Launch Slideshow

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Wet weather tools

Wet weather tools

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    Storm channels at treatment plants are activated during wet weather events to split excess flows and prevent flooding of biological or advanced treatment units. Photo: Pam Broviak

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    Typical sewage treatment systemThis diagram illustrates a typical treatment process at a wastewater plant. Blending takes place when flows are intercepted after primary treatment and conveyed directly to disinfection—bypassing secondary treatment. Source: U.S. EPA

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    Top: Some cities have constructed storage basins to collect CSO discharges. Primary treatment and disinfection are provided for these overflow volumes and when storm events pass, the water can be conveyed to the POTW for further treatment. Bottom: This tank normally provides aeration for plant effluent. During wet weather events, automated controls empty this tank and redirect final effluent flows to another tank. Storm flows are then conveyed to this tank for primary treatment and disinfection during the storm. Photos: Pam Broviak

Alternatives to Blending

Opponents to blending offer several solutions to alleviate or eliminate the need to continue this practice. In Sewage Warning! What the Public Doesn't Know about Sewage Dumping in the Great Lakes, the U.S. Public Interest Research Group (PIRG) Education Fund proposes several steps to achieve full biological treatment of sewage in all but the most extreme circumstances.

This group reports that “the most efficient way to stop sewage overflows is to stop water from flooding treatment systems when it rains. Efficiency in controlling overflows decreases the farther downstream in a wastewater system that the control occurs.” They suggest using soft-path or low-impact development to stop flooding of the system. Soft-path approaches are defined in the publication as “onsite wastewater treatment technologies; stormwater retention and filtration such as rain gardens, constructed wetlands, and native species plantings; and stream buffers, water conservation fixtures, rain barrels, reuse of ‘greywater,' and other low-impact development designs.”

U.S. PIRG's next suggestion echoes the cries of many others associated with the stewardship of our nation's infrastructure—increased funding. The group recognizes that prevention of overflows will involve major capital investments. They feel that much of the current state revolving loan fund is spent subsidizing urban sprawl or industrial agricultural operations instead of supporting infrastructure designed to limit overflows.

Finally, the PIRG proposes stricter enforcement of laws. The group feels that environmental agencies have been lax in enforcing regulations on overflows and suggest that permit holders be pushed harder toward complete elimination of overflows.

Citizens Campaign for the Environment, a non-profit environmental protection organization, proposed the following solutions in their comments submitted to the EPA on the proposed blending policy: reduce inflow and infiltration, manage capacity, properly maintain the system, construct retention facilities, install backup equipment, provide public education, eliminate illicit hook-ups and discharges, and strive for technical innovation.

Many cities have begun construction and implementation of overflow elimination projects. In some cases, cities have decided to leave CSOs in place and route the discharge to a collection basin or underground tunnel where the wastewater is temporarily stored. When wet weather flows diminish, this water is then routed through the treatment plant and receives full treatment.

Construction of the Tunnel and Reservoir Project, or “deep tunnel,” in Chicago is an example of an effort to provide storage in the collection system. According to literature from the Metropolitan Water Reclamation District of Greater Chicago, this plan involved construction of 109 miles of large, underground tunnels beneath the city designed to intercept CSO and convey it to large storage reservoirs. After the storms subside, the overflow is then conveyed to treatment plants for treatment before discharge to a waterway.

In the end, any final solution is still dependent on the available funding. As with all public infrastructure, those funds will come from taxpayers. The right balance must be found between the necessity to protect the environment and the costs required to achieve this protection.

In the meantime, the EPA will continue to study blending. “There is still some work to be done with respect to evaluating emerging high-efficiency physical and chemical treatment systems,” said Weiss. “Right now, typically the first priority should be to minimize overflows, even if it means having primary treatment and disinfection for excess flow at the treatment plant for an interim time period.”

Broviak is city engineer/public works director for in LaSalle, III.