Factors affecting effective disinfection include turbidity and resistant organisms

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Reynard Bryant
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1 DISINFECTION! refers to operations in water treatment that kills or renders harmless pathogenic microorganisms but does not refer to sterilization.! sterilization; the complete destruction of all living mater is Typhoid Treated Water usually not the objective of disinfection 30 20! see trend in Figure 1: NOTE: Typhoid and Treated Water 20 Symbols switched. Characteristics of Disinfectants! destroys pathogens 0! non-toxic to human beings and domestic animals! safe and easy to store, transport and dispense! reasonable cost! easy to analyze in water! reasonable residual protection < protects far points in distribution system Death Rate per 100,000 People Year Figure 1: Water Disinfection (Peavy, Rowe and Tchobanoglous, 1985) 10 0 Million of People with Treated Water! does not sterilize Important to note that other treatment operations are effective in reducing pathogens! coagulation and filtration remove 90% of bacteria and viruses! excess-lime softening is good for disinfection Factors affecting effective disinfection include turbidity and resistant organisms! turbidity producing colloids offer sanctuary to organisms, shielding them from disinfectant! particulate matter may adsorb disinfectant! viruses and cysts are more resistant to disinfectants than bacteria! additional contact time and exposure are required Chlorination Primary objectives of chlorination are as follows:! disinfection, taste and odour control of the finished water as it leaves the plant! taste and odour control in the distribution system! prevention of algae growth and other microorganisms that might interfere with coagulation and flocculation disinfection.wpd Page 1 of 9

2 ! elimination of slime growths and mud balls in filter media and the prevention of possible build-up of anaerobic bacteria in the filter media! destruction of hydrogen sulfide and control of sulfurous taste and odour in finished water! removal of iron and manganese! organic colour bleaching For chlorine, typical equation to relate residual concentration (HOCl) to kill (99%) is: C n t = K where K n C t = constant = constant = concentration, mg/l = contact time in min " if n equal to 1, contact time and conc. of equal importance " when n>1, contact time of more importance! ideally place injection near intake, but not possible as the formation of THM s increases! THM s are carcinogenic compounds that are formed if organic matter present < higher demand < larger amounts of by-products! up to 50 mg/l can be ingested! kills most viruses! at low concentrations chlorine kills microorganisms by penetrating the cell and reacting with the enzymes and protoplasm! at higher concentrations, oxidation of the cell wall occurs and the organism is destroyed! factors affecting processes include: < form of chlorine < ph < dosage < contact time < type and concentration of organism < temperature < reducing agents in water < ammonia! being a strong oxidant, chlorine will react with almost any material that is in a reduced state < Fe 2+, Mn 2+, H 2 S, and organics consume chlorine leaving less available for disinfection < products of organics (precursors) oxidized by chlorine are undesirable < organic acids (humic, fulvic) form chlorinated hydrocarbon compounds that are suspected of being carcinogenic " trihalomethanes disinfection.wpd Page 2 of 9

3 CHC1 3 chloroform CHC1 2 Br bromodichloromethane CHC1Br 2 dibromochloromethane < phenolic compounds react with chlorine and form severe taste and odour problems < important to remove the organics before chlorination; possible with: Chlorine can be used as: " activated carbon " chloramines, which do not react with organics or phenols for free chlorine! free chlorine (Cl 2 gas) Cl 2 + H 2 O -----> H + + HOCl < chlorine gas is the preferred species for chlorination, but it has difficulties associated with it " chlorine gas is very strong oxidant that is toxic to humans " heavier than air; consequently it spreads along the ground " has been the result of mass excavations like the one in Mississauga < gas is liquified at five to ten atmospheres and shipped in steel cylinders < available as 99.8 % Cl 2 < solubility up to about 700 mg/l, good for super chlorination! hypochlorites < ionized form < more expensive than chlorine gas but much easier and safer to handle < practical problem is in dissolving the hypochlorite < sodium hypochlorite NaOCl " commercial NaOCl contains 3 to 15% available chlorine NaOCl ----> Na + OCl - " domestic bleach contains 3 to 6% NaOCl ES&E (1996) reported that world market for bleach is 4 mil tonnes low cost and safe; good for 3 rd world countries hospitals use it to disinfect surfaces against HIV and Hepatitis B < calcium hypochlorite Ca(OCl) 2 " calcium hypochlorite contains on 70 to 80% available chlorine Ca(OCl) > Ca OCl - Hypochlorous acid (HOCl) and the hypochlorite ion (OCl) are related: disinfection.wpd Page 3 of 9

4 HOCl <=====> H + + OCl -! which vary with temperature and ph dependent! hypochlorous acid (HOCl) is more effective than the hypochlorite ion (OCl - ) by two orders of magnitude, but for safety reasons the hypochlorites are winning out, especially in densely populated cities! sum of HOCl and OCl - is the free residual chlorine < try to maintain this between 0.5 and 1 mg/l < desirable free residual chlorine is at least 0.2 mg/l at the farthest tap on the distribution system! hypochlorite also reacts with ammonia to form chloramines < are disinfectants but less effective that the HOCl and OCl - < referred to as combined residual! are persistent and provide protection in the distribution system HOCl + NH 3 <====> NH 2 Cl(monochloramine) + H 2 O HOCl + NH 2 Cl <====> NHCl 2 (dichloramine) + H 2 O HOCl + NHCl <====> NCl 3 (nitrogen trichloride) + H 2 O! reactions are dependent on several factors, most importantly ph and temperature < at ph greater than 6.0 monochloroamine will be the predominate species < dichloroamine predominates at ph of 5! since combined residuals are less effective as a disinfectant, concentration of 2 to 3 mg/l with contact time exceeding 30 min is required! chloramines are persistent and provide continued protection against regrowth in the distribution system! effective against bacteria but not so effective against viruses! chlorine gas, hypochlorous acid and hypochlorite ion remaining after demand is the free chlorine residual! chloramines and other reactive chlorine forms after demand are combined chlorine residuals! free are faster acting and have greater disinfecting capacity than combine residuals, especially for viruses Breakpoint Curve! curve shows that an increase in chlorine dosage results in an equivalent increase in residual up to the molar ratio of chlorine to ammonia nitrogen of 1:1 < residual is predominantly mono- and dichloramine < oxidation reaction is essentially complete at the breakpoint, where the chlorine dosage is about 1.5 to 2 moles of chlorine per 1 mole of ammonia and represents the dosage where the chloramines have been converted to nitrogen gases < continued chlorination results in the formation of free chlorine (HOCl + Ocl - ) disinfection.wpd Page 4 of 9

