First Flush Disinfection: Challenges of chlorinating wet weather primary effluent to meet a stringent E. coli target

A study to assist with the design of a chlorination facility for the Missouri River Wastewater Treatment Plant in Omaha, Nebraska is discussed. The facility will disinfect wet weather primary effluent flows to meet an effluent E. coli of 126 MPN/100mL as a geometric mean for an individual storm event. Samples were collected for seven events. Testing for samples collected during the first two wet weather events determined the presence, duration, and magnitude of a first flush. Subsequent wet weather samples were collected for three wet weather events. Paired primary influent and effluent samples were collected at the start of the event and at a time corresponding to the first flush. Samples were chlorinated at doses of 5, 15, 30, and 60 mg/L Cl2 and tested at various batch reaction times. Testing indicated that a free chlorine residual was needed to achieve a reduction of E. coli to 126 MPN/100mL. Further testing was conducted using dry weather samples. Raw, bench settled, and bench chemically enhanced settled samples were disinfected with chlorine to ammonia ratios of 3, 5, 12, and 16:1 and various batch reaction times. Results of the dry weather sample testing confirm that a free chlorine residual is needed to meet the required disinfection; the chlorine dose required is directly related to the ammonia concentration of the wastewater. During a storm event, the ammonia concentration was determined to vary from approximately 15 mg-N/L (during the first three hours of an event) to 2 mg-N/L (once the first flush dissipates). The chlorination facility will be designed with a high turndown capacity to meet the high range of chlorine doses required to achieve a free chlorine residual. Options for storing the initial high-ammonia wastewater for later dilution and/or sending as much of the high-ammonia wastewater through secondary treatment will also be considered during final design of the chlorination facility.