The discharge of ammonia and nitrogen to receiving bodies of water is becoming more strictly regulated at municipal wastewater treatment plants throughout North America. The dewatering of anaerobically digested biosolids creates a reject water stream (often referred to as centrate or dewatering filtrate) that can contribute to an additional 15 to 20 percent of the ammonia load to a facility. If not managed properly, the reject water stream can have a significant impact on secondary treatment performance in terms of nitrification and nitrogen removal. The objective of this paper is to document and summarize reject water treatment experience at facilities in North America. Much of the groundbreaking work in this field has been lead by the City of New York Department of Environmental Protection (NYCDEP) and a summary of their work over the past 15 years is provided herein. In addition, this paper provides a summary of other North American facilities that have carried out or are currently carrying out pilot or full-scale application of these technologies. The information presented in this paper will be useful to practitioners in the municipal wastewater industry who are considering the implementation of centrate/reject water treatment for their facilities and wish to gain a better understanding of where it has been practiced at North American facilities.
The discharge of ammonia and nitrogen from point sources such as wastewater treatment plants (WWTPs) is becoming more strictly regulated in a number of North American jurisdictions due to the sensitivity of the receiving body of water with respect to these parameters. The free or unionized form of ammonia can be toxic to aquatic life at concentrations typical of municipal wastewater effluents that do not practice nitrification. The discharge of nitrogen has been linked to eutrophication of receiving bodies of water.
Provision for sufficient secondary treatment tankage and oxygenation capacity for year round nitrification at municipal WWTPs is quickly becoming the norm. Provision for nitrogen removal, on the other hand, has been less common historically in North America. Most plants that practice some degree of nitrogen removal via denitrification do so only to facilitate biological phosphorus removal. In some cases, however, the need to provide nitrogen removal to very high levels is becoming more common. In the United States, nitrogen has been identified in the nutrient-related declines of shellfish and aquatic plant life in the Chesapeake Bay, Florida coastal areas, and the hypoxic ‘dead zone’ in the Gulf of Mexico and Long Island Sound. Many of the treatment plants discharging to these sensitive bodies of water have already instituted nitrogen removal or have planned to do so. In Canada, the eutrophication of Lake Winnipeg (Manitoba) has lead to plans to implement nitrogen removal at all three of the City of Winnipeg’s WWTPs.
At WWTPs that practice anaerobic digestion and dewatering, a key contributor to the ammonia and nitrogen load is the dewatering recycle stream, commonly referred to as centrate or reject water. Although the reject water typically contributes only one percent of the flow to the plant or less, this ammonia laden stream can contribute anywhere from 15-30 percent of the nitrogen load.