The results of the first four years' operation of a downflow submerged packed bed reactor treating distillery waste with a COD of 30 g/l (95 % in soluble form) are presented. The digester contains 4300 m3 of packing, and is operational six months a year. It is one of Europe's largest units using this technique.

After a few problems with fixing the biomass on start-up, organic loading has been regularly increased, reaching an average of 6 kg COD/m3.d during the 2nd year and 10 to 12 kg COD/m3.d during the 4th year, with soluble COD reductions of 90 to 95 %. The biomass released into the treated effluent was in the region of 0.15 kg SS/kg COD eliminated in the latter campaign. Overall performance in the 2nd, 3rd and 4th seasons are presented and discussed. The stability of digestion was studied over the whole of the 2nd and 4th seasons using measures of VFA at various levels within the reactor. Tracer study was conducted at the end of 2nd year to characterize the hydraulic behavior of the reactor. Results indicated a flow pattern close to a complete mixing and dead space was in the region of 20 % of liquid volume.

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