Experimental Study On Enhancing Methane Production In Anaerobic Digestion By Exogenous CO₂stimulation

As’Carbon Neutral’received increased attention among countries, excess sludge treatment has become a global issue again. It can not only reduce the sludge volume, but more importantly also release the chemical energy contained within the sludge in the form of methane (CH4), resulting in a recovery of energy consumed in sewage treatment. Thus, less or even no external power is needed and a net zero carbon emission or carbon neutral can be achieved.The traditional way of anaerobic digestion has a low production efficiency and small yield of CH4, which prevent people from achieving carbon neutral. Apart from internal technical measures, external approach, like addition of extra CO2or hydrogen (H2) into the anaerobic digester, can also be an option. The idea of increasing CH4yield by addition of exogenous CO2, a green house gas, into the anaerobic digester is based on the fact that the latter one is a substrate in biological processes of homoacetogenesis and autotrophic methanogenesis. Upon that, carbon capture and energy transformation are connected, making the entire process sustainable and worthy of further investigation.With small-scale tests, by observing how exogenous CO2introduced into the anaerobic digester would stimulate CH4production and evaluating the impacts of various processing parameters, including CO2input, initial pH and sludge retention time (SRT), on CH4yield, we identified the optimal condition for CH4production, which can then be referred to the design and processing of treatment units in a pilot or mass production.The results indicated that the optimal yield of CH4, showing a42.9%increase compared with control group (without exogenous CO2input), could be achieved when we set CO2input as69.7ml CO2/g VSS, initial pH as6.9±0.1and SRT as24d.From the observation and analysis, we infer that the enhancement of CH4yield with an addition of exogenous CO2may involve the following three pathways:1) exogenous CO2promotes the hydrolysis of fermentative bacteria and accelerates the transformation of organic matter in sludge;2) CO2combines directly with H2produced in acetogenesis, and then with autotrophic methanogens, CH4is produced;3) CO2combines with H2to produce acetic acid (HAc) with homoacetogens, and then be converted to CH4in heterotrophic methanogenesis.