Application of the microbial decay factor to maintain chloramine in large tanks

Microbial acceleration of chloramine decay (including nitrification) exacerbates the difficulty of maintaining an adequate disinfectant residual in chloraminated systems with long retention times. An adaptive management strategy based on the microbial decay factor (F_n) was developed to improve chloramine residual in a service reservoir (tank). The reservoir studied has a capacity of 404 ML and is not mechanically mixed. It undergoes continuous chemical, physical, and microbial stratification in summer but in winter is only microbially stratified. At the end of summer 2002-03 (southern hemisphere), signs of severe nitrification were observed. After careful consideration of control options, serial dilution was adopted, particularly in winter. The F _m value resulting from this serial dilution was predictable from previous measurement of F_m in the reservoir and its inflow. Subsequently, an adaptive control strategy to maintain low F_m ensured that the reservoir was clear of severe nitrification for the next two years of evaluation.