CT Setpoint Control for Disinfection of an 8.6 Billion Gallon Per Day Combined Sewer Overflow
Authors:
- Curtis D. Courter, P.E. - Hazen and Sawyer
-
CFD modeling of the innovative disinfection process at the Conner Creek CSO Control Facility. Read more about this facility's impact on the environment here.
Prior to construction of the Detroit Water and Sewerage Department’s (DWSD) Conner Creek CSO Control Facility (Facility), three combined sewer outfalls regularly discharged large volumes of untreated combined sewage into Conner Creek, a ½-mile long tributary to the Detroit River. On average, these combined sewer overflows resulted in discharges of over 2 billion gallons per year to the receiving waters, with a peak rate of flow in excess of 13,000 cfs for the 10-year, 1-hour storm event.
In 1994 the Michigan Department of Environmental Quality (MDEQ) adopted a presumptive treatment design criteria, which required skimming, settling, and disinfection, with thirty minutes of detention time, to treat the 10-year, 1-hour storm. Concurrently, the MDEQ allowed permittees to propose alternative levels of control provided that the facility “demonstrates” that it is capable of achieving water quality standards at times of discharge. Therefore, in lieu of the presumptive approach, the DWSD negotiated an agreement with the MDEQ to study, design, and construct a pilot CSO treatment facility to achieve daily and monthly fecal coliform limits of 400 and 200 (average geometric mean) colony forming units per 100 milliliters (cfu/100 ml), respectively, with only five minutes of contact time at the 10-year, 1-hour peak flow of 13,262 cubic feet per second (cfs). In 1997, the DWSD’s National Pollution Discharge Elimination System (NPDES) Permit was revised to reflect this agreement, and require construction of an approximately 27-million gallon retention basin, to provide settling, skimming, and disinfection of all transported combined sewage discharged from the three existing Outfalls to the Conner Creek.
Field pilot testing of the high-rate disinfection was completed on actual combined sewer overflows from the three outfalls to determine the dosing requirements to meet the effluent fecal coliform limit with a five-minute contact time. Based on the test results, the effluent fecal coliform limits could always be achieved with a maximum Cl2 dose of 25 mg/l. However, due to the extreme range of flows to the Facility, and the infrequency of the design storm, a much lower dose can be utilized for most events. Therefore, the disinfection system was designed to feed based on a concentration times contact time (CT) setpoint of 125, where the required dose is directly linked to the influent flow rate. Through this dosing method the permitted effluent standards are being met while reducing chemical usage and effluent TRC.
The operating Facility’s disinfection process is controlled by a computerized process control system using the CT setpoint identified during the study and design phases. Furthermore, due to the demonstrative nature of the Facility, stricter monitoring and reporting requirements were mandated for a period of two years after start up of the Facility. This intensive monitoring of operations confirmed successful operation of this facility and it was subsequently accepted by the MDEQ.
This presentation will focus on operational and process control considerations based on design phase process piloting testing and facility operational observations.
To request a copy of the full presentation, please contact the author at .(JavaScript must be enabled to view this email address).
