Enhancing biogas production using ultrasound-assisted thermal pretreatment technology for anaerobic co-digestion of sewage sludge and microalgae substrates

The study aimed to assess the effects of ultrasound-assisted thermal pretreatment on biogas production by co-digesting stress-adapted microalgae (Synechocystis sp.) with a combination of mixed municipal and industrial sewage sludge under anaerobic conditions. The pretreatment process involved subjecting the microalgae biomass to thermal pretreatment at temperatures of 70, 90, and 110°C while utilizing ultrasound pretreatment for the sludge (with an average solids content of 16.7 g/L) at a frequency of 25 kHz and power output of 400 W for durations of 3, 9, and 15 minutes before the main digestion process. The experiment was designed using response surface methodology (RSM) with a central composite design. The results revealed that the model exhibited statistical significance, with a probability value (Prob F) of less than 0.05. Pre-treated samples demonstrated a substantial increase in biogas production compared to untreated samples, showing an average 1.4 to 5.6-fold enhancements. Optimization analysis indicated that the highest cumulative biogas production, amounting to 706 NmL, could be achieved after two weeks by pre-heating the microalgae cells at 110°C for one hour and subjecting the sewage sludge to ultrasound pretreatment for three minutes. These findings highlight the potential of ultrasound-assisted thermal pretreatment technology as a strategy to enhance biogas production through the co-digestion of stress-resilient microalgae and sewage sludge.