CE 370CE 370

DisinfectionDisinfection

DisinfectionDisinfection

Definition: is the process of destruction of Definition: is the process of destruction of living pathogenic microorganismsliving pathogenic microorganisms

Disinfection depends on:Disinfection depends on:The physico-chemistry of the disinfectantThe physico-chemistry of the disinfectantThe cyto-chemical nature and physical state of the The cyto-chemical nature and physical state of the

pathogenspathogensThe interaction of the aboveThe interaction of the aboveTemperatureTemperaturepHpHElectrolytesElectrolytesInterfering substancesInterfering substances

Classification of DisinfectantsClassification of Disinfectants

Oxidizing agents (ozone, halogens)Oxidizing agents (ozone, halogens) Cations of heavy metals (silver, gold, mercury)Cations of heavy metals (silver, gold, mercury) Organic compoundsOrganic compounds Gaseous agentsGaseous agents Physical agents (heat, UV, pH)Physical agents (heat, UV, pH)

Disinfection RateDisinfection Rate

Disinfection rate by a chemical agent obeys Chick’s Disinfection rate by a chemical agent obeys Chick’s Law:Law:

dN/dt = rate of cell destruction (number / time)dN/dt = rate of cell destruction (number / time) k = rate constantk = rate constant N = number of living cells remaining at time tN = number of living cells remaining at time t

k depends on:k depends on: Microorganisms speciesMicroorganisms species Disinfectant natureDisinfectant nature Disinfectant concentrationDisinfectant concentration Environmental factors (pH and temperature)Environmental factors (pH and temperature)

kNdt

dN

Disinfection RateDisinfection Rate Another equation is:Another equation is:

C = concentration of disinfectant at time t = 0C = concentration of disinfectant at time t = 0 tc = time of contact required to kill a given percentage of the tc = time of contact required to kill a given percentage of the

microbesmicrobes K, n = exponential constantsK, n = exponential constants

n depends on the nature of disinfectantn depends on the nature of disinfectant If n > 1, then disinfection greatly depends on the concentration of If n > 1, then disinfection greatly depends on the concentration of

the disinfectantthe disinfectant If n < 1, then disinfection greatly depends on the time of contactIf n < 1, then disinfection greatly depends on the time of contact

K depends on type of microorganism and K depends on type of microorganism and environmental factors such as pH and temperatureenvironmental factors such as pH and temperature

KtC cn

Disinfection MethodsDisinfection Methods

ChlorinationChlorination Chlorine dioxideChlorine dioxide OzonationOzonation UV irradiationUV irradiation High pHHigh pH Other halogens (iodine and bromine)Other halogens (iodine and bromine)

ChlorinationChlorination

Chlorine is widely usedChlorine is widely usedEffective at low concentrationEffective at low concentrationCheapCheapForms residual if applied in sufficient dosagesForms residual if applied in sufficient dosages

Chlorine is applied as:Chlorine is applied as:Gas (most common)Gas (most common)hypochloritehypochlorite

Chlorine is a strong oxidizing agentChlorine is a strong oxidizing agentIt oxidizes enzymes necessary for metabolismIt oxidizes enzymes necessary for metabolism

Chlorination ReactionChlorination Reaction

Chlorine gas reacts with water to formChlorine gas reacts with water to form Hypochlorous acid (HOCl)Hypochlorous acid (HOCl) Hydrochloric acid (HCl)Hydrochloric acid (HCl)

Hypochlorous acid dissociates to hypochlorite ionHypochlorous acid dissociates to hypochlorite ion

The dissociation of the HOCl is a function of pHThe dissociation of the HOCl is a function of pH

HClHOClOHCl 22

OClHHOCl

Chlorination ReactionChlorination Reaction

Hypochlorite saltsHypochlorite salts Are available in dry formAre available in dry form They should be dissolved in waterThey should be dissolved in water

OClOCl-1-1 seeks equilibrium with H seeks equilibrium with H++ ion ion Therefore, acid may be needed to be addedTherefore, acid may be needed to be added

Both hypochlorous acid and hypochlorite ions are Both hypochlorous acid and hypochlorite ions are effective disinfectantseffective disinfectants

