High loading anaerobic co-digestion of food waste and grease trap waste: Determination of the limit and lipid/long chain fatty acid conversion

In order to boost the economics of biogas utilization in those small facilities for food waste (FW) digestion, proximate grease trap waste (GTW) was employed to co-digest with FW. A bench-scale continuous stirred tank reactor was set up and operated at mesophilic temperature to investigate the co-digestion limit and lipid/long chain fatty acid (LCFA) conversion by increasing lipid loading stepwise. Mixing FW and GTW at lipid/total solids (TS) 55%, where the lipid loading was 1.61 g/(L·d), was proved to be appropriate, and the maximum methane yield reached 68% higher than the mono-digestion of FW, with a similar lipid reduction at approximately 90%. In addition, since acetic acid accumulation above 100 mg HAc/L had been observed under the optimal loading, the threshold value for anaerobic co-digestion of FW and GTW was confined to the lipid loading around 1.61 g/(L·d). The rapid recovery strategy of mixing the inhibited sludge with fresh inoculum at 1:4 (V/V) took effect after process failure resulted from further increasing lipid/TS to 70%. Lipid/LCFA concentrations in the effluent rose with the increased lipid loading, while palmitic acid accounted for the most percentage among the residual LCFAs, as high as 73.7% at lipid/TS 55%. Acetoclastic methanogens played an important role in accelerating process conversion, due to the fact that raising GTW addition led to the increased methanogenic activity, with a value of 26.9 mL-CH₄/(g-VS·d) at the optimal loading.