solids but
cannot remove dissolved solids.

Those solids that are not dissolved in wastewater are called suspended solids. When suspended
solids float, they are called floatable solids or scum. Those suspended solids that settle are called
settleable solids, grit, or sludge. Very small suspended solids that neither float nor settle are
called colloidal particles. Colloidal particles are often removed in the biological treatment units.
They may also be removed by chemical treatment followed by sedimentation.

TEMPERATURE
The temperature of wastewater is commonly higher than that of the water supply because of the
addition of warm water from households and industrial activities. The temperature of water is a
very important parameter because of its effect on the aquatic life, the chemical reactions and
reaction rates and the suitability of the water for beneficial uses. Increased temperature, for
example, can cause a change in the species of fish that can exist in the receiving water body.
Industrial establishments that use surface water for cooling-water purposes are particularly
concerned with the temperature of the intake water.

In addition, oxygen is less soluble in warm water than in cold water. The increase in the rate of
biochemical reactions that accompanies an increase in temperature , combined with the decrease
in the quantity of oxygen present in surface waters ,can often cause serious depletion in
dissolved
oxygen concentration .When significantly large quantities of heated water are discharged to
natural receiving waters, these effects are magnified. A sudden change in temperature can result
in a high rate of mortality of aquatic life. Abnormally high temp. can foster the growth of
undesirable water plants and waste water fungus.

ODORS
Odors in wastewater usually are caused by gases produced by the decomposition of organic
matter. Fresh wastewater has a distinctive, but is less objectionable than the odor of septic
wastewater.
The most characteristic odor of stale or septic wastewater is that of hydrogen sulfide, which is
produced by anaerobic microorganisms that reduces sulfates to sulfides.

COLOR
Fresh wastewater is usually grey; however, as organic compounds are broken down by
bacteria,the

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dissolved oxygen in the wastewater is reduced to zero and the color changes to black. In this
condition the wastewater is said to be septic(or stale).some industrial wastewater may also add
color to domestic wastewater.

CHEMICAL CHARACTERISTICS:

The chemical characteristics of wastewater of special concern in wastewater engineering are pH,
DO (dissolved oxygen), oxygen demand, nutrients, and toxic substances.

PH

The term pH is used to describe the acid or base properties of water solutions. A scale from 0 to
14 has been established where pH value of 7 is neutral. A pH value less than 7 is acidic. A pH
value above 7 is alkaline or basic. A pH value less than 7 in the wastewater plant influent may
indicate septic conditions of wastewater. The pH values less than 5 and more than 10 usually
indicate that industrial wastes exist and are not compatible with biological wastewater
operations. Pre-treatment of these wastes at the source is usually required since extreme pH
values may damage biological treatment units.

DISSOLVED OXYGEN
Dissolved oxygen (DO) in wastewater has a great effect on the characteristics of the water.
Wastewater that has DO is called aerobic or fresh. Aerobic raw sewage is usually grey in colour
and has a stuffy odour. Wastewater that has no DO is called anaerobic or septic. Anaerobic raw
sewage is usually black and has an offensive hydrogen sulphide or rotten egg odour.

Dissolved oxygen is required for respiration of aerobic bacteria. Rate of biochemical reactions
increase with temperature but level of dissolved oxygen decrease with temperature. Problem is
compounded by reduced stream flow in hot weather. Overall DO is important is control of
obnoxious smells.

OXYGEN DEMAND
Oxygen demand is the amount of oxygen used by bacteria and other wastewater organisms as
they
feed upon the organic solids in the wastewater. Chemical tests such as the BOD (biochemical
oxygen demand), the COD (chemical oxygen demand) and the TOC (total organic carbon)
measure the "strength" of sewage. This topic will be discussed details later) .It is important that
organic wastes be removed to protect the receiving body of water into which the wastewater
plant
is discharging. Sludge deposits, odors, and fish kills may occur if removal is not adequate

NUTRIENTS

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The most important nutrients in wastewater treatment are life-supporting nitrogen and
phosphorus. They stimulate excessive growths of algae and other aquatic plant life. They are
always present in domestic wastewaters and are not removed during conventional primary and
secondary treatment. Removal is accomplished by processes in addition to normal wastewater
treatment or tertiary treatment, when specific reuse requirements require it.

TOXIC CHEMICALS OR PRIORITY POLLUTANTS

Many industrial installations use various types of toxic chemicals, the discharges of which can be
harmful to wastewater treatment processes. Toxic chemical are undesirable in trace quantities,
and they include mercury, lead, zinc, copper, cadnium etc. These toxic chemicals should be pre-
treated or removed before the wastewater enters the collecting system.

BIOLOGICAL CHARACTERISTICS

Biological organisms present in wastewater are bacteria, algae, Fungi, viruses, and parasites.

Bacteria
They are single celled organisms, useful for decomposition processes. Sewage consists of vast
quantities of bacteria, most of which are harmless to man. However, pathogenic (disease-
causing) organisms such as typhoid, dysentery, and other intestinal disorders may be present in
wastewater.

Indicator Organism
Concept.
Rather than testing for each pathogen, it is easier test for only one group of microorganism
whose presence is as assured evidence/indication that the wastewater has been polluted by feaces
of humans or warm blooded animals. This microorganism may be called an indicator organism.
The indicator organism that is commonly used is Escherichia coli.(E.coli)
Characteristics of E.coli that makes it suitable indicator
 Non-pathogenic faecal coliform that reside in the human intestinal tract.
 Excreted in large numbers in feaces, often amounting to 50millions per gram
 Untreated domestic sewage contains upto 3million coliform per 100ml
 E.coli persists in the environment outside the human intestine for a longer duration than
the other pathogenic bacteria (but note that viruses, cysts and helminth eggs are more
persistent than E.coli.)
 Its absence means the faecal contamination is absent
 It is easier to detect E.coli

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Bacteria can also be classified according to their dissolved oxygen requirement. Aerobic bacteria
are bacteria that require dissolved oxygen to live. Anaerobic bacteria cannot live if dissolved
oxygen is present. Facultative bacteria can live with or without dissolved oxygen.

Fungi
These are aerobic multi-cellular and non-photosynthetic organisms. Fungi obtain food form dead
organic matter and play an important part in decomposition of organic matter. Fungi have
advantage over bacteria in that they can grow under low-moisture condition and also in low-pH

Algae
Algae are a nuisance in surface water. Under right conditions, algae reproduce rapidly forming
floating colonies called algae bloom. Effluent wastewater is rich in nutrient hence causes nutrient
enrichment and increase rate of eutrophication. Algae cause colour, taste odour and light
penetration problems. It also renders surface water unsuitable for recreational use.

Viruses
Viruses are parasitic organisms. They don’t have ability to synthesise new compounds and
therefore invade living (host) cell where viral genetic material re-direct cells activities to
generate new viruses at expense of the host cell. When the infected cell dies, a large number of
viruses are produced. Viruses are excreted by human beings and become a major hazard to
public health, Viruses have a long life in water of up to 41 days in wastewater and 6 days in
natural stream. Wastewater often contains viruses that may produce diseases. Outbreaks of
infectious hepatitis have been traced through water systems because of wastewater entering the
supply. Sedimentation, filtration, and disinfection, if used efficiently, usually provide acceptable
virus removal.

Parasites
There are also many species of parasites carried by wastewater. The life cycle of each is peculiar
to the given parasite. Some are dangerous to man and livestock, particularly during certain stages
of the life cycle. Amoebic dysentery is a common disease caused by amoebic parasites.
Chlorination, chemical precipitation, sedimentation, or sand filtration is used to ensure treatment
against parasites.

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