Sludge dewatering through H2O2 lysis and ultrasonication and recycle for energy by torrefaction to achieve zero waste: An environmental and economical friendly technology.

This study investigated activated sludge (AS) dewatering and reuse for solid biofuel by novelty combining and a low H 2 O 2 dose and pulse ultrasonication followed by torrefaction to achieve zero-waste with the supernatant also recycled for nutrient supply to the AS unit. Mass and volume reduction rates are employed to evaluate the AS dewatering efficiency, and the biochar characteristics were systematically evaluated using higher heating value (HHV), mass and energy yield, proximate and ultimate analyses, and combustibility. The energy return on investment (EROI), greenhouse gas (GHG) emissions, and cost-effectiveness of the proposed technology were also systematically evaluated. Results showed that the mass and volume reduction rates of AS using an H 2 O 2 concentration of 0.8 % with ultrasonication at 70 W for 20 s was higher than 50 % and promoted the release of protein and carbohydrate content in the supernatant. Torrefaction of residual AS produced biochar with HHV of 17.8–18.4 MJ/kg. The biochar with 57.4 % less GHG emissions than burning bituminous coal, high EROI (4.7–7.3), but a low fuel ratio. Thus, we propose co-firing 10–20 % biochar with coal to enhance the FRs to that of bituminous coal while simultaneously mitigating GHG emissions. The production cost of biochar was 56.3 % lower than the traditional AS disposal cost. Therefore, the proposed technology is environmentally and economically friendly and can solve sludge disposal problems, produce biochar with reasonable cost for renewable energy, mitigate global warming, and recover supernatant as a nutrient feedstock. It is worth developing practical applications in this field. • The integrated technology involving low H 2 O 2 dose, pulse ultrasonication, and torrefaction achieved biofuel production and zero-waste. • Rich protein and hydrocarbon can be recovered as a nutrient source for the activated sludge unit. • Substituting the produced biochar for bituminous coal reduced 57.4 % CO 2 emission. • The EROIs of the biochar were high (4.7–7.3) and suitable for sustainable reuse. • An economic perspective with 56.3% less cost than the traditional sludge disposal. [ABSTRACT FROM AUTHOR]