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This past fall, Lake Champlain Sea Grant partnered with Burlington’s Main Wastewater Treatment Plant (WWTP) to organize a tour of the WWTP for city residents and interested community members. The tour focused on stormwater and Combined Sewer Overflows (CSOs), which can impact Lake Champlain’s water quality. Burlington has a wet-weather treatment system which reduces the volume of un-treated CSOs that occur. An untreated CSO occurs when wastewater and stormwater sent through the same pipes to the WWTP overwhelm the volume that the pipes can convey. This causes a mixture of stormwater and untreated wastewater to overflow into Lake Champlain. The primary pollutant of concern with untreated CSOs is E. coli bacteria. While not required by the State, Burlington posts notices at beaches adjacent to CSO discharge locations for 48 hours after a CSO to ensure that recreational water users are aware of the increased risks.   

Amelia McClure, Wastewater Treatment Plant Operator, and Eleonore Walker, Water Resources Policy and Programs Advisor with the City of Burlington, led over 30 tour attendees through the treatment process of wastewater and stormwater before it’s released into Lake Champlain.

“Burlington has both a Combined Sewer System (CSS) - accounting for less than 30% of the system - where some of the city’s stormwater drains into the same pipes as sewage to the Wastewater Treatment Plant, and Municipal Separate Storm Sewer Systems (MS4s) where stormwater flows separately and directly into waterways without treatment,” said Walker. “There are pros and cons to both of these systems. If the combined sewer flow can fit through the pipes and get to the plant, portions of the storm water and wastewater get full wastewater level treatment, with the remainder going through wet weather treatment which reduces pollutants and disinfects the water prior to it entering the Lake. However, intense rainstorms that cause combined sewer flows to exceed the pipe capacity can lead to overflows that contain a mix of stormwater and untreated wastewater.”

The outlines which areas of Burlington drain to the CSS and which areas drain to separate MS4 systems.

During dry weather, wastewater passes through the following steps:

1. Water starts by passing through the screening process where a screen and mechanical rake remove large solids.
2. The second step of the process removes small solids like grit, sand, egg shells, and coffee grinds.
3. After large debris is removed, clarifying tanks allow any remaining solids smaller than a grain of sand to settle at the bottom of the tank. From there they are pumped to the biosolids treatment process.
4. Secondary treatment tanks use microbes and aeration to eat any dissolved pollutants in the wastewater. . These helpful microbes are settled out and returned to the aeration tank where they can remove more pollutants. Other chemicals, such as alum which treats phosphorus, are added to the water after this stage.
5. Before being released into the lake, the treated wastewater is mixed with enough chlorine to ensure unsafe bacteria and pathogens are disinfected.

In dry weather, wastewater goes through the full physical, biological, and chemical treatment process in 18–24 hours. During storm events, this “premium treatment” is also applied to stormwater– up to a peak of 13 million gallons a day. At these higher flow rates water passes through the same process in 5–6 hours.

During intense rains, the volume of stormwater is too high to fit in the City’s existing pipes. There are overflow sites along the shore that release the stormwater and a small portion of untreated wastewater from the combined sewer system into the lake, acting like pressure release valves during a CSO event. However, all water making it to the WWTP receives some treatment.

Stormwater from the combined sewer system is sent to the “CSO Vortex” where underground pumps can send up to 100 million gallons per day into the wet weather treatment system. The vortex circulates, causing dirty water to sink to the bottom of the tank. The dirty water is sent back through the entire treatment process while the remaining cleaner portion of the water is disinfected with chlorine and bromine. 

Prior to the wet weather improvements made in 1994, the plant was not designed to treat more than the typical dry weather flows and there were numerous (11) CSO locations throughout the City.  The 1994 improvements increased the dry weather system capacity to 13 million gallons per day and created the Vortex separation and wet weather disinfection system.  Since its installation, the CSO Vortex has disinfected on average 170 million gallons per year of combined stormwater and wastewater that would have otherwise been directly discharged to Burlington Bay. As climate change leads to more frequent and intense storms, the CSO Vortex will likely treat more and more water each year.

Capturing stormwater on-site and reducing the volume of water being sent to the Wastewater Treatment Plant is crucial for the health of Lake Champlain. That is why the City has invested in Green Stormwater Infrastructure projects like rain gardens in City Hall Park and along Saint Paul Street and leveraged grants to build bioretention and infiltration systems in the South End that reduce flow to the Pine Street CSO. Private properties can also reduce stress on the Wastewater Treatment Plant by participating in BLUE or other stormwater programs like to install rain barrels, rain gardens, and infiltration trenches.