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

Abstract

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 110oC 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 110oC 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.

Keywords

• biogas
• anaerobic co-digestion
• pre-treatment
• ultrasound
• microalgae