Effect of feeding frequency on the anaerobic digestion of berry fruit waste.

On-site anaerobic digesters for small agricultural farms typically have feeding schedules that fluctuate according to farm operations. Shocks in feeding, particularly for putrescible waste can disrupt the stable operation of a digester. The effect of intermittent feeding on the anaerobic digestion of rejected raspberries was investigated in four 3L reactors operated in semicontinuous mode for 350 days at 38 °C with a hydraulic retention time of 25 days and an organic loading rate (OLR) of 1gVS/L/d. During the acclimatisation period (147 days) the organic loading was 5 feeds per week. The feeding regime of two reactors was then changed while maintaining the same OLR and HRT to one weekly feed event in one reactor and 3 equal feeds per week in another. The feeding regime did not significantly affect specific methane yield (369 ± 47 L/kgVS on average) despite very different weekly patterns in methane production. Volatile fatty acids (VFA) comprised >83 % of the organics in the effluent, while the rest included non-inhibitory concentrations of phenolic compounds (515-556 mg gallic acid/L). The microbial composition and relative abundance of predominant groups in all reactors were the archaeal genera Methanobacterium and Methanolinea and the bacterial phyla Bacteridota and Firmicutes. Increasing the OLR to 2gVS/L/d on day 238 resulted in failure of all reactors, attributed to the insufficient alkalinity to counterbalance the VFA produced, and the pH decrease below 6. Overall results suggests that optimal digestion of raspberry waste is maintained despite variations in feeding frequency, but acidification can occur with OLR changes.

Bottom ash from smouldered digestate and coconut coir as an alkalinity supplement for the anaerobic digestion of fruit waste.

Anaerobic digestion (AD) of readily hydrolysed substrates such as fruit waste requires the addition of a pH buffering agent. This study evaluated the use of bottom ash from the combustion of spent coconut coir and the digestate produced from rejected berry fruit and plant waste as a buffering agent. The performance of the ash was compared with using an equivalent amount of NaHCO3 as a buffering agent. Digestions of berry fruit waste were performed in 160 mL serum vials using anaerobic wastewater sludge as an inoculum. The methane yield at the optimum levels of buffering with NaHCO3 as the buffering agent was 233 ± 12 NmL CH4/g VS. The methane yield at an equivalent addition of alkalinity as ash was significantly less, (124 ± 3 NmL CH4/g VS), but still beneficial compared to the methane yield obtained from experiments with no added alkalinity (40 ± 1 NmL CH4/g VS). Further dosages of ash in systems containing optimal level of NaHCO3 buffer also revealed a decrease in the methane yield proportionally to the added ash concentration, which is suggested to be caused by soluble concentrations of Fe in the ash supplemented systems that are higher than reported inhibitory levels.