High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates

• Use of rice husk as co-substrate for food waste digestion.
• Continuous high solids mesophilic anaerobic co-digestion at different organic loading rates.
• VFA inhibition of co-digestion of food waste and rice husk due to overfeeding and overloading.

Low C/N ratio and high biodegradability are major limitations of anaerobic digestion of food waste which results in process inhibition due to rapid accumulation of volatile fatty acids. In the present study, C/N ratio of food waste was adjusted by mixing with rice husk which has low biodegradability. Co-digestion of the two wastes was performed in continuous pilot scale anaerobic reactor operated at organic loading rates (OLR) of 5, 6 and 9 kg VS/m3/d and mesophilic (37 °C) temperature under plug flow mixing mode. At organic loading rate of 5 and 6 kg VS/m3/d, the volatile fatty acids/alkalinity ratio ranged from 0.15 to 0.24 which indicated higher buffering capacity of digester. While at OLR of 9 kg VS/m3/d, volatile fatty acids/alkalinity ratio of 0.94 was recorded. Daily biogas production and gas production rate were 196 L/d and 2.36 L/L/d respectively at OLR of 6 kg VS/m3/d. However at loading rate of 9 kg VS/m3/d, daily biogas production drastically decreased from 196 L/d to 136 L/d. Highest volatile solids removal of 82% was achieved at 5 kg VS/m3/d. Biogas production, reactor stability and volatile solids removal efficiency decreased with increase in organic loading rate and decrease in hydraulic retention time.