Operation of a tubular anaerobic digester on piggery waste
Three_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 c6ntains 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 differingextents 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 Vm of the cells.