South Essex Sewerage District
The South Essex Sewerage District was established in 1925 to build, maintain and operate a sewerage system to collect, convey, treat and dispose of residential, commercial and industrial wastewater. The District currently serves approximately 180,000 people and businesses located in the geographical area that includes Beverly, Danvers, Marblehead, Peabody, Salem, a portion of the Town of Middleton, and a portion of the Town of Wenham. The District does not directly bill individual users. The District annually assesses its costs to the entities that are located within the District based on their use of District facilities as measured by wastewater flow in Millions of Gallons, and charging each community based on the costs of processing of that flow.
The South Essex Sewerage District wastewater collection and conveyance system includes 29 miles of interceptor and force main pipelines, pumping stations, and associated metering and sampling facilities. The District serves approximately twenty (20) significant industrial users (SIUs) that contribute approximately 1.8 MGD. The largest industrial user discharges approximately 1.5 MGD on average.
Our District facility is a secondary wastewater treatment plant designed to treat up to 29.7 million gallons per day (mgd). Treatment processes include pumping, screening, de-gritting, settling, biological treatment, disinfection, odor control and biosolids management. The District also uses best management practices to conserve and reduce energy at the facility.
The entire flow at the treatment plant is covered – meaning wastewater is not exposed in open tanks – in order to control odors. SESD effluent water quality is excellent and meets or exceeds the required permit values. Plant effluent is discharged through an ocean outfall into Salem Sound.
(Fig. 1) Wastewater is conveyed to our facility in Salem, MA through a combination of gravity interceptors and three (3) offsite pumping stations located in Beverly, Danvers, and Marblehead. Screenings are removed or reduced in size at the offsite pumping stations before reaching the plant. Two (2) additional pumping stations are onsite: the main influent pump station from Peabody/Salem and the Salem Neck Pump Station.
The Peabody/Salem Pump Station is also equipped with mechanically-cleaned bar screens which automatically remove any sticks, rags, and other large debris in the wastewater.
The screened wastewater then joins the gravity interceptor flow and is directed into aerated grit tanks for grit removal (Fig. 3). The settled grit is removed for disposal at a landfill.
After passing through the aerated grit tanks, the wastewater flows through pipes and then enters the rectangular primary settling tanks where some of the solid material in the wastewater is removed (Fig. 4).
The settled wastewater then flows into the rectangular high-purity oxygen reactors (Fig. 5).
The high-purity oxygen generation system supplies oxygen to the reactors by stripping nitrogen from compressed air to concentrate the oxygen. Alternately supplemental liquid oxygen can be added. The reactors use biological organisms in the wastewater and combine it with activated sludge to create mixed liquor. The biological solids in the mixed liquor consume the organic matter in the wastewater, breaking it down to carbon dioxide and water.
The flow then enters the rectangular secondary clarifiers for separation of the biological organisms from the mixed liquor (Fig. 6). The secondary clarifiers are stacked on top of each other and flow is split between upper and lower tanks.
A portion of the solids – called return-activated sludge – is removed, and the flow is pumped back to the high-purity oxygen reactors.
The remaining solids – referred to as waste-activated sludge – are collected and pumped to the operations building for further processing.
The clarified effluent then flows to our two (2) chlorine contact tanks to disinfect the wastewater (Fig. 7). Sodium hypochlorite is mixed with the treated effluent as it flows in to the chlorine contact tanks.
Then, de-chlorination occurs through adding sodium bisulfite to reduce the residual chlorine as it flows out of the chlorine contact tanks. This protects sensitive aquatic species, prevents algae blooms, and other environmental hazards. The treated effluent is pumped by one or more effluent pumps to the outfall diffuser in Salem Sound (Fig. 8).
There are six (6) separate odor control systems at the treatment plant. These odor control facilities use chemical scrubbers followed by activated carbon to remove odorous compounds from the wastewater. These systems are located to treat air from Salem Neck Pump Station, the aerated grit tanks, primary settling tanks, oxygen reactors, secondary clarifiers, influent pumping station, and the sludge-processing areas.
The biosolids management at the treatment plant takes waste-activated secondary sludge and thickens it with gravity belt thickeners (Fig. 9) and blends it with primary and secondary solids (Fig. 10).
Solids are stored prior to processing with belt filter presses for further de-watering (Fig. 11). De-watered biosolids are loaded and trucked to landfill(s), composting facilities, or for incineration.