*7.2. Inoculation*

Inoculating microorganisms into a MEC in which they would survive and grow, producing a microbial biofilm on the electrodes, is known as system inoculation. Because microorganisms are inoculated into the system, inoculum selection is essential for organic matter degradation [108]. Because the response of an MEC system is governed by the microorganisms involved, inoculation directly controls the start-up time of the system. The start-up period refers to the time it takes for a system to begin producing H2 or biogas. Long start-up durations diminish system proficiency because it takes longer for the system to begin generating energy; consequently, choosing the correct inoculum source is critical to maximizing system efficiency. Methods for reducing the time it takes for microbial electrochemical systems (MESs) to start up have been investigated. Selection of inoculum could be helpful to reduce start-up time while simultaneously improving efficiency. Biogas production was significantly increased by 18.5% when a mixture of inoculum sources containing a 1:4 ratio of activated sludge and municipal sewage was used as compared to a mono-inoculated treatment, implying that diversification is required for nutrient enrichment, which is essential for digestion [108]. In other words, the development of the methanogenic consortium in the system was also more rapid, which indicates that this mixed bacterial consortium helps enhance the hydrogenotrophic methanogen population. The amount of inoculum at the start of the experiment is equally critical. In the study conducted by Escapa et al., the authors discovered that the reactor failed to start when residential wastewater of low strength (230 mg COD·L−1) was used as the inoculum [51]. It is said that, with the addition of gas production, the start-up time for a large pilot system might be anywhere between 50 and 90 days. A simple technique to shorten the time it takes to set up a large-scale system is through "pre-acclimatization" of electrodes by applying a beneficial microbe to the electrodes before putting them into the system. Carbon electrodes that were inoculated with hydrogenotrophic bacteria extracted from natural bog sediment and pre-charged with hydrogen-rich water started up faster and produced more methane than those that were not [109].

A significant start-up time can also provide problems if repaymen<sup>t</sup> is not completed within a certain time frame. Pre-inoculation of electrodes can give a low-cost option for lowering start-up time, but this method comes with the added benefit of avoiding expensive modifications. Additional research into the best beginning circumstances will increase the technology's appeal for industrial use, providing the basis for MECs to be commercialized for worldwide wastewater treatment.
