Commercial RO System Dubai
Commercial RO System Dubai
Commercial RO System Dubai
http://aqua-pro.ae
ENVR 890
Mark D. Sobsey
Spring, 2007
http://aqua-pro.ae
Water Sources and Water Treatment
• Drinking water should be essentially free of disease-causing microbes,
but often this is not the case.
– A large proportion of the world’s population drinks microbially contaminated water,
especially in developing countries
• Using the best possible source of water for potable water supply and
protecting it from microbial and chemical contamination is the goal
– In many places an adequate supply of pristine water or water that can be protected
from contamination is not available
• The burden of providing microbially safe drinking water supplies from
contaminated natural waters rests upon water treatment processes
– The efficiency of removal or inactivation of enteric microbes and other pathogenic
microbes in specific water treatment processes has been determined for some
microbes but not others.
– The ability of water treatment processes and systems to reduce waterborne
disease has been determined in epidemiological studies
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Summary of Mainline Water Treatment Processes
Storage
Disinfection
Physical: UV radiation, heat, membrane filters
Chemical: Chlorine, ozone, chlorine dioxide, iodine, other
antimicrobial chemicals
Filtration
Rapid granular media
Slow sand and other biological filters
Membrane filters: micro-, ultra-, nano- and reverse osmosis
Other physical-chemical removal processes
Chemical coagulation, precipitation and complexation
Adsorption: e.g., activated carbon, bone char, etc,
Ion exchange: synthetic ion exchange resins, zeolites, etc.
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Water Treatment Processes: Storage
Reservoirs, aquifers & other systems:
store water
protect it from contamination
Factors influencing microbe reductions (site-specific)
detention time
temperature
microbial activity
water quality: particulates, dissolved solids, salinity
sunlight
sedimentation
land use
precipitation
runoff or infiltration
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Water Storage and Microbial Reductions
Microbe levels reduced over time by natural
antimicrobial processes and microbial death/die-off
Human enteric viruses in surface water reduced 400-
1,000-fold when stored 6-7 months (The Netherlands)
Indicator bacteria reductions were less extensive,
probably due to recontamination by waterfowl.
Protozoan cyst reductions (log10) by storage were 1.6 for
Cryptosporidium and 1.9 for Giardia after about 5
months (The Netherlands; G.J Medema, Ph.D. diss.)
Recent ICR data indicates lower protozoan levels in
reservoir or lake sources than in river sources; suggests
declines in Giardia & Cryptosporidium by storage
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Typical Surface Water Treatment Plant
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Chemical Coagulation-Flocculation
Removes suspended particulate and colloidal substances
from water, including microorganisms.
Coagulation: colloidal destabilization
Typically, add alum (aluminum sulfate) or ferric chloride
or sulfate 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.
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Coagulation-Flocculation, Continued
Flocculation:
Slow mixing (flocculation) that provides for for a period
of time to promote the aggregation and growth of the
insoluble particles (flocs).
The particles collide, stick together abd grow larger
The resulting large floc particles are subsequently
removed by gravity sedimentation (or direct filtration)
Smaller floc particles are too small to settle and are
removed by filtration
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Microbe Reductions by Chemical Coagulation-Flocculation
Considerable reductions of enteric microbe concentrations.
Reductions In laboratory and pilot scale field studies:
>99 percent using alum or ferric salts as coagulants
Some studies report much lower removal efficiencies (<90%)
Conflicting information may be related to process control
coagulant concentration, pH and mixing speed during flocculation.
Expected microbe reductions bof 90-99%, if critical process
variables are adequately controlled
No microbe inactivation by alum or iron coagulation
Infectious microbes remain in the chemical floc
The floc removed by settling and/or filtration must be properly
managed to prevent pathogen exposure.
Recycling back through the plant is undesirable
Filter backwash must be disinfected/disposed of properly.
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Cryptosporidium Removals by Coagulation
(Jar Test Studies)
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Water Softening and Microbe Reductions
”Hard" Water: contains excessive amounts of calcium
and magnesium ions
iron and manganese can also contribute to hardness.
