Dr. Prabha Joshi, Asst. Professor Department: B.E. Civil Engineering Disinfection of Water Dr. Prabha Joshi, Asst. Professor Department: B.E. Civil Engineering Subject: Environmental Engineering - I Semester: VI eCourseware@AIKTC Teaching Aids Service by KRRC Information Section

Key Terms Cleaning Disinfection Sanitation Sterilization Removal of Visible physical dirt and stains. A clean surface is defined as being free from soil (e.g. food residues), free from bad odours, be non-greasy to the touch and have no visible oxidation (e.g. rust). Disinfection Removal of harmful bacteria / microbes. A sanitized clean surface is defined as a clean surface that is substantially free from pathogenic microorganisms and undesirable numbers of spoilage microorganisms. Sanitation Process in which most or nearly all micro organisms (whether or not pathogenic) killed through use of chemicals, heat, ultraviolet rays e.g. Milk is disinfected by heating up to 100degree C for atleast 10 sec. to kill most microbes (but not necessarily their spores) to make it more stable than pasteurized milk Sterilization Total destruction of all microorganisms ( whether or not pathogenic) and their spores, usually through the use of drastic methods such as concentrated toxic/ non toxic chemicals (Chlorine, formaldehyde, glutar-aldehydes, etc.), very high temperatures, or intense radiation. A sterilized item cannot support life in any from.

WHO on alternative disinfectants: Iodine, silver, copper, quaternary ammonium compounds none of them are considered suitable for long-term use to disinfect drinking water Iodine is difficult to deliver to water and can cause adverse health effects However, iodine, either dissolved in water or in the form of an iodinated exchange resin, has been used for short-term water treatment Silver and copper are difficult to deliver to water and are only bacteriostatic. Quaternary ammonium compounds are limited in availability, costly and not effective against viruses and parasites. http://www.who.int/water_sanitation_health/dwq/wsh0207/en/index6.html

Ozone: O3 Very strong oxidant (very low C▪t values) but has no residual disinfection power Generated by passing high voltage through the air between two electrodes More expensive than chlorination but does not produce trihalomethanes which are suspected carcinogens Widely used in Europe, limited use in U.S.

Ozone (advantages and disadvantages) Highly effective against all type of microbes Disadvantages Expensive Unstable (must produced on-site) High toxicity High chemical hazards Highly sensitive to inorganic and organic loads Formation of harmful disinfection by-products (DBP’s) Highly complicated maintenance and operation No lasting residuals

Ox idant Advantages Disadvantages Chlorine Chloramines Strong oxidant Persistant residual Chlorinated by - products Taste and odor problems pH influences effectiveness Chloramines No trihalomethane formation Weak oxidant Some organic halide Taste, odor, and growth problems Chlorine dioxide Relatively persistant residual No trihalomethane prod. No pH effect Total organic halide formation ClO3 and ClO2 by products On site generation required Hydrocarbon odors possib le Ozone No trihalomethane or organic halide formed No taste or odor prob. Little pH effects Coagulant aid Some by products biodegradable Short half life Energy intensive Some by products biodegradable Complex generation Corrosive

Ultraviolet irradiation has been used in wastewater disinfection for more than 50 years Increased interest after the discovery of its remarkable effectiveness against Cryptosporidium parvum and Giardia lamblia in late 1990’s

Ultraviolet irradiation C G T DNA physical process energy absorbed by DNA pyrimidine dimers, strand breaks, other damages inhibits replication UV

UV Disinfection Optimum ultraviolet light wavelength range for germicidal effect: 250 nm - 270 nm Low pressure mercury lamps emit 253.7 nm Damages microbial/viral DNA and viral RNA by blocking nucleic acid replication Does not produce toxic by-products Higher costs than chemical disinfection, no residual disinfection

UV disinfection (advantages and disadvantages) Very effective against bacteria, fungi, protozoa Independent on pH, temperature, and other materials in water No known formation of DBP Disadvantages Not so effective against viruses No lasting residuals Expensive

Disinfectants Bromine Bromine Bromine can be used for the disinfection of swimming pool and cooling tower water. It is not used for the disinfection of drinking water. What are the characteristics of bromine? Bromine has the atomic number 35. Like chlorine, it is a halogen and it easily reacts with other elements. In nature bromine can only be found in compounds. These combinations are called bromides. Bromides are used to obtain pure bromine and to produce bromine products. After fluorine, bromine is the most reactive element. It reacts with many different substances, is very corrosive and destructive on organic material. Bromine is a bleach. It is poisonous in fluid form and bromine vapor is destructive for the human skin, eyes and respirational tract.

