Responding to an SOS with CSOs

The Capital Regional District (CRD) in British Columbia, Canada, continues to distinguish itself as one of North America's most environmentally proactive government entities. In the last decade alone, the CRD has cleaned up Greater Victoria's beaches, reducing pollutants. But perhaps one of the CRD's crowning achievements is its successful implementation of the Liquid Waste Management Plan of 2000, which includes developing guidelines for and enforcing compliance with source water, wastewater and stormwater quality management, especially in regard to combined sewers.

An Overview of CSOs
Collection system capacity is a global concern. Europe, Canada, the United States and other countries are pursuing various approaches to better control pollution outspills and increase collection system capacity.

According to the U.S. Environmental Protection Agency (EPA), combined sewer systems are sewers designed to collect rainwater runoff, domestic sewage and industrial wastewater in the same pipe. Combined sewer systems usually transport all of the wastewater to a sewage treatment plant, where it is treated and then discharged to a water body. During periods of heavy rainfall or rapid snowmelt, however, the wastewater volume in the combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers or other water bodies.

The combined sewer overflows (CSOs) contain not only stormwater but also untreated human and industrial waste, toxic materials and debris.

A type of "urban wet weather" discharge, these CSOs contain not only stormwater but also untreated human and industrial waste, toxic materials and debris. These contaminants present major pollution concerns for the approximately 772 U.S. cities and more than 200 Canadian cities that have combined sewer systems.

Although no longer built in the United States or Canada, combined sewer systems are still an extensive part of the existing infrastructure in both countries. According to Metcalf & Eddy's Wastewater Engineering (McGraw Hill), CSOs sometimes contaminate the receiving waters into which they discharge, causing municipalities to be out of compliance with water quality standards. Bacteria, nutrients, solids, biochemical oxygen demand (BOD), metals and other potentially toxic constituents are just a few of the contaminants discharged from CSOs that can adversely affect receiving waters.

According to an EPA survey, in the early 1990s U.S. municipalities spent more than $16 billion to comply with CSO pollution control regulations. Engineers are using a variety of CSO control methods to correct this problem, including construction of new separate sewers and storm drainage systems (sewer separation), treatment at the combined sewer outlet and storage followed by treatment at dry-weather facilities.

Using Overflow Screens to Handle CSOs
As part of its stormwater quality management program, the CRD is in the process of installing a StormGuard™ overflow screen at the Macaulay Point Pumping Station in Victoria, British Columbia. The 14-ft-8-inch long screen, manufactured by USFilter John Meunier Products of St. Laurent, Quebec, will help keep debris inside while the station overflows -- a common occurrence during Greater Victoria's frequent rain and storm events. The first of its kind installed in North America, the screen is scheduled to come online in the next few months.

The overflow screen is an upward flow, horizontal fine filtration band screen that fits any straight-edged, single- or double-sided weir overflow, up to 26 feet long. As water reaches the unit, a level detector activates the screening removal mechanism. Water enters the bottom of the unit through ΒΌ-inch diameter perforated elements and then overflows through a weir to the outfall.

EPA's CSO Policy

A submersible motor moves the filtering elements from upstream to downstream, propelling solids to the end of the main channel section. There, a rotating polypropylene brush cleans the dirty filtering elements while they move from the submerged side to the topside of the unit. The clean plates then travel back to the upstream channel section.

If the water level exceeds the screen capacity during a CSO event, the top of the screen frame can also serve as an emergency overflow. Additionally, the screen's optimized hydraulic design and maximum hydraulic footprint ratio enable the screen to successfully

If the water level exceeds the screen capacity during a CSO event, the top of the screen frame can also serve as an emergency overflow.

remove debris. With a surface area that is much larger than the overflow section of the weir, the overflow screen can handle flows up to 115 million gallons per day (mgd). The inner water circulation generates secondary flow to help further clean the perforated elements at peak flow. Another of the product's key success factors is the screen's strong, compact, all-stainless steel, single-piece frame. The StormGuard screen design is currently pending patent in various countries.

Advantages of the Technology
"We accepted the StormGuard screen over a previously specified design, based on competitive pricing as well as favorable third-party reviews," said G. Arthur deMeulles, project coordinator at Hartland & Core Area Engineering for the CRD. "We also liked the screen's low power consumption and easy maintenance."

"The chamber is designed to eventually accommodate a second StormGuard screen," deMeulles concludes.

The new overflow screen is expected to help personnel at the Macaulay Point Pumping Station confine debris from CSOs to the station during heavy rain and storm events. No doubt this new technology will also be popular in the United States given the large number of combined sewer systems still in use.


  • U.S. Environmental Protection Agency National Pollutant Discharge Elimination System (NPDES) Program --
  • Water Environment Federation --
  • Association of Metropolitan Sewerage Agencies --

  • EPA's CSO Policy
    EPA's Combined Sewer Overflow Control Policy is a national framework for control of CSOs through the National Pollutant Discharge Elimination System (NPDES) permitting program. The policy resulted from negotiations among municipal organizations, environmental groups and state agencies. It provides guidance to municipalities and state and federal permitting authorities about how to meet the Clean Water Act's pollution control goals as flexibly and cost-effectively as possible. The CSO Policy was published April 19, 1994, at 59 Federal Register 18688. There are currently no proposed rules or final rules published by EPA concerning CSOs, only the agency's policy.

    The first milestone under EPA's CSO Policy was the January 1, 1997, deadline for implementing nine minimum technology-based controls. These are measures that can reduce the prevalence and impacts of CSOs and that are not expected to require significant engineering studies or major construction.

    According to EPA, communities with CSOs are now in various stages of developing and implementing their long-term control plans, including characterizing their combined sewer systems, monitoring the impacts of CSOs on waterways and discussing water quality and CSO control goals with permitting authorities, water quality standards authorities and rate payers. EPA encourages municipalities to take advantage of the flexibility in the agency's CSO Policy as they embark on this process, particularly where opportunities exist to evaluate water pollution control needs on a watershed management basis and to coordinate CSO efforts with other point and nonpoint source control activities.

    This article originally appeared in the October 2003 issue of Environmental Protection, Vol. 14, No. 9.

    This article originally appeared in the 11/01/2003 issue of Environmental Protection.

    About the Author

    Lawrence H. Keith, PhD, is a vice president and senior corporate fellow at the Waste Policy Institute, Blacksburg, Va.

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