Reduced Hydraulic Detention Time for Complete Nutrient Removal with Innovative Biological Reactors

To limit pollution discharges through wastewater into surface waters, increasingly stringent effluent standards particularly for nutrient removal require extensive upgrading of conventional wastewater treatment plants. Large sewage treatment plants in Western Europe are often located in densely urbanised areas where land is unavailable. To avoid additional unit processes for nitrogen and phosphorus removal, innovative solutions are demonstrated. A compact reactor is presented that achieves low pollutant residuals (carbonaceous matter, suspended solids, nitrogen and phosphorus) in reduced space and a few examples of full scale applications are given. Through extensive pilots tests, the limiting parameters for nutrient removal are established on sewage after pretreatment and lamella settling. The biological treatment by two granular filters in series combines biodegradation and suspended solids retention without further particle separation. The anoxic reactor removal rates varied between 0.3 and 1.5 kg N-NO3/m3 d, depending on temperature and available carbon. Nitrification on the aerobic filter increases from 0.3 kg to 1 kg N-NH4/m3 d with temperature in a range between 5 to 20 °C. Total nitrogen residuals below 10 mg/l and effluent phosphorus below 1 mg/l can be achieved with hydraulic retention times of around 3 hours.