Critical evaluation of different mass transfer equations to model NO emissions from water resource recovery facilities with diffuse aeration.

NO measurements by liquid sensors in aerated tanks are an input to gas-liquid mass-transfer models for the prediction of NO off-gas emissions. The prediction of NO emissions from Water Resource Recovery Facilities (WRRFs) was evaluated by three different mass-transfer models using Benchmark Simulation Model 1 (BSM1) as a reference model. Inappropriate selection of mass-transfer model may result in miscalculation of carbon footprints based on soluble NO online measurements. The film theory considers a constant mass-transfer expression, while more complex models suggest that emissions are affected by the aeration type, efficiency, and tank design characteristics. The differences among model predictions were 10-16% at dissolved oxygen (DO) concentration of 0.6 g/m, when biological NO production was the highest, while the flux of NO was 20.0-24 kg NO-N/d. At lower DO, the nitrification rate was low, while at DO higher than 2 g/m, the NO production was reduced leading to higher rates of complete nitrification and a flux of 5 kg NO-N/d. The differences increased to 14-26% in deeper tanks, due to the pressure assumed in the tanks. The predicted emissions are also affected by the aeration efficiency when depends on the airflow instead of the . Increasing the nitrogen loading rate under DO concentration of 0.50-0.65 g/m increased the differences in predictions by 10-20% in both alpha 0.6 and 1.2. A sensitivity analysis indicated that the selection of different mass-transfer models did not affect the selection of biochemical parameters for NO model calibration.