WATER QUALITY AND RISK-BASED PROCESS FOR HYDRAULICALLY SIZING SANITARY SEWER SYSTEMS COMPLIES WITH DRAFT SSO RULE'S LONGTERM REMEDIATION EVALUATION PLAN AND WATERSHED APPROACH
Abstract:Since the early 1990s, municipalities have tried to respond to regulatory expectations that:
Construction programs to increase the size of sanitary sewer systems which result in bigger and bigger sewers and treatment plants can eliminate sanitary sewer overflows (SSOs);
Capacity of a system is a technology requirement of the Clean Water Act; and
Engineering designs to ensure water quality in site-specific water bodies is a treatmentonly decision not related to capacity of sanitary sewers.
The draft SSO Rule requires a Capacity, Management, Operations and Maintenance (CMOM) program for each owner and operator of a sanitary sewer system. The wastewater industry understands the role and extent of how MOM can provide for effective capacity in the existing system and prevent SSOs. But the “C” in CMOM stands for capacity and the fundamental issue is: how much capacity is enough?
Under the draft SSO Rule SSOs are prohibited. For the U.S. Environmental Protection Agency (EPA) to consider enforcement discretion or affirmative defenses to an SSO discharge, the cause of the SSO must be attributed to an exceptional circumstance. This protectionist extreme sounds good from a regulatory perspective, but it does not match the real world risk-based decisions that both EPA and the business world believe that public utilities should adopt.
Capacity decisions should be site-specific in nature and based on water quality concerns in the specific water body where and when an overflow may occur. The view that there should never be an overflow from a sanitary sewer is not only physically infeasible (because pipes do leak and there will be a time that groundwater or rainfall will result in an overflow), but the SSO prohibition is also not necessary for every water body across the country in order to protect beneficial uses.
Municipalities should carefully consider the risks of using a capacity standard not related to water quality standards. The model of basing the capacity determinations of the collection and the treatment system on site-specific water quality parameters is consistent with the existing national water quality program. It is recognized that this will require extensive data, analysis and negotiations with the regulators.
A conceptual framework for capacity determination based on water quality in a site-specific water body, at the time that a SSO discharge could likely occur, has been discussed by members of the municipal community. Such a framework could be risk-based, watershed based and provide a basis for permitting SSOs that would not be considered risks.
Another option would be to develop industry-accepted default criteria. Under this mechanism, hydraulic and water quality modeling and analysis to determine how and when a default would be applied to the utility-specific system would be required. Much like the “Presumption” approach of the CSO Policy, however, the industry default would be rebuttal should there be water quality issue in a site-specific location caused by SSOs.
A risk-based, water quality-based approach to “C” coupled with a MOM program will meet the technology and water quality requirements of the Clean Water Act and will focus the long-term remediation evaluation planning leading to cost-effective capacity increases on a site-specific basis.
Document Type: Research Article
Publication date: January 1, 2003
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