Study On Formic-acetogenic Pathway Of VFA Fermentation In ASBR

In this paper, the hydrogenic-acetogenic and formic-acetogenic processes were studied and the effects of pH, Alkalinity, Oxidation Reduction Potential (ORP), organic loading and matrix type on them were investigated. The results of the study provided insight to the VFA’s formic-acetogenic metabolism processes, expecting to solve the volatile fatty acid accumulation and accelerate the digestion rate of the ASBR can also be accelerated.In this study, the sludge inoculated in the ASBR was got from the mud store poll of a wastewater treatment plant. After 220 days’operation, a steady state was established and its organic loading was 6kgCOD/(m3d). The effluent chemical oxygen demand (COD), pH, volatile fatty aid concentration and effluent suspended solid (SS) were 75mg/L,6.72,50mg/L, 65mg/L, respectively. The sludge in the ASBR is mainly flocculent. The MLSS, MLVSS and MLVSS/MLSS of the active sludge were 22.586g/L, 20.513g/L and 0.913, respectively.The results of the study of formic-acetogenic pathway showed that, when the hydrogenic-acetogenic processes with glucose as substance was inhibited, part of the VFA in the reactor was degraded. When CHCb was added into the reactor as inhibitors, the methanogenesis was inhibited completely and formic was accumulated. When the hydrogenic-acetogenic process was inhibited at the same time, the accumulation of formic was higher. The results suggested that there was formic-acetogenic process in the ASBR. In sludge digestion system, the same phenomenon occurred. The existence of homoacetogenesis was also proved in this system. The influencing factors of formic-acetogenic process were invested. The results showed that pH had great effect on glucose fermentation. When pH=4.0, the typical ethanol-type fermentation occurred. But when pH=5,6,7, the fermentation pattern was propionic acid-type fermentation and the accumulative formate production was less than that of ethanol-type fermentation system. Therefore, ethanol-type fermentation was more beneficial to formic-acetogenic process and it could also be concluded that pH was propitious to formic-acetogenic process when propionic acid-type fermentation was occurred. Alkalinity had significant effect on formic-acetogenic process. The accumulation of formic was increased with the increasing of Alkalinity. When Alk/COD<1, the concentration of formic was less than 100mg/L. But when Alk/COD=1,2,4,6, the concentration of formic were 157.2mg/L,245.3 mg/L,371.5 mg/L and 469.8mg/L, respectively. ORP was likely to influence the formic-acetogenic process and the improvement of ORP was not favorable to formic-acetogenic process. The substance fed had effect on formic-acetogenic process. In the reactor with butyric fed, the formic-acetogenic process held the absolute predominance, but the hydrogenic-acetogenic process had the contrary result. In the reactor with propionate fed, the hydrogenic-acetogenic process was predominate and propionate was degraded primarily through hydrogenic-acetogenic process, so the production of acetate acid by hydrogenic-acetogenic process was almost two times than that of formic-acetogenic process. In the case of normal load, the improving of organic loading was favorable to formic-acetogenic process. But in the case of shock load, with the increase of organic loading, the VFA accumulation was more and more serious, and as a result, the low pH had an adverse effect of formic-acetogenic process.