A Critical Review on Sludge Anaerobic Digestion Progress from 2012 to 2015

Large amount of sludge has been a serious problem due to its great environmental risk. Anaerobic digestion (AD) is a commonly used method to treat sludge. Timely review of research developments of sludge AD is needed to gain insight for its future research and industrial applications. This paper summarized the main research developments of sludge AD from 2012 to 2015. The research hotspots of sludge AD from 2012 to 2015 were divided into three parts: sludge pretreatments, sludge co-digestion with other substrates, and the fate of emerging contaminants during sludge AD. Publications on pretreatments have shown an increasing trend from 2012 to 2015. Physical, chemical, biological, and combined pretreatments could all improve sludge AD efficiency, and physical pretreatment was the most reported group, accounting for 45.9%. Removing extracellular polymer substances has been a new pretreatment method in the last four years. As for sludge co-digestion with other substrates, publications have increased by 112% from 2012 to 2015. Other organic waste was the most widely reported co-substrate, followed by biowaste, fat, oil and grease, and combined waste. All of these co-substrates could improve biogas (methane) production with suitable mixed ratio and organic load rate. As for the fate of emerging contaminants during sludge AD, pharmaceuticals have been the most researched group, which accounted for 52.7% of total publications. The removal efficiencies of various emerging contaminants had a great difference during sludge AD. Future perspectives of above three research hotspots were also discussed in details.