Biofilms as an aid to nitrogen removal in sewage treatment lagoons

A pilot study of sewage treatment lagoons near Melbourne indicated that the best way to enhance nitrogen removal in such lagoons was to promote nitrification. Combined algal/bacterial biofilms grown on surfaces immersed in the lagoons showed potential for greatly increasing the extent of nitrification. A laboratory study was undertaken to determine the effect of pH, dissolved oxygen concentration, temperature and other factors on the nitrifying capabilities of such biofilms; effective nitrification was observed at temperatures down to 10°C, at pH's down to 5.5 and at dissolved oxygen levels down to 2.5 mg/L. Tanks containing biofilm-coated plates nitrified much more effectively than tanks containing only suspended microbial growths. At dissolved oxygen levels of 2-3 mg/L, up to 30% of the ammonia-nitrogen fed to tanks containing biofilm-coated plates was removed by denitrification. Photosynthetic activity in the biofilm greatly enhanced nitrification efficiencies at low dissolved oxygen levels.