Guest Column | May 18, 2020

WWEMA Window: Peak Wet Weather Flow Management — A Slow And Painful Process

By John D. Dyson


The water and wastewater industry is not like most businesses. Our industry is driven by many factors, including environmentalist concerns, federal and state regulatory water quality requirements, and utilities being good stewards of our waterways and providing clean water for drinking and recreational use. We have accomplished a lot of great work to clean up our waterways and provide safe water since the Clean Water Act (CWA) was first passed. Many of our wastewater treatment plants produce effluent water quality that could not be imagined 50-plus years ago with very low total suspended solids (TSS), biochemical oxygen demand (BOD), nutrients, etc. One major issue that still remains is the discharge of untreated waste from many collection systems or treatment plants during peak wet weather flow events. According to the U.S. EPA, the number one contaminant that impairs our waterways is pathogens, which consist of many types of bacterial organisms, viruses, and parasitic pathogens. These organisms enter our waterways from stormwater runoff and overflows in our collection networks or at the treatment plants. During typical dry weather conditions, our utilities do a great job of eliminating this risk to waterways, allowing them to be enjoyed for recreational use.

Management of peak wet weather flows and prevention of overflows are complex issues because it starts in the collection systems with the prevention of inflow and infiltration (I&I), whether to a combined sewer system (CSS) or sanitary sewer system (SSS). Depending on the age of a town, repairing and replacing the collection network is a NEVER-ending task. This is without even discussing the fact that collection laterals from our homes or business are a large portion of the I&I issue and whether the home/business owner or the utility is responsible for these repairs/replacement.

The discussion on how to handle this issue of peak wet weather flows has been debated since the passage of the CWA and the position of EPA has changed multiple times over the years. There have been a couple attempts at developing a rule on how to handle peak wet weather flows at a treatment facility in CSS and SSS communities. Presently, EPA is working on a third attempt to create a peak wet weather flow management rule and the draft is expected in 2020 for public comment. One of EPA’s goals is for utilities to repair and replace collection networks to reduce dramatically and/or eliminate the peak flow conditions. This is a great goal, but the timeframe to accomplish task is decades. Many utilities have been working on their networks for decades and have decades more work to be done. It is a never-ending task, because as fast as we repair some parts of the network, other parts of the network break or leak.

Generally, the first solution has been to build storage for the peak wet weather flow events and feed the stored volumes back to the treatment plant. This solution works in some but not all cases because the volumes to be stored can be very large and not practical for all events. The term “very large” is relative to the size of the town when you talk about the cost for storage. We have found in many cases that utilities have added enormous amounts of storage, but overflow events still occur at the storage facilities and treatment plants.

Recently, I watched a news report on my local news channel about the difficulties handling the peak flows and overflows that were occurring. Because of large amounts of recent rainfall, the utility had discharged over 3 billion gallons of untreated wastewater within one year. The utility has spent hundreds of millions of dollars over the last couple decades on storage in tunnels and basins and repairs to the collection network, but overflows of untreated wastewater are still occurring. This situation is very common for many utilities.

How can we solve the issue of reducing or eliminate the discharge of untreated wastewater during peak wet weather flow events? The solution is the use of a combination of technologies to manage and control these peak wet weather flows and treat these volumes of wastewater. The advancement in different monitoring and treatment technologies offers utilities new solutions to gain control of these peak wet weather flow conditions.

What are the technologies available to utilities now for peak wet weather flows?

  • Active flow management in the collection network using advanced monitoring, providing storage in the network
  • Classic storage solutions, optimized
  • Enhanced High Rate Treatment Technologies (EHRT) – filtration and clarification

The combination of the technologies noted above, along with repair of the collection network, can help any utility manage and eliminate untreated wastewater overflows. The solutions above do not eliminate the need for a utility to continue to do collection network repairs. If this work is not done, then flows will continue to increase during these wet weather events.

In the last 30 years, several technologies have been developed for the treatment of the wet weather flows and have been proven with many operating installations. ERHT has many advantages. One advantage of EHRT treatment is the ability to continue to treat throughout an event, whether at a treatment plant or remote site. In many cases, storage can fill up, then discharges of untreated wastewater can occur. EHRT treatment continues to remove solids throughout, allowing for effective disinfection and reduction in parasitic pathogens.

A second advantage is the fact that these EHRT technologies can be used for tertiary treatment at the plant during dry weather conditions. Because of this capability, there is beneficial use of the capital investment by a utility along with providing even better effluent quality during dry weather conditions.

A third advantage is peak wet weather flow management with auxiliary EHRT technologies allow utilities to meet weekly and monthly permit criteria because they provide event effluent water quality close to or better than secondary treatment standards. When the peak wet weather flow is combined with the effluent from the biological treatment train, the effluents are well below secondary treatment levels for the wet weather event. Then, combined with the rest of the dry weather flow for the week or month, the average treatment plant is very low for the secondary treatment standards.

A fourth advantage is EHRT technologies can be constructed now and provide an immediate impact on water quality to our waterways while the reduction of wet weather volumes and repair work continues to the collection network.

In summary, we have debated for decades about how to handle peak wet weather flows and legal issues regarding whether treatment of peak weather flows by auxiliary EHRT technologies is acceptable. As an industry, we have lost sight of the real goal, which is to prevent overflows of untreated wastewater from entering our waterways. While we have spent years debating how it should be done, millions, if not billions of gallons of untreated effluent have continued to flow into our waterways. Many EHRT technologies produce effluents close to or better than secondary treatment standards without biological treatment during wet weather events. The use of auxiliary EHRT technologies as part of the solution can allow us to dramatically reduce the number of untreated overflows, provide improved effluent quality to our waterways, and make them safe for recreational use. The use of technology gives us time to do the necessary and ongoing collection network repairs.

John Dyson is Product Manager – AquaPrime/AquaStorm for Aqua-Aerobic Systems, Inc., headquartered in Loves Park, IL. He is the Immediate Past Chairman of the Water and Wastewater Equipment Manufacturers Association (WWEMA) and active in legislative issues including peak wet weather flows.  WWEMA is a non-profit trade association that has been working for water and wastewater technology and service providers since 1908. WWEMA’s members supply the most sophisticated leading-edge technologies and services, offering solutions to every water-related environmental problem and need facing today’s society. For more information about WWEMA go to