Wastewater treatment plants rank amongst the largest municipal power consumers, yet produce energy-rich residues, which represent an enormous energy potential. During the purification process a big amount of sewage sludge accrues, which can be exploited for thermal utilization, after it has undergone suitable treatment. Hence it is possible to regain electricity and heat to increase the level of self-sufficiency of the purification plant. By creating a calculation model, it is possible to compare different variations of treatment plants, establish energy balances and draw conclusions on the best combination of sewage works and sludge utilization. Based on the usage of digester gas, which occurs in anaerobic stabilizing waste water treatment plants, a high production of electricity can be achieved in comparison to other procedures of wastewater treatment. By means of additional incineration or gasification of digested sludge, a large share of the heat and electricity demand of the sewage plant can be covered.