OClCaOClCa OH 2)( 22

2

Chlorination ReactionChlorination Reaction

Reaction with ammoniaReaction with ammonia

At pH > 6.0, the monochloramine predominatesAt pH > 6.0, the monochloramine predominates At pH about 5.0, the dichloramine predominatesAt pH about 5.0, the dichloramine predominates Chloramines are effective against bacteria but not Chloramines are effective against bacteria but not

virusesviruses

etrichloridnitrogen

OHNClHOClNHCl

edichlora

OHNHClHOClClNH

emonochlora

OHClNHHOClNH

232

222

223

33

min

22

min

Chlorination ReactionChlorination Reaction

Reaction with organicsReaction with organicsReaction with phenol produces chlorophenolsReaction with phenol produces chlorophenolsReaction with humic substances produoces Reaction with humic substances produoces

trihalomethanestrihalomethanes Chloroform (CHClChloroform (CHCl33))

Bromodichloromethane (CHClBromodichloromethane (CHCl22Br)Br)

Dibromochloromethane (CHClBrDibromochloromethane (CHClBr22) )

Dosages, Demand and ResidualsDosages, Demand and Residuals

Dosage: the amount of chlorine addedDosage: the amount of chlorine added Demand: the amount of chlorine needed to Demand: the amount of chlorine needed to

oxidize materialsoxidize materials Residual: the amount of chlorine remaining Residual: the amount of chlorine remaining

after oxidationafter oxidation

Dosages, Demand and ResidualsDosages, Demand and Residuals

Contact timeContact timeWhen increased, more microorganisms are killedWhen increased, more microorganisms are killedWhen increased, the demand increasesWhen increased, the demand increasesWhen increased, the amount of chlorinated by-products When increased, the amount of chlorinated by-products

increase (if precursors are available)increase (if precursors are available)

Free chlorine residualsFree chlorine residualsChlorine gas + hypochlorous acid + hypochlorite ionChlorine gas + hypochlorous acid + hypochlorite ion

Combined chlorine residualsCombined chlorine residualsChloramines + other chlorine reactive formsChloramines + other chlorine reactive forms

Dosages, Demand and ResidualsDosages, Demand and Residuals

Free chlorine residuals are faster acting than Free chlorine residuals are faster acting than combined residualscombined residuals

Free chlorine residuals have greater Free chlorine residuals have greater disinfecting capacity than combined residuals disinfecting capacity than combined residuals (especially for viruses)(especially for viruses)

DechlorinationDechlorination

Chlorinated effluents can have negative Chlorinated effluents can have negative impacts on receiving environmentimpacts on receiving environment

Chemicals used for dechlorination include:Chemicals used for dechlorination include:Sulfur dioxide (most common)Sulfur dioxide (most common)Sodium sulfiteSodium sulfiteSodium bisulfiteSodium bisulfiteSodium thiosulfateSodium thiosulfateHydrogen peroxideHydrogen peroxideammoniaammonia

DechlorinationDechlorination

Activated carbon is:Activated carbon is: Well established in the dechlorination processWell established in the dechlorination processIt has limited use due to its high costIt has limited use due to its high costRequires long contact timesRequires long contact timesIt can be used for dechlorination and removal of organic It can be used for dechlorination and removal of organic

compounds at the same timecompounds at the same time

Sulfur Dioxide (SOSulfur Dioxide (SO22))

General characteristicsGeneral characteristics GasGas ColorlessColorless Has strong pungent odorHas strong pungent odor Neither flammable nor explosiveNeither flammable nor explosive With sufficient moisture, it is corrosive to most metalsWith sufficient moisture, it is corrosive to most metals Has solubility in water of 18.6% at 32Has solubility in water of 18.6% at 32 F (0 F (0 C) C) When reacts with water, it forms a weak solution of sulfurous acid When reacts with water, it forms a weak solution of sulfurous acid

(H(H22SOSO33))

Sulfurous acid dissociates as follows:Sulfurous acid dissociates as follows:

233

332

SOHHSO

HSOHSOH

Sulfur Dioxide (SOSulfur Dioxide (SO22))

