In practice, it is not always operable to add carbonous organic matters into the carbon-defect digestion solutions to adjust its C/N ratio. Instead, the accumulated ammonia during digestion can be adsorbed using porous materials such as powdered phosphorite ore (Krylova et al., 1997), glauconite and activated carbon (Hansen et al., 1999), as well as zeolite in other literature (Ho and Ho, 2012) and this study. This, however, usually increases the treatment cost and brings about difficulties in engineering operation and utilization of digestion wastes. Thus, the removal of free ammonia by inert gas purging should be more practical in terms of technical maturity and cost.