Hardness ions are removed by adding lime (CaO) and
sometimes soda ash (Na2CO3) to precipitate them as
carbonates, hydroxides and oxides.
This process, called softening, is basically a type of
coagulation-flocculation process.
Microbe reductions similar to alum and iron
coagulation when pH is <10
Microbe reductions >99.99% possible when pH is >11
microbial inactivation + physical removal
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Microbial Reductions by Softening Treatment
Softening with lime only (straight lime softening); moderate
high pH
ineffective enteric microbe reductions: about 75%.
Lime-soda ash softening
results in the removal of magnesium as well as calcium hardness
at higher pH levels (pH >11)
enteric microbe reductions >99%.
Lime-soda ash softening at pH 10.4, 10.8 and 11.2 has produced
virus reductions of 99.6, 99.9 and 99.993 percent, respectively.
At lower pH levels (pH <11), microbe removal is mainly a
physical process
infectious microbes accumulate in the floc particles and the
resulting chemical sludge.
At pH levels above 11, enteric microbes are physically
removed and infectivity is also destroyed
more rapid and extensive microbe inactivation at higher pH
levels.
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Granular Media Filtration
Used to remove suspended particles (turbidity) incl. microbes.
Historically, two types of granular media filters:
Slow sand filters: uniform bed of sand;
low flow rate <0.1 GPM/ft2
biological process: 1-2 cm “slime” layer (schmutzdecke)
Rapid sand filters: 1, 2 or 3 layers of sand/other media;
>1 GPM/ft2
physical-chemical process; depth filtration
Diatomaceous earth filters
fossilized skeletons of diatoms (crystalline silicate);
powdery deposit; few 10s of micrometers; porous
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Slow Sand Filters
Less widely used for large US municipal water supplies
Effective; widely used in Europe; small water supplies;
developing countries
Filter through a 3- to 5-foot deep bed of unstratified sand
flow rate ~0.05 gallons per minute per square foot.
Biological growth develops in the upper surface of the sand is
primarily responsible for particle and microbe removal.
Effective without pretreatment of the water by
coagulation-flocculation
Periodically clean by removing, cleaning and replacing the
upper few inches of biologically active sand
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Microbial Reductions by Slow Sand Filtration
Effective in removing enteric microbes from water.
Virus removals >99% in lab models of slow sand filters.
Up to 4 log10; no infectious viruses recovered from filter effluents
Field studies:
naturally occurring enteric viruses removals
97 to >99.8 percent; average 98% overall;
Comparable removals of E. coli bacteria.
Virus removals=99-99.9%;
high bacteria removals (UK study)
Parasite removals: Giardia lamblia cysts effectively removed
Expected removals ~ 99%
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Rapid Granular Media Filter Operation
Sometimes multiple
layers of different media
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Roughing Filter
•Used in developing
countries
•inexpensive
•low maintenance
•local materials
•Remove large solids
•Remove microbes
•1-2 log10 bacterial
reduction
•90% turbidity
reduction
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Microbe Reductions by Rapid Granular Media Filters
Ineffective to remove enteric microbes unless preceded by
chemical coagulation-flocculation.
Preceded chemical coagulation-flocculation & sedimentation
Enteric microbe removals of 90->99 % achieved.
Field (pilot) studies: rapid sand filtration preceded by iron
coagulation-flocculation: virus removal <50% (poor control?).
Giardia lamblia: removals not always high; related to turbidity
removal; >99% removals reported when optimized.
Removal not high unless turbidity is reduced to ~0.2 NTU.
Lowest removals shortly after filter backwashing
Microbes primarily removed in filter by entrapped floc
particles.
Overall, can achieve 90% microbial removals from water
when preceded by chemical coagulation-flocculation.
