Comprehensive analysis of the factors for propionic acid accumulation in acidogenic phase of anaerobic process

The accumulation of propionic acid in the anaerobic process often leads to a low efficiency of the methanogenic phase due to the low acetogenic rate of propionic acid, and hence the low wastewater treatment efficiency. Three acidogenic reactors were used to study the reasons for propionic acid accumulation in the acidogenic phase and its control strategy. The experimental results indicated that in the micro-aerobic circumstance (ORP>-100 mV) the accumulation of propionic acid in the acidogenic reactor always occurred at any pH value. Under strict anaerobic circumstances (ORP < -150 mV), typical propionic acid-type fermentation commonly occurred at pH about 5.5 regardless of ORP value. However, at pH about 5.0, either propionic acid-type or butyric acid-type fermentation might occur depending on whether ORP was high or low. Higher hydrogen production rate was found to have no direct relationship to the accumulation of propionic acid in the acidogenic reactors. In fact, great yield of NADH maybe the major reason for simultaneous accumulation of propionic acid and higher biohydrogen generation rate in the traditional anaerobic process. The highest biohydrogen generation rate occurred during the ethanol-type fermentation without the accumulation of propionic acid. Ethanol-type fermentation is a better choice when using an anaerobic acidogenic reactor of a two-phase separated anaerobic process to efficiently produce biohydrogen simultaneously with organic wastewater pre-treatment.