Sewage Treatment – Small System in a Watershed

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ABSTRACT:
Water purveyors, in examining their watersheds, have raised concerns over small community pollution sources, particularly over phosphorus, nitrogen, pathogens and suspended solids. As our population has increasingly moved further upstream in watersheds, the application of classic sewage treatment plants (STPs) have been pushed to their limit of design, especially in light of the need by water purveyors to “gain control” of pollutant sources and emerging issues. Needs for upgraded and/or new STPs are readily evident. Typical downsized “big system” designs are being applied. However, many of the existing STPs that need upgrading or replacement are located on sites that have residents screaming in anger and the costs of treatment are causing some to be concerned about their family budgets and the “sellability” of their homes.

O'Brien & Gere Engineers, Inc. (OBG) was retained by the Town of Beekman, New York to design a STP to replace the existing plant at the Dover Ridge residential community. The Town was required to take over the facility following default of the former owner.

The existing STP, which was not meeting several of the requirements of its NYSDEC SPDES permit, discharged to a waterbody tributary to the water supply of a correctional facility. The existing STP consisted of intermittent sand filter beds and sodium hypochlorite disinfection. Such designs are common for small communities, schools, and camps. These systems are typically unable to handle increasingly more stringent effluent requirements that are being imposed by regulating agencies, especially for those STPs that discharge to drinking water supplies.

Under pressure of an order on consent, an initial solution consisting of an MBR was proposed. However, the community demanded a lower cost facility to keep sewer fees at affordable levels. In addition, the project site was “in the backyards” of homes and a “zero” profile facility was important to residents. To meet these requirements, OBG proposed a recirculating sand filter (RSF) based STP. Advantages of RSFs over conventional STPs (e.g., activated sludge, fixed film) are as follows: Simpler operation and lower O&M costs, less visual impacts, Improved solids removal and enhanced nutrient removal.

RSFs offer superior performance and have lower footprint requirements than traditional intermittent sand beds. Their improved suspended solids removal allows for the use of UV disinfection, which does not form disinfection by products, as does chlorine. Other improvements at Dover Ridge included additional septic tankage, UV disinfection, and cascade aeration.

The plant’s SPDES permit effluent requirements are as follow:

  • CBOD5 – 5.0 mg/l (Daily Maximum)
  • TSS – 10.0 mg/l (Daily Maximum)
  • Fecal Coliform – 200/100 ml (30-day geometric mean), 400/100 ml (7-day geometric mean)
  • TRC – 0.1 mg/l (Daily Maximum)
  • pH – 6.5 to 8.5 SU
  • Settleable Solids – 0.1 ml/l
  • Dissolved Oxygen – 7.0 mg/l (Daily Minimum)
  • Monitoring of effluent ammonia.

The construction of the new Dover Ridge STP is complete. To date, the above stringent effluent limits are easily being met by the new treatment facilities. Effluent ammonia concentrations have typically been less than 1.0 mg/l. Actual operating data will be presented.

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