The figures 3 and 4 illustrate exemplarily the influence of the selected technology and the size of the plant on operation costs. In principle, low-tech systems as wetland systems or stabilization ponds with high land requirements have very low energy consumption and result in relatively low total operation costs. On the opposite, high-tech systems like activated sludge systems or chemical treatment can only be operated with high expenditures of energy and other operation costs. It can be noticed that the specific operation costs of small wastewater treatment plants are remarkably higher than of larger treatment plants. This is a result of economy of scale of all categories achieved in larger sizes of installations.

Operation Costs in Relation to total Costs of WWTP !

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 2 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.

Figure 2: Composition of total annual costs for wastewater systems in Germany [ATV, 2003]

Figure 3: Comparison of operation costs operation (O&M) for different types of secondary treatment options [Kampet, 2000]

Figure 4: Operation costs for wastewater treatment plants related to the size of the plant [Kampet, 2000]