Pretreatment of robusta coffee hulls and co-digestion with cow-dung for enhanced anaerobic digestion

The effect of alkali pre-treatment on anaerobic digestion of Robusta coffee hulls with a lignin content of 29% was studied in batch culture at 55˚C. In order to investigate the effect of cow dung in minimising the effect of coffee-based inhibitory compounds, digestion experiments were performed with blends of pre-treated and non-treated coffee hulls and cow dung with 0,5,10,20,30,50,75 and 100% coffee hulls at a total volatile solids concentration of 1.8g/l fermentation volume. Methane production increased with increase in coffee hulls concentration up to 30% for both substrates. The lowest production was observed at 100% non-treated coffee hulls. Methane production was significantly higher (13-164%) for the pre-treated substrates compared to the non treated one. The highest increase was obtained at 100% coffee hulls. Degradation of volatile solids decreased with increase in coffee hulls concentration for both substrates. The extent of degradation of the pre-treated substrate (59-94%) was significantly higher than that obtained for the non-treated substrate (30-91%). The lowest extent of degradation was measured for the non-treated hulls at 100%. It is possible that there was an inhibition of methanogenesis by coffee based compounds with increase in the coffee hulls concentration in the digestion mixture.