Durham, N.C., facility installs sidestream deammonification system to meet strict TN limits
The City of Durham is located in the Research Triangle Region of North Carolina. It operates two wastewater treatment plants – the North Durham Water Reclamation Facility (WRF) and the South Durham WRF, both of which are permitted to treat 20 mgd. In 2011, the city completed a comprehensive wastewater master plan that evaluated different treatment techniques for meeting strict total nitrogen (TN) limits at the South Durham WRF. As a result of the evaluation, Durham selected Kruger’s ANITA Mox sidestream deammonification system for ammonia and total nitrogen removal. The decision was based on a combination of capital and operating costs. The ANITA Mox facility at the South Durham WRF began operation in the spring of 2016.
The South Durham WRF will need to meet a TN limit of 3 mg/L at its design flow to comply with the total maximum daily load (TMDL) in the Jordan Lake Watershed, which serves as a source of drinking water in the region. The WRF uses anaerobic digesters to break down the plant’s sludge. Downstream of the digesters, the plant uses belt filter presses for dewatering. The resulting liquid – the pressate from dewatering, or what is referred to as “sidestream” flow – historically accounted for about 20% of the nitrogen load in the plant’s biological nutrient removal (BNR) process. While this sidestream nitrogen contribution sounds high, it is typical for many plants with anaerobic digestion.
The city studied mainstream and sidestream treatment alternatives to meet its TN limits. In its cost comparisons, ANITA Mox was calculated to be approximately three times lower in cost per lb of nitrogen removed when capital and operating costs were considered. ANITA Mox was estimated to cost $0.93 per lb of nitrogen removed, while the most cost-effective mainstream BNR solution was estimated at $2.66 per lb of nitrogen removed. The city thus selected ANITA Mox as a cost-effective nitrogen removal solution.
ANITA Mox is Veolia’s sidestream deammonification technology for short-cut nitrogen removal. It is based on the moving-bed biofilm reactor (MBBR) platform. As such, the system consists of engineered polyethylene carriers – in this case AnoxKaldnes K5 media – to provide ample protected surface area for biofilm to thrive. The K5 media (approximately the diameter of a quarter) host two types of bacteria in the same reactor. The outer layer consists primarily of ammonia oxidizing bacteria (AOB), which convert about half of the ammonia to nitrite. The inner layer consists mainly of anaerobic autotrophic ammonia oxidizer (anammox) bacteria. These bacteria utilize the resulting nitrite and much of the remaining residual ammonia and convert them to nitrogen gas, which is released harmlessly to the atmosphere.
Since ANITA Mox has a high removal rate and treats the smaller sidestream flow at a wastewater plant, it has a compact treatment footprint. At many plants, the system can fit into a spare or abandoned tank on site. At the South Durham WRF, for example, the MBBR system was constructed in an abandoned aerobic digester.
ANITA Mox is offered in both MBBR and integrated fixed-film activated sludge configurations, depending on site conditions. Kruger Inc. is Veolia’s subsidiary in the United States that serves the municipal market.
At the South Durham WRF, the ANITA Mox system was started up in 12 weeks – an efficient time, given the slow growth of anammox bacteria. Now operating full-scale, the system is achieving greater than 80% ammonia removal and 70% total inorganic nitrogen removal – both exceeding guaranteed values. The system is thus helping the facility meet its strict effluent nitrogen limits using a cost-effective solution.