Green sludge dewatering and recycling technology for generating renewable energy and liquid nutrients: Bench- and pilot-scale studies

Abstract

In this study, ultrasonication was performed under a low H2O2 dosage for dewatering and reducing activated sludge. Moreover, torrefaction was performed to convert the sludge remaining after ultrasonication into biochar, which is a renewable energy source. Factors including the H2O2 concentration, ultrasonication time and power, and torrefaction temperature and reaction time were systematically evaluated. The results of bench-scale tests indicated that high sludge volume and mass reductions (>50%) as well as high-carbohydrate and -protein content in the supernatant could be achieved through 20-s ultrasonication under a H2O2 dosage of 1.5% and ultrasonication power of 70-W. In these tests, the produced biochar exhibited a HHV of 12.8–14.4 MJ kg−1, a return of energy investment (ROEI) of 7.7–14.2, and 65.4% lower greenhouse gas (GHG) emissions upon combustion than did bituminous coal. In the pilot-scale tests, the produced biochar exhibited an ROEI of 10.4, and its GHG emissions on combustion were 74.8% lower than those of bituminous coal. The cost of the developed method is 84.6% lower than that of the conventional dewatering and disposal method. In summary, the proposed method is an energy-efficient, environment-friendly, and economically feasible method for producing biochar and recovering nutrients from supernatants without considerable GHG emissions.