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