Bromine can easily be dissolved in water (35 g per L water), carbon disulfide and other organic solutions. When added to water, bromine forms hypobromous acid. Hyprobromous acid is a weak acid. It partly dissociates to form hydrogen ions and hypobromite ions. T he rate of hypobromous acid and hypobromite ions is determined by the pH value of the water. When the pH value is between 6.5 and 9 both hypobromous acid and hypobromite ions can be found in water.  If water contains ammonia nitrogen, bromamines will be formed (NH2Br, NHBr2 and NHBr3). For disinfection bromamines are as effective as hypobromous acid.

Disinfection with bromine In the United States bromine has been used since the 1930's for the disinfection of water. Bromine substances are disinfectants and can be used as an alternative for chlorine. In swimming pools, bromine is used against the formation and growth of algae, bacteria and odours in swimming water Bromine can be applied in fluid form or in a mixture. When bromine is applied in fluid form, the following equilibrium is established: Br2 + 2H2O « HOBr + H3O+ + Br-  HOBr + 2H2O « OBr- + H3O- This equilibrium strongly depends on the pH value. At the pH value that is usually found in swimming pools, bromine is mainly present as hypobromous acid (HOBr). Bromine has to be used combined with an oxidizing agent (for example chlorine or ozone).

Table 1: influence of the pH on the formation of hypobromous acid. Hypobromous acid (HOBr) hypobromite ion (OBr-) % bromine as HOBr pH % bromine as OBr- 100 6,0 0,0 99,4 6,5 0,6 98,0 7,0 2,0 94,0 7,5 83,0 8,0 17,0 57,0 8,5 43,0

Silver as a Disinfectant… Silver is used as a bacteriostatic agent for point-of-use or household water treatment by storing water in vessels composed of silver or passing water through porous or granular filter media impregnated with silver Many microbes including viruses, protozoan cysts, oocysts, and bacterial spores, are not inactivated at silver concentrations employed for point-of-use drinking water treatment Bacteria may develop silver resistance Therefore, silver is not recommended for routine disinfection of household water http://www.who.int/water_sanitation_health/dwq/wsh0207/en/index6.html

Key Words Chlorine Residual- Measurable chlorine remaining after the demand is satisfied. Chlorine Demand- The amount of chlorine destroyed by reaction with Fe, Mn, turbidity, organics, and microorganisms in the water. Chlorine Demand= Chlorine Dose- Chlorine Residual Free Chlorine Residual- point past breakpoint where HOCl (hypochlorous acid) and OCL- (hypochlorite ion) form. 25 times more powerful than combined chlorine for disinfection Combined residual chlorine: chlorine combined with organics or ammonia. NH2Cl (monochloramine) and NH(Cl)2 (dichloramine) N(Cl)3 (nitrogen trichloride)

Key Words Breakpoint chlorination: The point at which near complete oxidation of nitrogen compounds is reached . Any point beyond breakpoint is mostly free chlorine (HOCL and OCL-) C•T (concentration and contact time): Effectiveness of chlorination is dependant on chlorine concentration and contact time. Sterilization: The destruction of all living things in a sample Disinfection: The removal or inactivation of disease causing (pathogenic) organisms

Chlorine Disinfection Mechanisms Oxidation of membrane-bound enzymes for transport and oxidative phosphorylation Oxidation of cytoplasmic enzymes Oxidation of cytoplasmic amino acids to nitrites and aldehydes Oxidation of nucleotide bases Chlorine substitution onto amino acids DNA mutations DNA lesions Rittmann, B. E. and P. M. Huck (1989). "Biological Treatment of Public Water Supplies." Critical Reviews in Environmental Control 19(2): 119-184.

Chick’s Law The death of microorganisms is first order with respect to time Thus, the remaining number of viable microorganisms, N, decreases with time, t, according to: where k is an empirical constant descriptive of the microorganism, pH and disinfectant used. Integrating with respect to time, and replacing limits (N = No at t = 0) yields: http://www.swbic.org/education/env-engr/disinfection/kinetics.html

Kinetics of Disinfection Inactivation is a gradual process involving a series of physicochemical and biochemical steps. Inactivation is described by the equation: Nt/N0 = e-kt Where: N0 = number of microorganisms at time = 0 Nt = number of microorganisms at time = t k = a decay constant (1/time) t = time Ideally, inactivation follows first-order kinetics (blue line), but often non-ideal behaviors occur resulting from clumping of cells or multiple hits of critical sites before inactivation

Concentration and Contact Time Effectiveness of chlorination depends primarily on the concentration used and the time of exposure Disinfectant effectiveness can be expressed as a C ▪ t value where: C = disinfectant concentration t = time required to inactivate a 99% of the population under specific conditions The lower the C ▪ t, the more effective the disinfectant In general, resistance to disinfection is in the following order: vegetative bacteria < enteric viruses < spore-forming bacteria < protozoan cysts

