The wet weather experienced in 2011 further demonstrates the significant impact that precipitation has on treated discharges from retention treatment basins (RTBs) serving combined sewer systems. Southeast Michigan experienced record-setting levels of rain and snow, and RTBs set records for treated discharges. Wastewater generated from our homes and businesses could not have contributed the volumes of flows that taxed combined sewer systems. Storm water runoff was generated in record volumes from our streets, parking lots and roofs during frequent and intense rain storms this past year.
RTBs are used to treat a mixture of storm water and sewage during large storms when combined sewers in the area become overloaded with storm water and the Detroit Wastewater Treatment Plant (WWTP) reaches its maximum treatment capacity. These treated discharges meet federal and state National Pollutant Discharge Elimination System (NPDES) permit requirements and are deemed protective of public health.
The Detroit WWTP services 77 communities in southeast Michigan; 26 of these communities have older combined sewer systems that have implemented controls to handle these increased wet weather flows. Since 1959, 15 RTBs have been constructed in the sewer system tributary to the Detroit WWTP to treat combined sewage through screening, settling and disinfection. Three screening and disinfection facilities (SDFs) have also been constructed that treat flows using fine screening with disinfection contact time provided in downstream pipes rather than a basin structure. This past year, 3 new treatment shafts, a type of RTB that stores and treats flows in a vertical shaft rather than a horizontal basin, were placed in service. These 15 RTBs, 3 SDFs and 3 treatment shafts are operated by the Detroit Water and Sewerage Department (DWSD), the City of Dearborn, and Macomb, Oakland and Wayne Counties. All of these facilities must meet similar NPDES permit requirements. While each facility operates independently, operators coordinate efforts when dewatering stored flows back into the sewer system and work together to develop and implement best practices for operations. This report summarizes the performance of these facilities and their operators.
A Historic Year for Weather
The year 2011 stands out as the wettest year for the Detroit Metropolitan area since the National Oceanic and Atmospheric Administration (NOAA) began keeping records in 1870. Figure 1 shows the precipitation recorded each month as well as monthly and seasonal records established. Figure 2 compares 2011 precipitation and snowfall to the average from NOAA’s records.
According to Danny Costello, Hydrologist and Meteorologist with NOAA’s White Lake office that compiles data for our region, there was a significant increase in widespread storm systems across the Detroit Metropolitan Area during 2011. “Every year there are heavy rains in the US but it doesn’t always occur in the same spot,” explains Costello. “Northern Indiana was hit for a few years in a row, and this past year the Detroit area saw increased precipitation. In fact, the Detroit area received 13.9 more inches of rain than Saginaw.”
“Our weather differed from other parts of the country. The South and Midwest were in drought,” continues Costello. “This year was significantly different from 2010 when the Detroit area was really dry for six months and only received above average precipitation during May, June and July. Overall, precipitation in the Detroit area was 32.28 inches in 2010 compared to 47.70 inches in 2011.”
This localized wet weather required our sewer infrastructure to perform at a higher level than in years past. RTB operators worked long hours for days and weeks at a time to successfully capture and treat the increased flows storms sent down sewer pipes at increased frequency and volume this year.