THE DEVELOPMENT OF DISINFECTION DESIGN CRITERIA FOR A 13,262 CUBIC FEET PER SECOND COMBINED SEWER OVERFLOW TREATMENT FACILITY

The Detroit Water and Sewerage Department (DWSD) developed a Long-Term Combined Sewer Overflow (CSO) Control Plan (Plan). The Plan addresses control of discharges from CSO outfalls, which discharge to the Detroit River and the Rouge River. One component of the Plan is the Conner Creek Pilot CSO Control Facility (Facility), which discharges to the Detroit River.In lieu of following the State of Michigan Department of Environmental Quality's (MDEQ's) presumptive criteria of 30 minutes of retention time for the disinfection of CSOs for the 10-year, 1-hour storm, DWSD negotiated with MDEQ to study, design, and construct a pilot CSO treatment facility to achieve daily and monthly fecal coliform limits of 400 and 200 colony forming units per 100 milliliters (cfu/100 ml), respectively, with five minutes of contact time at the ten-year, one-hour peak flow of 13,262 cubic feet per second (cfs). If this 30-million gallon “pilot facility” can meet the fecal coliform limits, it may allow for significant cost savings in CSO treatment.The objective of this investigation was to determine the appropriate design criteria for storage, feeding, and mixing of sodium hypochlorite with CSO to achieve the monthly fecal coliform limit of 200 cfu/100 ml with 5 minutes of contact time. Both field scale tests and computer modeling were performed to determine the design criteria.