Statement of the Problem
Wastewater chloramination is practiced when the effluent must be free of THM’s and other disinfection byproducts. It is also practiced at plants that have substantial contact time available (such as in a long discharge pipe) and at plants where chloramine is used to back flush or clean membranes in the treatment process.
Chloramination of wastewater is difficult to control because of the variable concentration of ammonia that may be present in wastewater that has not been fully nitrified during the treatment process. The ammonia concentration is rarely proportionate to the flow rate, thus making flow paced control an unreliable strategy. Nitrate, organics and microbial demand may compete for chlorine, making calculated feed rates unpredictable.
Process Control Strategy
Under variable ammonia conditions, one strategy is to monitor incoming ammonia (prior to a chlorine addition point) and use this information to feed forward a desired addition value for chlorine which has been stochiometrically calculated to produce a desired monochloramine concentration. Some plants choose to fully nitrify the wastewater, then add both ammonia and chlorine. A second sample point at or after the point of chlorine addition is used to verify that the desired monochloramine concentration is being achieved and that the set point for free ammonia following chloramine formation is also as desired (zero or at some minimum value). The desired monochloramine concentration is selected based on the concentration and contact time necessary to assure disinfection of the wastewater.
ChemScan Process Analyzers are used to measure incoming nutrients prior to chloramine formation and are used to detec free ammonia, monochloramine and totoal chlorine downstream from chloramine formation.