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Disinfection
Dr. Akepati S. Reddy
Associate Professor, Thapar University
Adjunct Scientist, TCIRDPatiala (Punjab) – 147 004
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Biological water quality
• Assessed by MPN test (Multiple tube fermentation technique andmembrane filtration technique)
• Improved by disinfection: deactivation (render harmless), not sterilization,of pathogens (causing water borne diseases)
Disinfection two types
• Primary disinfection: achieving desired level of microbial kills orinactivation
• Secondary disinfection: maintaining disinfectant residual in finished waterto prevent regrowth of microorganisms
Disinfection can be brought about by
• Chemical agents (disinfectants)
• Chlorine and hypochlorite; Chlorine dioxide; and Chloramines
• Ozone
• Mixed oxidants
• Irradiation (UV radiation) and Heating
• Sonification, electrocution,
• Filtration (and membrane filtration, nano-filtration!), Coagulation-flocculation, and Settling also assist
Disinfection
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Disinfection
• Inactivation processes includedenaturation of
– proteins (structural proteins, enzymes,transport proteins)
– nucleic acids (genomic DNA or RNA,mRNA, tRNA, etc)
– lipids (lipid membranes, other lipids)
• Disinfection kinetics is described byChick’s law (1908)
– Follows first order kinetics - when thedisinfectant level is constant thenumber of microbes surviving is
function of contact time – Equal susceptibility and uniform
dispersion of the microbes is assumed
– Reduction of microbes is expressed inlog reductions (1 log reduction = 10%survival, 2 log reduction = 1% survival, 3
log reduction = 0.1% survival and so on.
kt N
N
kN dt
dN
0
ln
N is the number of microbes
‘t’ is contact time
‘k’ is disinfection rate constant
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Disinfection• Disinfection is temperature sensitive
– Usually observed to double for every 10°C
raise in temperature – This can be used find out the typical
activation energy (Ea) value
• Disinfection is pH sensitive
– OCl- is less effective than HOCl and low pH is
more effective
– Ozone is more effective at lower pH – athigher pH ozone may convert into OH-
– ClO2 is more effective at higher pH
• Disinfection is function of both time (t) anddisinfectant concentration (C)
• Chick-Watson law
– Here K0 is extent disinfection per unitconcentration of the disinfectant and per unit
time (L/mg.min) – Higher K0 indicates more effective disinfection
t C
N
N
k
t C k N
N
law sChick from
lawWatsonChick
k t C
K
K T RT
T T E
K
K
n
n
n
T
T
a
0
0
0
0
10
21
12
2
1
ln
ln
)'(
2
ln
R is gas constant
(8.314 J/mol.K)
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Disinfection
K 0 for 99% kills at 5°C
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CT values for inactivation using ClO2
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Ideal Disinfectants• Versatile (effective against all types of
pathogens)
• Fast-acting (effective within short contacttimes)
• Robust (effective in the presence ofinterfering materials such as particulates,suspended solids and other organic andinorganic constituents)
• Easy to handle – non-toxic, soluble, non-flammable, non-explosive
• Compatible with various materials/surfacesin WTPs (pipes, equipments)
• Economical• A good disinfectant must be toxic to
microorganisms well below the toxicthresholds to humans and higher animals
– Should have fast rate of kills and shouldpersist enough to prevent re-growth in thedistribution system
Oxidants used in the disinfection
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DisinfectionFactors influencing disinfection
• Type of disinfectant used
• Concentration of disinfectant
• Contact time
• Temperature and pH
• Concentration and type of microorganisms
• Presence of interfering materials (pathogens can be embedded in theorganic particles
• Other environmental variables!
