A Pilot-Scale Comparison of IFAS and MBBR to Achieve Very Low Total Nitrogen Concentrations

Integrated Fixed-film Activated Sludge (IFAS) is an option that is being considered for advanced nutrient control in many facilities in North America. In Northern Europe, the Moving Bed Biofilm Reactor (MBBR) process, which uses almost identical equipment to IFAS but has only an attached growth biomass, has proven to be a good nutrient removal choice for facilities with weak waste strength and low operating temperatures. Pilot testing of both processes was carried out at the Mamaroneck wastewater treatment plant in Westchester County, New York, where conditions similar to those encountered in northern Europe, weak wastewater strength and cold operating temperatures, are encountered. The program was intended to test the ability of both MBBR and IFAS for meeting future low effluent total nitrogen limits of approximately 4 mg/L. The MBBR option would require a tertiary denitrification stage, whereas the IFAS option would be required to achieve complete nitrification and denitrification in the existing tankage with a total hydraulic retention time of only three hours. In order to provide adequate denitrification in such a limited volume, IFAS media was added to the post-denitrification stage with the thought that it would facilitate the growth of slower growing methanolotrophic (methanol-loving) bacteria on the media, enhancing denitrification and preventing their washout. This approach proved successful, effectively doubling the denitrification rate from 14 to 28 mg/L/hr. Nitrification rates and biomass growth on the media were measured through the test period.The loading to the pilot was pushed to its maximum during the coldest winter temperature enabling the two processes to be assessed under the most stressed conditions. Results from the pilot testing confirmed that either IFAS or the MBBR would be capable of achieving the target total nitrogen concentration.