Bioenergy recovery from waste engine oil through anaerobic co-digestion with sewage sludge: Effects of total solids and inoculum to substrate ratio

Considering their various benefits, such as pollution control, waste management, and eco-friendly energy production, much more attention is currently devoted to utilization of some urban, industrial, and agricultural wastes in the course of anaerobic digestion (AD) across the world. From this perspective, the present study aimed to evaluate prospects of biogas production from waste engine oil (WEO) through its co-digestion (CoD) with activated municipal sewage sludge (SS). In order to do this, AD was carried out in batch-mode 1-L reactors at a mesophilic temperature of 35 degrees C, with consideration for total solids (TS) levels of 3% and 6% and inoculum-to-substrate ratios (ISRs = 0.5, 1, and 2), which were based on volatile solids (VS), over a period of 85 days. The treatment with TS = 6% and ISR = 2, or 6874 mL, showed the maximum biogas generation rate, according to the trials, with TS and VS removal values of 23% and 28%, respectively. As well, reactor monitoring during AD revealed that the parameters of potential of hydrogen (pH) and alkalinity recovered following acidification after 7 days, and the relatively low efficiency of the given process (0.233 m3/kg-VSadded) was attributed to the highly complicated and difficult-to-degrade structure of WEO. The kinetics of anaerobic CoD consistently established that the maximum cumulative biogas production rate (Rm = 5.61 m3/kg-VSadded.d) and the shortest biogas production lag time (lambda = 0) occurred when there were TS = 3% and 6%, in that order. Moreover, kinetic models, i.e. the Modified Gompertz and Logistic Function ones, were found to be in very good agreement with experimental data, whereas the first-order model failed to offer ideal predictions of biogas production trend in most cases.