To find out, which measures will be most favorable for the reduction of energy costs, an energy analysis can be carried out [NRW, 1999]. In Germany and in Swiss such analysis are successfully applied since many years on more than 100 wastewater treatment plants.

The energy analysis is carried out as a double-level analysis. In a first step, overall data of the plant as well as relevant energy consumption data of plant units like aeration step, sludge treatment etc. are recorded, evaluated and compared with best practice data from similar treatment plants. The aim of this first step is to find out in an “low-budget� way, whether there is some potential for energy saving existing. If so, the second step comprises the detailed analysis of each relevant electrical consumer of the plant and the sludge digestion, the gas utilization process (if existing) and the electrical power tariff system. The analysis will result in detailed recommendations for single optimization measures. The recommendations are divided up into short-termed measures, medium-termed measures and depending measures (measures which are connected e.g. to overall changes in process technology or renovation strategies).

Figure 9: Energy savings related to proposed measures in 47 wastewater treatment plants in Germany [Kaste, 2003]

Detailed analysis of 47 wastewater treatment plants in Germany of 2,500 – 1,000,000 p.e. identified an energy cost saving potential between 5 and 100 % (average 34 %). Providing the realization of all proposed measures, about 2.3 m € could be saved per year [Kaste, 2003]. As expected, the main reduction in energy costs can be achieved by optimizing the aeration step (Figure 9) . Energy being produced of a renewable source or biological material like Sewage sludge and being sold to the power utility is financially promoted by the state of Germany. Thus, in Germany all measures in this area will extremely contribute to the energy savings as to be seen in the figure. But also optimizing the operation of the electrical consumers under consideration of the tariff system and contract negotiations with the power utility can be very profitable.