Special Structures

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 wastewater is diverted by regulator structures to Combined Sewer Overflows (CSO) that flow directly into local bodies of water.

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 to divert a portion of the storm-related flow into receiving waters. Special manholes 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 inspects and maintains its regulators frequently to ensure they are operating properly, and cleans them when needed, to prevent any overflows during dry weather, in accordance with the National Pollutant Discharge Elimination System (NPDES) permit.

Stormwater pipes and some combined sewers (during large wet weather events) can flow into receiving waters. Tide gates are installed near many outfalls flowing into waterways with fluctuating surface elevations in order to prevent the flow of these waters from entering BWSC’s collection system. A tide gate allows for water to flow outward toward an outfall but closes when then water level on the outfall side of the gate is greater than that on the other side.

BWSC inspects, exercises, and maintains its tide gates frequently to ensure they are operating properly and cleans them when needed.

Siphons are used to allow a liquid, in this case wastewater, to flow through a pipe under low lying areas or obstructions such as rivers, streams, dips in elevations, railroads, or other utilities, where flow by gravity is impossible. Siphons achieve flow due to the difference in hydrostatic pressure, eliminating the need for pumping. The flow changes from upstream gravity flow to pressurized flow through the siphon sections.

BWSC inspects, exercises, and maintains its siphons regularly to ensure they are operating properly, and cleans them when needed.