Improvement of Solid-State Anaerobic Digestion of Yard Waste by Co-digestion and pH Adjustment

To make clear the feasibility of controlled anaerobic digestion (AD) of organic fraction of municipal solid waste being implemented at transfer stations prior to their final disposal, the performance of solid-state AD (SSAD) of yard waste (YW) as well as the improvement by its co-digestion with food waste (FW) in combination with the aid of pH adjustment were investigated by conducting batch SSAD experiments. The results showed that AD of YW with high solid contents easily caused difficulties in the initiation stage due to accumulation of volatile fatty acids. Even with the aid of alkali to successfully initiate the initiation stage, the AD of YW with the high solid content of 29% generated such a low biogas yield as 207 mL/g VS. By contrast, co-digestion of YW and FW with pH adjustment gave the highest biogas and methane yields of 306 and 180 mL/g VS, 47.8 and 40.3% higher than those for AD of YW alone, respectively. A sufficiently long acidogenesis duration was helpful for biogas production, while the inhibitions from excessively high ammonia nitrogen concentrations and organic loading rates were apt to happen after extremely long acidogenesis durations. For co-digestion of YW and FW, adding proper amounts of alkali (CaO) on the 3rd days resluted in the best performance.