Revealing hydrodynamics and energy efficiency of mixing for high-solid anaerobic digestion of waste activated sludge

Anaerobic digestion is a feasible and promising technique to deal with emerging waste activated sludge issues. In this work, the hydrodynamics and digestion performance of horizontal anaerobic systems equipped with double-bladed impeller and ribbon impeller were investigated. Simulation using computational fluid dynamics technique visually showcased the favorable mixing status implementing ribbon impeller. The mixing modes were considered as the major motivation for the difference of mixing efficiencies. Tracing experiment indicated that the minimum thorough mixing time with ribbon impeller was 20 min at a rotation speed of 50 rpm, whereas it was 360 min for the double-bladed impeller under similar conditions. The superior mixing performance of ribbon impeller resulted in better anaerobic digestion and energy efficiency outputs. The digester employing ribbon impeller obtained an ultimate biogas yield of 340.38 ± 15.91 mL/g VS (corresponding methane yield of 210.34 ± 7.55 mL/g VS) and produced a surplus energy of 16.23 ± 0.76 MJ/(m³·d). This study thus ascertained that ribbon impeller was proficient for high-solid anaerobic digestion and it will prominently benefit future system designs.