Thickened wastewater from a fish hatchery recirculating water system (RAS) was successfully treated by anaerobic digestion, showing a high methane potential in brackish and marine conditions (Chiumenti et al., 2021). To optimize the process and establish operational criteria for a full-scale transfer, pilot-scale tests were carried out in a BioReactor Simulator System (BRS, Bioprocesscontrol, Sweden). The BRS consists of 6 reactors with a volume of 1.8 L each: 3 reactors operated in conventional configuration (CSTR, continuous stirred tank reactor), while the other 3 were modified with an up-flow fixed bed layout, with floating plastic filling elements (UAFF, upflow anaerobic floating filter). The present paper describes the initial colonization batch process, performed in 72 days by treating aquaculture sludge inoculated with brackish digestate, for the CSTRs, and with the biofilm of the plastic media for the UAFFs. The batch tests were followed by a continuous loading phase, characterized by start-up and steady phases. The tests were performed in mesophilic conditions. The start-up in the UAFF reactors resulted significantly faster and more productive than in conventional reactors, with specific yields reaching 188.0 NmL CH4/g VS compared to 100.0 NmL CH4/g VS. During the following steady phase, with the retention time of 20 days, the specific yields increased in all digesters but the differences between the 2 types of reactors were not significant, even if favorable for the UAFF (190.5 and 180.9 NmL CH4/g VS, respectively for UAFF and CSTR). Further experimental tests are ongoing with shorter retention times (12 and 8 days), to evaluate the best compromise between methane yields and reduction of digestion volumes, in the optic of a full-scale transfer of this process.
