Effect of pH and Temperature on Ozone Disinfection

Abstract

The present study was undertaken to observe the disinfection efficiency of ozone in different water samples at different pH and temperatures. The water samples including distilled water, well water and tap water were collected from Rawalpindi / Islamabad area. Pure cultures of Escherichia coli (E. coli) and Salmonella were isolated from waste water collected from Nullah Lai and stored in nutrient agar slants at 4 oc after isolation and purification. The microbial cultures were inoculated in water samples for inactivation study. Three different ozone dosages, i.e., 1.5, 1.7 and 2 mg/l in gas phase were applied for 3 minutes. The ozonation was carried out at 15, 25 and 35 oc temperature and pH of 6, 7 and 8. During ozonation, samples were taken at different intervals of time such as 5, 10, 20, 30, 45, 60 seconds and 3 minutes to determine their survival using spread plate count (SPC) method. The ozone residual concentration for each sample was also determined. Ozone was found to be a potent disinfectant which reduced bacterial counts from initial count of 107 CFU/ml to almost non-detectable level after contact time of 60 seconds. Among the three applied ozone dosages, highest microbial inactivation was observed in distilled water with an applied ozone dosage of 2 mg/l, i.e., a 5 log decrease of E. coli just after 20 seconds. While similar disinfection was observed in tap water and well water samples after 45 and 60 seconds respectively. Disinfection study of E. coli was carried out at three different pH values. It is evident from the results that the survival of E coli was high at pH 8, than survival of E. coli observed at pH of 7 and 6. After ozonation of 60 seconds, the E. coli counts were 5.80x10 1 at pH of 6, while it was and at pH of 7 and 8 respectively. When disinfection studies of E. coli were carried out at different temperatures, it was revealed that disinfection efficiency of ozone was higher at 35 oc as compared to lower temperatures of 15 oc and 25 0C.