Effect Of HRT On The Performance And Propionate-oxidizing Bacteria In Anaerobic Digester Systems

Organic loading rate (OLR) is an important engineering control parameter for operating anaerobic digestion system. Controling the OLR is often by adjusting the HRT for particular wastewater treatment. High OLR will lead to the accumulation of VFA, especially the accumulation of propionic acid, which is happened when the HRT is too short. Propionate is one of the most important intermediate products of the anaerobic digestion process, and also hard to be oxidated. The accumulation of propionate in anaerobic system can often lead to the deterioration of operational stability of the system, and even lead to the acidification of the anaerobic digestion or the decline in performance or loss of the system. Therefore how to increase the OLR, and protect the transformation between metabolic intermediates which include propionic acid in the system at the same tiome, is the key to advance the effect of anaerobic digestion, and a hot spot to study in the area in wastewater treatment. This paper studied two types of anaerobic reactors in engineering, which are the most common reactor, one of them is UASB and the other is CSTR. This paper studied the impact of HRT for the effect of operation and discussed the phylogeny and functional diversity of propionic acid-oxidating bacteria through analyzing the biofacies of sludge and modern molecular biology. All these studies provide a guide of engineering operation for improve the effect of anaerobic digestion system.This paper studied the impact of the effect of operation about UASB when changing of HRT under the following condition: the sludge is 7.75 gMLVSS/L, influent COD 1000 mg/L (diluted molasses), the influent alkalinity is 2000 mg/L, hydrolic retention time (HRT) 48 h, pH 6.5～7.5 and a temperature of (35±1)℃. The results indicated the pH and alkalinity changed little, gas production increased as the HRT changed from 48h to 24h, 20h, 16h, 12h and 8h. The strainaway rate of COD was still above 80% when the HRT was 8h. The shortening of HRT leaded to the transient increasing of propionate and acetate, then continued decreasing as time gone, and both of them were 100 mg/L when the HRT was 8h.The rate of declining in acetate is faster than that of propionate as the operational time running. This result indicates that the ability of using acetic about anaerobic sludge is stronger than that of using propionate, and acetogenic bacteria which can using propionate grows slower and the ability of transforming substrate lower than methanogenus which can using acetate. As the increasing of OLR, there are obvious succession process appears between acetogenic bacterias which can degradate propionate in anaerobic activated sludge. They are successively Syntrophus gentianae, Syntrophobacter sulfatireducens, Pelotomaculum schinkii and Desulfotomaculum benzoicum, which proves that these acetogenic bacteria has extensive biodiversity and function diversity. This paper also studied the impact of the effect of operation about CSTR when changing of HRT under the following condition: the sludge is 12.9 gMLVSS/L, influent COD 2000 mg/L (diluted molasses), the influent alkalinity is 2000 mg/L, hydrolic retention time (HRT) 48 h, and a temperature of (35±1)℃. The results indicated the pH and alkalinity changed little, gas production increased as the HRT changed from 48h to 36h, 24h, and 18h. The strainaway rate of COD always stayed above 97% in this process. The concentration of acetate and propionate in effluent fluctuated and increased as the shortening of HRT. When the HRT was 18h, they remained 90 mg/L and 30 mg/L respectively. There are obvious succession process appears between acetogenic bacterias which can degradate propionate in anaerobic activated sludge. They are successively Syntrophus gentianae,Pelotomaculum schinkii and Desulfotomaculum benzoicum. The results about comparing the biodiversity and functional diversity of acetogenic bacteria which can degradate propionate between UASB and CSTR shows that there was a correlation between the high-low of this diversity and the disposal efficiency of the reactor.