Controlling On The Biofilm And Nitrification In Drinking Water Distribution System

In response to new limits on disinfection byproducts (DBPs), drinking water utilities in many countries are implementing chloramination for the purpose of distribution system disinfection. Although the use of chloramines resulted in inactivation bacteria, chloramines will react with organic matter in bulk water and biofilm. The chloramines consumed in the bulk and biofilm will release microbial assimilated organic carbon (AOC) which can serve as a source of energy for heterotrophic bacteria localized and reproduced in the bulk water and biofilm. Another significant concern associated with this practice is nitrification caused by the growth of ammonia-oxidizing bacteria (AOB) in the distribution, which is associated with an increase in heterotrophic plate count (HPC) bacteria. In this study, bacteria growth, nitrification, chloramines decay and the controlling method of microbial safety was investigated. The primary objective of this research was to develop scientific basis regarding the microbial safety in model distribution system and site sampling using chloramines for secondary disinfection.Two Rotating Annular Bioreactors (RABs) with copper and stainless steel pipe materials were adopted to investigate the effects of these two pipe materials and chloramines disinfection on biofilms formation. The result indicated that the heterotrophic bacteria concentration increased in the absence of chloramines, but the bacteria concentration in biofilm can be controlled in the presence of chloramines. The count of bacteria in copper slide was lower significantly than in steel slide in the absence and presence of chloramines. The control of HPC in bulk water and biofilm was achieved by chloramines, but low level chloramines (0.5-0.75mg/L) cannot prevent the formation of biofilm and bacteria regrowth in bulk water.Take Escherichia coli biofilm as the research objective, the comparative study was conducted regarding to the inactivation effect on the biofilm applying chlorine and chloramines. Two kinds of disinfectant oxidation on biofilm were observed for the first time. The released oxidation product from biofilm will stimulate heterotrophic bacteria regrowth. Chlorine inactivation effect on biofilm increased with the chlorine concentration increasing on the same CT value, but chloramines inactivation effect highly depend on CT value on the same chloramines concentration. Using chlorine and chloramines resulted in release AOC and microbial assimilated phosphorus (MAP) from biofilm and increase the bacteria regrowth potential (BRP).The study used a bench-scale approach to evaluate the impact of organic matter level and chlorine to ammonia-N ratio on the incidence and potential establishment of nitrifying biofilm within simulated water distribution system. The temperature and pH effect on nitrification also studied insimulated distribution system. The finding from this study clearly indicated that organic matter level influence significantly on nitrification, but chlorine to ammonia ratio is not the factor for nitrification. Obvious symptom of nitrification occurred at high level organic matter. Increasing pH level can stabilize and increase the chloramines concentration, which can control AOB concentration to some extent. Chloramines controlling for nitrification depend on chloramines dosage and temperature.Relationship was observed between heterotrophic bacteria growth and metabolic product produced by AOB. The formation of soluble microbial products (SMP) by AOB and dead AOB can provide a supplementary organic substrate for heterotrophic bacteria.It was found that auto-decomposition rate of chloramines increased with temperature increasing. Chloramines show unstable at high chlorine to ammonia ratio. Chloramines were more stable at higher pH level. Nitrite is typically found in distribution system due to nitrification. The presence of nitrite accelerates the chloramines decay. The chloramines decay rate caused by biofilm is 50% of total decay rate in simulated distribution system, which indicated that chloramines decay largely attribute to biofilm in distribution system.Because of disinfectant can react with organic matter in bulk water and biofilm and auto-decompose. The disinfectant concentration decreased will lead to bacteria growth in distribution system. It is important that maintain appropriate concentration disinfectant to guarantee microbial safety of drinking water.