Current increase in waterborne diseases: Giardiasis Cryptosporidium Infectious Hepatisis ( viral infection ) Protozoa

Scales of the Embedded Virus Location Deactivation rate Dispersed Very fast Inside cell with disrupted cell wall Slow Inside intact cell Very slow 1000 nm 1 mm Virus particles are about 20 nm, HOCl are about 0.2 nm Viruses and bacteria are embedded within organic matter in turbid water

Mass Transport and Chlorine Protection Chlorine must diffuse through cell contents to reach virus Organic material inside the cell reacts with chlorine before it gets to the virus

Scale this up to a Fecal Aggregate Turbid water could easily have organic particles that are 10 or even 100 mm in diameter The amount of organic matter in a small particle and the slow diffusion would provide long term protection for embedded pathogens 10 mm

Getting the Right Dose: WHO on Chlorination Chlorine compounds usually destroy pathogens after 30 minutes of contact time, and free residual chlorine (0.2–0.5 mg per liter of treated water) can be maintained in the water supply to provide ongoing disinfection. Several chlorine compounds, such as sodium hypochlorite and calcium hypochlorite, can be used domestically, but the active chlorine concentrations of such sources can be different and this should be taken into account when calculating the amount of chlorine to add to the water. http://www.who.int/water_sanitation_health/hygiene/om/en/linkingchap6.pdf

Getting the Right Dose: WHO on Chlorination The amount of chlorine that will be needed to kill the pathogens will be affected by the quality of the untreated water and by the strength of the chlorine compound used. If the water is excessively turbid, it should be filtered or allowed to settle before chlorinating it http://www.who.int/water_sanitation_health/hygiene/om/en/linkingchap6.pdf

Chlorine Reactions Cl2 + H2O  HOCl + HCl- HOCl  H+ + OCl- Charges +1 -2 +1 -1 Cl2 + H2O  HOCl + HCl- HOCl  H+ + OCl- The sum of HOCl and OCl- is called the - Hypochlorous acid Hypochlorite ion free chlorine residual

Chlorine and pH HOCl  H+ + OCl- low HOCl is the more effective disinfectant Therefore chlorine disinfection is more effective at ________ pH Dissociation constant is 10-7.5 HOCl and OCl- are in equilibrium at pH 7.5 low

Ammonia Reactions NH3(aq) + HOCl  NH2Cl+ H2O -3 +1 -3 +1 +1 NH3(aq) + HOCl  NH2Cl+ H2O Combined chlorine NH2Cl + HOCl  NHCl2+ H2O Rittmann, B. E. and P. M. Huck (1989). "Biological Treatment of Public Water Supplies." Critical Reviews in Environmental Control 19(2): 119-184. Substitution reactions… The combined chlorine maintains its oxidizing potential

Removal of ammonia by chlorination Breakpoint Chlorination: The addition of chlorine in such amount that it Oxidizes the organic matter, reducing matters and free ammonia in raw water. And leaves free residual Cl which disinfect Water. Removal of ammonia by chlorination Oxidizing equivalents of chlorine are consumed -3 +1 -1 2NH3(aq) + 3HOCl  N2+ 3Cl- + 3H2O Rittmann, B. E. and P. M. Huck (1989). "Biological Treatment of Public Water Supplies." Critical Reviews in Environmental Control 19(2): 119-184.

Generalized curve obtained during breakpoint chlorination

Break Point Chlorination

Advantages of Breakpoint Chlorination It will remove taste and odour of water. The desired residual chlorine will remain in water. It will remove organic matter and Mn. It will complete oxidation of Ammonia from water.

Does Chlorine Completely oxidize organic matter? 4HOCl + CH4  CO2 + 2H2O + 4Cl- + 4H+ Oxidation states Carbon in organic matter (-4) Carbon in carbon dioxide (+4) Chlorine in HOCl (+1) Chloride (-1) Therefore should take 4 moles of chlorine (Cl2) per mole of organic carbon 23.6 g chlorine/g organic carbon

Common Disinfectants in Water Treatment Chlorine Chloramines Chlorine dioxide Ozone Ultraviolet light

Chlorine Most commonly used disinfectant In water chlorine undergoes the following reaction: Cl2 + H2O HOCl + HCl HOCl H+ + OCl- HOCl and OCl- is defined as free available chlorine HOCl more effective than OCl- due to lack of charge Presence of HOCL and OCl- is determined by pH In drinking water 1 mg/L of chlorine for 30 min is generally sufficient to reduce bacterial numbers. In wastewater with interfering substances up to 20-40 mg/L may be required

