Filtralite is used to manage strict discharge limits at Bekkelaget WWTP
The Bekkelaget wastewater treatment plant in Oslo is designed to treat wastewater from the eastern parts of the city, with a total load of 280.000 pe and a maximum hydraulic load of 260.000 m3/d.
Due to local regulations and environment protection standards, the plant was built to manage restrict discharge limits on phosphorous and nitrogen, including an average discharge limit for phosphorous of 0.2 mg P/L.
The treatment line is based on an activated sludge process with simultaneous precipitation and pre-denitrification. To these steps is added a tertiary filtration with a dual media made of sand and Filtralite MC 2,5-4. The reduction of phosphorous and suspended solids over the filters is about 80% with a limited need for coagulants.
The dual media filters are designed with 1.8 metres of filter material in each filter: 1.2 metres of Filtralite® MC 2.5-4 mm.(top-layer), and 0.6 metre of fine-grained sand 1.2 – 1.8 mm (bottom-layer). Each of the 16 filters has a surface area of 50 m2, total area is therefore 800 m2. At the bottom of the filters, there are 55 nozzles per square metre with 0.7 mm slits.
Those filters treat the total incoming flow, both biologically treated and direct-precipitated water. Normal hydraulic load is 5.7 m/h, and max load is 14.4 m/h with 15 filters in operation (one filter is always in backwash mode during max load periods).
With those dual media filters as a final polishing step, the levels of both phosphorus and suspended solids are reduced by approximate 80% under normal conditions and about 60% during high flow situations. The need for coagulants is limited, only a small dose, 0.5-1.0 g Fe/m3 is needed during rainy weather and increased flows.