Factors that prevent effective disinfection also include
• Turbidity – offer sanctuary and shield from the full action of thedisinfectant
• Resistance to disinfectant (cysts, encysted bacteria, bacterial spores,ova, viruses)
Disinfection byproducts (DBPs)
• The disinfectant can react with water and its constituents (metals,ammonia, organics, etc.) and form DBPs
• Taste and odour problems
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• Chlorine gas is highly oxidizing, toxic, corrosive and hazardousyellow-green gas (supplied as liquid chlorine in bullets)
• At 0.1% (volume) concentration in air lethal to human beings
• Heavier than air and spreads slowly at ground level
• Effective against all types of microbes as both primary and
secondary disinfectant• Leaves combined and free residual chlorine in the treated water
and this prevents microbial regrowth during supply
• Chlorine handling requires specialized equipment, care and skill
• Storage in a separate room (not connected to other rooms), doors
opening to outside, windows facilitating visual inspection needed
• Install chlorinator in the rooms with direct emergency access tooutside air
• Self contained breathing apparatus and chlorine cylinder repair kitmust be readily accessible
• Masks, air tanks, chlorine detection devices etc. needed
Disinfection (Chlorination)by Chlorine gas
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• Inexpensive and large scale applications prefer its use
• Liquid chlorine is drawn out, vapourized (should take care of the
latent heat of vapourization) and dosed into water by an injector• Highly pressurized water is passed through a venturi – vacuum created
sucks chlorine gas into the water stream
• Provisions are made for proper mixing of the chlorine dosed and forthe requisite contact time
• pH control may be necessary for effective disinfection• Alternatively the chlorine gas is dissolved in water to form chlorine
solution and this inturn dosed/injected into water
– Solubility is 750 mg/l at usually encountered pH and temperature
• Chlorine gas on dissolution in water forms hypochlorus acid (HOCl)
and reduces pH – HOCl dissociates to form OCl- (this is limited by the lower pH)
– HOCl is more effective as disinfectant than OCl-
• Dechlorination of chlorinated wastewater prior to disposal may be
needed (SO2, Na2SO3, sodium metabisulfite, activated carbon)
HOCl H O H Cl 22
OCl H HOCl
Disinfection (Chlorination) by Chlorine gas
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Chlorine gas application system
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Chlorine gas application system
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• More expensive than Cl2 and Highly corrosive
• Available as a solution with 5-15% available chlorine level• Decomposes on storage
– Should not be stored beyond a month
– Should be stored in cool, dark and dry areas
• Easier to handle than calcium hypochlorite and chlorine gas
• Preferred for highly populated areas and small scale applications
• The sodium hypochlorinator includes a solution tank, dosingpumps, tubing and diffuser
– Diluted hypochlorite solution is injected into water supply pipe atcontrolled rate
• On dissolution sodium hypochlorite forms OCl- (less effective asdisinfectant than HOCl)
• Sodium hypochlorite can be generated onsite by electrolysis of
sodium chloride solution in specialized proprietary equipment – Hydrogen is given off here as a byproduct
OCl Na NaOCl
Disinfection (Chlorination) by sodiumhypochlorite
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• Calcium hypochlorite is a white solid (easily soluble in water) and
has about 65% available chlorine – Can be purchased in granular, powdered and tablet forms
• Very corrosive, has a strong odour and readily absorbs moistureand generates chlorine gas
• Should be kept away from organic materials (can generate heat and
cause fire or explosion)• Packed calcium hypochlorite is very stable
• Calcium hypochlorite can be dosed as
– Dissolution in water to prepare a solution with 1-2% available chlorineand dosing/injecting as solution
Diaphragm pumps are used for the dosing – Tablets of calcium hypo can be directly dissolved in water at
atmospheric pressure
• On dissolution calcium hypochlorite forms OCl- (less effective asdisinfectant than HOCl)
OCl CaOCl Ca 2)( 22
Disinfection (Chlorination) by calciumhypochlorite
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• Very strong oxidant (comparable to Cl2) and very expensive
• Highly soluble in water (10 times to Cl2)
• Can react with household materials and produce offensive odours• Can transfer from solution to gaseous form and become explosive
• Volatile and subjected to photo-decomposition
• Unstable at higher concentrations (>15%) and under pressure
• Used for disinfection, as primary disinfectant (alternative to Cl2)
• Disinfection is brought about by oxidation - Better for Giardia andCryptosporidium kills
• Does not react with ammonia - Forms halogenated organics andchlorite (toxic to humans), but not THMs and HAAs
• Superior for manganese oxidation
• Insensitive to pH over a broard range (4-8) – pH >9.0 is avoided• ClO2 is also used in a pretreatment (>5.0 min contact time)
• Destructs TTHM and HAA precursors, oxidizes manganese andcontrols taste and odour (from algae – diatoms)
• Typical dose: 0.6 to 1.7 ppm (2.0 to 5.0 ppm is typical dose for Cl2)
• Simultaneous application of NH2Cl and ClO2 are avoided
Disinfection by Chlorine Dioxide
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Chlorine dioxide application system
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Chlorine Dioxide Generation SystemsClO2 is unstable and hence produced onsite (as 1-2% solution!)
– Stabilized ClO2 solutions of 3000 mg/L strength are also
available – can prove ideal for small water systems
Cl2 gas/25% sodium chlorite solution or solid sodium chlorite
(NaClO2) systems and solid based systems
– 95% purity of CLO2 is attainable
– Overdose of Cl2 can make the process very efficient
– Sodium chlorite storage/leaks are the biggest safety concerns
– Sodium sodium chloride produces pure ClO2 (low chlorite!)
