Environmental and economic life cycle assessment of energy recovery from sewage sludge through different anaerobic digestion pathways

Anaerobic digestion is recognized as a good and promising method for energy recovery from sewage sludge, but it is difficult to select a suitable process from various conventional and emerging technical options. In this study, five processes including mesophilic and thermophilic anaerobic digestion (CAD and TAD), mesophilic and thermophilic high-solids anaerobic digestion (HSAD and THSAD) and anaerobic digestion with thermal hydrolysis pretreatment (THPAD) are compared using life cycle environmental and economic assessment. Particularly, the uncertainty derived from variable sludge organic content and biogas production is analyzed. The results showed that energy output should be the most sensitive factor determining the assessment results. For common high-organic-content sludge, thermophilic processes like THSAD and TAD lead to the least environmental impact while THSAD and THPAD exhibit the best economic performance. Compare with CAD, THSAD have 44% less environmental impact and 118% higher net present value (NPV) for a project with treatment capability of 100 t dry solids per day. However, for low-organic-content sludge, high-solids processes like THSAD and HSAD are much better than the others mainly owing to their less consumption of thermal energy. Using this kind of feed sludge, THSAD can bring 40% less environmental burden and 31% more NPV than CAD.