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

Abstract

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.