5 < breakpoint chlorination is dependent on water " domestic water; 2-20 mg/l " wastewater; between 50 and 70 mg/l (making the effluents highly toxic to aquatic life) disinfection.wpd Page 5 of 9

6 8 CONSUMPTION 7 6 FORMATION OF CHLORAMINES DESTRUCTION CHLORAMINES CHLOROORGANICS FORMATION OF FREE RESIDUAL & CHLOROORGANICS COMBINED RESIDUAL BREAKPOINT FREE RESIDUAL (HOCl + OCl) COMBINED RESIDUAL (CHLORAMINES) Figure 2: Breakpoint Curve BRKPNT.pre disinfection.wpd Page 6 of 9

7 Ozonation! ozone, the allotropic from of oxygen is the most powerful oxidant used in water treatment! the oxidant is the oxygen atom which results in an environmentally acceptable compound; i.e. no THMs are produced < after oxidant demand has been met, ozone reacts vigorously with bacteria and viruses < more effective in inactivating resistant strains of bacteria and viruses than chlorine! used extensively in Europe < with out it they could not use the heavily polluted waters in the Seine and Rhein Rivers as drinking water supplies! up till now not popular in North America because raw water quality was extremely high < believed that Montreal has the biggest ozone facility in the world < becoming more popular here because of the THM problem < as of 1999, 260 plants existed in USA! excellent for taste and odour control! ideal when water contains large amounts of precursors! produces a high energy oxidation where the bacteria cells literally explode! must be produced on site and is expensive < cannot be stored; used immediately < 10 to 20 kwh/kg of ozone < costs 2 to 3 times that of chlorination < half-life if approximately minutes in distilled water at 20 o C with oxidant demanding materials present, the half life is even shorter! produced by passing dry air between oppositely charge plates under high voltage < high electric discharge is applied to O 2 6,000 to 30,000 V 3O > 2O 3! solubility at mg/l, with a K L a of 0.25 to 0.45 min-1! naturally unstable in water and readily decomposes in water < has no residual protection < Europeans add about 0.1 to 0.3 mg/l of chlorine for residual! contact time of minutes at 0.5 to 1.0 mg/l sufficient for most pathogens < French use a two contactor system on waters from the Seine " 8 to 12 minutes with a residual of 0.4 mg/l (ozone demand step) " 4 to 8 minutes, with a residual of 0.4 mg/l (disinfection step) < Germans use one contactor system with contact time of 20 minutes! typical Ct=2.9 to 0.48 for 99 % kill! creates no noxious and harmful compounds in treated water! slowly gaining acceptance in wastewater applications disinfection.wpd Page 7 of 9

8 disinfection.wpd Page 8 of 9

9 Chlorine Dioxide! ClO 2 has many of the same properties as ozone! strong oxidant which neither forms chloroforms or chloramines! effective in taste and odour control, iron and manganese removal! highly soluble but does not react with water! contact with the atmospheres results in loss of ClO 2 as it decomposed by ultraviolet irradiation < consequently must be produced onsite! ClO 2 residual is longer lasting than HOCl, but possible reduction to chlorate, substance toxic to humans, makes it questionable for potable water < typical Ct = 63 to 11 for 99.9 % kill UV Irradiation! irradiation with ultraviolet light is a promising method! provides no residual! can kill both bacteria and viruses, but not effective against Giardia cysts! most effective band is 2000 to 3000 A (angstrums) [5 to 400 nm)! usually generated with low-pressure mercury vapour lamps! power out put of 30 W/cm 2 over thin sheets of turbidity water free water works best, allowing UV to reach organism! inactivation rate constants (k in m2/j) from Droste, 97 pg. 533) < E. Coli = < fecal strepococci = < viruses = < coliphages = < bacteriophages = 0.011! turbidity of wastewater is inhibiting its use in wastewater, but advancements made such as self cleaning tubes! from a cost perspective in $1000 per year for WW (WQI #2, 1009 pg 7) < 1 MGD: 19.6 to to 206 < 10 MGD: 153 to to 781 < 100 MGD: 1132 to to 2826 disinfection.wpd Page 9 of 9

CE 370. Disinfection. Location in the Treatment Plant. After the water has been filtered, it is disinfected. Disinfection follows filtration.

CE 370. Disinfection. Location in the Treatment Plant. After the water has been filtered, it is disinfected. Disinfection follows filtration. CE 70 Disinfection 1 Location in the Treatment Plant After the water has been filtered, it is disinfected. Disinfection follows filtration. 1 Overview of the Process The purpose of disinfecting drinking