Characterisation and anaerobic batch degradation of materials accumulating in anaerobic digesters treating poultry slaughterhouse waste.
We characterised materials accumulating in two failed mesophilic semi-continuous anaerobic digesters treating poultry slaughterhouse waste and, for reference, materials in the two well-performing digesters, to find the anaerobic degradability of these materials and the factors affecting their degradation. We also studied materials accumulating and stratifying in various layers in one of the two well-performing digesters. The material from the most severely failed digesters produced methane sluggishly and did not improve appreciably even with 33 percent dilution suggesting that the recovery of failed process is slow. The methane production was apparently affected by the accumulated long-chain fatty acids, totalling 8.1 g l-1, which degraded slowly. However, the material produced methane in the end, which shows that the failure was reversible. In the well-performing digester, considerable amounts of long-chain fatty acids already floated on top of the digester after 20 hours without mixing, a phenomenon which may have affected their bioavailability and toxicity. However, materials from the top, middle, and bottom layers of the digester were readily and largely methanised by the microbial populations present in them and additional inocula did not markedly enhance the methanation. The results indicate that long-chain fatty acids are apparently the main factor affecting both the failure and recovery of a poultry slaughterhouse waste digester. Thus excessive feeding of lipids into the digester should be avoided.