Over the past ten years, multiple end-of-pipe control facilities have been constructed within theSoutheastern Michigan region to control CSO flows. The National Pollutant Discharge Elimination System (NPDES) permits issued for these facilities contains a requirement to provide disinfection
of the treated effluent discharged from these facilities. Sodium hypochlorite has been the disinfectant of choice at all CSO control facilities within the region. This chemical has been selectedbecause it is relatively safe, cost effective, and can achieve satisfactory disinfection performance
if dosed correctly. The primary concern over the use of sodium hypochlorite has been with theability to meet the permit requirement while limiting the discharge of residual chlorine. Facility operators have been challenged to satisfy their NPDES permit requirements by the highly variable
nature of CSO flows. CSO flow rates are highly dynamic and very variable. The chlorine demand of the flow varies through the storm event and is likely correlated to the total suspended solids concentration. As every storm event is different, the facility operators have over the years developed
an historical basis to modify their disinfection protocols "on the fly" in response to the conditions they encounter during each and every CSO event. This paper will review the process control techniques being implemented at several facilities within the region to control the application
of sodium hypochlorite. Data collected over the years will be used to demonstrate the effectiveness of each sodium hypochlorite application technique. It is anticipate that, over time, the procedures to control the application of sodium hypochloriteat CSO control facilities will evolve. At
2 AM during a storm event, the ability of a facility to satisfactory meet the disinfection requirements contained within its NPDES permit will ultimately determines the long-term operational success of a facility. Operators are challenged to protect the environment for the general public,
while at the same time trying to operate a facility under a wide range of flow and chlorine demand conditions. Operators at newly constructed CSO control facilities can benefit from the operational experiences at the Southeastern Michigan CSO control facilities over the past ten years.
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