The Hybrid Membrane Biofilm Process (HMBP): A Novel Approach to Achieving Biological Nitrogen Removal in Activated Sludge

The hybrid membrane biofilm process (HMBP) is a novel technology for removing total nitrogen (TN) from municipal wastewater. Air-filled, hollow-fiber membranes are placed in an activated sludge basin, and the bulk liquid is left anoxic. The membranes support an aerobic, nitrifying biofilm, while produced nitrite and/or nitrate is exported to the bulk liquid and reduced with BOD as the electron donor. A bench-scale HMBP was tested for 100 days with synthetic wastewater and a bulk-liquid SRT of less than 5 days. The nitrification rate was 0.85 kgN-1000m−-2-day−-1, and over 80% TN removal was achieved. Microbial community investigations confirmed that the biofilm was dominated by nitrifying bacteria. Modeling suggested that adequate detachment is key to maintaining a thin, highly active nitrifying biofilm, and that increasing the membrane surface area would result in over 90% TN removal. A second bench-scale HMBP investigated the impact of C:N loadings and bulk liquid BOD concentrations on nitrification in the HMBP. Bulk liquid BOD concentrations had a large impact on the nitrification rates; however the C:N loading had little impact. The HMBP also accumulated more nitrite than nitrate, suggesting “short-cut” denitrification was occurring. These results suggest the HMBP is a promising technology for TN removal, especially for existing plants designed with a short SRT.