CONVERSION OF NITRATE TO NITRITE AS A RATE LIMITING STEP FOR ANAEROBIC AMMONIA OXIDATION IN SOIL SYSTEMS

Soil Aquifer Treatment has been used extensively for the recharge of groundwater using reclaimed water in arid areas. During Soil Aquifer Treatment (SAT), reclaimed water is percolated through the vadose zone and biological removal mechanisms remove contaminants of concern including organic carbon and nitrogen. This study evaluated anaerobic ammonia oxidation as a sustainable nitrogen removal mechanism in SAT systems. During anaerobic ammonia oxidation, ammonia serves as the electron donor to reduce oxidized nitrogen to nitrogen gas. Four soil columns were operated for a period of two years to evaluate nitrogen removal. In addition, a column with a non-woven media and a membrane bioreactor were also operated. The columns were fed an inorganic matrix consisting of inorganic carbon and different nitrogen species including ammonia, nitrate or nitrite. Nitrogen removal was sustained in columns fed a mixture of nitrate and ammonia throughout the experiments, however, the nitrogen removal rates decreased with time. Nitrogen removal was also sustained in a soil column fed nitrate as ammonia adsorbed on the columns was available to sustain nitrogen removal. When the non-woven media and membrane bioreactor were fed nitrate and ammonia, the nitrogen removal in these systems was negligible although they were seeded with effluent from the soil columns for several months. After over 500 days of operation, the columns were fed a mixture of nitrite and ammonia and the nitrogen removal rates increased by over an order of magnitude. The increase in removal rate was accompanied by an increase in the growth of microbial community. After feeding with a mixture of nitrite and ammonia, one column was switched back to a feed mixture of ammonia and nitrate. Subsequently, the nitrogen removal rate returned to the previous lower rates although a significant increase in the microbial community had occurred. The accepted stoichiometry for anaerobic ammonia oxidation is with nitrite as the electron acceptor. During this study, negligible removal was observed in non-soil systems when nitrate and ammonia were used as feedstock. A component of the soil appears to facilitate the conversion of nitrate to nitrite allowing for anaerobic ammonia oxidation to proceed at a limited rate. Attempts to evaluate the exact nature of this soil component were not successful, however, several common soil components were identified as potential candidates.