The combined sewer conundrum

By Betsy Frederick and Ray Ferrara

In 1994, the U.S. Environmental Protection Agency (USEPA) published the Combined Sewer Overflow (CSO) Control Policy, which provided a national framework for management and mitigation of impact from combined sewer discharges during wet weather events. Combined sewers – like wastewater treatment discharges and some stormwater discharges – are controlled through the National Pollutant Discharge Elimination System (NPDES) permitting program. Under the umbrella of the Clean Water Act (CWA), the CSO program was intended to provide a cost-effective and flexible means to eventually meet all water quality standards for the receiving body of water through the development of a Long-Term Control Plan (LTCP) specific to the combined system operation. 

Combined sewers are a vestige of an earlier era in the country’s urban development, and are no longer an allowable approach to wastewater and stormwater collection. Nevertheless, when USEPA reviewed the NPDES program in 2001, there were 859 active CSO permits, covering 9,463 outfalls discharging to surface waters in 32 states. Today, there are roughly 770 communities with CSOs, with the majority in communities with a population of less than 10,000. 

A majority of the active CSOs are found in the Northeast, Great Lakes Region, and the Pacific Northwest. Unfortunately, under most climate change model predictions, these are precisely the areas more likely to experience greater precipitation and a greater frequency of heavy storm events (e.g. models predict that a current 20-year storm event may occur every 4–15 years by the end of the century). This is the “perfect storm” for increased CSOs, and a significant threat to water quality.

The Old Paradigm

Combined sewer overflow reduction, or elimination of overflows entirely, is a very expensive proposition. Many urban systems adopted a sewer separation approach, which is essentially a capital investment in an entirely new wastewater or stormwater infrastructure. Costs associated with capital improvement in the hundreds of millions of dollars are not unusual. Depending on the total number of overflows a system might experience in any given year, however, that expenditure does not always result in a particularly compelling environmental return on investment. Nevertheless, the CSO permit program has an equivalent force—and equivalent regulatory liability--to other CWA programs (such as wastewater treatment plant and stormwater discharge permits) where the greater proportion of potential pollutant loadings may reside.

Many of the communities subject to CSO requirements do not have the human and financial resources to allocate to this program. Therefore, the challenge to meet all CWA obligations is a steep climb and heavy burden.

A New Approach

USEPA recently launched, and is actively promoting, an Integrated Planning Model for municipal management of wastewater and stormwater infrastructure. The goal is to allow a community to assess the entirety of its water resource infrastructure (and its impacts) more holistically, rather than through the narrow lens of the respective permits under which each of these systems operates. The rationale for a capital improvements program must be demonstrated through a risk-based asset management approach that prioritizes capital, as well as operation and maintenance investments, on the basis of the “biggest bang for the environmental buck.” 

Under this model, CSO-critical criteria--including cost, benefits to the users, environmental improvement and compliance with other regulatory requirements--is part of the evaluation, but not the exclusive consideration. For instance, under a traditional approach, an LTCP-required project might reduce the number of overflows from 10 events to nine per year (with a consequent positive but incremental impact on water quality). Because the community is forced to spend those limited resources on that project, they might not be able to repair or rehabilitate an at-risk wastewater asset, failure of which has much greater environmental consequences. USEPA is not waiving any obligation of the community to meet standards under the CWA, but they are acknowledging that a “separate but equal” approach to different infrastructure systems is not the most efficient, cost-effective or sustainable manner to achieve the shared objective of the suite of NPDES permit programs – the attainment and protection of locally-specific water quality standards. 

Toe in the Water

Municipalities have never had a better opportunity to address this CSO “elephant in the room” than now. The Integrated Planning Model promotes use of asset and risk management systems, and advocates for an adaptive management approach more suitable to this period of climate impact variability. With technological advances, public works officials will be able to make informed (and defensible) decisions to address critical needs in the appropriate sequence and in a financially sustainable manner. Regulators may not be offering complete relief, but this integrated planning framework is a much needed and welcome compromise. It represents a significant step forward as cities and towns across the country face continually evolving infrastructure management challenges in the face of accelerating change.

Betsy Frederick is a senior client manager at Kleinfelder. She can be reached at

Raymond (Ray) Ferrara is the vice president of corporate business development at Kleinfelder. He can be reached at


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