Effects Of Azo Dyes/Nitrate On Conversion Of Organic Acids In Anaerobic Digestion

Organic acids as important intermediate products usually accumulated during anaerobic fermentation process, which led to the failure operation of anaerobic reactor. High hydrogen partial pressure was reported as the key factor for the accumulation of organic acids thermodynamically. It was recognized that the low hydrogen partial pressure could be achieved through the syntrophism growth between hydrogen-utilizing methanogens and hydrogen-producing acetogenic bacteria in anaerobic reactor. However, methanogens were sensitive to the environmental condition and once the environment condition changed, the activity of methanogens decreased and further influenced the anaerobic fermentation process. On the other hand, hydrogen was a favourable electron donor for the reduction of some pollutants (azo dyes, nitrate etc.). Therefore, by combining reduction of pollutants with the anaerobic fermentation, it would be an optional method to realize both the degradation of pollutants and enhancement of anaerobic fermentation via the hydrogen transfer between pollutants degradation bacteria and hydrogen-producing acetogenic bacteria.In this study, some kinds of pollutants (azo dyes, nitrate) were added into anaerobic reactor to explore their effects on decomposition of organic acids with different organic acids as substrates. The conclusions were as follow:1. Azo dyes as the electron accepters consumed the hydrogen released by organic acids, to accelerate the anaerobic decomposition of organic acids. When propionate was used as the substrate, adding azo dyes of35mg/L decreased the effluent propionate from2400mg/L to689mg/L and increased the effluent acetate from180mg/L to519mg/L. Fluorescence in situ hybridization (FISH) indicated that addition of azo dyes improved the amount and activity of hydrogen-producing acetogenic bacteria. Moreover, azo dyes could also accelerate the anaerobic fermentation of glucose. When glucose was used as the substrate, the COD removal and decomposition of organic acids were accelerated by23.3%and320mg/L, respectively. So adding azo dyes could enhance the anaerobic fermentation process by remitting the accumulation of organic acids.2. Denitrification not only consumed the hydrogen produced by organic acids, but also accelerated the formation of granular sludge. After addition nitrate, the granulation of sludge were accelerated by288.6%,325.0%and788.9%, respectively. The level of granulation had directly relationship with the denitrification efficiency. Due to the easier release of hydrogen in butyrate decomposition process, the granulation of sludge and the denitrification were the best with butyrate as substrate. Scanning electron microscope (SEM), denaturing gel gradient electrophoresis (DGGE) and transmission electron microscope (TEM) analyses indicated that the cytoplasm in the cells were the most abundant in butyrate digester. Therefore, addition of nitrate could enhance the anaerobic fermentation process by accelerating the granulation of sludge and remitting the accumulation of organic acids.