CEL 795- Water and Wastewater Treatment Unit Processes 1 st -Semester 2011-2012 Disinfection Dr. Arun Kumar (arunku@civil.iitd.ac.in)
Courtesy: Dr. Irene Xagoraraki (MSU, USA) Disinfection Water is often disinfected before it enters the distribution system to ensure that potentially dangerous microbes are killed. Chlorine, chloramines, or chlorine dioxide are most often used because they are very effective disinfectants, not only at the treatment plant but also in the pipes that distribute water to our homes and businesses. Ozone and ultraviolet radiation are effective disinfectants for relatively clean source waters, but neither of these are effective in controlling biological contaminants in the distribution pipes. October 17, 2011 (C) Arun Kumar, IIT Delhi 2
Courtesy: Dr. Irene Xagoraraki (MSU, USA) Disinfection Disinfectants: NaOCl Ca(OCl) 2 Cl 2 gas Chloramines Ozone UV irradiation October 17, 2011 (C) Arun Kumar, IIT Delhi 3
Courtesy: Dr. Irene Xagoraraki (MSU, USA) Disinfection (chlorination) Full-scale Bench-scale October 17, 2011 (C) Arun Kumar, IIT Delhi 4
The Chick s Law: Number of pathogens at time t (N t ) for initial N 0 pathogens: N t =N 0 exp (-kt m ) [1] Where k is disinfection constant and depends on pathogendisinfectant interaction (unit=1/time unit); m is interaction constant The Watson s Law (relationship between chemical concentration and contact time) C n t p =constant [2] Here, C is disinfectant concentration (mg/l) and t p is time required to effect a constant % kill (mg/l) If n>1 => contact time is more important than dosage and n=1, both parameters contribute equally. October 17, 2011 (C) Arun Kumar, IIT Delhi 5
Intensity and Nature of Physical Agent:(Effect of heat and light; function of intensity) N t =N 0 exp (-kt) [1] Here, N is number of microorganisms, t is time (minutes) and k is reaction velocity of constant (1/min) Chlorination Reactions in water Reactions with ammonia Breakpoint reaction October 17, 2011 (C) Arun Kumar, IIT Delhi 6
Example: Chlorine Demand? For a 1-h contact time, chlorine dose to achieve a combined residual of 0.4 mg/l is 0.6 mg/l. Determine the daily amount of NaOCl to be applied to treat a wastewater to produce 0.4 mg/l combined residual and 0.5 mg/l free residual (flow=24000 m 3 /day) October 17, 2011 (C) Arun Kumar, IIT Delhi 7
Example: Chlorine Demand: Solution NaOCl Na + + OCl - Molecular weight of NaOCl=75.5 g/mole As 1 mole of Cl 2 gives 1 mole of OCl - (reversible reaction) So 1 mole of OCl - also produces 1 mole of Cl2 in water during use of NaOCl. Amount of NaOCl added to meet the required demand of 0.6 mg/l chlorine =(24000 m 3 /day)(0.6mg chlorine gas/l)(74.5 mg NaOCl/71 mg Cl 2 )(1000L/m 3 )(1kg/10 6 mg)=15.1 kg/day October 17, 2011 (C) Arun Kumar, IIT Delhi 8
Example: Chlorine Demand: Solution Amount of NaOCl added to meet the required demand of 0.5 mg/l free chlorine (i.e., total required = 1.6 mg/l) =(24000 m 3 /day)(1.6mg chlorine gas/l)(74.5 mg NaOCl/71 mg Cl 2 )(1000L/m 3 )(1kg/10 6 mg)=40.3 kg/day Say NaOCl is 80% pure. So, NaOCl required =40.3kg/day /(0.80) October 17, 2011 (C) Arun Kumar, IIT Delhi 9
Chlorine Demand (Chlorine decays in system) Say required free chlorine residual = C residual But chlorine decays at the rate of k decay (for the first order decay behavior with time). Chlorine conc. need to be increased to take care of the decay in duration (t). Thus, required free chlorine residual C modified = C residual *exp (k decay *time) (i.e., increasing the original free residual chlorine by the term [exp (k decay *time)] October 17, 2011 (C) Arun Kumar, IIT Delhi 10
1. Chlorine Dioxide as a Disinfectant Chlorine dioxide an unstable and explosive gas and thus it is generated on-site. 2NaClO 2 + Cl 2 ----> 2ClO 2 + 2NaCl Here, 1.34 mg Sodium chlorite reacts with 0.5 mg chlorine to yield 1 mg chlorine dioxide. As Sodium chlorite is only 80% pure, thus we need = 1.34/0.80= 1.68 mg to produce 1 mg chlorine dioxide. October 17, 2011 (C) Arun Kumar, IIT Delhi 11
Chlorine Dioxide Effectiveness Agent responsible: Free dissolved chlorine dioxide (ClO 2 ), which has a very high germicidal effect. It doesn t react with ammonia and thus no additional dose is required. Possible bactericidal mechanism: Inactivation of critical enzyme systems or disruption of protein synthesis. By-product formation Environmental impacts Chlorite and chlorate are toxic in nature. Not yet known, probably less adverse than that from chlorine. It remains as chlorine dioxide in solution and persist in environment. October 17, 2011 (C) Arun Kumar, IIT Delhi 12
2. Ozone as a Disinfectant Ozone: Used for controlling odor, taste and color producing agents from water (also in lieu of the adsorption process). Unstable => generated onsite (electrical discharge method) Ozone Chemistry O 3 + H 2 O ---> (HO 3+ ) + (OH - ) (1a) (HO 3+ ) + OH - ----> 2(HO 2 ) (1b) O 3 + HO 2 -----> (HO) + 2(O 2 ) (1c) HO + HO 2 ----> (H 2 O) + (O 2 ) (1d) Here, (HO 2 ) and (HO) are free radicals and have great oxidizing powers and are active disinfecting agents. October 17, 2011 (C) Arun Kumar, IIT Delhi 13
Ozone Effectiveness By-product formation Environmental impacts Agent responsible: Free radicals, which are high oxidizing agents. Possible bactericidal mechanism: It disintegrates cell wall (i.e., cell lysis). In addition, it is highly virucidal as well. It does not produce any ion/solids and does not react with ammonia. Beneficial impacts on environment; Ozone toxic to aquatic species but not much as it is short-lived. DO levels reaches to its saturated levels as ozone decomposes into oxygen only and thus it does not require any re-aeration of water for meeting the oxygen demand. It does not produce any residual and thus it doesn t require any special handling. October 17, 2011 (C) Arun Kumar, IIT Delhi 14
3. Ultraviolet (UV) Light as a Disinfectant Radiation emitted from the ultraviolet source: Used for disinfecting water supplies Low-pressure mercury arc lamp-the primary source of generating UV energy. The mercury lamp is preferred as 85% of light output is monochromatic at wavelength of 253.7 nm which is within the optimum range (250 to 270 nm). October 17, 2011 (C) Arun Kumar, IIT Delhi 15
UV Radiation (A physical disinfecting agent) Effectiveness By-product formation Environmental impacts Agent responsible and mechanisms: Radiation at 254 nm penetrate the cell wall of the microorganisms and is absorbed by cellular materials including DNA and RNA which prevents the replication or causes death of the cell to occur. Water must be turbidity free for killing whole bacteria otherwise UV radiation might not able to completely kill bacteria. This disinfection process has been described as a first-order decay kinetics. None No residual formation, but additional information about organic compounds degraded are required. October 17, 2011 (C) Arun Kumar, IIT Delhi 16