SOLID WASTE TREATMENT FOR SALTWATER RAS: ANAEROBIC DIGESTION AND BIOMETHANE PRODUCTION

The recirculating aquaculture system (RAS) treats and eliminates fish farming wastes such as ammonia and carbon dioxide for reuse of water for fish culture. Solid wastes are also removed from system water but usually for the discharge without the treatment. Collected solid wastes should be treated to reduce organic load in RAS effluent to increase RAS sustainability and promote RAS aquaculture. Options that have been applied in some facilities are, for example, to generate fertilizer by composting the wastes and to use for landfill; however, it won't be the case for salt-containing sludge of marine RAS. Another alternative is on-site conversion of the solid waste to biomethane, called anaerobic digestion. This approach reduces sludge organic volume to fuel grade biogas that can be used as a heating source at the facility offsetting the operational costs. One of the problems for this approach with fish sludge from aquaculture systems is the low C:N ratio resulted from the high protein content of both the uneaten feeds and the fish excreta reducing the efficiency of bioconversion.

Our group at IMET has successfully developed consortium of anaerobic microorganisms that are capable to degrade low C:N fish wastes and effectively convert to biomethane using selective enrichment approach. Solid waste treatment and biomethane production using an integrated up-flow anaerobic sludge blanket (UASB) reactor in marine RAS have been demonstrated using enriched consortium. In this report, we describe the development of methanogenic consortium that efficiently converts biomass to methane by sequential transfers of bacteria with the target substrate under the defined anaerobic conditions. Enriched halotolerant microbial consortium is optimal for low COD:N ratios typical of RAS solid wastes and does not require supplemental materials such as organic carbon or nutrients for aquaculture solids bioconversion. We characterized five predominant phylotypes identified in the microbial consortium and isolated them in pure culture. Of which, two isolates are anaerobic fermentative bacteria Dethiosulfovibrio and Fusobacterium spp. Both species hydrolyze and ferment proteins, peptides and amino acids. The other three isolates are an acetate-utilizing methanogenic archaeron, Methanosarcina sp., and two hydrogen-utilizing methanogenic archaea, Methanogenium and Methanoplanus spp. We evaluated bioconversion rates of re-constituted microbial consortium with five isolates that resulted in equivalent to the original enriched consortium, suggesting those isolates can be used as seeding inoculum for marine RAS solid waste digester. In addition, evaluation of inoculum in the integrated anaerobic digester in marine RAS to accelerate start-up phase and maintain highly active and stable microbial community in UASB during intensive European sea bass production will be discussed.