toxicity of free ammonia from the degradation of protein (Chen et al., 2008).
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