Regular articleThe influence of sludge retention time on mixed culture microbial fuel cell start-ups

•SRT drastically affects the air-cathode MFC performance.•The higher the SRT, the higher is the electricity production during the start-up of MFCs.•Both long and short term responses in the production of electricity are affected by SRT.•In air-cathode MFC, the electricity produced by direct mechanism is the predominant.

In this work, the start-ups of air-cathode microbial fuel cells (MFCs) seeds with conventional activated sludge cultivated at different solid retention times (SRTs) are compared. A clear influence of the SRT of the inoculum was observed, corresponding to an SRT of 10 days to the higher current density exerted, about 0.2 A m−2. This observation points out that, in this type of electrochemical device, it is recommended to use high SRT seeds. The work also points out that in order to promote an efficient start-up, it is not only necessary to use high SRT seeds, but also to feed a high COD concentration. When feeding 10,000 ppm COD and keeping SRT of 10 d differences of current densities up to 0.1 A m−2 were observed within a cycle. Additionally it was observed that SRT influences direct and indirect electron transfer mechanisms, being the direct mechanisms the most relevant ones, accounting for more than 95% of the total electricity production.