Comparative evaluation on municipal sewage sludge utilization processes for sustainable management in Tibet

In recent years, a sharp increase in the amount of municipal sewage sludge (MSS) in Tibet has posed serious threats to the fragile ecological environment. Tibetan sludge, with a high content of volatile and low heavy metals, has re-utilization advantages, and thus, the selection of appropriate utilization processes for Tibetan MSS is of great importance. In this study, not only the processes themselves, but also other factors including legislations and environmental pollution were investigated. This study introduced the current waste management legislation situation (especially for MSS) in China and Tibet, China. Moreover, a series of SWOT (strength & weakness and opportunity & threats) analyses were conducted to compare anaerobic digestion (AD), incineration, pyrolysis (PY), gasification, and anaerobic digestion coupled with pyrolysis. The results showed that anaerobic digestion coupled with pyrolysis was the optimal treatment option, because anaerobic digestion was suitable for the low oxygen content in Tibet. Although only 50-60% of the organic matter in MSS could be degraded by anaerobic digestion, the residual organic matter (energy) could be further decomposed by pyrolysis, converting it into pyrolytic gas, bio-oil, and biochar, as valuable products. Sludge digestate pyrolysis could reduce environmental risks, save energy, recover materials, and produce high value-added materials. Moreover, it provides a "zero waste" solution for sludge disposal and promotes a "Circular Economy." The challenges and obstacles of MSS anaerobic digestion coupled with pyrolysis in Tibet were also investigated. This study provides an important technical reference for the comprehensive utilization of Tibetan MSS.