Using biochemical methane potential results for the economic optimization of continuous anaerobic digestion systems: the effect of substrates' synergy

During the processing of raw agricultural materials from the primary sector, large quantities of by-products with high chemical energy content are produced. Some of these are olive mill wastewater (OMW), cheese whey (CW), and liquid cow manure (LCM). Anaerobic digestion is a promising, environmentally friendly process for the energy valorization of agro-industrial and livestock by-products. The purpose of this study was the mathematical modeling of anaerobic digestion in batch systems, aiming to quantify possible synergies occurring between complementary substrates. The analysis was based on using biochemical methane potential data to predict the optimum hydraulic retention time (HRT) under steady-state conditions of continuous systems, for different substrate ratios. The objective function for HRT optimization was based on maximizing the process profit and included technoeconomic parameters, allowing the comparison of the different substrate ratios that can be used in the feedstock. The co-digestion with a mixture ratio of OMW:LCM equal to 70:30 proved very satisfactory for the sustainability of the process, exhibiting a maximum net profit of 16 euro/m3reactor/d for a plant capacity of 10 m3feed/d.