According to the polluter pays principle, wastewater dischargers should be charged for the environmental and social costs that result from disposing wastewater - such as downstream impacts of sewage discharges. Therefore, pollution charges are often levied by local or national governments on the discharge of water into the Environment, i.e. mostly into surface waters. They are usually imposed on operators of treatment plants and industrial dischargers. The charges are generally calculated based on actual quantities and/or pollution loads of the effluent. There is a variety of charging systems into place to determine the pollution charges on wastewater discharge. For treatment plants the pollution charge is often calculated based on the number of inhabitants served by the plant. Further on, charges are calculated based on specific chemical, biological and biochemical parameters determining the pollution load, such as content of phosphorus, nitrogen, biological oxygen demand, heavy metals, etc. pollution charges are therefore of special interests for industries who discharge wastewater of high pollution loads into sewer systems or directly into nature. High pollution charges will encourage reduction in effluents produced or in-house treatment by industry.