Sulfuric acid, sodium chlorate/ H2O2 solution system
– Bulk storage of sodium chlorate and of 73% H2SO4 are the
concerns
Electrochemical systems and acid-chlorite systems
NaCl ClOCl NaClO 222 222
2422422232 OSO NaClOSO H O H NaClO
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SAF-T-CHLOR
SOLID
SODIUM
CHLORITE
CHLORINE
AIRAIR
FILTER
E J E C T O R
RAW WATER
IN
SOLUTION
OUTCHLORINE
SAF-T-CHLOR
SOLID
SODIUM
CHLORITE
CHLORINE DIOXIDE + AIR
SCALESCALE
Cl2 + 2NaClO2 2ClO2 + 2NaCl
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Disinfection by TwinOxide• Developed and made in Netherlands
• An advanced delivery system of aqueous ClO2 solution (0.3%)• Delivered as a powder kit of two components having very long
shelf life (5 years)
– Component A: 64% - sodium chlorite and 36% - other ingradients
– Component B: sodium bisulfate
• Components A & B are mixed onsite in specified volume of tapwater at neutral pH , and left for 3 hrs to produce aqueous ClO2 solution (concentrate)
– 4 kg product produces 100 L of 0.3% of the concentrate
– The concentrate should be stored in UV proof sealed container, in
cool dark room (half-life when stored in dark at 22C is 30-60 days)
• Works as disinfectant in the pH range of 4 to 10
– Imparts no smell, taste or colour
– Generates no chlorine, chlorate, chlorite or chloride
– Non-explosive and lightly corrosive
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• Chlorination is highly sensitive to inorganic and organic loads andforms harmful disinfection byproducts (DBPs), like, tri-halo-
methanes (THMs), halo-acetic acids (HAAs), etc.• Oxidation products of organics, some are carcinogenic, and some cause
taste and odor problems
• IS 10500: 2012 prescribes limits for total tri-halo-methanes (TTHMs) indrinking water
•Limits prescribed: 0.08 ppm for TTHM, 0.06 ppm for HAA and 1.0 ppmfor Chlorite
• Factors affecting the DBPs formation
• Types and concentrations of organic materials
• Dose of chlorine and reaction/contact time
• temperature and pH of water• Solutions to the disinfection byproducts (DBPs)
• Reducing the organics concentration in the water prior to chlorination(adsorption on activated carbon)
• Use of alternate disinfectants that form no undesirable DBPs(substitution of chloramines, a less effective disinfectant, to chlorine)
• Removal of the DBPs formed from the water after chlorination
Disinfection/Chlorination byproducts (DBPs)
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Disinfection by Chloramines• Weak disinfectant – effective bactericide but less effective against
viruses and protozoans – Produces fewer disinfection byproducts (DBPs)
– An effective and appropriate secondary disinfectant
• Formed by chlorinating ammonia containing water or by addingammonia (anhydrous ammonia, ammonium sulfate, or ammonium
hydroxide) to the water containing chlorine – Into the water supply main chlorine is injected and then ammonia is
injected and adequate mixing and contact time is provided
– Chloramines formation reactions are 99% complete within a fewminutes
• Formation of nitrogen trichloride is undesirable – NCl3 is harmful to humans and it imparts disagreeable taste and
odour
– Chlorine to ammonia ratio of 5:1 is not exceeded and pH of water isnot allowed to drop below 5 do not allow NCl3 formation
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Disinfection by Ozonation• Formed by passing dry air or oxygen through a system of high
voltage electrodes
• Unstable (half life is <15 min.) hence generated onsite• Ozonation system includes
– Air (or oxygen) preparation as feed (pure oxygen use has higherproduction density and requires relatively lesser energy)
– Electrical power supply
– Ozone generation by using a corona discharge cell – Ozone contact chamber - requires shorter contact time than Cl2
– Ozone exhaust gas destruction
• Used as a primary disinfectant (leaves no disinfecting residue andhence requires a secondary disinfectant)
– Preferred for waters containing colour and organics – Has low solubility in water hence needs rigorous mixing
• Capital cost of ozonation systems is higher; Operation andmaintenance is complex; Electricity amounts to 26-43% of O&M cost
• Forms no undesirable products with organics – can produce
halogenated organics provided bromide is present
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Disinfection by UV radiation• Special lamp is used for UV radiation and disinfection uses reactors
– Thin sheets of turbidity free water are exposed to about 30
microwatts/cm2 power
– Equipment is easy to operate and maintain
• Destroys genetic material (energy intensity and contact time areimportant) - effective wavelengths of the radiation are 200-300 nm
– Damage of genetic material is not sufficient – hence higher than therequired dose of UV radiation is used
• Used as primary disinfectant (attractive for small water systems)
– May not be effective in inactivating protozoan cysts – hencepreferably used with groundwater systems not in direct influence ofsurface waters
– Not suitable for water with high levels of suspended solids, turbidity,colour and/or soluble organic matter
– A secondary disinfectant must be used to prevent re-growth ofmicrobes
• Requires shorter contact times - produces no known toxic residuals
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Chemistry of Chlorination• Objectives of chlorination: Disinfection, H2S control, sludge bulking
control, odour control, etc.
