The cumulative CH4 production in four digesters, as illustrated
in Fig. 1C, exhibited significantly different results. The highest
cumulative CH4 yield was obtained in digester PPW-FR-1 (273 L/
kg VSfed), which was 14% higher than that using PPW as feedstock
in digester PPW-1. A similar study found that a higher CH4 potential
was obtained from the lactic acid fermentation residue of municipal organic solid (Dreschke et al., 2015). Coats et al.
(2012) studied the biogas production from dairy manure with a
two-stage AD process, and found that the additional prefermentation
in the first stage with short solid retention time of
4 days increased CH4 content and enhanced the biogas production
as compared to single stage AD process. It is postulated that the
pre-fermentation process can loosen (or disrupt) the recalcitrant
components in the biomass, which make them susceptible to
microbial attack and assimilation by AD. Insufficient digestion
effects were obtained in digester PPW-FR-2 and PPW-FR-3, which
had cumulative CH4 yields of 150 and 118 L/kg VSfed, respectively.
It should be noted that the CH4 contents in these two digesters
reached a similar level with the other two digesters (PPW-1 and
PPW-FR-1).