Ultrasound and microwave pretreatments promote methane production potential and energy conversion during anaerobic digestion of lipid and food wastes

Highlights: • Ultrasonic/microwave strongly degraded lipids (>36%) prior to anaerobic digestion. • Increasing pretreatment energy input led to increased methane yield from lipids. • Coating of methanogens by lipids was substantially relieved after pretreatment. • ECE of waste lipids of anaerobic digestion was increased from 48.78% to 69.89%. • 1000 kJ/kg-TVS of ultrasonic/microwave pretreatment showed the best energy conversion. To alleviate difficult degradation of lipids and its inhibition on anaerobic digestion of food waste, ultrasound and microwave heating were used to pretreat lipid and food wastes for promoting methane production. The soluble COD (10,130 mg/L) of lipid waste pretreated with ultrasound of 50,000 kJ/kg-total volatile solids (TVS) was higher than that (1910 mg/L) with microwave heating of the same energy input. This was because the thermal and mechanical effects of the ultrasound pretreatment were stronger than the thermal and solid dissolution effects of the microwave heating pretreatment. The methane yield of lipid waste pretreated with ultrasound during anaerobic digestion increased by 43.3%–927.97 mL/g-TVS, higher than that of 738.63 mL/g-TVS with microwave heating. Fourier transform infrared spectrometer (FTIR) and Scanning electronic microscopy (SEM) analysis demonstrated that the residual lipids were reduced and microorganism coating was relieved after anaerobic digestion of lipid waste pretreated with ultrasound. The energy conversion efficiency of lipid waste pretreated with ultrasound to methane product was 69.89%, which was higher than that of 58.98% with microwave heating. Ultrasound efficiently degraded lipid waste and released more energy to be converted into methane.