It was reported that the nitrate in anaerobic sludge reduced to ammonia (ammonification) by nitrate reducing bacteria. But recent researches found that when denitrifiers exist in anaerobic sludge, denitrification could occur and that denitrification and ammonification occurred differently depending upon the substrate in anoxic anaerobic sludge. Volatile fatty acid was favored for denitrification, while the fermentative substrates such as glucose were preferred for ammonification. Since early 90's, the combined process of denitrification and methanogenesis in a single anaerobic reactor has been studied for simultaneous removal of carbon and nitrate. However, the relations between denitrifier, acidogenic bacteria, and methanogenic bacteria were not defined.
In this study, hybrid type upflow sludge baffled filter (UBF) reactor was adopted. Sludge bed in the bottom of reactor was intended to remove carbon and nitrate by denitrification and methanogenesis. And floating media in the upper of reactor were intended to capture the floating fine biomass from disintegration of granular sludges and also to remove remaining carbon which was not removed due to the inhibition of nitrogen oxide on methane producing bacteria. To investigate the effect of different substrate on denitrification and ammonification, acetate and glucose as substrates were used. It was attempted to remove nitrate and carbon in a single-stage hybrid type reactor.
All the reactors removed over 96% of COD and most of nitrate. Nitrate in anaerobic sludge was converted to nitrogen gas(denitrification) or ammonia(ammonification) according to the environmental conditions. When acetate was used as a substrate, COD removal efficiency was very easily changed by the change of volumetric loading rates. And nitrate was removed by ammonification and denitrification according to pH. When influent pH was about 4.7, most nitrate was changed to ammonia and when influent pH was about 7.0, most nitrate was denitrified independent to COD/Nitrate-N ratio. 4000 mgCOD/L and 266 mgNitrate-N/L was removed over 94 and 99% in 24 hours of hydraulic retention time. Most granule had gray color and some had black and in gray-colored granule, black inner side was covered with gray substance and SEM illustrated sarcina type microorganisms which were compact spherical shape.
When glucose was used as a substrate, that reactor showed stable COD removal efficiency due to diversity of microorganisms. And in nitrate conversion, influent alkalinity played important role in the ratio of denitrification and ammonification of nitrate. When influent alkalinity was 600 mg$CaCO_3$/L, most nitrate was converged to ammonia but when influent alkalinity was proper, high denitrification ratio was obtained. As COD/Nitrate-N ratio was reduced, denitrification was dominant. Especially, below COD/Nitrate-N=10, most nitrate was denitrified. 4000 mgCOD/L and 700 mgNitrate-N/L was removed over 95 and 99% in 24 hours of hydraulic retention time. There were 3 type in granule according to surface color of granule - gray, yellowish gray, and black. In gray and yellowish gray-colored granules, black inner side was covered with gray and yellowish gray substances and SEM illustrated rod-type type microorganisms which were composed of rod-shaped bacteria resembling Methanothrix.sp..
Till this study, nitrate removal in anaerobic sludge was thought to have problems due to ammonification, but this study showed that most nitrate could be converged to nitrogen gas by adjusting of influent alkalinity and COD/Nitrate-N ratios. Therefore carbon and nitrate removal were carried out in a single stage UBF reactor
