Operating strategies for acid phase digestion: an industrial case study

Acid phase digestion was investigated for enhanced operation in an industrial wastewater treatment plant. In particular, sludge retention time (SRT), temperature and pH were assessed for determining optimal conditions under operating constraints. Volatile fatty acid (VFA) production and soluble chemical oxygen demand (SCOD) were the key process parameters used to assess system performance. Increase in SRT from 0.8 to 1.4 days (at 22 degrees C) had moderate effect on VFA production (approximately 15% increase), achieving a maximum VFA production of 3600 mg/L. High VFA production rate (0.101 +/- 0.035 mg/mgVS.d) was obtained at the highest operating retention time during summer (27 degrees C). The degree of solubilisation of particulate organic matter increased with temperature and retention time. Despite an increase in SCOD (6472 +/- 873 mg/L max.), a corresponding increase in VFA was not observed. SCOD showed a linear correlation with decrease in pH, while acidogenesis (SCOD conversion to VFA) was found to be favourable at a pH of 4.5.