Interactions Between Chloramine Decay And Microbial Behaviors During Mild Nitrification Phase In Drinking Water

With the widespread attention to the disinfection by-products of drinking water,chloramine as a secondary disinfectant is increasingly replacing chlorine disinfection.However,the complex physical,chemical and microbial behavior processes（especially nitrification）in water distribution systems would lead to the decay of chloramine loss seriously threatening the safety of drinking water quality.Previous researches and control of chloramine decay in drinking water mostly focused on the late-stage nitrification reaction,while the underlying mechanisms of early-stage microbes-induced chloramine decay and associating accelerated nitrification are not fully understood yet.Therefore,this study aimed to investigate the effects of physical and chemical conditions on the chloramine stability and decay process,and to unravel its interactions with microbial behaviors in drinking water.This study would establish the connection among chloramine decay,microbial behaviors,resulting nitrification and water quality,thus providing theoretical guidance for nitrification control in chloramine disinfection system and thereby improvement of drinking water quality.Firstly,the effect of physical-chemical parameters of drinking water on chloramine components and stability was studied.The results showed that the Cl/N ratio of chloramine,water p H,turbidity and Br^-concentration posed significant impacts on the proportion of monochloramine（P<0.001）.Increasing initial chloramine dosage（1.0-4.0 mg/L）and Cl/N ratios（2-5）,or decreasing p H values（4-9）promoted the decay of chloramine.In addition,the Cl/N ration was observed to have the greatest influence on the stability of chloramine through orthogonal analysis,with its decay rate increasing from 0.0001 h^-1to 0.0003 h^-1in response to the Cl/N ratio elevated from 2:1 to 5:1.Secondly,the effect of drinking water microbes on chloramine decay process was studied.The results showed that the decay rate of monochloramine and total chlorine was magnified by increased chloramine dosage.The presence of microbes accelerated the chloramine decay course,which became more significant with inoculation density increasing.The microbial decay factor analysis of chloramine showed that the promoting effect of microbial-induced chloramine loss was more substantial at 2.0 mg/L chloramine and the Cl/N ratio of 2:1 with the decay rate of 0.015 h^-1and microbial decay factor of 2.520.However,the decay process was accelerated after 5 d incubation,meanwhile with the conversion of ammonia nitrogen to nitrate nitrogen,indicating that microbes-triggered chlorine decay yielded subsequent nitrification reaction.In contrast,the contribution of microbes to chloramine decay was inhibited at 3.0 mg/L chloramine.Thirdly,the effects of initial chloramine dosages on the microbial growth and associating water quality were studied.Augmenting chloramine dosage significantly inhibited microbial growth in bulk water as expected.As incubation time progressing,planktonic microbes decreased followed by a sudden elevation after 7 d incubation,which was accompanied by increased turbidity and decreased p H value.The results indicated that the decay of chloramine led to rapid microbial growth and water quality deterioration.Additionally,boosting chloramine dosage（1.0-2.0 mg/L）significantly encouraged biofilm formation,yet inhibited at 3.0 mg/L chloramine.Fourthly,the interactions between extracellular polymeric substances（EPS）-mediated chloramine decay and microbes were examined to further reveal the mechanism of microbes-induced chloramine decay at early stages.An elevation in chloramine concentration（1.0-2.0 mg/L）significantly promoted the EPS secretion（especially extracellular proteins）,while further increasing the concentration to 3.0mg/L inhibited the EPS production.Further investigation showed that EPS removal impeded microbial activities and thus delayed chloramine loss with prolonged nitrification occurrence,which demonstrated the crucial role of EPS on the chloramine decay course.In addition,extracellular proteins stimulated chloramine loss through sacrificing themselves with chloramine.Finally,the effects of water quality conditions on chloramine decay,microbial behavior and nitrogen compound release in water were studied.Elevating the ratio of source water encouraged microbial growth,EPS release and rapid chloramine decay,while the concentration of ammonia nitrogen remained in the gradual increase and the nitrate nitrogen at low levels（<0.05 mg/L）after 7 d incubation,indicating no obvious nitration reaction occurring.This further evidenced the crucial contribution of microbes and water quality to chloramine loss in drinking water.