The biofilm process differs from the activated sludge process in that the latter operates with the activated biomass suspended in the system while it attaches to the carriers in the biofilm process, which means that biomass grows and adheres to the su...
The biofilm process differs from the activated sludge process in that the latter operates with the activated biomass suspended in the system while it attaches to the carriers in the biofilm process, which means that biomass grows and adheres to the surface of the carriers. Recently, the concerns on biofilm process are increasing because of its high nitrification efficiency and the convenience of operation.
To characterize nitrifying bacterial communities, the routine bacteriological enumeration by spread-plate or most-probable-number count, has been studied. However, these methods are not suitable because these need some weeks of incubation before these bacteria are enumerated, as well as these techniques often detect only a minor portion of biofilm that may increase the statistical uncertainty of the enumerations. Thus, better monitoring of the content of bacteria in the microbial community, polymerase chain reaction (PCR)-based techniques, such as denaturing gradient gel electrophoresis (DGGE) and cloning, fluorescence in situ hybridization (FISH) and INT-dehydrogenase activity (DHA) test have been utilized and proven as an effective method for analysis of microcolonies and biofilm without the need for disruptive or labour consuming procedures
In this study, to assess the effect of DO concentration andtemperature in aerobic biofilm reactor, the combination of PCR-based techniques, such as DGGE and cloning, FISH and INT-DHA test was used in this study. And for the purpose of evaluating the independent anoxic reactor, the pilot-scale upflow Biobead? process, one of the biological aerated filters (BAF), was set up. To study the community of the denitrifying microorganisms, PCR-DGGE was performed.
To assess the effect of DO concentration in aerobic biofilm reactor, four reactors with different DO concentrations (1, 3, 5 and 7 mg/L, respectively) were set up in the thermostat and acclimated. The optimal DO concentration with stable nitrification efficiency in aerobic biofilm reactor was above 5.0 mg/L. Low DO concentration led the the growth of filamentous microorganism such as type 021N. The results of DGGE and cloning based on PCR targeting 16S rRNA and amoA gene showed that the community of ammonia oxidizing bacteria (AOB) and the ratio of Nitrosomonas sp. changed little in spite of different nitrification efficiencies. And the result of FISH showed that higher DO concentration led the increase of AOB and nitrite-oxidizing bacteria (NOB) ratios and the decrease of heterotrophs ratio. The INT-DHA showed that only active biomass influenced the nitrification efficiency.
To assess the effect of temperature in aerobic biofilm reactor, three reactors with different temperature (5, 10 and 30℃, respectively) were set up in the thermostat and acclimated. Nitrification efficiency was sharply decreased at low temperature, whereas a little COD removal efficiency was decreased. The results of DGGE and cloning based on PCR targeting 16S rRNA and amoA gene showed that the ratio of Nitrosomonas sp. increased as temperature was increased. So, it was estimated that different temperature changed the community of microorganisms. And the result of FISH showed that lower temperature condition led the decrease of AOB and NOB. As temperature was decreased, the INT-DHA and the attached biomass was decreased.
To assess the independent anoxic reactor of the modified upflow Biobead? process which was used commercially, PCR-DGGE was performed. Two types of nitrite reductase genes were selected. One is nirS represented cytocrome cd1 nitrite reductase gene and the other is nirK represented Cu-containing nitrite reductase gene. Denitrifier community in the independent anoxic reactor was analyzed with PCR-DGGE using these two denitrifying functional genes.
As the result of the PCR, only nirS gene was detected between nirS and nirK. With the result of the DGGE, specific bands became strong, as the operating days were longer, nitrate loading rate was increased. otherwise those of the initial activated sludge showed various bands. In the consequence of the sequence of the DGGE bands, various denitrifiers were sequenced in the initial activated sludge, while specific denitrifiers like Alcaligenes faecalis were predominant in the anoxic reactor. Consequently, introduction of the independent anoxic reactor made it possible to achieve 96% denitrification efficiency, and was proper for the modification of BAF process.
The molecular biological techniques give us the fundamental knowledge of microbial community. These method will be useful for the design of the aerobic biofilm reactor and the operational diagnosis method for the wastewater treatment plant.