The effects of waste-activated sludge pretreatment using hydrodynamic cavitation for methane production

• Hydrodynamic cavitation (HC) was designed and proposed to disintegrate the sludge.
• Energy consumption of HC was compared with the ultrasonic-based method and even thermal pretreatment.
• Cumulative yield and percentage of CH4 indeed depended on the degree of sludge disintegration.

Disintegration of waste-activated sludge (WAS) is regarded as a prerequisite of the anaerobic digestion (AD) process to reduce sludge volume and increase methane yield. Hydrodynamic cavitation (HC), which shares a similar underlying principle with ultrasonication but is energy-efficient, was employed as a physical means to break up WAS. Compared with ultrasonic (180–3600 kJ/kg TS) and thermal methods (72,000 kJ/kg TS), HC (60–1200 kJ/kg TS) found to consume significantly low power. A synergetic effect was observed when HC was combined with alkaline treatment in which NaOH, KOH, and Ca(OH)2 were used as alkaline catalysts at pH ranging from 8 to 13. As expected, the production yield of CH4 gas increased proportionally as WAS disintegration proceeded. HC, when combined with alkaline pretreatment, was found to be a cost-effective substitute to conventional methods for WAS pretreatment.