In Norway’s largest sewage plant, Veas in Oslo, Filtralite* has proven extremely durable as both bio substrate and sludge filtering medium. After 25 years of operation with daily back flushing there is no need for replenishing.
A river of sewage of maximum 11 m3 second runs in a 42 kilometre underground tunnel, collecting sewage from Oslo centre to Slemmestad in Asker, where the plant’s large treatment basins hide inside the mountain.
Annual volume of sewage is 100 million m3 emoval of nitrogen is a major objective before leaving the effluent to the popularOslo fjord, and Filtralite plays a leading role in the nitrification and de-nitrification. These two steps of bio filters work downstream of chemical flocculation with sedimentation.
The unique biofiltering process consist of eight «nitrification» bio-filters, each followed by «de-nitrification» bio filters. The nitrification step is an aerobic bio filter which consume dissolved organics, converting to sludge, carbon-dioxide and nitrate. The substrate is Filtralite HC 2,5-5 (crushed fragment). In the subsequent aerobic bio filter the nitrates are converted to nitrogen gas. In this process Filtralite HR 3-6 (whole extruded spheres) acts both as substrate and filtering medium.
These filters are a result of great innovation in the past, and we are proud that this design has proven very successful for us. Filtralite was designed especially for this task by Norwegian researchers, trying to imitate and multiply natural processes. The special porosity of expanded clay enables the bio filters both to host bacteria and act as filter at the same time, explains senior advisor Øystein Moursund of Veas. All filters are back flushed
daily and the sludge goes to processing and agricultural uses. Veas has decided to engage more filters of the same kind to keep up with an increasing load.
It is the porous nature of Filtralite thatenables the dual function, working both as aparticle trap and housing bacteria inside the pores maintaining a robust bio culture.