Deammonification, based on partial nitritation combined with anammox, is a more environmental friendly nitrogen removal technology compared to conventional nitrification/denitrification, due to decreased energy need, less use of chemicals and low emissions of CO
2 and N2O. Treatment of supernatant from dewatering of digested sludge with this technology can decrease nitrogen load to wastewater treatment plants and gives better possibilities to meet future stringent effluent requirements.
The objective of this study was to investigate strategies for controlling and monitoring the deammonification process in a moving bed biofilm reactor (MBBR) and evaluate the influence of different aeration strategies on the process performance and efficiency. Experiments were carried out, both in laboratory scale and in a pilot plant at Hammarby Sjöstadsverk.
A significant parameter influencing the nitrogen removal rate and activity of different microorganisms is dissolved oxygen (DO) concentration which has to be high enough to allow ammonia oxidizing bacteria (AOB) to produce a sufficient amount of NO
2-N for anammox reaction, but not too high to cause inhibition of anammox bacteria and increased activity of nitrite oxidizing bacteria (NOB).
The investigations were made with continuous and intermittent aeration with different DO concentrations and different ratios between non-aerated and aerated phase duration (R). The activity of different microorganisms in the biofilm was measured by specific anammox activity (SAA), oxygen uptake rate (OUR) and nitrate utilization rate (NUR) tests. On-line measurement of pH, conductivity and redox potential were useful to monitor the process.
The highest nitrogen removal rate and efficiency was obtained when DO was 3.5 mg/l and R equaled to 1/3. Activity tests showed that anammox bacteria and AOB played the dominating roles in the biofilm. The average and maximum values of SAA were 3.0 g N/m
2 × d and 4.3 g N/m2 × d, respectively. An average value of 4.0 g O2/m2 × d and the maximum value of 5.1 g O2/m2 × d was obtained in the OUR test for AOB activity.
Intermittent aeration reduced energy usage and improved process efficiency. Introduction of anaerobic phases and high nitrogen load enhanced the activity of anammox bacteria and NOB activity was limited. Nitrite production was the bottleneck of the one stage deammonification process.
Monitoring of N
2O emissions in the pilot plant showed that 0.4–2 % of nitrogen load was converted into N2O gas. N2O production was related to the nitrogen load, DO concentrations and ratio between non-aerated phase and aerated phase (R).
The results showed that redox potential (ORP) can be used as control parameter for operation of one stage deammonification process. At experiments using ORP for controlling air flow, the highest nitrogen removal rate and efficiency were obtained when the set point for ORP was 0 mV.