In New York’s Ulster County, the heavy rain of August 4, 2026 caused the discharge of over 62,000 gallons of combined sewer overflow (CSO) in six hours into the Rondout Creek and the surrounding beach. That same day, 54 miles away in Dutchess County, the sewer at 37 Corbin Road overflowed in the Pawling Joint Sewer Commission Wastewater Treatment plant into the main influent pump station.

Normally all wastewater from sewers collects into a wastewater treatment plant where it is treated and is then discharged into a nearby water body that could be a recreational water body. However during heavy rainfall (known as wet weather), huge amount of wastewater could enter the wastewater treatment plant – more than the capacity that the plant normally handles – causing untreated sewage to spill directly into nearby areas, a scenario known as sewer overflow. When this overflow occurs from combined sewer, meaning all sewage runs in the same pipe(s) to the water treatment facility, the overflow is known as CSO.

According to the New York Department of Environmental Conservation, CSO discharges, because they are untreated sewage and thus may contain harmful microorganisms and toxic pollutants, pose health risk to humans and animals, lead to reduced oxygen level in the water and thus impact water quality leading to water pollution which in turn could cause algae growth.

Because around 60 percent New York’s sewers are combined sewers, totaling more than 20 billion gallons of sewage annually, environmental groups are pressing the government to notify the public when CSOs occur so that people can reduce their time in polluted recreational water bodies due to CSO’s health and environmental hazards. However, this is challenging because of the frequency of CSO events, an example being that ten CSO events occurred last year alone in Newtown Creek, New York according to a recent report by the City of New York Department of Environmental Protection.

To reduce the occurrence of CSO events, wastewater treatment plants use wet flow management. What strategies are used in wet flow management? This question is addressed in this article on the fundamentals of wet weather flow in wastewater systems.

Overview of water flow in sanitary sewer system

Before discussing wet weather system, it is important to understand that all sewage – domestic and industrial sewage – flows into sanitary sewer system which is a collection of pipes leading to the wastewater treatment facility. Pertaining regional regulations govern the design and number of pipes in the sanitary sewer system.

The flow in the sanitary sewer system could be wet weather flow or dry weather flow. While wet weather flow includes runoff from storms, dry weather flow includes flow in the absence of wet weather (rainfall and snowmelt).

Combined sewer system and separate sewer system

When wastewater from domestic, commercial, and industrial sources and stormwater flow through the same pipes, the sewer system is known as a combined sewer system. In a separate sewer system, separate set of pipes separately collect sewage and stormwater. One set of pipes deliver sewage to the wastewater treatment facility while another set of pipes collect stormwater and delivers it to a local waterbody.

According to the EPA, most communities in the United States have separate sewer systems. However as of 2026, approximately 700 communities in the US have combined sewer systems and the water pollution from these systems, due to CSOs, are major health and environmental issues for these communities.

What is wet weather flow in wastewater systems?

Wet weather flow is sewage runoff into sewer systems during a rainstorm. According to the wet weather flow management guidelines by the City of Toronto, wet weather flow includes stormwater runoff.

Based on guidelines, there are variations in how different states measure wet weather flow. For example, Spotsylvania County, Virginia includes peak wet weather flow in their formula for calculating sewer overload as follows:

Peak wet weather flow = average dry weather flow + rainfall-dependent infiltration flow

The average dry weather flow in the above formula is the average flow that occurs daily without rainfall and it is composed of sanitary base flow and groundwater infiltration. In Spotsylvania, groundwater infiltration is an allowance that is added to the sanitary base flow and is estimated to be 500 gallons per day per inch diameter mile.

Why is wet weather flow management important?

The surge in the flow of wastewater containing sewage mixed with rainwater in combined sewer systems causes sewer overflows. Because this overflow contains untreated sewage which means it may contain harmful microorganisms and toxic chemicals, exposure to this overflow may pose serious health issues and thus, it is important to manage wet weather flow.

Chemical constituents and environmental impacts of sewer overflow

The following are some of the main chemical constituents in CSOs:

• BOD₅/COD (5-day biochemical oxygen demand/chemical oxygen demand): Although these concentrations are diluted due to the mixing of untreated sewage with stormwater, the concentrations could be high, similar to the concentration of the influent at wastewater treatment facilities.
• TSS (total suspended solids): High concentration of TSS is due to the presence of microorganisms and chemicals in the untreated sewer water.
• Nitrogen and phosphorus: The stormwater could contain nitrogen and phosphorus originating from land runoff. When this mixes with the sewage that also contains nitrogen and phosphorus, the sewer overflow has high concentrations of nitrogen and phosphorus.
Metals: Similar to the origin of nitrogen and phosphorus in stormwater, metals can also wash off from land into the stormwater and eventually into the sewer overflow.

How does wet weather flow impact treatment plants?

Wastewater treatment plants are affected by wet weather flow because the excess water – sewage water mixed with stormwater – particularly from CSOs, causes high influent flow, referred to as peak flow. Exceeding the capacity that could be treated by the wastewater treatment plant, peak flows could result in:

• Increase in volume of sewer overflow including backup of sewage
• Impair biological treatment units within the wastewater treatment facility because these units are sensitive due to changes in wastewater characteristics from peak flow.

What strategies are used to manage wet weather flow?

Separate sewer systems containing separate pipes to deliver sewer to wastewater facilities and other pipes to collect and discharge stormwater into water bodies is a common strategy used to manage wet weather flow.

The use of large tanks in which the mixture of sewer and stormwater flows and is stored followed by subsequent treatment is a CSO management approach due to the 1994 US EPA CSO control policy for capture and treat. Some of the other treatment approaches for wet weather flow are as follows:

High rate-solids separation: This includes clarification processes that rely on gravity to settle solids. Flocculation and vortex are other techniques for separating solids in the overflow.

Filtration: For treating wet weather flow, compressible media filters have been tested. When using compressible media filters, the spherical synthetic media are compressed between two porous steel plates to form smaller pores capable of capturing more solids than without compression during the filtration process for the overflow. After the filtration period, the compression is released to open larger pores in the filters to remove accumulated solids.

Smart data infrastructure

The US EPA updated an existing report in 2021 on smart data infrastructure that support wet weather control. In simple words, it focused on how municipalities and utilities can use smart data infrastructure to support wet weather control and decision making in real time or near real time.

This report defined smart data infrastructure as a network of technology tools including probes and solutions that are focused on the collection, storage, and potentially the analysis of water-related data. Identifying variables in smart data infrastructure that affect influent collection and system performance can be useful for wastewater treatment facilities to implement solutions for wet weather management and to minimize operational costs.

Real time monitoring provided by smart data infrastructure is made possible by recent developments in artificial intelligence, machine learning, and decision support systems. Scalability – meaning the ability to handle large wastewater treatment facilities efficiently – and solutions for rapidly changing climatic conditions have made these technologies promising in managing wet weather flow for wastewater. These recent developments make for an exciting era to see what is next in wet weather flow management for wastewater.