CSOs

What is a Combined Sewer System (CSS)?

Combined Sewer Systems are very common in industrialized cities such as Lawrence, Lowell, Haverhill and 16 other communities across the Commonwealth. They collect surface runoff from rain and snowmelt, i.e. stormwater and wastewater in a shared system. At GLSD, wastewater from households, office buildings, and industries, as well as stormwater is collected in large pipes called interceptors along the Merrimack and Spicket Rivers. This system works well during dry weather days but may become taxed during rain and heavy snowmelt. These systems prevent sewage backup into homes and businesses by automatically releasing combined sewerage that is beyond the capacity of the system.

Because Combined Sewer Overflows can have significant environmental impact, these systems are no longer considered state-of-the-art, but they do serve an important purpose in protecting residents during extremely harsh weather events – events which are happening more and more frequently in the face of climate change.

GLSD has invested over 60 million dollars towards CSO reduction by bolstering its treatment processes, increasing maximum flows pumped to the plant and by operating a high flow treatment system. All of these improvements have helped to reduce the frequency and duration of CSO events. In addition, to ensure our facilities are able to continuously operate during power outages beyond our control, full backup power was added to the main treatment plant in 2019 and the Riverside Pumping Station in 2020. (For details on these and other past capital projects click here.)

We also partner with municipalities along the Merrimack, advocating for improvements to sewer infrastructure that will reduce in number and minimize the impact of future CSO events on both the River and its surrounding communities. As extreme weather patterns grow in frequency, these infrastructure improvements become more and more essential to protecting the health of the Merrimack River, and we are continually investigating new innovations that will allow GLSD to treat more flow.