Strategies and Tools for Managing Wet Weather Flow Issues

Strategies and Tools for Managing Wet Weather Flow Issues

September 15, 2023
Bob Dengler, PE; Ben Briston, PE

You probably don’t have to think too far back to remember seeing your last gully washer. The rain fell hard, causing the streets to flood or the water to pool in your yard.

Wet weather events like hard rains can challenge wastewater collection and treatment facilities that must manage the increased flow rates. Stormwater inflow and infiltration (I&I) into the sanitary sewer system, which is meant to collect wastewater, can cause sanitary sewer overflows (SSOs). SSOs release untreated sewage from a sewer into homes, streets, or bodies of water.

Let’s journey through the evolution of sewer systems, explore tools for wet weather flow studies, and understand the strategies for effective SSO management. We’ll discuss several solutions a sewer utility can use to manage wet weather flows and reduce and prevent SSOs, including a novel flow management strategy that doubles a wastewater treatment plant’s peak flow capacity.

The Evolution of Sewer Systems

Sewer systems trace their origins back to ancient Rome, with the first sewers constructed in the sixth century B.C.

Fast forward to the 1800s, when rapid urbanization in the U.S. led to sewer system development for public health and flood control. Notably, the first comprehensive sewer system was designed for Chicago in 1858, and the first sewage treatment plant was built in Worcester, Mass., in 1890.

Congress passed the Federal Water Pollution Control Act in 1948 to address water pollution. Growing public awareness and concern for controlling water pollution led to sweeping amendments in 1972. Known as the Clean Water Act, the amendments established regulations for pollutant discharges, water quality standards, and sewage treatment plant construction.

From the first sewers in ancient Rome to the Clean Water Act of 1972, the complexity of sewage collection and treatment has evolved.

Understanding Wet Weather Flow Systems

Wet weather flow systems can be categorized as combined sewer systems (CSS) and separate sewer systems.

Combined Sewer Systems

CSSs collect and convey sanitary wastewater (domestic sewage from homes and industrial and commercial wastewater) and stormwater in a single pipe. When rainfall or snowmelt cause the flow to exceed the collection system’s capacity, the system overflows, resulting in a combined sewer overflow (CSO) that discharges to nearby water bodies. Urban growth and development increase the frequency of CSO events and the amount of released sewage.

During dry weather, combined sewer systems send sanitary waste to the wastewater treatment facility.

During wet weather, sanitary waste and stormwater stream together and can overwhelm combined sewer systems, resulting in a combined sewer overflow that discharges to nearby water bodies.

CSSs serve approximately 40 million people in 32 states, mainly in the Northeast and Great Lakes regions, and CSOs present concern in these states. The U.S. Environmental Protection Agency (EPA) estimates that CSSs release about 850 billion gallons of untreated wastewater and stormwater as CSOs annually.

CSOs make contact with river and stream water dangerous during and after overflow events. Combined raw sewage and stormwater can also back up in basements and surcharge through utility holes when pipes are overwhelmed, causing public health issues. The principal pollutants found in CSOs include:

  • Microbial pathogens.
  • Oxygen-depleting substances.
  • Total suspended solids.
  • Toxics.
  • Nutrients.
  • Floatables.
  • Trash.

Separate Sewer Systems

Sanitary sewer systems (SSS), or separate sewer systems, are wastewater collection systems that collect and convey only sanitary wastewater. Areas served by SSSs often have a municipal separate storm sewer system to collect and convey rainfall and snowmelt runoff. SSSs can overflow when wet weather exceeds the collection system capacity because of excessive stormwater infiltration, when an obstruction blocks normal dry weather flow, or when mechanical failures prevent proper operation.

During dry weather, separated sewer systems send sanitary waste to the wastewater treatment facility through pipes dedicated to sanitary waste while separate pipes deal with stormwater.

During wet weather, separated sewer systems send sanitary waste to the wastewater treatment facility through one set of pipes and stormwater to a storm sewer outfall through a different set of pipes.

The EPA estimates approximately 75,000 SSOs occur yearly, resulting in 3 to 10 billion gallons of untreated wastewater spilling into the environment. For context, there are 850 billion gallons of CSOs.

As communities recognized the importance of wastewater treatment in the mid-1800s, the difference between the combined and separate systems became apparent. Combined sewers offered an efficient and cost-effective means of collecting and conveying stormwater and wastewater in a single pipe. However, the significant flow variations between dry and wet weather conditions made treatment difficult. Growing cities using combined systems often neglected to upsize their pipes, resulting in increased flow.

Separate systems simplified and lowered the cost of wastewater treatment. However, a separate system’s capacity may diminish when the service population increases, but the utility may not initiate corresponding system upgrades.