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Microbe Reductions by Chemical Coagulation-Flocculation and Filtration
of River Water by Three Rx Plants in The Netherlands
Reduction
Type Rate (M/hr) Coagulation % (log10)
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Cryptosporidium Removal by Coagulation and Direct Filtration
Log10 Reduction of
Run No. Cryptosporidium Turbidity
1 3.1 1.3
2 2.8 1.2
3 2.7 0.7
4 1.5 0.2*
Clarification by:
Coagulation flocculation-sedimentation
<1 - 2.6
or Flotation
Rapid Filtration (pre-coagulated) 1.5 - >4.0
Both Processes <2.5 - >6.6
Slow Sand Filtration >3.7
Diatomaceous Earth Filtration >4.0
Coagulation + Microfiltration >6.0
Ultrafiltration >6.0
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Cryptosporidium Reductions by Coagulation and
Filtration
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Cryptosporidium Reductions by Membrane Filtration
Log10
Membrane, Pore Size Cryptosporidium
Type Reduction
A, MF 0.2 µm >4.4
B, MF 0.2 µm >4.4
C, MF 0.1 µm 4.2->4.8
D, UF 500 KD >4.8
E, UF 300 KD >4.8
F, UF 100 KD >4.4
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Disinfection
Any process to destroy or prevent the growth of microbes
Intended to inactivate (destroy the infectivity of) the microbes
by physical, chemical or biological processes
Inactivation is achieved by altering or destroying essential
structures or functions within the microbe
Inactivation processes include denaturation of:
proteins (structural proteins, enzymes, transport proteins)
nucleic acids (genomic DNA or RNA, mRNA, tRNA, etc)
lipids (lipid bilayer membranes, other lipids)
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Properties of an Ideal Disinfectant
Broad spectrum: active against all microbes
Fast acting: produces rapid inactivation
Effective in the presence of organic matter, suspended
solids and other matrix or sample constituents
Nontoxic; soluble; non-flammable; non-explosive
Compatible with various materials/surfaces
Stable or persistent for the intended exposure period
Provides a residual (sometimes this is undesirable)
Easy to generate and apply
Economical
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DISINFECTION AND MICROBIAL INACTIVATION KINETICS
First
Multihit
Order
Log Survivors
Retardant
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Disinfection of Microbes in Water: Conventional Methods
used in the Developed World
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Factors Influencing Disinfection Efficacy
and Microbial Inactivation, Continued
Microbial strain differences and microbial selection:
Disinfectant exposure may select for resistant strains
Physical protection:
Aggregation
particle-association
protection within membranes and other solids
Chemical factors:
pH
Salts and ions
Soluble organic matter
Other chemical (depends on the disinfectant)
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Some Factors Influencing Disinfection Efficacy
and Microbial Inactivation - Bacteria
Surface properties conferring susceptibility or resistance:
Resistance: Spore; acid fast (cell wall lipids); capsule; pili
Susceptibility: sulfhydryl (-SH) groups; phospholipids; enzymes;
porins and other transport structures, etc.
Physiological state and resistance:
Antecedent growth conditions: low-nutrient growth increases
resistance to inactivation
Injury; resuscitation and injury repair;
disinfectant exposure may selection for resistant strains
Physical protection:
Aggregation; particle-association; biofilms; occlusion (embedded
within protective material), association with or inside eucaryotes;
corrosion/tuberculation
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Some Factors Influencing Disinfection
Efficacy and Inactivation - Viruses
Virus type, structure and composition:
Envelope (lipids): typically labile to disinfectants
Capsid structures and capsid proteins (change in
conformation state)
Nucleic acids: genomic DNA, RNA; # strands
Glycoproteins: often on virus outer surface; typically labile
to disinfectants
Physical state of the virus(es):
Aggregated
Particle-associated
Embedded within other materia (within membranes)
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Factors Influencing Disinfection Efficacy and
Microbial Inactivation - Parasites
Parasite type, structure and composition:
Protozoan cysts, oocysts and spores
Some are very resistant to chemical disinfectants
Helminth ova: some are very resistant to chemical
disinfection, drying and heat.
