The Detroit Water and Sewerage Department (DWSD) Wholesale Customer Outreach Public Education Work Group is continuing to monitor and share information about the performance of the wastewater and storm water infrastructure system tributary to the Detroit Wastewater Treatment Plant (WWTP).
The 15 combined sewer overflow (CSO) retention treatment basins (RTBs) that operate throughout the Detroit wastewater service area play a major role in protecting water quality. Based on a review of initial data, RTBs in the Detroit sewer system prevented more than 15.6 billion gallons of combined sewage from overflowing into our waterways from April through October 2011. Of this total, 4.8 billion gallons were captured, transported to and treated at the Detroit WWTP, and 10.8 billion gallons were treated and discharged from RTBs.
The months of April through October are considered the growing or recreation season in Michigan. RTBs are required to operate under more stringent discharge requirements during this time. Precipitation levels can also be high this time of year. During spring 2011, the area received 14.6 inches of rain, a record-setting amount of rain that is second only to the spring of 1947 when 16.3 inches of rain fell on the area. Three of the seven months also experienced enough rain to be counted among the top four wettest months on record in Southeast Michigan – April, July and September.
RTBs went into operation on 62 out of 214 days, or 29% of the time as shown in Figure 1. Treated and captured volumes of combined sewage compared to the same period in 2010 are shown in Figure 2. The dramatic difference in precipitation between these two years is evident. It is important to remember that storms trigger RTB operation – the volume of discharge is dictated by the volume of rain water, not the volume of dry weather sewage.
Figure 2 RTB Volumes in Million Gallons 2010 – 2011 (April – October)
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The volume of combined storm water and wastewater stored and treated in RTBs for the recreation season of 2011 contrasts sharply with the same time last year. Frequent and intense rain storms dramatically increased the volumes handled by RTBs, setting records in many facilities during May. While complete data was not available, it is estimated that 31% of the flows captured in RTBs were sent to the WWTP after storms subsided. The remaining 69% of flows were treated in RTBs before being discharged to receiving waters. |
The largest single storm event began on May 25 when 3.7 inches of rain fell in some areas on soils still saturated from a previous storm. Every RTB in the area went into operation; many RTBs received and treated flows for three days before halting operations and sending remaining flows to the Detroit WWTP. Two of Detroit’s RTBs that have always fully captured flows, received volumes large enough to require a treated discharge for the first time since going into operation. The area’s 15 RTBs and 3 screening and disinfection facilities captured, treated and discharged 4.6 billion gallons of flow and sent an additional 734 million gallons of flow to the Detroit WWTP when RTBs were drained, or dewatered, back into the sewer system.
“This particular event was very intense for 9 hours while maximum rates of flow were successfully processed and treated at the Chapaton and Martin RTBs,” explains Brent Avery, Operations Manager for Macomb County Public Works. “Fecal coliform counts were very low as a result of our treatment and disinfection process. Chapaton was 22 per 100 ml and Martin was 34 per 100 ml, both well below the State NPDES permit allowable limit of 400 per 100 ml. Our staff were once again pushed to exhaustion handling the entire event cycle.”
“The Seven Mile and Puritan-Fenkell RTBs located along the Rouge River discharged for the first time in their 12 years of operation,” stated Kerry Rudolph, DWSD’s CSO Supervisor. “These basins have fully captured wet weather flows in the past but have never had to discharge flows.”
“April and May were very active months,” adds John Stange, Supervisor of RTBs for the Oakland County Water Resources Commissioner. “We had to use two different suppliers for the sodium hypochlorite disinfectant we use, to keep up with the rain. In May, we processed 20 tank truck loads of disinfectant at our four RTBs. There is a significant amount of labor in loading and diluting the tanks to achieve the right concentration of chemical. Our staff kept busy operating during storms and preparing for the next storm.”
Ancell Noel, CSO Supervisor for the Wayne County Department of Public Services noted there were more isolated storms this spring. “There was very little time to prepare for some of the storms because they were so hard to predict. Several times we received more rain than we expected and very isolated storms would move in and drop a significant amount of rain over a smaller area. Operators had to mobilize quickly and keep their equipment in top shape, ready to perform.”
Each of the 15 RTBs and 3 screening and disinfection facilities tributary to the Detroit sewer system discharged during the recreation season. The performance of the five largest facilities that handled 93% of the combined sewage flows during this time is shown in Figure 3. These facilities require the greatest effort to operate and yield the biggest benefit in terms of volume of treated CSO.
Figure 3 Performance of Area’s Five Largest RTBs from April – October 2011
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In summary, the recreation season of 2011 represents a milestone effort in protecting our waterways. More than 15 billion gallons of combined sewage were captured and treated in RTBs or at the Detroit WWTP. The investment communities have made in these treatment facilities delivers high returns in wet years like 2011. The dedication of operators to keep these facilities achieving a high level of treatment storm after storm is commendable.
Source: The Detroit Water and Sewerage Department (DWSD)