Wastewater Treatment Facility Upgrade and Expansion, Derry Township Municipal Authority, Hershey, PA

Derry Township Wastewater Treatment Facility

Since the mid-1990s, BH has worked with the Authority in completing long-term facilities planning, design, and construction management for the entire collection, conveyance, and treatment system.

The DTMA Clearwater Road (CWR) Wastewater Treatment Facility (WWTF) provides complete treatment of the wastewater generated in the Township of Derry and surrounding communities. The service area includes more than 8,000 customers including two significant industrial users. The WWTF, which was originally constructed in 1976, has had numerous improvements constructed over the past few years. It is designed to treat 5.02 MGD of wastewater to a level meeting or exceeding all current PA DEP, EPA, and Chesapeake Bay requirements.

Influent wastewater is pumped from the wet well to the headworks using dry pit centrifugal pumps controlled by redundant level transmitters and operated by state of the art variable frequency drives. The headworks equipment includes two self-cleaning cylindrical screens with dewatering and conveyance conveyors, metering equipment, and two induced vortex grit removal units. Screenings and grit removed in the headworks are discharged to a dumpster for disposal at a landfill.

Immediately adjacent to the headworks is a two-lane septage receiving station and grease trap pretreatment facility. The WWTF is one of the few facilities accepting trucked grease trap waste for disposal.

Following preliminary screening and grit removal, all settleable materials are removed as a primary sludge in the subsequent primary clarification step. Wastewater then flows to the biological treatment processes. The WWTF uses a three-phase biological treatment process that includes primary anoxic, oxic, and secondary anoxic followed by reaeration to remove pollutants including total nitrogen to very low levels. During the biological step, ferric chloride is added to what is now known as mixed liquor to remove phosphorous. This mixed liquor then flows into the final clarifiers where the biological (or activated) sludge is settled and removed. A portion of the removed activated sludge is returned to the head of the biological process and a portion is wasted. The resulting high quality final clarifier effluent is disinfected using ultraviolet light and discharged to the Swatara Creek.

The treatment of the wastewater results in the production of waste solids or sludges. The primary, waste activated, and industrial sludges (piped directly from an industrial pretreatment plant) are processed in a 1.2-million-gallon, egg-shaped anaerobic digester. The anaerobic digestion process involves heating to 100°F and mixing the digester contents for approximately 30 days. The digestion process reduces the amount of solids by more than 50% and transforms the sludge into biosolids.

A byproduct of the digestion process is the production of large quantities of methane or biogas. The biogas produced by the digestion process is conditioned into a high quality fuel used to: heat the contents of the digester, produce steam for the biosolids dryer, and fire an internal combustion engine for the cogeneration of electric power and heat. Annually, the cogeneration system will generate more than 1.5 million KwH of electric power and in addition, save more than 18,000 gallons of fuel oil through the recovery of waste combustion heat.

The liquid, digested biosolids are physically dewatered into a cake-like form using a high speed solids separator known as a centrifuge. This cake is then fed to an indirect, steam-heated paddle dryer that heats the biosolids cake to more than 300°F in a pasteurization-like process. The resulting product is a dry granular, high quality fertilizer product known as STEADIGRO™.