Wanying Ren

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1. Introduction Over the last few decades, the global production of waste-activated sludge (WAS) has been continuously increasing, which poses a potential threat to the environment []. According to the statistics, China produces 60 million tons of sewage sludge (80% water content) with an annually increasing rate of 10%, while the European Union produces approximately 50 million tons of sewage sludge per year and the United States produces 40 million tons [].