During a heavy storm, many cities cannot handle the large flow of stormwater entering their wastewater systems. This leads to untreated wastewater being released into local waterways, causing pollution and illnesses.
A combined sewer system collects sewage and stormwater runoff in the same pipes and tunnels. When the weather is dry, local water treatment facilities can usually treat all of the incoming water, but during storms these systems can cause big problems.
According to the American Society of Civil Engineers, combined sewer overflows are one of the leading sources of water pollution in the US. In 2014, for example, there were 1,482 combined sewer overflow events in the Great Lakes Basin. Every year in New York City, about 20 billion gallons of combined sewer overflows are discharged into waterways.
These overflows cause significant damage to the local environment, and several studies have shown that gastrointestinal distress increases after heavy rains. These illnesses, affecting millions of Americans annually, are caused by people swimming in contaminated water, or consuming contaminated water or food.
Des Moines gets a smarter solution for combined sewer overflows
Des Moines, Iowa, is one of the hundreds of cities in the US with a combined sewer system. In the past, severe storms caused untreated combined sewer overflows to be discharged directly into the Des Moines River, putting the city’s 500,000 residents at risk. The Des Moines Metropolitan Wastewater Reclamation Authority (MWRA) decided it needed a better solution.
The city’s previous system sent all wastewater from 17 metro area municipalities to a wastewater treatment plant. The MWRA decided to construct a Combined Sewer Solids Separation Facility (CSSSF) that could treat overflows at a high rate during wet periods.
The MWRA engaged Xylem, a leading global water technology provider, to help design the new facility. Xylem’s engineers recommended a single sump design and the construction of two adjacent pump stations. Three centrifugal low-flow submersible pumps were sized to provide a total capacity of 90 million gallons per day (MGD).
Six axial flow submersible pumps with a combined capacity of 300 MGD were placed in the high flow side of the pump station to provide the remainder of the required 390 MGD flow capacity. A Formed Suction Intake (FSI) concept was implemented for each of the axial flow pumps to enable smaller channel width, a smaller wet well and the removal of separation walls between adjacent pump tubes.
Successful operation and $1.2 million saved in construction costs
The innovative two-stage wet well design delivered multiple benefits to the Des Moines MWRA. A minimized wet well footprint saved them $1.2 million in construction costs, and reduced projected operating and maintenance costs.
Since its commissioning, the CSO treatment facility has successfully operated during multiple large storm events. During a severe storm on June 24, 2015, the system ran for over six hours uninterrupted, pumping and discharging more than 390 MGD of treated water into the Des Moines River.