Combined Sewer Overflows

What is a combined sewer overflow?

Combined sewers collect both wastewater and stormwater and convey it to the MWRA for treatment. During large wet weather events, combined sewers may reach capacity and the untreated combined flow is diverted to CSOs that flow directly into local bodies of water.

Identifying Overflows

Overflows cannot be measured directly because they are influenced by a number of factors. Sensors were placed strategically in a pipeline system to help determine when an activation occurs.

It becomes impossible to quantity an overflow when tide from Boston Harbor or stormwater entering the outfall pipe downstream of the regulator is present before the overflow occurs. The presence of water in this pipe reduces the velocity of the overflow to a level that is very difficult to measure. Identifying when an overflow begins and ends depends upon having credible level and flow measurements.

To avoid missing data during an overflow, BWSC installed depth and velocity sensors in each inflow pipe in addition to the overflow pipes. The pipes carrying water into the regulator usually have flows that are easier to measure.

Most of the time, regulators route combined flows, a mixture of sanitary wastes and stormwater, into a much larger pipe that conveys the combined flows to the MWRA's wastewater treatment facility on Deer Island. During the larger rain storms, it can be necessary for a combined sewer system to allow a portion of the storm-related flow into receiving waters. Special structures referred to as regulators are in place to route the excess flows: making it possible to avoid water backing up onto the streets or into buildings. The diversion is accomplished without any mechanical equipment. Where the flow is directed depends upon water levels; the higher water level flows towards the lower water level. If the water level in the regulator is higher than the overflow weir then an overflow occurs.

BWSC has 82 regulators which it inspects each month to make sure they are operating properly. If they find any material restricting the flow at the bottom of the regulator they will remove it. The NPDES Permit prohibits any overflows during dry weather.

As long as the water level in the regulator is below the overflow weir then the incoming water will be conveyed to the MWRA's treatment facility. The sketch, demonstrates how the water in the inlet pipe is directed into the interceptor.

The normal conditions graphic also shows a typical regulator piping configuration; the overflow pipe is located on the manhole wall opposite the inlet pipe. In this graphic, the flow is routed from the entrance point on the sidewall of the regulator to the exit point at the bottom of the manhole. Each overflow pipe contains a weir to control the level at which an overflow occurs. As part of the MWRA's System Optimization Plan, the height of each weir was adjusted to make sure that as much flow as possible is conveyed to treatment. Generally, the conditions in the interceptor dictate the water levels in the regulator; and more importantly whether an overflow occurs.

The graphic also shows an underflow baffle which was installed as a way to intercept floatable materials before they are discharged during an overflow.

A large storm can affect the water levels in the interceptors. If these level rise high enough then it is possible for the water level in the regulator to rise above the overflow weir. When this happens the water coming into the regulator will be directed out into Boston Harbor. How much water is discharged will depend upon how freely the water can flow into the receiving water. The figure, Overflow Conditions with Rainfall, shows the conditions in a regulator during an overflow. The water level in the inlet pipe is above the overflow weir.

Combined Sewer Overflow Map

The Commission's CSO permit requires that BWSC submit an annual report. These reports have been submitted to the US EPA and the Massachusetts Department of Environmental Protection since 1990.

CSO Annual Report 2022

CSO Annual Report 2021
CSO Annual Report 2020
CSO Annual Report 2019

MWRA's CSO Control Plan

BWSC reports CSO activations to the Department of Environmental Protection when they occur. The CSO activation reports can be viewed here: https://eeaonline.eea.state.ma.us/portal/dep/cso-data-portal/