Biogas production from co-digestion of Phyllanthus emblica residues and food waste

Phyllanthus emblica residues (PER) are unsuitable as a single substrate for biogas production because they usually contain residual tannins which are methanogenic inhibitors. In addition, the ratio of total chemical oxygen demand (tCOD) to nitrogen of 14:1 w/w in the PER is much lower than the value that is usually recommended. It is shown that a mixed substrate of PER and food waste has a tCOD to N ratio that is closer to recommended values and that co-digestion with this mixed substrate improves the yield of biogas. In this study, three lab-scale bioreactors were used to investigate (1) the single digestion of food waste, (2) the single digestion of the PER and (3) the co-digestion of food waste and PER at a mixing ratio of 11:1 g tCOD food waste/g tCOD PER. The organic loading rates (OLRs) of the co-digestion were 2.13, 2.83 and 4.26 g tCOD/L·day at hydraulic retention times (HRTs) of 40, 30 and 20 days, respectively. The OLR of the co-digestion was chosen to be equal to the sum of the OLRs of the single digestions. The methane production of the co-digestion was found to be greater than the sum of the methane production of the two single digestions by 12% at HRTs of 40 days and by 22% at HRT of 30 days. In addition, the methane yield of the co-digestion was higher than that of each of the single digestions. The co-digestion strategy was also applied to a pilot-scale 2000 liter-bioreactor that was usually used for treating food waste. It was found that the methane production was increased by 34% when the substrate was switched from food waste to the mixed food waste and PER co-substrate, and no signs of inhibition were observed.