AbstractThree inclined tubular digesters of 13-15.3L volume were operated successfully at 20-35 C with hydraulic retention times (HRT) of 10-50 days on pig slurry of 10,,5,.2.5 and 1% Total Solids content. Tube inclination of 16-20 overcame the scumming problems encountered with horizontal tubular digesters. Performance compared favourably with completely mixed digesters, gas yields showing similar variations with operational parameters and reaching 0.46L gas gVS added ~ .
Tracer studies show that soluble components of slurry mix throughout the digester (which contains minimal dead space) in 25% of the HRT or less. Solids move in well dispersed plug flow and are retained longer than in completely mixed digesters with a consequent improvement in gas yields. Solids retention is affected by the size of the digester exit and is greater with slurry of low solids content. There appears to be a link between the movement of solids and gas along and out of the digesters which can lead to unstable 'oscillating' gas production.
Gas production is most rapid in the central section of the digester tubes and relatively slow at the lower (influent) end where sediment accumulates. Acetoclastic methanogens are not closely associated with slurry solids. Gas production from acetate is restricted both by the population of acetoclastic methanogens and by the acetate concentration, to differing 'extents in different parts of the digesters.
Extractable exocellular endoglucanase activity against carboxymethyl cellulose decreases and cell associated activity increases as slurry enters and passes along the digesters. Detectable activity is associated with particulate solids and does not disperse widely. Electron microscopy showed that digester bacteria degrading cellophane are predominantly rods with activity localised to within 0.5 ym of the cells.
|Date of Award||Aug 1984|