Biological treatment . Step one ( Equalization and removal of COD and toxic matter) The wastewater is pumped into the first aeration tank, which has a volume of 2000m ³ . The tank serves as a buffer tank for equalization of flow and pollutants. At the same time the tank is used as an aeration tank where micro- organisms are grown to oxidize a great part of the organic matter and sulphide in the wastewater. From the aeration tank the wastewater flows to a deaeration tank where iron salts and polymer can be added to improve performance of the first settling tank. In the first settling tank most of the suspended solids in pre-treated wastewater are removed. Excess sludge is pumped to sludge dewatering in the existing sludge dewatering building. Sludge can also be pumped back into the aeration tank to improve removal of organic matter in the system. Pre- treated wastewater is collected at an intermediate pumping station. The wastewater is pumped into the second aeration tank at intervals depending on the operation mode in the aeration tank.
Biological treatment . Step two The final purification of the wastewater takes place in the second aeration tank. The tank is designed with a big volume ( 5100m ³ ) so biological nitrogen removal can take place. Nitrogen is mainly present in the wastewater as ammonia. The nitrogen removal is a biological process performed by two processes: nitrification and denitrification. In the first biological process the ammonia nitrogen is oxidized into nitrate. This process takes place under aerobic conditions, i. e. in the presence of oxygen. In the second process ( denitrification), the nitrate is reduced to gaseous nitrogen, which escapes into the surrounding atmosphere. The denitrification takes place under anoxic conditions, which means that oxygen is not present or in anoxic zones of the flocks. From the aeration tank, the wastewater flows to a deaeration tank where iron salts and polymer can be added to improve performance of the final settling tank. In the final settling tank, the suspended solids settles and the treated wastewater is transported to a disc filter, which has a maximum size of 10µm to ensure that levels of remaining suspended substances are low. A flow- controlled sampler is used to check the pollution levels of the treated water. After this process, the treated water is discharged. The major part of the sludge will be pumped back to the second aeration tank, while a minor part of the sludge is pumped to sludge dewatering in the existing sludge dewatering building. It is important for the stability of the nitrification/ denitrification that micro- organisms from the sludge are transferred back to the aeration tank. Environmental performance The wastewater treatment process has been monitored since the opening of the plant, and tests showed that the process was stable, even under different weather conditions. The project's results have been impressive: A reduction in nitrogen of 90% was achieved - higher than the expected 80%. The nitrogen load from the plant is nine tons a year ( 2008), a marked decrease on the previous Incoming ( mg/ l) Outgoing ( mg/ l) Reduction BOD448310,599,8 % COD978048895,0 % Nitrogen3043189,8 % Chromium5,50,1497,4 % The results of some key parameters from January - June 2008