The Study On Nitrification And Its Control Measures In Chloraminated Drinking Water Distribution Systems

The nitrification occurs widely in the chloraminated drinking water distribution systems, and it leads many adverse effects, including decay of disinfection, the regrowth of HPC and decline of water quality. Taking nitrification in simulated water distribution system in laboratory as study object, and Guarantee of safety drinking water quality as the study purpose, the paper worked systemicly and profoundly on nitrification in chloraminated drinking water distribution system and its control.Based on the demand of research, experimental schemes of static beaker, static pipe wall and dynamic distribution simulated system were established. The effects of different pH, sequence of chlorination and ammoniation and water temperature on nitrification under conditions of long-term static are studied. The results show that suitable pH range for nitrifying bacteria growth is 7.0 ~ 8.5, when pH below 7.0, the rate of nitrification decreased significantly. Using firstly chlorination and then ammoniation can be effective to reduce the possibility of nitrification episode; the higher the water temperature, the larger possibility of nitrification occurred.Based on simulated water distribution system dynamic experiment, water temperature, initial chlorine concentration, pH, chlorine-to-ammonia ratios, hydraulic retention time, pipe material and other factors that influence nitrification are studied, which supplied experimentation basis for building water quality model of nitrification in water supply network. Changing the above water quality conditions can control and prevent the occurrence of nitrification, but when nitrification of water distribution system has been very serious, changing the above water quality parameters, nitrification can not be well controlled.Using Monod kinetics, a steady state plug-flow kinetics model was developed to describe the variations of ammonia, nitrite and nitrate-N concentrations in a chloraminated distribution system. Active AOB and NOB biomass in the distribution system was determined using predictive equations within the model. The kinetic model was solved by goal seek programme in Excel modified by VBA. The model predicted ammonia and nitrate concentrations adequately but overestimated nitrite, which may have been caused by the lack of consideration of nitrite oxidation by chloramines.Finally, the nitrification control practices of booster chlorination, temporary free chlorination and breakpoint chlorination are studied. The results show that the use of free chlorine disinfection inactivated AOB or alternating to use free chlorine disinfection and chloramine disinfection maybe an effective measure to control nitrification and inhibit bacteria growth in water distribution system.