Anaerobic digestion for the improvement of the energetic sustainability of dairy farms: lab-scale assessment of the BMP of dairy manure and whey, and on-site pilot scale stress test

Abstract. The present study aimed to improve the energetic sustainability of a small dairy farm located in Northern Italy. The research was conducted at laboratory and pilot scales. The lab-scale test, performed employing the BRS System (Bioprocesscontrol, Sweden) in batch for 40 days, at 38°C, aimed to determine the optimal biogas and biomethane potential of manure and whey. Different inoculum/whey ratios were tested to verify possible acidification. At pilot scale, a 24 m3 bag-type reactor with hydraulic mixing (AD Bag®, Demetra Group, Italy) was employed to evaluate the potential in non-optimal conditions (stress test). No inoculum was used to start-up the digester, which was filled only with manure. At continuous operation, input manure was collected in a non-mixed tank reflecting real operative conditions. Different hydraulic retention times (HRT), and organic loading rates (OLR) were tested, with temperatures between 23.8 and 25.4°C. The lab test showed biogas yields of 0.50 NL/g VS for manure, 0.52 NL/g VS for a manure and whey mixture, and 0.89 – 0.95 NL/g VS for whey; methane yields of 0.26 NL/g VS for manure, 0.25-0.26 NL/g VS for manure whey mixture, and 0.40 – 0.42 NL/g VS for whey were recorded. The pilot scale test showed biogas yields between 0.174 and 0.217 NL/g VS, with the highest achieved with the highest OLR: these results could be of interest considering the non-optimal conditions of the stress test. With 8 m3/day of manure the farm could power a 16.2 kWe CHP biogas unit, covering the entire energy demand.