Strain differences and selection:
Disinfectant exposure may select for resistant strains
Physical protection:
Aggregation; particle-association; protection within other
solids
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Factors Influencing Disinfection Efficacy and
Microbial Inactivation - Water Quality
Particulates: protect microbes from inactivation;
consume disinfectant
Dissolved organics: protect microbes from inactivation; consumes or
absorbs (for UV radiation) disinfectant; Coat microbe (deposit on
surface)
pH: influences microbe inactivation by some agents
free chlorine more effective at low pH where HOCl predominates
neutral HOCl species more easily reaches microbe surface and
penetrates)
negative charged OCl- has a harder time reaching negatively charged
microbe surface
chlorine dioxide is more effective at high pH
Inorganic compounds and ions: influences microbe inactivation by
some disinfectants; depends on disinfectant
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Factors Influencing Disinfection Efficacy and Microbial
Inactivation - Reactor Design, Mixing & Hydraulic
Conditions
Disinfection kinetics are better in plug-flow (pipe) reactors
than in batch (back-mixed) reactors
Disinfectant Disinfectant
Flo
w
Retardant
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Disinfection
Types is a kineticKinetics
of Disinfection process
Increased inactivation with increased exposure or
contact time.
Chick's Law: disinfection is a first-order reaction.
(NOT!)
Multihit-hit or concave up kinetics: initial slow rate;
multiple targets to be “hit”; diffusion-limitions in
reaching “targets”
Concave down or retardant kinetics: initial fast rate
that decreases over time
Different susceptibilities of microbes to
inactivation; heterogeneous population
Decline of of disinfectant concentration over time
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Disinfection Activity and the CT Concept
Disinfection activity can be expressed as the product of disinfection
concentration (C) and contact time (T)
Assumes first order kinetics (Chick’s Law) such that disinfectant
concentration and contact time have the same “weight” or
contribution in disinfection activity and in contributiong to CT
Example: If CT = 100 mg/l-minutes, then
If C = 10 mg/l, T must = 10 min. in order to get CT = 100 mg/l-min.
If C = 1 mg/l, then T must = 100 min. to get CT = 100 mg/l-min.
If C = 50 mg/l, then T must = 2 min. to get CT = 100 mg/l-min.
So, any combinationof C and T giving a product of 100 is
acceptable because C and T are interchangable
The CT concept fails if disinfection kinetics do not follow Chick’s
Law (are not first-order or exponential)
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Factors Influencing Disinfection of Microbes
Microbe type: disinfection resistance from least to most:
vegetative bacteria viruses protozoan cysts, spores and eggs
Type of disinfectant: order of efficacy against Giardia from best to worst
O3 ClO2 iodine/free chlorine chloramines
BUT, order of effectiveness varies with type of microbe
Microbial aggregation:
protects microbes from inactivation
microbes within aggregates not be readily reached by the disinfectant
Particulates: protects from inactivation; shielded/embedded in particles
Dissolved organics: protects
consumes or absorbs (UV radiation) disinfectant; coats microbes
Inorganic compounds and ions: effects vary with disinfectant
pH: effects depend on disinfectant.
Free chlorine more biocidal at low pH where HOCl predominates.
Chlorine dioxide more microbiocidal at high pH
Reactor design, mixing and hydraulic conditions; better activity in "plug
flow" than in "batch-mixed" reactors.
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Inactivation of Cryptosporidium Oocysts in Water by Chemical
Disinfectants
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Effect of pH on Percentages of HOCl and OCl-
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Free Chlorine and Microbial Inactivation
Greater microbial inactivation at lower pH (HOCl) than at high pH
(OCl-)
Probably due to greater reactivity of the neutral chemical
species with the microbes and its constituents
Main functional targets of inactivation:
Bacteria: respiratory activities, transport activities, nucleic
acid synthesis.
Viruses: reaction with both protein coat (capsid) and nucleic
acid genome
Parasites: mode of action is uncertain
Resistance of Cryptosporidium to free chlorine (and
monochloramine) has been a problem in drinking water supplies
Free chlorine (bleach) is actually used to excyst C. parvum
oocysts!
