OHM Cheers End of the "Build it Bigger" Directive: Metropolitan Communities Look to New Methods to Evaluate, Rehabilitate Aging Sewer Systems

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In an effort to crack down on sewer overflows that cause untreated sewage to flow into nearby waters, the EPA has filed lawsuits against a number of major municipal wastewater systems. Rather than building bigger systems to prevent overflows, OHM has helped a few cash-strapped metropolitan communities to combine traditional civil engineering practices with state-of-the-art concepts from the aerospace engineering field. The results? Hundreds of millions in cost savings for taxpayers.

OHM's innovation, ideas, and tenacity convinced Michigan Department of Environmental Quality that a more expensive solution was not needed. OHM saved Wayne County's taxpayers $175 million." Kurt Heise, Director, Wayne County Department of Environment.

Officials in Kansas City, Missouri, announced today that the city will spend $2.5 billion to repair the city's broken sewer system. Sewer systems in major metropolitan areas are aging and deteriorating at an alarming rate. These crumbling systems allow inflow and infiltration, sanitary sewer overflows (SSOs) and combined sewer overflows (CSOs) - often causing untreated sewage to flow into nearby lakes, rivers and streams. A recent New York Times article, cites state and federal studies indicating that "thousands of water and sewer systems may be too old to perform properly." OHM, an engineering, architectural and planning firm committed to advancing communities, has successfully partnered with dozens of communities to rehabilitate sewers and comply with EPA mandates.

The Environmental Protection Agency (EPA) has filed lawsuits against a number of major municipal sewer systems to enforce the Federal Clean Water Act. These consent decrees require the offending systems to put measures in place to prevent future overflows. The regional utilities then look to the participating satellite communities to fund the necessary sewer upgrades and expansions.

The standard engineering solution to eliminate sewer system overflows is to simply increase capacity - often with a hefty pricetag. Recently though, communities surrounding Detroit, Columbus, Milwaukee and Chicago pursued an innovative approach to meeting the EPA's mandate. Instead of building bigger and bigger pipes and tunnels, satellite communities in these systems combined investigation, analysis and state-of-the-art modeling to rehabilitate and increase capacity in existing wastewater collection systems at a much lower price tag than standard worst-case infrastructure expansions.

Components of this cost-effective approach include:

Inflow & infiltration investigations (I&I)
Sanitary sewer evaluation studies
Flow monitoring & hydraulic analysis
CCTV investigations
Hydrologic modeling

In the field of hydrologic modeling, a new methodology is proving that the best new ideas come from outside standard accepted practices. Traditionally in civil engineering, wet weather inflow and infiltration (I&I) into sewer collection systems has been analyzed using storm water techniques. Experience shows that those techniques often don't work well for wastewater. The complex components that contribute to the amount of I&I that enters a system vary according to how wet or dry the cumulative weather has been. This is referred to as "antecedent moisture". Antecedent moisture conditions change continuously and can have a very significant effect on the flow responses in systems during wet weather.

i3D is the marriage of traditional hydrologic modeling with technology borrowed from aerospace engineering. The idea for i3d was sparked when Robert Czachorski, a civil engineer with OHM, an engineering, architecture and planning firm, and his college roommate, Tobin Van Pelt, an aerospace engineer with Lockheed Martin, were talking about the challenges of predicting how sewer systems would function in wet weather situations. The result was like the old Reese's Peanut Butter Cup commercial - two great ideas that worked great together.

Using a modeling approach like i3D that more accurately predicts a collection system's response to weather changes eliminates overly conservative "build it bigger" recommendations and allows decision makers to evaluate the risks of overflows versus the costs of improvements.

Fast forward nearly ten years and the i3D modeling method has helped to save communities hundreds of millions of dollars. In one project alone, the director of Wayne County's Department of Environment credited i3D with saving Michigan taxpayers $170 million dollars.

Ms. Josephine Bradley, President of Board of Public Affairs for the Village of Urbancrest, a satellite community of the City of Columbus' wastewater collection system commends OHM. "OHM's advisors are very knowledgeable about Findings and Orders. Their work is outstanding and they have saved us lots of money."

The American Council of Engineering Companies gave the i3D Antecedent Moisture Model a http://blog.ohm-advisors.com/2010/02/i3d-antecedent-moisture-model-why-it.html [national award for innovation in 2008.


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i3D Antecedent Moisture Model White Paperi3D Antecedent Moisture Model for Hydrologic Modeling of Sanitary Sewer Systems uses a new methodology based on systems identification theory, rather than storm design to comply with EPA mandates to prevent SSOs and CSOs.Regulatory, Technical and Modeling Challenges to Developing a Frequency Based Approach to SSO ComplianceWayne County’s North Huron Valley / Rouge Valley (NHVRV) interceptor system collects sewage from 15 communities located in Southeast Michigan and transports flows to the Detroit Water and Sewerage Department (DWSD) for treatment and discharge. The County is evaluating a regional approach to controlling wet weather sanitary sewer overflows (SSOs). A new methodology called the i3D antecedent moisture (AM) model, was used to perform the hydrologic modeling. The i3D model is a continuous model that produces a good match to observed flow data over time. The accuracy of the model resulted in a high level of confidence in the frequency analysis for SSOs and will serve as the basis for recommending improvements to control wet weather SSOs. The use of the AM model combined with a frequency analysis for sizing improvements eliminated the need to select a design storm event based on “average” conditions. This reduced many of the conservatisms that are frequently included in event models such as the capture coefficient and seasonal effects. The use of spatially varied rainfall also improved the accuracy of the analysis over the use of a point rain gauge. This paper presents the modeling and analysis innovations used and the preliminary development of a regional project.