Anaerobic co-digestion of animal manures and lignocellulosic residues as a potent approach for sustainable biogas production

Anaerobic digestion is a versatile biotechnology for conversion of organic wastes to valuable biogas. Anaerobic digestion of manure is making the most of it, as the process allows simultaneous bio-energy generation, production of nutrient rich soil amendment, reduced emission of greenhouse gases and odor control; it is thus in line with climate friendly farming practices. Despite the enumerated advantages, the potential of manure for biogas production is not fully utilized due to the low and imbalance carbon to nitrogen (C/N) ratio in animal manures. To satisfy the anaerobic digestion requirements and to compensate the carbon deficiency of manure, there should be another carbon-rich substrate to be co-digested with manure to improve its characteristics for anaerobic digestion; lignocellulosic biomass residues seem promising for this purpose. This work presents a review on anaerobic co-digestion of animal manure and lignocellulosic feedstock for biogas production. Several research studies conducted on co-digestion of these organic wastes are described and reviewed. The impact of numerous parameters including temperature, pH, organic loading rate (OLR), hydraulic retention time (HRT), C/N ratio, alkalinity and concentration of volatile fatty acids (VFAs) on the performance and stability of the co-digestion process is extensively discussed. The influence of various pretreatment methods including physical, chemical and biological pretreatments on providing well-prepared substrate for anaerobic co-digestion and thus enhancement of biogas production is discussed. An overview of the most significant factors and intermediates which can inhibit or even cease the process is also presented.