Anaerobic co-digestion of food waste and cardboard in different mixing ratios: Impact of ultrasound pre-treatment on soluble organic matter and biogas generation potential at varying food to inoculum ratios

The aim of the present work is to investigate the impact of acetic acid assisted ultrasound pre-treatment of food waste (FW) and cardboard (CB) to overcome the barriers of substrate characteristics. Anaerobic co-digestion (ACoD) experiments were conducted with FW and CB in the mixing ratios of 100:0, 80:20, 60:40 and 50:50 at a food to inoculum (F/I) ratio of 1:1 and 1:2 to investigate its impact on biogas yield, volatile solids (VS) removal, solubilization of organic matter and the specific energy (SE) required. Results revealed that the biogas yield obtained from ultrasound pre-treated samples regardless of mixing ratio was higher at a F/I ratio of 1:2 compared to 1:1. The formation of bubbles, cavities and floc matrix significantly increased and the particle size decreased from 100 μm to 10 μm for FW:CB of 60:40 and 50:50 after pre-treatment. Ultrasound pre-treatment for 45 min was adequate for FW:CB of 100:0 and 80:20 while it was 60 min for 60:40 and 50:50 for increased sCOD between 37 % - 53 %. The VS removal efficiencies increased between 3–15 % from pre-treated samples compared to untreated. The CB mixing ratio is inversely related to SE while directly related to sCOD. Overall, during ACoD of FW and CB, the FW:CB of 80:20 gave a higher biogas yield of 471 mL/g VSadded followed by 60:40 and 50:50.