AbstractPassing whole cattle slurry (WCS) through a mechanical separator removed a large proportion of fibrous solids and produced separated cattle slurry (SCS). SCS gave a higher gas yield (per VS added) at 35oC and a 1O day, 25 day and infinite retention time (RT) than WCS, and three contributing factors were identified. Firstly, the higher solids concentration of WCS (39X greater) resulted in digester overloading at a 1O day RT but had little effect at longer retention times. Secondly, holocellulose represented a majority of destroyed solids in WCS, whereas in SCS over half the degraded solids were attributed to lipids, VFA and crude protein. These differences accounted for the higher gas yield (per VS destroyed) and richer quality of biogas from SCS, and the more rapid fall in the gas yield from WCS as the retention time was dropped. A third factor, particle size did not significantly influence the digestion process.
Shock treatments were performed by lowering the temperature of digesters operated at a 1O and 25 day RT on SCS from 35oC to 3-6oC over several days, and employing different recovery strategies. Measurement of gas composition, gas volume, pH, individual and total VFA concentrations revealed that steady-state conditions were re-established more rapidly when the temperature was restored to 35oC in a single day rather than over a longer period. The most sensitive Indicator of instability was a large but unequal rise in the VFA concentration, particularly of branched VFA and propionate.
A higher gas yield (per VS added) was produced at a 5 day RT (35oC) from SCS in an upflow anaerobic filter than a conventional digester. This was attributed to the retained biomass preventing overloading in the upflow anaerobic filter.
|Date of Award