The role of sludge in renewable energy production and the final disposal of sludge are topical issues. The cost of sludge treatment account for a significant share of the total running costs of a WWTP. By increasing the efficiency of sludge treatment, significant savings can be achieved. Savings can be gained e.g. by more efficient energy utilisation of sludge, combined with optimised wastewater treatment processes. In addition to cost efficiency, environmental sustainability of the applied solutions is crucial. The energy balance and green house gas emissions can be used as tools to evaluate the sustainability of sludge treatment options.

This paper presents a case-based comparison of energy balances, green house gas emissions and costs of municipal wastewater sludge process chains based on anaerobic digestion or incineration. Information from existing plants was used for conducting the study. Several utilisation options were covered for biogas energy as well for energy from sludge incineration.

Based on the results, sludge incineration was the most expensive option of the studied scenarios, but justified if the heat generated can be fully utilised e.g. in district heating. Based on costs and energy balances as well as green house gas emissions it is most preferable in anaerobic digestion chains to feed the generated biogas to a gas engine to produce electricity and heat. Moreover, thermophilic anaerobic digestion can improve the overall economy of a WWTP provided that the biogas yield is sufficiently higher than in a mesophilic process. However increased odour is a risk in thermophilic digestion.

The study showed that the most energy efficient process modifications are always very case bound. However the tools developed in this study are generally applicable to waste water treatment plants for similar analyses.

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