The decreasing of pH revealed that the digester was in the acidogenic phase, whereas after the 9th
week, the digester started the methanogenesis phase. Yet, a stable and neutral pH was not easy to reached,
showing that the digester needed a longer time to be in the methanogenesis phase. In an anaerobic system,
the acetogenic bacteria convert organic matter to organic acids, possibly decreasing the pH, reducing the
methane production rate and the overall anaerobic digestion process unless the acids were quickly
consumed by the methanogens. Basically, pH in the range of 6.8 to 7.4 should be maintained in the
anaerobic digestion process, which is the optimum range for methanogens growth. If the pH could not be
controlled, the appearance of volatile organic acid and carbonic acid will increased the acidity of digester.
In this research, the decreasing of pH was significantly occurred and reached 5.3. Maintaining pH of the
reactor was found difficult in the initial period up to 9 weeks. Thus, in the research, an appropriate amount
of 5M NaHCO3 has added to maintain the pH.