Bettendorf, Davenport likely to avoid multi-million dollar overflow basin for sewage treatment plant; upgrades have led to lower stormwater infiltration

Owners of the Davenport Waste Water Treatment Plant – the cities of Davenport, Bettendorf, Riverdale and Panorama Park – now appear likely to avoid building a multi-million holding basin designed to handle overflows of sewage and stormwater to the plant during flooding and after heavy rains.

The equalization basin – estimated to cost $25 million back in 2012 – was the most expensive requirement of a 2013 consent agreement between the Iowa Department of Natural Resources (IDNR) and the cities, which jointly operate the treatment facility along the Mississippi River on Concord Street, Davennport.

The huge storage basin initially was scheduled for completion in 2021, but that date was later pushed back to 2025. The IDNR now is considering amending the agreement to remove the equalization basin requirement.

Completion of the equalization basin was delayed to give the communities time to repair their systems to reduce the inflow and infiltration of stormwater into sewer lines.

When large amounts of stormwater enter sewer lines during flooding of the Mississippi River and after heavy rains, the treatment plant is unable to process all the inflow.

When that occurs, the plant closes gates to the plant, forcing Bettendorf to pump sewage/rainwater from the main sewer interceptor line along the riverfront into stormwater pipes that empty directly into the river.

After the latest heavy rains March 28 Bettendorf pumped 6.7 million gallons of sewage and stormwater into the Mississippi River, according to Sanitary Sewer Overflow (SSO) reports filed with the IDNR. It was the first time this year, the city has was forced to pump waste/stormwater into the river.

City officials have lobbied for years to avoid building the equalization basin saying inflow and infiltration improvements would lower the size of an equalization basin or eliminate the basin need altogether.

Also addressing the overflow issue was the recent completion of a $7-million upgrade to the treatment plant, substantially increasing the amount of sewage the plant can process daily.

The lower Infiltration shown in the sewage system reports also has led the IDNR to reconsider the equalization basin requirement.

The January minutes of the joint committee overseeing the sewage treatment plant operations stated the IDNR legal staff was drafting an amendment to the administrative order, but IDNR officials late last month said no "draft" report has yet been prepared.

Another major bottleneck and source of infiltration into the sewage system has been the old (1930) interceptor sewer along the riverfront.

The old interceptor was built to move sewage and stormwater from upstream of the water treatment intake (Iowa-American Water) to below Lock & Dam 15, where it would flow directly into the Mississippi River. Before the the sewage treatment plant was built the late 1950's, raw sewage was simply piped into the river and most sewer lines handled both raw sewage and stormwater runoff.

Most sewer and stormwater lines have now been separated so sewage is directed to the treatment plant, while only stormwater lines empty into the river.

In the 1970's, a large sewage interceptor line was installed along the riverfront, but the old line was left in service and at some locations it interconnects with the newer line.

Now a 4-year, $10.8-million project will abandon large segments of the old 1930 interceptor, rehabilitate other sections and reconnect them to the newer 1970 interceptor line. The project has begun along River Drive at McClellan Boulevard in Davenport.