Anaerobic fixed-bed reactors with small buoyant supports: influence of media design on biomass retention, fluidization and reactor performance
Résumé
Anaerobic fixed bed reactors with random supports are bound to have clogging as the major issue during long-term operation. So, apart from having good biomass retention capability, the support media used should be easily fluidized for occasional unclogging and removal of excess biomass. The effect of media design on biomass retention, fluidization and process performance of anaerobic fixed bed reactors with small buoying supports was investigated in this study. Five supports of the same size (3 cm) but with varying shape and geometry were used in reactors fed with distillery vinasse. The reactors were operated with a classical start-up strategy starting at a low OLR of 0.25 g.L-1.d-1 followed by gradual increase to a maximum of 22 g.L-1.d-1, with 80% COD removal as the threshold limit. Quite high quantities of solids could be entrapped/attached on the supports (1.5-2 g/support) indicating that these supports are good means to increase the quantity of biomass inside the reactors. Biomass in the supports was present as entrapped solids in the void space (46-77% of total solids) and the rest as biofilm. Biofilm represented an average attached TS per m2 support area in the range 18-52 g TS.m-2. As the reactors with the different supports behaved quite closely, the main criteria to take into account for the selection of the best support is the ability for fluidization. Important differences were found for the fluidization of the five supports. Indeed, supports with a plain outer design with minimum ridges were found to be easily fluidized thus favouring easy unclogging conditions while supports with outer ridges formed a cramped structure hindering fluidization.