disinfection and chlorination

8/8/2019 Disinfection and Chlorination

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Disinfection

Dr. Akepati S. Reddy

Associate Professor, Thapar University

Adjunct Scientist, TCIRDPatiala (Punjab) – 147 004



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



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



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)



Disinfection

K 0 for 99% kills at 5°C



CT values for inactivation using ClO2



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



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



• 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



• 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



Chlorine gas application system



• 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



• 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



• 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



Chlorine dioxide application system



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



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



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



• 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)



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



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



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



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



C at

C at pK a

082.7

2058.7



Chlorine reacts with reduced materials (Fe2+, Mn2+, H2S,

organics, NH3, etc.) – consumes the dosed chlorine



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



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




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


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



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


disinfection and chlorination

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