Biological removal of nitrogen to improve the quality of reclaimed wastewater for groundwater recharge

Nitrogen removal was studied in a pilot scale continuous flow system consisting of two reactors in series for nitrification and denitrification. The system was used to eliminate ammonia, nitrate and nitrite from a secondary wastewater effluent. The first stage of the process consisted of an aerobic upflow bioreactor with an autotrophic nitrifying population for ammonia oxidation, which was grown on silica sand granules and produced during the growth process nitrite and nitrate. During the second stage, these oxidised forms of nitrogen were successfully reduced to nitrogen gas in an anoxic denitrification fluidised bed reactor. At this stage, methanol was added as an external carbon source for the heterotrophic organisms growing on a silica sand support. The optimum nitrification efficiency was 91% for the highest ammonia concentration at the influent (51 mg NH 3 -N/1) and a retention time of 3.7 hours. With a lower ammonia concentration (23 mg NH 3 -N/1), the highest nitrification efficiency was 95% corresponding to 1.1 mg NH 3 -N/1 in the nitrified effluent. In the denitrification process, a 95% removal efficiency of nitrite and nitrate for 55 mg NO x -N/1 at 2.3 hours and a concentration of microorganisms in the reactor of approx. 6,000 mg VSS/1 was obtained. As a complementary stage of the nitrogen removal process, a silica sand and powder-activated carbon filter was installed in order to improve the quality of the final effluent in terms of other properties like turbidity, colour and the content of organic matter.