Disinfection Objective: To inactivate pathogens Common Methods Free chlorine Ozone UV radiation Disinfectant Advantages Disadvantages Free chlorine Strong, cheap, residual THMs formation Chloramines No THMs Very weak Chlorine dioxide Very strong Expensive, Chlorate/Chlorite formation Ozone Very strong Cost, no residual UV radiation Strong Expensive, no residual Factors Affecting Microorganism Inactivation Type of disinfectant Concentration (C): Higher inactivation is reached with higher C Contact time (t): Longer contact time results in higher inactivation Temperature: For each 10 oC increase in temperature, the inactivation rate doubles pH: Increasing pH generally increases (C.t) required to achieve the same inactivation Turbidity C.t product Each disinfectant has distinctive characteristics that results in different C.t values for the same microorganism and same conditions C.t (mg.min/l) values for 1.0-log in activation of Giardia lamblia cysts Disinfectant pH Water Temperature 0.5 oC oC 10 oC 15 oC 20 oC Free chlorine (based on residual of mg/l) 49 70 101 35 50 72 26 37 54 18 25 36 13 18 27 Performed chloramine 6-9 1300 740 620 500 370 Chloride dioxide 6-9 21 8.7 7.7 6.3 5.0 Ozone 6-9 0.97 0.63 0.48 0.32 0.24 1-log inactivation= 90% inactivation 2-log inactivation= 99% inactivation 3-log inactivation= 99.9% inactivation Example To inactivate 99% of Giardia Lamblia cysts a free chlorine dose of 3.6 mg/l is needed for 10 minutes What will be the time to reach the same inactivation rate with a chlorine concentration of mg/l? Solution Since the inactivation rate is the same, C.t will be constant Thus, C.t = 36 mg.min/l For C= 2mg/l, t will be 18 minutes Chlorination Cl2(g)+H2O HOCl pH < Hypochlorous acid HOCl +H+ +ClH+ +OCl- pH > HOCl and OCl- are called free available chlorine OCl- is weaker than HOCl Cl2(g)+NH3 NH2Cl, NHCl2, or NCl3 (Chloramines) Chloramines are called combined available chlorine and are weaker than free available chlorine but more persistent Increasing the chlorine dose results eventually in the oxidation :of ammonia or chloramines as 3Cl2(g)+ 2NH3 N2(g) + 6HCl Ammonia destruction )Chlorine demand (mg/L Chlorine residual )(mg/L Breakpoint chlorination Combined available chlorine Free available chlorine )Applied chlorine dose (mg/L