technology on tannery wastewater with Elmo Leather. These trials were very successful and the results from the trials were used for upscaling to demonstrate that the technology can be used on wastewater from tanneries. Examples of important factors needed to obtain a stable cleaning process are: the plant has a strict process control ( pH and balance of nutrients is especially important); the plant is a two- step plant ( toxic compounds degraded in the first step) and the plant has the possibility of hydraulic equalization ( in order to avoid fluctuations in, for example, chloride concentration). Technical description An overview of the plant is given below and followed by a description of the plant. The principle of the WWTP ( wastewater treatment plant) is as follows: The wastewater flows from the tannery to the new inlet pumping station through a 2mm screen. The wastewater is then pumped to biological treatment.
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.