Biogas from anaerobic co-digestion of food waste and primary sludge for cogeneration of power and heat

The anaerobic digestion (AD) process is being increasingly recognised as a technology for clean energy generation. However, in Ecuador, there is a little application of this technology due to lack of adequate research, economic incentives and the current relatively low price of electricity. This study examined the feasibility of biogas production using anaerobic co-digestion of food waste (FW) and primary sludge (PS) under thermophilic (55 °C) and mesophilic (35 °C) conditions. The biogas is then used for power and heat generation. Using case study approach, data were collected from Riobamba vegetable market and Penipe waste treatment plant in Ecuador. Three different mixing ratios of FW: PS were used (1:2, 1:1 and 2:1) with volatile solids (VS) content of 84.53%, 86.99%, and 89.6% respectively. Furthermore, the organic loading rates (OLR) used were 2.08, 2.49 and 3.34 gvs 1^-1 day^-1 for the above mixing ratios with a hydraulic retention time (HRT) of 21 days. Computational models of biogas production and a combined heat and power (CHP) system were developed using Aspen Plus software. Results indicated that a mixing ratio of 1:2 and mass flow of 132.42 tonnes/day, the maximum specific methane production obtained was 270 and 205 ml CH₄/ g_VS at thermophilic and mesophilic conditions respectively. The power production with the aforementioned values were 188.42 and 137.79 kW for both thermal conditions. Finally, an economic analysis for both scenarios was carried out using Ecuadorian renewable energy tariffs. A positive NPV values of £147,580 and £186,307 with a discounted payback period of 20.97 and 17.33 years were obtained for both scenarios respectively, assuming that the interest rate was 4.89% and a lifetime of 25 years.