Peirce Island Wastewater Facility
The Peirce Island WWTF is the larger of the City’s two treatment facilities and is located on Peirce Island. The treatment facility was originally constructed in 1965 and was designed to treat an average daily flow of 1.5 million gallons per day (MGD) of wastewater. The original treatment facility consisted of grit removal, primary clarifiers, and gas chlorination. The treatment facility was upgraded in 1993 with the addition of new larger primary clarifiers, primary effluent sand filtration, and dechlorination. This system was designed to treat an average daily flow of 4.8 MGD of wastewater and a peak flow of up to 22.0 MGD during wet weather events. The sand filtration system added in the 1993 upgrade was unable to efficiently operate and was abandoned in the late 1990’s. The primary clarifiers were upgraded in 2002 to provide chemically enhanced primary treatment (CEPT) and the WWTF has operated with this technology since that time. The treatment facility’s current operations include grit removal, chemically enhanced primary treatment, disinfection with liquid chlorine, and dechlorination. Treated wastewater effluent is discharged to the Piscataqua River through a 24” diameter single port outfall. Sludge is removed from the primary clarifiers then thickened using a gravity thickener and stored in tanks. The stored thickened sludge is dewatered using a rotary press and then disposed of at a landfill.
Municipal wastewater treatment facilities that provide only Primary Treatment are seldom allowed under current regulatory standards. A minimum of Secondary Treatment is required by the 1972 Clean Water Act. The use of Primary Treatment as the sole treatment technology was only allowed for certain facilities where a waiver from Secondary Treatment under Section 301(h) of the Clean Water Act was provided under the National Pollutant Discharge Elimination System (NPDES) permits issued by the Environmental Protection Agency (EPA). The City had received a NPDES permit from EPA with a waiver from Secondary Treatment standards that expired in 1985, and this permit remained in effect pending issuance of a renewed permit. A draft NPDES permit renewal with a 301(h) waiver was issued to the City in 2002 by EPA , but after appeal was rescinded and a permit requiring Secondary Treatment was received from EPA in 2007. The City was required to enter into an Administrative Order with the EPA to upgrade the treatment facility because it could not meet this permit. In 2009 the City entered into a Consent Decree with EPA that outlined the steps and timeframe to upgrade the existing WWTF to meet Secondary Treatment standards. This Consent Decree has been modified since 2009 with the First Modification in 2012 and Second Modification in 2017. The Consent Decree Second Modification outlines the current schedule for meeting Secondary Treatment permit levels by April 2020. The Consent Decree further requires seasonal treatment for total nitrogen.
The treatment facility upgrades which are currently under construction include building a biological treatment system capable of meeting secondary level treatment requirements and providing nitrogen removal. The City took extensive measures to select a treatment technology that both had a small footprint to minimize the impact on Peirce Island, and that provides flexibility. The ongoing WWTF upgrade includes the addition of a two stage biological aerated filter (BAF). This system will treat carbonaceous organics and will convert ammonia to nitrogen gas. There are a number of additional treatment processes being upgraded and added as part of the project. The final treatment system will include raw wastewater screening (new), aerated grit removal, primary clarification (CEPT during wet weather events), BAF influent pumping, BAF Stage 1 (carbon removal and nitrification), BAF Stage 2 (denitrification), disinfection with liquid chlorine and dechlorination. Sludge generated in the upgraded treatment facility will be thickened in two gravity thickeners, stored in aerated storage tanks, dewatered on rotary screw presses, and then disposed of at a landfill.