October 1, 2012
When a combined sewer system reaches capacity during heavy rainfall, stormwater and sewage is discharged into local waters. Communities across the United States spend a lot of time and money trying to treat wastewater and combined sewer flows. Now, several cities are focusing on ways to keep water from coming into contact with pollution in the first place.
About 770 communities in the United States have combined sewer systems, serving about 40 million people. Mitigation of overflows can take many forms, including improving collection system performance, increasing downstream treatment facility capacity and establishing dedicated wet weather treatment facilities.
Combined sewer overflows, or CSOs, were built as part of sewer collection systems designed to carry both sewage and stormwater in the same pipe. When a combined sewer system reaches capacity during heavy rainfall, stormwater and sewage is discharged into local waters. The EPA estimates that about 850 billion gallons of untreated wastewater and stormwater are released as CSOs each year. In contrast, between 3 billion and 10 billion gallons of untreated wastewater are released as sanitary sewer overflows, or SSOs, the unintentional discharge of raw sewage from municipal sewers.
As thousands of water quality professionals gather at WEFTEC 2012 this week, many attendees will learn about the latest wet weather technologies and solutions. From green infrastructure to integrated planning to stormwater retrofits, a few communities will showcase their innovative and cost-effective techniques to solve these problems.
Wet weather treatment studies
The Milwaukee Metropolitan Sewerage District, for example, wants to increase its South Shore Water Reclamation Facility’s wet weather treatment capacity from 300 mgd to 450 mgd. Don Esping, a process and design engineer in BC’s St. Paul, Minn., office, is leading a study to evaluate three auxiliary wet weather treatment technologies to treat primary influent or primary effluent during wet weather flow events: biological contact treatment, chemically enhanced sedimentation and compressible media filtration.
“Based upon wet weather testing completed to date on primary influent, all three technologies have produced good to excellent effluent quality with biological contact treatment achieving the lowest effluent concentrations,” Esping said.
In Cleveland, the Northeast Ohio Regional Sewer District is exploring the use of chemically enhanced primary treatment and high-rate disinfection of wet weather flows at its three wastewater treatment facilities. Henryk Melcer, senior process engineer in BC’s Seattle office, led the CEPT-HRD study, which concluded that CEPT-HRD was technically feasible in meeting proposed effluent quality limits.
“As a result, the district is completing pilot and full-scale demonstrations of the CEPT-HRD process,” Melcer said, “and, together with implementation of CSO control measures and combined with work already under way including green infrastructure improvements, reduce CSO discharges to about 494 million gallons in a typical year and result in capture for treatment of more than 98 percent of total wet weather flows.”
Esping also contributed to a study for NEORSD, using computational fluid dynamic modeling to evaluate primary settling tank performance and a novel wet weather chemically enhanced high-rate treatment tank configuration. “CFD modeling of wet weather treatment tanks with chemically treated raw influent showed that CE-HRT can meet project requirements for wet weather flow discharges,” he said.
Before completing a massive sewage overflow reduction program in the early 1990s, communities around Milwaukee released on average about 8 billion gallons of sewage into Lake Michigan every year. The Milwaukee Metropolitan Sewerage District responded by developing its Water Pollution Abatement Program, which included the 19.4 mile-long Deep Tunnel storage system to reduce overflows. Whereas other large cities are dealing with dozens of overflows a year, only one overflow of combined sewers took place in central Milwaukee and eastern Shorewood in 2011 and none in 2012.
“Milwaukee is a shining example of what other wet weather programs would like to accomplish,” said Andy Lukas, Brown and Caldwell’s national wet weather practice leader. “A lot of communities in the Midwest and Northeast that are of the size of Milwaukee are still dealing with combined and sanitary sewer overflows. They are looking at having to spend billions to try to get to the place where Milwaukee is right now.”
An important part of MMSD’s strategy to reduce SSOs is addressing private property infiltration and inflow. Infiltration occurs when water enters a sewer system through defective pipes, pipe joints, connections or manhole walls. Inflow is stormwater runoff that enters the sewer system through connections such as foundation drains, connected downspouts and catch basins.
“Infiltration and inflow doesn’t really contribute to water quality issues, but it puts so much water into the system during large events that it contributes to basement backups and sanitary sewer overflows,” Lukas said. “The private pipes represent half of the system and have not been touched for many decades in a lot of cases.”
To help resolve this problem, MMSD has set up a regional program to provide money to the 28 communities it serves to address the worst parts of their systems. The goal is to reduce the risk of basement backups by cutting down on the excess water that leaks into privately owned sanitary sewers as well as from older homes that have foundation drains connected directly to sanitary sewers. Remedies include downspout disconnection, foundation drain disconnection, lateral repair, lateral rehabilitation and improved surface water drainage.