Tubular Digester Research Articles

Operation of a laboratory-scale tubular digester on piggery waste

Three inclined tubular digester of 13- to 15-litre volume were operated at temperatures between 20 and 30°C with hydraulic retention times of 10–50 days on pig slurry of 10%, 5% and 2·5% Total Solids content. Tube inclination of 16–20° gave no scumming problems. Solids were retained, giving improved gas yields when compared with literature values for conventional stirred digesters. Solids retention is affected by the size of the digester exit and appears to be greater with slurry of low solids content. There is a link between the movement of solids and gas along, and out of, the digester which can lead to unstable oscillating gas production. Tracer studies showed soluble components of the slurry mixed throughout the whole volume of digester in 25% of the hydraulic retention time or less. Solids moved in well dispersed plug flow. Gas production (both intrinsic and after the addition of acetate) was most rapid in the central section of the digester. Extra-cellular carboxy-methyl cellulose activity was not uniform throughout the digester, suggesting the activity was particle associated.