Sludge digestion instead of aerobic-stabilisation – a cost-benefit analysis

The consequences of a worldwide increase of energy costs and an efficient use of the renewable energy resource sewage sludge have to be more and more considered also for smaller WWTP within design capacities between 10 000 and 50 000 PE. To find out the lower limit for an economical conversion into a digester plant, cost functions for specific capital costs as well as operating cost savings were derived. By means of those tools it is possible for the operator of a plant to evaluate if further examinations concerning a refitting of the existing plant to digestion are promising. By comparing capital costs with operation cost savings a break-even point for process conversion could be determined. The break-even point varies depending on project specific constraints as well as assumptions related to future energy and operation costs and variable interest rates. A 3 % increase of energy and operation costs leads to a cost efficient conversion for plant larger than 7 500 PE. Such a conversion of WWTP results in different positive effects on energy turn-around and the operation process of the plant: Increased efficiency by process optimization and energy savings as well as on-site renewable power generation by digester gas which can be used on the plant. Besides, the optimisation of energy efficiency results in an important reduction of primary energy consumption.