Interfering Substances Turbidity can prevent adequate contact between chlorine and pathogens Chlorine reacts with organic and inorganic nitrogenous compounds, iron, manganese, and hydrogen sulfide. Dissolved organic compounds exert a chlorine demand Knowing the concentrations of interfering substances is important in determining chlorine dose

Chloramines Chloramines are produced by combining chlorine and ammonia NH3 + HOCl NH2Cl + H2O monochloramine NH2Cl + HOCl NH2Cl2 + H2O dichloramine NH2Cl2 + HOCl NCl3 + H2O trichloramine breakpoint reaction Used mainly as secondary disinfectants, e.g., following ozone treatment, when a residual in the distribution system is needed

Chloramines (advantages and disadvantages) Less corrosive Less toxicity and chemical hazards Relatively tolerable to inorganic and organic loads No known formation of DBP (disinfection byproducts) Relatively long-lasting residuals Disadvantages Not so effective against viruses, protozoan cysts, and bacterial spores

General Characteristics Chlorine Dioxide: ClO2 General Characteristics Highly soluble in water (10x more than Cl2) Volatile & subject to photo-decomposition Unstable at high concentrations (>15%) and under pressure ~ must be generated onsite 2NaClO2 + Cl2 2ClO2 + 2NaCl Oxidizes, does not chlorinate, using selective one- electron transfer Does not react with ammonia Unaffected by pH over broad range (4-8) 41

Chlorine dioxide (advantages and disadvantages) Very effective against all type of microbes Disadvantages Expensive Unstable (must produced on-site) High toxicity 2ClO2 + 2OH- = H2O + ClO3- (Chlorate) + ClO2-(Chlorite) (in alkaline pH) High chemical hazards Highly sensitive to inorganic and organic loads Formation of harmful disinfection by-products (DBP’s) No lasting residuals

Chlorine Advantages Chlorine provides a strong residual in the distribution system. Chlorine can be easily converted to chloramines which also provide a strong residual and do not produce by-products. Chlorine is easy to apply. Can use it in liquid, solid, or gas form Chlorine is a relatively inexpensive disinfecting agent. Chlorine is effective at low concentrations.

Chlorine Disadvantages When chlorine reacts with organic material its' concentration is reduced and trihalomethanes(THM's), haloacetic acids (HAA5), chlorite (when chlorine dioxide is used) and bromate (when ozone is used) are disinfection-by-products (DBP's). These compounds are carcinogenic. Chlorine provides poor Cryptosporidium and Giardia control. Effectiveness varies depending on turbidity, [ammonia], pH, etc. Chlorine is a dangerous and potentially fatal chemical if used improperly

TREATMENT OF WATER - CONTD Methods of chlorination Plain chlorination Pre-chlorination Post chlorination Double chlorination Breakpoint chlorination Super chlorination De chlorination

Chlorination levels If a system does not allow adequate contact time with normal dosages of chlorine, superchlorination followed by dechlorination (chlorine removal) may be necessary. Superchlorination: It is adopted during outbreak of epidemics and in heavily polluted waters. It provides a chlorine residual of 3.0-5.0 mg/l, 10 times the recommended minimum breakpoint chlorine concentration. Retention time for superchlorination is approximately 5 minutes.

Dechlorination: Dechlorination is the process of removing residual chlorine from disinfected wastewater prior to discharge to the distribution system. Sulfur dioxide is most commonly used for dechlorination . Other methods of dechlorination are uses of Sodium thiosulphate and Sodium biosulphate and prolonged storage and absorption on charcoal. Prechlorination: Application of chlorine prior to any unit treatment process. This helps in controlling biological growth in raw water pipes and increasing the efficiency of further treatment process. Post Chlorination: application of chlorine to treated water before it enters in the distribution system. This helps to maintain residual chlorine in treated water. Re- Chlorination: When distribution system is long and complex, the residual chlorine of 0.2 mg/l may not be available at the consumer end. To achieve this stage-wise application of chlorine in distribution system is carried out and is called re-chlorination.

TREATMENT OF WATER - CONTD Plain chlorination →Treatment in which only chlorination is done Used when the water is more or less pure with less turbidity (20-30mg/lit) → Lake water Dosage → 0.50mg/lit Pre-chlorination → Applied before filtration/ even sedimentation Reduce load on filter bed Reduce the odour, colour, algae, organic impurities Applied such that 0.20-0.50 mg/lit as residual

TREATMENT OF WATER - CONTD Post chlorination → Ordinary method Applied to the filtered water Applied such that 0.20-0.50 mg/lit as residual Double chlorination → Pre and post chlorination is done simultaneously