The Great Lakes Water Authority (GLWA) Customer Outreach Wastewater Best Practices Work Group is continuing to share information about the performance of the wastewater and stormwater infrastructure system tributary to the Detroit Wastewater Treatment Plant (WWTP). The 22 combined sewer overflow (CSO) retention treatment basin (RTB) facilities that operate throughout the GLWA wastewater service area play a major role in protecting water quality.
From May through October 2016, these RTBs prevented more than 9.5 billion gallons of untreated combined sewage from overflowing into our waterways. Of this total, 3.2 billion gallons were captured, transported to and treated at the WWTP, and 6.3 billion gallons were treated and discharged from RTBs. These treated discharges meet federal and state National Pollutant Discharge Elimination System (NPDES) permit requirements and are deemed protective of public health.
The Detroit area experienced the warmest summer on record. Monthly records were set for the 2nd warmest August and 4th wettest September. Overall, RTBs went into operation on 46 out of 184 days. Operational days for area RTBs are shown in Figure 1. Treated and captured volumes of combined sewage are shown in Figure 2.
From a volume perspective, the largest storm to hit the area occurred over a several day period at the end of September and beginning of October. This storm generated more than 3.1 billion gallons, or 50%, of the total RTB volumes that were treated and discharged during the entire recreation season. Two localized, intense storms also occurred in July and August delivering high volumes of rain over short periods of time on the east side of Detroit and in adjacent communities.
Figure 1 – RTBs operated on 25% of the days in the 2016 recreation season based on a review of initial data. The remaining 75% of the time, RTBs were on standby ready to go into operation when a storm event hit the area.
Figure 2 – Based on a review of initial data, it is estimated that 34% of the combined stormwater and wastewater flows captured in RTBs during the recreation season were sent to the WWTP after storms subsided. The remaining 66% of flows, or 6.3 billion gallons, were treated in RTBs before being discharged.
Source: Great Lakes Water Authority