Free and combined chlorine forms react readily with Free and combined chlorine forms react readily with sulfite ion (SOsulfite ion (SO33

-2-2) as follows:) as follows:

Required mass ratio of sulfur dioxide to chlorine is Required mass ratio of sulfur dioxide to chlorine is 1.1:11.1:1

42

4222

3

24

23

NHClSOOHClNHSO

HClSOHOClSO

OzonationOzonation

General characteristics of ozoneGeneral characteristics of ozone OO33

Powerful oxidantPowerful oxidant More powerful than hypochlorous acidMore powerful than hypochlorous acid Unstable in aqueous solutionsUnstable in aqueous solutions Has a half-life of 20 to 30 minutes in distilled waterHas a half-life of 20 to 30 minutes in distilled water Widely used in drinking water treatmentWidely used in drinking water treatment Is produced on-site and can not be storedIs produced on-site and can not be stored

Ozone productionOzone production1.1. Air is refrigerated to remove moisture (-40 to -60Air is refrigerated to remove moisture (-40 to -60 C) C)

2.2. Air is dried through desiccants (silica gel and activated alumina)Air is dried through desiccants (silica gel and activated alumina)

3.3. Air is passed between oppositely charged plates Air is passed between oppositely charged plates

Ozonation PerformanceOzonation Performance

Some investigators reported total destruction Some investigators reported total destruction of polioviruses in distilled water at a dose of of polioviruses in distilled water at a dose of 0.3 mg/l and a contact time of 3 minutes0.3 mg/l and a contact time of 3 minutes

Presence of SS may increase the dose and Presence of SS may increase the dose and contact timecontact time

For good results, SS should not be very highFor good results, SS should not be very high Ozone is never used as a terminal treatment Ozone is never used as a terminal treatment

(no residual ozone)(no residual ozone)

Ozonation AdvantagesOzonation Advantages

When compared to chlorination:When compared to chlorination:Ozone degrades to OOzone degrades to O22, so no toxic residues, so no toxic residues

OO22 level in effluent is usually at saturation level in effluent is usually at saturation

Does no increase TDS in effluentDoes no increase TDS in effluentLess cost with pure oxygen activated sludge processLess cost with pure oxygen activated sludge processOzone has been used to remove phenols, cyanides and Ozone has been used to remove phenols, cyanides and

heavy metalsheavy metalsOzone can render some refractory organics Ozone can render some refractory organics

biodegradablebiodegradable

Current Research TrendsCurrent Research Trends

Identification of byproducts of the reaction of Identification of byproducts of the reaction of ozone with organic materialsozone with organic materials

Ozone reaction with pesticides may produce a Ozone reaction with pesticides may produce a more toxic material more toxic material

Chlorine Dioxide (ClOChlorine Dioxide (ClO22))

ClOClO22 was originally used to remove taste and was originally used to remove taste and

odor from waterodor from water General characteristics of ClOGeneral characteristics of ClO22

More powerful oxidant than chlorineMore powerful oxidant than chlorineDoes not react with waterDoes not react with waterCan be easily removed from water by aerationCan be easily removed from water by aerationReadily decomposed by exposure to UV radiationReadily decomposed by exposure to UV radiationDoes not react with ammoniaDoes not react with ammoniaMaintains a stable residual Maintains a stable residual

Preparation of ClOPreparation of ClO22

Acid and sodium chloriteAcid and sodium chlorite

Chlorine gas and sodium chlorite (excess chlorine)Chlorine gas and sodium chlorite (excess chlorine)

Sodium hypochlorite and sodium chloriteSodium hypochlorite and sodium chlorite

HNaClClOHClNaClO 22

OHNaClClONaClOHClHOCl

HClHOClOHCl

222

22

222

222 3222 HNaClClOHClNaOClNaClO

Applications of ClOApplications of ClO22 Disinfection Disinfection

ClOClO22 has longer lasting residual than HOCl has longer lasting residual than HOCl In wastewater, ClOIn wastewater, ClO22 use is limited to phenolic use is limited to phenolic

wastes and the control of sulfide in wastewater wastes and the control of sulfide in wastewater collection systemscollection systems

