Can We Avoid Nitrification? Tracking the In-Plant Fate of Nitrogen at the Hopewell Regional Wastewater Treatment
The allowable nutrient discharge loading for a publicly owned industrial treatment works (POITW) with high industrial loading treating approximately 27 million gallons per day (mgd) is being capped based on the Chesapeake Bay and its tributaries nutrient control regulations adopted by the Virginia Department of Environmental Quality. Based on an analysis of historical data, the facility has experienced an increase in effluent total nitrogen between 2000 and 2004 such that it cannot meet its proposed waste load allocation (WLA) at current flow conditions, without implementing source controls or inplant nitrogen removal. Approximately one-half of this increase is the result of an increase in influent organic-N loading to the plant from both domestic and industrial sources. The other one-half is attributed to expected variability in the amount of TKN (organic-N and ammonia-N) that the treatment facility removed. To explain this variability and to identify plant practices or in-plant sources that may contribute to periodic lower nitrogen removal, the POITW initiated development of an optimization study for TKN removal. The objectives of the study were to determine fate of nitrogen species through the plant, and identify in-plant sources of nitrogen or possible operational practices that could be addressed to reduce effluent nitrogen. The study developed historical mass balances to track the fate of nitrogen within the plant by combining historical data with more detailed results from a short-term intense sampling effort. Tracking internal nitrogen sources, such as high-TKN solids recycle streams, provided a clearer picture of the uncontrollable variability in TKN removal that was observed in the plant’s past performance. Understanding the causes of this variability will enable the facility to optimize TKN removal such that the plant removes the maximum amount of nitrogen possible for the system without nitrification, which may help the POITW to meet its proposed WLA at current flow conditions.
The Hopewell Regional Wastewater Treatment Facility (HRWTF) is a 50 million gallon per day (mgd) publicly owned industrial treatment works (POITW) located in Hopewell, Virginia that treats an annual average flow of approximately 27 mgd of combined wastewater from local industries and domestic sources. The treatment facility, illustrated in Figure 1, currently uses high purity oxygen (HPO) in its biological treatment tanks, and achieves denitrification of influent nitrates and nitrites in a denitrification basin, but not nitrification. TKN reduction is achieved only through assimilation that occurs as a result of cellular growth in the activated sludge process.
Municipal wastewater from the City of Hopewell is pretreated offsite at a satellite facility (Primary Plant). The wastewater undergoes primary treatment and disinfection at the Primary Plant prior to discharge to the HRWTF. The primary solids are lime stabilized then combined with Primary Plant effluent. Industrial loading comprises about 85 percent of the flow and 90 percent of the BOD loading to the plant, and is generated primarily by five local industries: Honeywell-Hopewell, Hercules Incorporated, Smurfit- Stone Container Corporation, Virginia American Water Company, and DeGussa- Goldschmidt Chemical Corporation. The high proportion of industrial loading in the influent creates a scenario not commonly encountered at publicly owned treatment works, which typically treat primarily domestic flow. Because of the large industrial contribution, HRWTF faces a number of unique challenges not typical of publicly owned treatment works. These challenges include: high influent temperatures that exceed 37oC (the upper temperature limit for stable nitrification), variable influent wastewater characteristics and frequent spike loadings, potential sources of chemical inhibition of nitrification, and high concentrations of volatile organic compounds (Bailey and Steidel, 1998; Bailey and Steidel, 1999; Bailey and Walker, 1999; Malcolm Pirnie, Inc., 1999).
The HRWTF’s allowable nutrient discharge loading is being capped based on the Chesapeake Bay and its tributaries nutrient control regulations adopted by the Virginia Department of Environmental Quality. The waste load allocations (WLA) limit HRWTF to an annual average effluent total nitrogen cap of 1.8 million lbs/year (Mlbs/yr). HRWTF has long-term plans to upgrade the treatment facility to achieve nitrification and biological nitrogen removal to meet WLA at design flows. However, because the high industrial contribution complicates achieving nitrification at HRWTF, the POITW voluntarily initiated a series of steps to optimize nutrient removal and to reduce effluent nitrogen as much as possible without initiating nitrification. To date, these steps have been taken:
- In 1997, the original sludge heat treatment and solids dewatering treatment system was replaced with a centrifuge dewatering facility. This measure reduced the ammonia loading from the solids handling operations at the HRWTF by an estimated 400 lbs/day (Malcolm Pirnie, Inc., 1999).
- In 1998, installation of an ammonia stripper at the Honeywell-Hopewell facility reduced the total influent ammonia loading to the HRWTF by more than half (Malcolm Pirnie, Inc., 1999).
- In 2002, implementing a permanent denitrification process at HRWTF by converting an existing primary clarifier to a denitrification basin enabled the HRWTF to achieve greater than 95 percent removal of influent nitrate and nitrite. This measure has reduced the annual average effluent total nitrogen loading by approximately 3,500 lbs/day.