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

The Nuances of Blending

So blending continues for many CSO communities as they progress toward full implementation of their long-term plans. However, in some EPA regions, communities have been prevented from performing blending operations. For many, blending had been a previously approved and permitted operation, and the change from approval to enforcement has created some confusion—particularly when blending continues as an approved practice in other regions.

This inconsistency led the EPA to propose a blending policy in November 2003 that would “clarify the National Pollutant Discharge Elimination System (NPDES) requirements for discharges from POTWs where peak wet weather flow is routed around biological treatment units and then blended with the effluent from the biological units prior to discharge.” This final discharge would have to meet “permit effluent limitations based on the secondary treatment regulation or any more stringent limitations necessary to attain water quality standards.”

“In the past we've had a mix between permits and enforcement,” said Weiss. “Different states were doing different things; we wanted to provide some consistency.”

This proposed policy drew a strong negative response. The EPA received more than 98,000 public comments, particularly from environmentalists and shellfish growers. In testimony submitted to the U.S. House of Representatives by John H. (Jack) Graham Jr., P.E., assistant director of water quality in Maryville, Tenn., an environmental publication was cited that stated the tradition of baptisms in natural settings was jeopardized due to the “sad fact that many rivers and streams are simply too dirty to wash away original sin.”

Congress also took on the issue of blending and attached a rider to the fiscal year 2006 Interior and Environmental Appropriation Bill to prevent the EPA from spending funds on a final blending policy based on the November 2003 proposal. As a result of this reaction, the EPA withdrew the November 2003 proposed blending policy from consideration.

Still, many individuals and groups supported the policy. In a press release issued on Jan. 24, 2005, the National Association of Clean Water Agencies supported the proposed blending policy and stated that “blended effluent fully meets Clean Water Act permit requirements, protects public utility infrastructure from ‘washout,' and prevents the release of untreated sewage into the environment and sewer backups into homes and businesses. In fact, a final blending policy will increase permitting consistency and make more information publicly available.”

There is still much disagreement over blending. Operators realize that without blending, plant capacity will be exceeded and either more untreated water will need to be diverted in the collection system or flooding of the plant will occur—leading to a decrease in water quality of all plant discharges. Environmental groups argue that the release of any partially treated sewage is unacceptable. In the end, the solution is dependent on available treatment technology and money.