EFFICACY OF CHLORINE AND CHLORAMINE DECAY WITHIN A PROTOTYPE DISTRIBUTION NETWORK

Abstract

One of the major responsibilities of drinking water supply agencies is to prevent faecal contamination of drinking water to ensure its safe provision. But upon disinfection to inactivate bacteria, the presence of organic and inorganic precursors in raw water leads to the formation of harmful disinfection by-products (DBPs). The present study compared the biocidal efficiency of chlorine and chloramine, two common disinfectants, to inactivate gram negative (K. pneumoniae & P. aeruginosa) and gram-positive (S. aureus) microorganisms along with monitoring the formation of DBPs i.e. Chloroform, Iodoform & NDMA in treated water. Bench-scale experiments were conducted to optimize chlorine dose and contact time for maximum efficiency of both disinfectants. Response surface methodology (RSM-CCD) was employed to design experiments for lab-scale distribution network. After a series of experimental runs, dose was found to be the most significant factor (p<0.01) in determining DBPs formation while contact time significantly (p<0.01) affected bacterial inactivation by both disinfectants. The quantification results for THMs revealed that chloroform was the predominant species in all post-chlorinated water samples, however, 88% samples surpassed USEPA limit of 0.07 mg/l. Whereas, iodoform speciation was less favourable. NDMA was predominant in chloraminated samples. A negative correlation between bacterial regrowth and disinfectant residual was found. Chloramines were more stable than free residual chlorine thus inhibiting bacterial regrowth. The model was further validated and results showed that the predicted values were near to the experimental values. The ANOVA test also showed that the model is significant (p<0.01), which indicated that the model is a good fit for this research study. PCR detection of ammonia oxidizing bacteria (AOB) in sludge spiked water revealed 2-log lesser bacterial inactivation as nitrification by AOB converted monochloramine to free chlorine thus reducing its disinfection efficiency.