Seneca Wastewater Treatment
Plant Expansion

The Washington Suburban Sanitary Commission (WSSC) upgraded the 25-year-old Seneca Wastewater Treatment Plant in Germantown, Maryland to meet current and future wastewater needs of upper Montgomery County residents. Using advanced technology to protect the environment and the Chesapeake Bay, the WSSC expanded the average plant capacity from 5 mgd to 20 mgd, with a future expansion capacity of 26 mgd. The new wastewater treatment plant uses state-of-the-art biological and chemical processes that remove 64 percent more nitrogen and 77 percent more phosphorous than the original 5-mgd plant. PBS&J provided design and construction services for the expansion of this facility, including extensive technical and cost evaluations that reduced construction costs from the original estimate of $80 million to $47 million. PBS&J also provided survey, geotechnical services, cost assessments, value engineering (VE) evaluations, and resident engineering services.

Construction of this advanced facility took over four years and included biological nutrient removal processes capable of removing nitrogen and phosphorous to very low levels; more than six miles of 1- to 60-inch diameter pipe; removal of nearly 2,000 truckloads of soil to create landscaped buffers; high-efficiency blowers and diffusers to reduce energy costs; and advanced chemical and biological odor control facilities. Specific upgrade or expansion design elements include:

• A 48-inch force main and influent pump station equipped with six 500-hp pumps with VFDs.
• Four activated sludge tanks incorporating a three-pass biological nitrogen removal process with a selector designed to control SVI and filaments. Fine bubble diffusers, new 800-hp, high-efficiency blowers, and an automated oxygen control system in each pass to minimize operational costs.
• Three 150-foot diameter secondary clarifiers with spiral rake mechanisms.
• Thirteen deep bed filters capable of being adapted for denitrification.
• Chlorine and sulfur dioxide disinfection facilities and controls.
• Centralized hypochlorite, caustic soda, alum, and polymer chemical storage and feed facilities.
• A comprehensive biosolids processing facility that incorporated two 3-meter gravity belt thickeners, WAS storage facilities, two high solids centrifuges, lime stabilization facilities, dewatered biosolids storage silo, scum collection and processing facilities, and two-stage biosolids processing odor control and biofilter.