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).