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Monochloramine - History and Background
First used in Ottawa, Canada and Denver, Co. (1917)
Became popular to maintain a more stable chlorine residual and to
control taste and odor problems and bacterial re-growth in
distribution system in 1930’s
Decreased usage due to ammonia shortage during World War II
Increased interest in monochloramine:
alternative disinfectant to free chlorine due to low THM
potentials
more stable disinfectant residual; persists in distribution
system
secondary disinfectant to ozone and chlorine dioxide
disinfection to provide long-lasting residuals
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Monochloramine: Chemistry and Generation)
Monochloramine formation:
HOCl + NH3 <=> NH2Cl + H2O
Stable at pH 7 - 9, moderate oxidation potential
Generation
pre-formed monochloramine:
mix hypochlorite and ammonium chloride (NH4Cl) solution at
Cl2 : N ratio at 4:1 by weight, 10:1 on a molar ratio at pH 7-9
dynamic or forming monochloramination:
initial free chlorine residual, folloowed by ammonia addition to
produce monochloramine
greater initial disinfection efficacy due to free chlorine
Dosed at several mg/L
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Reaction of Ammonia with Chlorine:
Breakpoint Chlorination
Presence of ammonia in water or wastewater and the addition of free
chlorine results in an available chlorine curve with a “hump”
Free chlorine present
Combined
Cl2
present
Chlorinethe
At chlorine doses between added,
humpmg/L
and the dip, chloramines are
being oxidatively destroyed and nitrogen is lost (between pH 6.5-8.5).
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Ozone
First used in 1893 at Oudshoon
Used in 40 WTPs in US in 1990 (growing use since then), but more
than 1000WTPs in European countries
Increased interest as an alternative to free chlorine (strong oxidant;
strong microbiocidal activity; perhaps less toxic DBPs)
A secondary disinfectant giving a stable residual may be needed
to protect water after ozonation, due to short-lasting ozone
residual.
Colorless gas; relatively unstable; reacts with itself and with OH- in
water; less stable at higher pH
Formed by passing dry air (or oxygen) through high voltage
electrodes to produce gaseous ozone that is bubbled into the water
to be treated.
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Chlorine Dioxide
First used in Niagara Fall, NY in 1944 to control phenolic tastes and
algae problems
Used in 600 WTP (84 in the US) in 1970’s as primary disinfectant and
for taste and odor control
Very soluble in water; generated as a gas or a liquid on-site, usually
by reaction of Cl2 gas with NaClO2 :
2 NaClO2 + Cl2 2 ClO2 + 2 NaCl
Usage became limited after discovery of it’s toxicity in 1970’s &
1980’s
thyroid, neurological disorders and anemia in experimental animals by
chlorate
Recommended maximum combined concentration of chlorine
dioxide and it’s by-products < 0.5 mg/L (by US EPA in 1990’s)
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Chlorine Dioxide
High solubility in water
5 times greater than free chlorine
Strong Oxidant; high oxidative potentials;
2.63 times greater than free chlorine, but only 20 % available at
neutral pH
Neutral compound of chlorine in the +IV oxidation state; stable free
radical
Degrades in alkaline water by disproportionating to chlorate and chlorite.
Generation: On-site by acid activation of chlorite or reaction of
chlorine gas with chlorite
About 0.5 mg/L doses in drinking water
toxicity of its by-products discourages higher doses
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Inactivation of Cryptosporidium Oocysts in
Water by Chemical Disinfectants
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UV Absorption Spectra of DNA: Basis for Microbial Activity
(pH 7 in 0.1M phosphate buffer)
0.8
0.7
0.6
Absorption
0.5
254 nm by low
Optical Density
0.4 pressure
mercury UV
0.3 lamps
0.2
0.1
200 220 240 260 280 300 320
0.0
200 220 240 260 280 300 320
Wavelength, nm
Wavelength (nm) http://aqua-pro.ae
Figure 3. Absorbance spectra of nonhydrolyzed deoxyribonucleic acid (DNA)
Low and Medium Pressure UV Technologies
• ••••• • • • • • • •
100 200 300 400 500 600 700 800 900 1000
Wavelength (nm)