varied between 0.13 and 0.16 g/L, which was probably due to the low P content (0.4%) in PPW (Mahmood et al., 1998). Previous studies reported that PPW contained 17–25% protein (Liang and McDonald, 2014; Liang et al., 2014). Protein degradation in the AD process releases NH4-N, and in this study, the NH4-N concentration increased from 0.56 to 1.48 g/L in digester PPW-1 and PPW-FR-1. It is known that the free ammonia (NH3-N)/NH4-N is one of the main factor causing reactor failure, whereas the threshold level of methanogenesis inhibition/toxicity varies with different substrates and conditions (Yenigun and Demirel, 2013). It is highly possible that at least the NH3-N/NH4-N concentration