Chloramination in a Partially Nitrified Effluent: Process Control Solutions and Case Study

The Westside Wastewater Treatment Plant (WWTP) was required to implement effluent disinfection as a result of its renewed NPDES permit issued in 2011. The new permit requires disinfection to be operational by December 31, 2013. The city chose bulk sodium hypochlorite, and an investigation into the plant's operating data showed that nitrification occurred a few days each year. Further research into similar WWTPs revealed that it may be difficult to maintain process control of chloramination in a low ammonia effluent and that a large chlorine demand could be exerted prior to the formation of free chlorine. The design team opted to provide continuous effluent ammonia monitoring. This would help alert the operators to implement process changes when ammonia concentrations dropped below 1-mg/L. In this condition a chloramine residual may not be possible so the chlorine dose should be increased to establish free residual chlorine. This paper provides a discussion of WWTPs with low effluent ammonia and discusses options that are available to improve disinfection. Additionally, a case study is provided describing the process control strategy for the Westside WWTP.