Current approaches and future potential for pretreatment of agricultural residues and industrial effluents to boost biomethanation

The burning of organic agricultural substrates such as crop residues, paddy straw, maize stalk, sugarcane straw and corn stover leads to severe air pollution. These wastes are essential feedstocks in the production of biogas. The above materials can be anaerobically digested to produce biogas, which in turn can be used to generate fuel, cooking gas, soil-conditioner and electricity as a sustainable alternative use for these residues. However, the presence of lignin and cellulose contents in these wastes, at concentrations ranging from 6-26 % and 5-50 %, reduce the effectiveness of biomethanation process. For optimum anaerobic digestion, appropriate pretreatment methods, such as physical (milling, grinding, ultrasonic), chemical (alkali, thermo-chemical pretreatment) or biological (enzymes, microorganisms) techniques, can be used to lower the lignin content. Untreated effluents containing organic matter, fertilizers, heavy metals and other contaminants are released into water bodies by agro-industries, resulting in the degradation of land and water ecosystems. Like agricultural substrates, agro-industrial effluents can be efficiently used to produce bioethanol and biogas, following the removal of these inhibitors by using effective pretreatment methods. When utilized as a feedstock, pre-treated wastewater can yield up to 2.8 % more biogas and 64 % more methane than untreated wastewater. The biodigested slurry improves soil health and enhances crop yield. This article describes several pretreatment techniques to improve biogas production from industrial effluents and agricultural wastes entrusting the soil health, sustainability and crop yield, consistent with the circular economy concept.