|Rapidan S.A.-Wilderness STP
||Northern Regional Office, Woodbridge
||71,270 lbs/yr TN; 16,800 lbs/yr TP
Brief Project Description
Grantee: Rapidan Service Authority
The Rapidan Service Authority owns and operates the Wilderness STP, with a current design flow of 0.715 million gallons per day (MGD). The facility exceeded 95% of its design flow for three consecutive months in 2007; therefore, the Authority has proposed an expansion to 2.0 MGD with installation of nutrient reduction technology to comply with the Chesapeake Bay Nutrient Discharge Control Regulations.
The upgraded facility will utilize a 5-stage modified Bardenpho process coupled with chemical addition followed by tertiary filtration to achieve state-of-the-art nutrient removal limitations. As the existing secondary treatment process consists of extended aeration in shallow oxidation ditch basins, upgrading to a 5-stage process will require construction of additional biological treatment basins. The activated sludge treatment improvements will comprise three treatment trains including nitrate recirculation, return activated sludge, chemical feed and storage facilities for supplemental carbon, anoxic mixing and aeration blowers with fine bubble diffusers. The treatment trains will be followed by three secondary clarifiers for solids/liquid separation.
Phosphorous removal will be accomplished primarily by tertiary filtration preceded by chemical treatment. The 5-stage Bardenpho process will be utilized for biological phosphorous removal. However, to achieve stringent phosphorous removal as required by the facility’s permit, chemical precipitation followed by filtration will be required. Tertiary treatment improvements include chemical storage and feed facilities for alum, injection and flocculation facilities followed by a continuously backwashing upward-flow sand filter system. Disinfection will be accomplished by chlorination using liquid sodium hypochlorite and de-chlorination using liquid sodium bi-sulfite. Disinfection improvements will include chlorine contact basins and chemical feed facilities for hypochlorite and sodium bi-sulfite. The solids treatment process will consist of waste activated sludge thickening, aerobic digestion and sludge dewatering. The existing plant utilizes a recessed plate filter press; improvements will include the addition of a second filter press and appurtenances.
The overall grant eligibility of the plant expansion to 2.0 MGD, as related to the nutrient reduction portion of the project, has been pro-rated at 90% due to a design flow exceeding what DEQ deems “reasonable and necessary” for a 20-year design life. Bids have been received for project construction; Notice to Proceed is scheduled for Sept. 2009 with Final Completion by May 2011.