ClOClO22 does not produce measurable amounts of does not produce measurable amounts of THMs (trihalomethanes) or TOXs (total THMs (trihalomethanes) or TOXs (total organic halogens)organic halogens)

Cost of equipment and sodium chlorite are Cost of equipment and sodium chlorite are highhigh

Ultraviolet (UV) IrradiationUltraviolet (UV) Irradiation

Was used in drinking water disinfectionWas used in drinking water disinfection In wastewater treatment, and when compared In wastewater treatment, and when compared

to chlorination and Ozonation, UV was foundto chlorination and Ozonation, UV was foundMore effectiveMore effectiveMore economicalMore economical

UV irradiation is gaining prominenceUV irradiation is gaining prominence

Advantages and Disadvantages of UVAdvantages and Disadvantages of UV

Advantages (in wastewater treatment)Advantages (in wastewater treatment)Effective in pathogen inactivationEffective in pathogen inactivationAbility to achieve disinfection goalsAbility to achieve disinfection goalsViable applications to wide range of wastewater Viable applications to wide range of wastewater

qualitiesqualitiesCost effectiveCost effectiveRelative simplicityRelative simplicityAbsence of residualsAbsence of residuals

Disadvantages (in water treatment)Disadvantages (in water treatment)Lack of residualsLack of residuals

Types of UV InstallationsTypes of UV Installations

There are two types:There are two types:Overhead bulb radiating downward through a shallow Overhead bulb radiating downward through a shallow

depth of water (2.5 to 5.1 cm)depth of water (2.5 to 5.1 cm)Submerged UV lamb encased in a quartz tubeSubmerged UV lamb encased in a quartz tube

The submerged type was found more effectiveThe submerged type was found more effective The efficiency of the process is not The efficiency of the process is not

significantly affected by turbidity and significantly affected by turbidity and suspended solidssuspended solids

UV performs well with virusesUV performs well with viruses

Performance of UV IrradiationPerformance of UV Irradiation

Dose-response for three organisms showed similar Dose-response for three organisms showed similar trends:trends:

y = log reductiony = log reduction x = log UV dosex = log UV dose

Dose (D) = radiation intensity Dose (D) = radiation intensity detention time detention time

).....(68.459.1

).....(48.462.1

).....(21.348.1

coliphagesxy

espoliovirusxy

coliformsfecalxy

Performance of UV IrradiationPerformance of UV Irradiation

For total and fecal coliforms:For total and fecal coliforms:D = 24,800 x – 60,000 (fecal coliforms)D = 24,800 x – 60,000 (fecal coliforms)D = 20,000x – 48,200 (total coliforms)D = 20,000x – 48,200 (total coliforms)

Where x = log reductionWhere x = log reduction

High pH TreatmentHigh pH Treatment

Lime can destruct bacteria at high pH valuesLime can destruct bacteria at high pH valuesIn this case, no residual will remain after In this case, no residual will remain after

neutralizationneutralization Studies showed that Studies showed that

higher removal of viruses was obtained with higher pH higher removal of viruses was obtained with higher pH valuesvalues

Optimum pH in the range of 11.2 to 11.3Optimum pH in the range of 11.2 to 11.3Optimum contact time in the range of 1.56 to 2.40 hoursOptimum contact time in the range of 1.56 to 2.40 hoursComplete destruction of viruses was obtained at pH of Complete destruction of viruses was obtained at pH of

11.0 and contact time of 5.0 hours and 10 minutes11.0 and contact time of 5.0 hours and 10 minutes

High pH TreatmentHigh pH Treatment The process requires an additional residual The process requires an additional residual

disinfectant (minimum amount)disinfectant (minimum amount) High pH treatment can also remove ammonia and High pH treatment can also remove ammonia and

phosphorousphosphorous Recarbonation of the treated water might be needed Recarbonation of the treated water might be needed

before dischargebefore discharge Lime stabilization of sludge is implemented at pH of Lime stabilization of sludge is implemented at pH of

12 for 30 minutes (higher destruction of bacteria at 12 for 30 minutes (higher destruction of bacteria at higher pH values)higher pH values)

No information about virusesNo information about viruses Little effect on parasites Little effect on parasites