Excess Flows: Past & Present
Improving collection system performance begins with a thorough understanding of its components
Municipalities are continually challenged in their management of excess flow conveyed in the sanitary collection system and treatment of peak flows when they reach the wastewater treatment facility. This is particularly true for extreme precipitation events. An understanding of how the collection system was developed, how it has been maintained and what service was intended to be provided is important for improving collection system performance, with respect to customer service and environmental protection.
In older cities, initially, there was no thought of treatment. Sewers were intended to convey storm water and wastewater away from houses, businesses and industries to the nearest water body. This practice degraded receiving waters and in many cases created a public health risk. A solution to this problem was to build interceptors to carry dry weather flow to treatment, with most storm water and wastewater wet weather flows discharged to a receiving stream, limiting peak flows at the treatment facility.
Communities stopped constructing combined sewers 50 to 60 years ago, opting instead for separate storm and sanitary sewers. Some communities separated their combined sewers by constructing new sanitary sewers with new building connections. Separation by installation of new storm sewers that capture public right-of-way inflow sources, however, has been more common. Separate sewers often are subject to wet weather flows as well. Reasons include:
- • Sources of clear water, such as foundation drains, often were connected to the sanitary sewer with the original plumbing.
- • Original lateral connections may have been installed with defects and laterals may be leaky.
- • Soil conditions may turn the sanitary sewer into a drain, causing infiltration defects below the groundwater level.
- • Separated sewers that continue to use the combined sewer as the sanitary sewer likely retain many private property clear water connections.
- • Separate sewers constructed next to streams or drainage swales may be subject to infiltration or inflow at a particular stream stage.
- • Sewer systems respond to differently to a given statistical rainfall event depending on antecedent moisture.
Many communities have a mixture of combined sewers, separated sewers and separate sewers. Even sewer systems originally constructed as separate systems will have sewers that perform differently under varying wet weather conditions in different parts of the system. Therefore, each collection system will respond uniquely to wet weather conditions, and different parts of the system will have different responses.
Management of peak flows is equally challenging at the treatment facilities. Primary facilities may be more forgiving of variable flows, but secondary biological processes are limited with respect to peak flow response. For this reason, diversions around secondary prcoesses to manage excess flows have been incorporated as an option at treatment facilities.
Discharges from combined sewers now are strictly limited and bypasses from sanitary sewers are not permitted, per the Clean Water Act. Enforcement by the U.S Environmental Protection Agency (EPA) and state regulatory agencies has increased, and proper interpretation of allowable management practices has become more difficult. But publicly-owned treatment works that hold the National Pollutant Discharge Elimination System permit may not control all elements of their collection system, having to deal with peak flows generated in portions of the system owned and operated by others.
Level of Service
So, what is a collection and treatment system owner to do? Every collection and treatment system will fail under some combination of rainfall intensity and depth and antecedent moisture conditions. Another way to state this is that each system provides a level of service to its customers. It is important to recognize that different parts of the system may have different levels of service. The level of service is quantifiable (for example, a probability of one bypass in a 10-year period) and measureable. Sewer monitoring and modeling are needed to determine level of service, which can be communicated to those served and to the regulators. Planning by the utility can then proceed to determine the level of service beneficial to its customers consistent with Clean Water Act requirements. An example of Milwaukee Metropolitan Sewerage District’s level of service concept is shown in Figure 1.
For a particular sewer, the probability of current peak flow values can be determined. Then, the flow controls or reductions required to provide a given level of service can be determined. Management tools include sewer maintenance, rehabilitation, added capacity, private property controls, and real-time control of flows. Recent years have seen more extreme rainfall events and more frequent periods of wet weather. Level of service planning should consider that facilities may need more capacity to maintain the same level of service going forward. Public understanding of the level of service being provided (frequency of basement backups or overflows) leads to understanding of costs of service. Most collection systems now are required to have a capacity management operation and maintenance (CMOM) plan. Properly prepared and implemented, the CMOM plan can be benchmarked to maintaining a level of service to customers. Assets will be managed to deliver reliable service.
Excess flow management at treatment plants also has been challenging. For the past decade, EPA has interpreted peak wet diversions within the treatment plant as prohibited under Clean Water Act 40 CFR Section 122.41(m). Recent court decisions, greater flexibility with respect to excess flow treatment and technology advances are providing more options for wastewater treatment plant operations. The treatment facility responds to flow delivered by the collection system. The wastewater treatment facility will only provide reliable treatment when peak flows are managed to be protective of the biological and other advanced treatment processes. Therefore, the utility should:
- • Understand which peak flows can be handled by biological and advanced processes;
- • Understand the frequency and duration of flows at the treatment facility;
- • Determine if peak flows should be attenuated in the collection system or at the treatment facility; and
- • Determine the method of peak flow management, whether it is equalization, high-rate physical or chemical treatment, adding a biological process to the high-rate treatment, recombining, or separate disinfection.
The challenges of system-wide peak flow management for extreme events are significant. Determining a level of service, having an informed understanding of how the collection and treatment system respond to extreme precipitation events, and having a plan for maintaining reliable system capacity will provide reliable and cost-effective service to wastewater utility customers while meeting obligations under the Clean Water Act.