Comprehensive evaluation of sewage sludge anaerobic digestion process with different digestate treatments

Anaerobic digestion is one of the most promising methods for reducing sewage sludge and recovering energy. In the present study, a comparative life cycle assessment (LCA) of sewage sludge anaerobic digestion processes with different digestate treatments, including mesophilic anaerobic digestion with digestate landfilling (CAD-1) and digestate incineration (CAD-2), thermophilic anaerobic digestion combined with thermal hydrolysis pre-treatment with digestate land use (THPAD-1), and digestate incineration (THPAD-2), was performed to evaluate their environmental, resource, economic, and comprehensive performances using the SimaPro software. Environmental impact analysis revealed marine ecotoxicity, freshwater ecotoxicity, and human carcinogenic toxicity as the most obvious impacts, resulting in the most significant damage to human health. Resource analysis indicated that anaerobic digestion combined with cogeneration and digestate incineration is advantageous to high energy recovery, but digestate incineration is disadvantageous to economic performance because of increased investment costs. Comparison of the four processes revealed that THPAD-2 results in the largest environmental damage, whereas CAD-1 has the smallest load. Meanwhile, THPAD-2 and THPAD-1 exhibit the best resource performance and net economic benefit, respectively. The comprehensive evaluation indices revealed that THPAD-1 and CAD-2 show better comprehensive performance. And the deep drying incineration process exhibited better comprehensive performance than sewage sludge anaerobic digestion processes.

Automated summary

Anaerobic digestion is a promising method for reducing sewage sludge and recovering energy. A comparative life cycle assessment was conducted to evaluate different anaerobic digestion processes. The results show that anaerobic digestion combined with cogeneration and digestate incineration is advantageous to high energy recovery, but incineration is disadvantageous to economic performance. The study also revealed that the environmental damage is largest in the process of thermophilic anaerobic digestion with digestate incineration, while the smallest load is in mesophilic anaerobic digestion with digestate landfilling. Overall, thermophilic anaerobic digestion combined with thermal hydrolysis pre-treatment exhibits the best resource performance, and digestate incineration and CAD-2 show better comprehensive performance. The deep drying incineration process has better comprehensive performance than sewage sludge anaerobic digestion processes.