Wet Weather Management Approaches

Managing wet weather flows involves three main strategies:

  1. Removing: Utilities can remove wet weather flows by rehabilitating sewer or utility hole infrastructure to keep wet weather flows from entering the separate system. Utilities can also achieve removal by separating stormwater from the combined system. The removal process can be long and arduous because there are many sources of I&I, and it is difficult to pinpoint where inflow enters the system.
  2. Storing: Storing excess flow in equalization tanks or tunnel systems provides temporary relief.
  3. Conveying and Treating: Conveying flows to treatment plants requires additional capacity at the plant to handle the further wet weather flows without affecting the treatment processes or overloading the various systems.

A combination of these strategies often proves most effective at reducing pollutant discharge.

Tools for Wet Weather Management

Municipalities use numerous technologies and operational practices to reduce the volume, frequency, and impacts of CSO and SSO events. The performance and cost-effectiveness of these technologies are often related to site-specific factors. Technologies deemed highly effective in one location may prove ineffective in another.

Effective wet weather management relies on various tools and practices, including:

  1. Existing Data: To understand system conditions, begin with existing data, reports, and institutional knowledge.
  2. Geographic Information Systems (GIS): Detailed GIS systems help manage assets, analyze data, and make informed decisions.
  3. Sewer System Investigations: Physical inspections, utility hole inspections, and cleaning or flushing can identify system issues.
  4. Closed-Circuit Television (CCTV) & Manhole Inspections: Remote inspections identify structural problems or blockages, aiding rehabilitation planning.
  5. Flow Monitoring & Micro-Monitoring: Monitoring programs evaluate flow patterns, I&I, and the success of rehabilitation efforts.
  6. Sewer Modeling: Calibrated sewer models identify bottlenecks and inform decision-making for rehabilitation and upgrades.
  7. Sewer Lining & Rehabilitation: Trenchless technologies like cured-in-place pipe lining, epoxy coatings, and pipe bursting enhance structural integrity.
  8. Private Lateral I&I: Addressing defects in private laterals can significantly reduce I&I contributions to the system.
  9. Alternative Analysis: Manage wet weather flows by evaluating storage tanks, interceptor upsizing, and wet weather treatment facilities.

A Novel Flow Management Strategy

The Pleasant Hills Authority (PHA) SSO elimination and capital improvements project offers an insightful example of efficient wet weather management.

The PHA serves portions of six municipalities south of Pittsburgh. The wastewater treatment plant, Lewis Run Pump Station, and the associated trunk sewers were constructed in 1959 with an average flow capacity of 3 million gallons per day (mgd). The PHA expanded the average flow capacity to 5 mgd in 1990 due to population growth and commercial development. Additionally, the PHA expanded the plant to a two-stage process to meet more stringent permit limits.

Although the PHA designed and permitted the system as an SSS, it acted like a CSS. During extreme wet weather events, the system overflowed 3 to 10 times per year. The WWTP met National Pollutant Discharge Elimination System (NPDES) permit limits during average flows.

However, high flows exceeded the raw sewage pumping station capacity during wet weather events. The flow would bypass the first stage and flow directly to the second stage, which the Pennsylvania Department of Environmental Protection considered slipstreaming.

Solids in the second stage final clarifier tended to wash out, causing permit exceedances. Further, the last major upgrade to the plant was in 1990, and many major unit processes were at or near the end of their life expectancy. The plant was a combination of a 55-year-old plant (stage one) and a 24-year-old plant (stage two).

Flow monitoring studies indicated that the plant’s wet weather peak capacity needed to increase to 25 mgd to prevent SSOs during a 2-year, 24-hour storm. To meet the system’s needs, the major project components included:

  • Constructing new headworks to handle up to 12.5 mgd, flow control chamber, and stage two pump station.
  • Upgrading and enhancing the aeration systems.
  • Modifying two stage one final clarifiers to wet weather clarifiers.
  • Adding a second ultraviolet disinfection system to handle up to 25 mgd.
  • Other various improvements to replace old and outmoded equipment.

A post-construction summary reveals the system’s ability to reduce overflows. From 2016 to 2017, 20 storm overflows resulted in over 3.7 million gallons. From 2021 to 2022, 17 storms resulted in 117,712 gallons. Most notably, average storms less intense than a 2-year, 24-hour frequency that produced over 760,000 gallons of overflow before construction produced 0 gallons post-construction.

The flow management strategy allowed the PHA to double its peak wet weather flow without the substantial capital cost of additional tanks. The approach saved the authority money, reduced the impact on ratepayers, and dramatically decreased the system’s SSOs.

Wet weather flow issues remain a crucial challenge in urban areas. Municipalities can work toward more effective solutions by understanding the history, causes, and management strategies. Leveraging innovative tools and techniques, like GIS, CCTV inspections, and sewer lining, enables communities to tackle wet weather flow challenges and safeguard public health and the environment.

Learn More About Wet Weather Flow Issues

To learn more about wet weather flow issues and earn a free professional development hour, watch our webcast, “So Long, SSOs! A Discussion on Managing Wet Weather Challenges.”


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