• Chlorine as gas, ClO2, and sodium/calcium hypochlorite (andchloramines!) are used
– Large systems use chlorine gas
• All the chlorine based disinfectants release HOCl or OCl
• pH and temperature determine the equilibrium relationship
– pKa at 20C is 7.58 and at 0C is 7.82 –indicates HOCl levels are higher atlower pH (at <5 pH all chlorine is HOCl and at >10 pH most of it is OCl-
76% HOCl at 7.0 pH and 33% at 7.8 pH
– pKa increases with decreasing temperature
Sum of HOCl and OCl- is known as free residual chlorine
HOCl H O H Cl 22
OCl CaOCl Ca 2)( 2
2
OCl Na NaOCl OCl H HOCl
HOCl
OCl H K a
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C at
C at pK a
082.7
2058.7
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Chlorine reacts with reduced materials (Fe2+, Mn2+, H2S,
organics, NH3, etc.) – consumes the dosed chlorine
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Design of chlorination
Dosage control
• Dose range should be known - depends on
– Peak flow
– Dose needed – depends on
• Characteristics of the water
• Residual chlorine needed in the treated water
– Can be estimated on the basis of laboratory experimentation
• Dose control can be
– Manual (to obtain the residual chlorine needed after 15
minutes of contact time) – involves pacing chlorine flow ratewith the water flow rate
– Automatic control – automatic measurement of residualchlorine and using it for dose adjustment
Loss of weight of the cylinder can be the basis for dosage
monitoring
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Injection and initial mixing
• Can be applied directly as gas or indirectly as aqueoussolution
• Can be withdrawn from storage facility as liquid or as gas
• Use black steel piping for dry chlorine (liquid or gas) andPVC piping (schedule-80) for chlorine solution
• For withdrawing as gas evaporation of the liquid in thecontainer is needed (can form frost)
– Withdrawal as gas is possible when dose is <18 kg/day for68 kg cylinders and <205 kg/day for 908 kg cylinders
– Evaporators are used if withdrawal is >180 kg/day (must
when total dosage is >680 kg/day) – Temperature control to avoid freezing is needed
• Addition of chlorine solution can be by diffuser – a plasticpipe with drilled holes – into the path of wastewater flow
• Mixing can be by inline static mixers, in-line mixers, high
speed induction mixers, pressurized water jets and pumps
Design of chlorination
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Chlorine contact basin design
• Basin configuration
– Long plug flow chambers are used – to eliminate formationof hydraulic dead zones that reduce effective HRT
– Length to width ratio is 20:1 to 40:1
– Often larger diameter sewer pipes are used
• Use of submerged baffles/guide vanes or combination ofboth for better hydraulic performance – Open area in the submerged baffle is 6-10% of the flow
cross section area
– Head loss across the baffle is
• Eliminate requirement of contact basin if travel time in thepipeline is greater than contact time needed
Design of chlorination
2
2
1
Cna
Q
g H L
Q is water flow rate
C is discharge coefficient (0.8)
‘n’ is number of openings
‘a’ is cross section area of opening
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Chlorine storage facilities
– Gas is toxic, very corrosive and heavier than air
– Ventilation at floor level to storage chamber with capacity of60 air changes per hour
– Fixed glass viewing window for checking leaks prior to entry
– Fan controls at the room entrance – Protect storage and feed facilities from fire hazards
– Provide leak detection equipment connected to alarmsystem
– Protect cylinders from direct sun light during summer warm
climates – Spill control and containment and emergency caustic
scrubbing system to neutralize leaks
• Containment vessels to provide total enclosure of cylinder
• in event of cylinder failure gas is contained within and
d t l t th h hl i ti f iliti
Design of chlorination