THE IMPORTANCE OF pH CONTROL DURING ANAEROBIC DIGESTION OF MUNICIPAL TREATMENT SLUDGES

In this study, the role of the pH on the anaerobic digestion of the primary sludge, taken from the primary sedimentation tank of a municipal wastewater treatment plant, at mesophilic temperature (35 degrees C) was evaluated. In the scope of this study, performances of the continuously stirred anaerobic reactors, operated as batch systems, were investigated with the changes in the production of soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFAs) as well as in the removal of volatile suspended solid (VSS). In the first anaerobic reactor, the pH was not controlled (R1) whereas the pH was kept above 6.5 by NaOH addition in the second reactor (R2). Results indicated that, VSS was removed with a corresponding production of VFAs and SCOD in both reactors. However, productions of VFAs and SCOD stopped earlier in the pH-controlled reactor. Thus, methanogenic phase started to operate at earlier times in the pH-controlled reactor whereas hydrolysis and fermentation were still operating in the pH-uncontrolled reactors. The complete consumption of VFAs was observed in both reactors. However, the complete reduction in VFAs in the pH-controlled reactor was observed almost one week earlier than it was in the pH-uncontrolled reactor The pH values changed between 4,7-6,7 in the pH-uncontrolled reactor in which VFA's production did not yield a sudden pH drop. This might be due to the buffering of ammonia released from amino acid fermentation due to the proteins present in the primary sludge. Reduced solid mass is one of the most important objectives of anaerobic sludge application. This is very important for the treatment sludges when they are disposed of in landfills especially in the countries like Turkey. By the decrease in the sludge solid amounts, the sludge volumes that should be transported to landfills will also decrease. This will also help to reduce the area requirement observed in most landfills recently.