Reaction mechanisms in solid-state anaerobic digestion - I. The reaction front hypothesis

The proposed mechanism applies to the solid-state anaerobic digestion of a bed of organic wastes or other substrates. It is initiated by a seed body above a minimum viable size, around which a multi-zoned, mobile, reaction front develops. Acetate is formed in the leading zone, then diffuses back to a separate methanogenic zone, which is protected from acid inhibition by an intervening, passive buffer zone. This front gradually advances until the whole waste mass has been stabilized. The front cannot cross voids, so good inter-particle contact accelerates the process. The number of seed bodies determines the number of fronts and thus the rate of stabilization. Moreover, seeding patterns largely determine the mean distance the fronts must cover in order to complete the stabilization process, so they have a major effect on process kinetics. In packed-bed digesters densely seeded with previously digested waste, viable seed bodies are plentiful and closely spaced. Consequently, numerous fronts are formed and the mean distance they must cover is short, so the process is quick. However, in a typical landfill, inadequate seeding greatly extends the distances the few fronts must travel and thus greatly prolongs the stabilization process. This solid-phase mechanism would be dominant in well-seeded, rich, organic wastes. However, in lean wastes or in the absence of viable seed bodies, conventional, liquid-phase mechanisms would be dominant instead. In some circumstances, the two mechanisms might also co-exist. In others, neither might be effective, so the raw waste could be preserved indefinitely. This hypothesis is proposed for evaluation and testing. No such mechanism seems to have been considered in process modelling, in the design of experimental studies or in the operation of packed-bed waste digesters and bioreactor landfills. However, it could have a major impact in all these applications.