Regarding the total costs of a wastewater system operation costs play an important role. Operation costs are the expenses related to the operation, maintenance and monitoring of the plant, once it was constructed. These costs will be incurred in a regular basis along the service life. Exemplarily, Figure 1 shows the cost composition of wastewater systems in Germany divided up in capital and operation costs [ATV, 2003]. Easily it can be seen, that operation costs can amount up to 50 % of the total annual costs. Therefore during the evaluation of process alternatives the consideration of operation costs is of crucial importance. Disregarding this conclusion will result in costs, which are neither adequate nor affordable for the customers. For example the treatment process may be interrupted or totally turned off due to e.g. high energy costs, which can not be paid to the local power authority.