The sewer pipes, which range in size from 6” to 10 feet in diameter, direct wastewater to the head works of the wastewater treatment plant. At several locations along the interceptor routes, siphons convey wastewater under the Concord and Merrimack Rivers.
There are nine diversion structures along the interceptor sewers that serve to control flow through the siphons and to divert, if necessary, a portion of wet weather flows.The wastewater treatment plant is a primary and secondary treatment plant. The primary treatment removes settleable solids, grit, and floatable materials. The secondary treatment principally deals with the removal and/or conversion of biodegradable organic contaminants.
Typically, the facility will remove a minimum of eighty-five percent (85%) of the contaminants that enter the treatment plant. The two primary contaminants that the plant is designed to remove are Total Suspended Solids (TSS) and Biochemical Oxygen Demand (BOD). The plant is designed to treat wastewater flows averaging 32 Million Gallons per Day (MGD). During wet weather conditions, the plant can treat as much as 75 MGD of combined sewage and stormwater.
After the primary and secondary treatment processes, a form of Chlorine (Sodium Hypochlorite) is introduced into the wastewater for disinfection. Any chlorine residual remaining in the treated water is then removed through the addition of Sodium Bisulphite before the plant effluent is discharged to the Merrimack River. The solids, or sludge, produced by the removal of pollutants go through a solids dewatering process and are transported to a remote location for land application.