A Study of Bacterial Regrowth in Chlorinated Water under Stagnant Conditions

Chlorination is a common disinfection method that has been applied for controlling pathogenic bacteria in the water treatment process to ensure consumer’s safety. However, some bacteria may survive after the chlorination and regrowth in distribution systems, especially under stagnant conditions. In this recent work, post-filtered water samples from a water treatment plant were used in chlorination with free chlorine concentration of 1 mg/L, and subsequently incubated at 20˚C in the stagnant conditions for 30 days to observe bacterial regrowth. It was found that 4-log or 99.99% reduction of heterotrophic bacteria was achieved with 1 mg/L as free chlorine in chlorination after 30 min of contact time and disinfection rate constant was 0.523 min-1. The regrowth of heterotrophic bacteria noted after 5th day and kept increasing at the concentration of 5.67 x 10^6 CFU/mL at 30th day. Therefore, it can be concluded that a 1 mg/L as free chlorine in chlorination was not enough for controlling bacterial regrowth in the stagnant conditions and may pose health risk to the consumer.