A critical review on the effects of pneumatic mixing in anaerobic digestion process

Abstract

With focus today shifting towards green-energy and green-fuel, anaerobic digestion is one of the valiant technologies, which is sought after. Many methods are employed to better the process of anaerobic digestion and one such strategy is pneumatic mixing. Pneumatic mixing is the process of introducing pressurized gas (raw biogas, air/oxygen, hydrogen) into the bioreactor used mostly to improve biogas yield and purity. Biogas mixing has been employed for its major physicochemical contribution in improving methane yield (up to 90%), bettering reactor stability, maintaining pH, and also in the removal of hydrogen sulphide (up to 77%) from biogas. Limited-aeration on the other hand is a biochemical process where the addition of air/oxygen improves the biodegradability of organic compounds. It is also used for the removal of hydrogen sulphide from biogas (up to 99%) thereby improving its purity. Stoichiometric hydrogen addition enhances methane conversion, as hydrogenotrophic methanogens convert hydrogen and carbon dioxide to methane. Numerous research works are scattered across the domain of biogas mixing, limited-aeration, and hydrogen addition, but there is a shortage of review articles on this topic. The effect of gas mixing on effluent and sludge characters are also scarcely represented in the research fraternity. Thus, this review aims to critically analyse the effects of pneumatic (biogas, hydrogen, and air/oxygen) mixing in the anaerobic digestion process parameters. This review article also sheds light on future research (using mathematical simulations), for studying the combined effects of these gases on anaerobic digestion process.