A decrease in the process pH was observed in the first few days of the digestion and this is due to high volatile fatty acid (VFA) formation [8]. In consequence, a large fluctuation was observed in ammonia nitrogen concentrations, achieving a maximum value of 1.66 g/l, 1.77g/l and 1.76 g/l at mixing ratios 0.5:5, 1:5 and 1.5:5, respectively during the batch operation. However, the pH