| Factors influencing the growth of nitrifying biofilms in chloraminated drinking water distribution systems were investigated through both field observations and laboratory experiments. The field study was designed to observe nitrification in a New Jersey water distribution system and its relation to water quality parameters over four seasons. Samples collected through hydrant flushing and coupons inserted in pipelines contained both ammonia-oxidizing and nitrite-oxidizing bacteria. The nitrifying bacteria most probable number counts (MPNs) depended on season, location, and method that samples were collected. Higher counts of nitrifying bacteria tended to occur more frequently in summer than in winter, and in the areas where chlorine residuals were relatively lower. Overall, the nitrifying bacteria counts were positively correlated with the heterotrophic bacteria counts observed in the hydrant flushing water samples.; To simulate a chloraminated water distribution system, an annular reactor with continuous feeding was built to observe nitrification and its relation to the particular water quality parameters disinfectant concentration, total organic carton (TOC) concentration, pH, substratum material, and sheer stress on the surfaces of reactor walls. With chlorine residual at 2 mg/L, more nitrifying bacteria were found at a 5:1 chlorine-to-ammonia-N mass ratio than at a 3:1 ratio, but more heterotrophs were found at the 3:1 ratio. More heterotrophic bacteria were found in reactors at a higher TOC concentration (2 mg/L vs 0.3 mg/L). Adding TOC seemed to increase the growth of nitrifying bacteria, if the chlorine-to-ammonia ratio was at 5:1. When the chlorine-to-ammonia ratio was higher, or when more TOC was added to the water, the decay of monochloramine was also faster. There were significantly (p < 0.01) higher nitrifying bacteria counts and lower heterotroph counts in chloramine at pH 7.5 than at pH 9.0.; In addition to chemical parameters, two physical parameters affecting the growth of bacteria biofilms in the water distribution system were also investigated. High-density polyethylene showed a much higher bacterial density than copper, stainless steel, and polycarbonate slides. There also was less biofilm accumulated on the higher sheer stress surface (that on the inner rotator) than on the lower shear stress surface (on the outer cylinder).; These results suggest that compared with 5:1, the 3:1 chlorine-to-ammonia-N ratio at pH 7.5 was preferable for controlling nitrifying biofilm formation. Minimizing TOC is also helpful. Hydrant flushing may be an essential practice for chloraminated utilities because of its capacity to remove sediment and biofilms from the pipelines. |
