Research On Rapid Formation Of Aerobic Nitrifying Granule And Its Stability

Aerobic granules are compact microbial aggregate that formed by activated sludge under various selection pressures through self-immobilization. Compared with conventional activated sludge, aerobic granules have good settling properties, dense microbial structure, and good capacity to deal with toxic medium. As one type of aerobic granules, aerobic nitrifying granules have attracted the attention from many researchers for its excellent ability in removing nitrogen. But the long granulation time of aerobic nitrifying granules and instability in long-time running severely limit the application of aerobic nitrifying granules in treating high strength ammonia wastewater. The present study has investigated the method that enhanced aerobic nitrifying granulation and the influences of toxic substance on the stability of aerobic nitrifying granules.Regard to the long granulation time of aerobic nitrifying granules, the present study enhanced aerobic nitrifying granulation by applying an intensity of48.0mT static magnetic field successfully. Scanning electron microscope/Energy Dispersive X-ray analyzer (SEM-EDX) results showed that the applied magnetic field could promote the accumulation of iron compounds in the sludge. And then the aggregation of iron decreased the full granulation time from41days to25days by enhancing the setting properties of granules and stimulating the secretion of extracellular polymeric substances (EPS). Aerobic granules cultivated in magnetic field hold dense and compact bacterial structures, with cocci bacteria spreading all over the granules. Meanwhile granules cultivated without magnetic field were dominant by bacillus bacteria, and structures of granules were loose. Three-dimensional excitation and emission matrix (EEM) fluorescence spectroscopy results show that magnetic field had not changed the component of EPS although it enhanced the secretion of EPS. Results of long-term and cycle experiments revealed that magnetic field could enhance the bioactivities of nitrite-oxidizing bacteria (NOB). Fluorescence In-Situ Hybridization (FISH) analysis also proved that an intensity of48.0mT magnetic field could enlarge the proportion of NOB. These findings suggest that magnetic field is helpful and reliable for accelerating the aerobic nitrifying granulation.The bioactivity and stability of partial nitrifying granules are easily affected by external factor for its simple microbial community. Phenol is widely utilized in many industries including petrochemical, pharmaceutical, coke conversion and resin manufacturing plants, thus phenol is considered as one common toxic substance in the wastewater. Considering the possible influences of phenol on partial nitrifying granules, the present study has investigated bioactivities, physicochemical properties and microbial communities of partial nitrifying granules.The results of short-term, long-term experiment and the measurement of specific oxygen utilization rates (SOUR) revealed that phenol can inhabit the bioactivities of all bacteria in partial nitrifying granules and the inhibitory effects aggravated as the concentration of phenol increased. The bioactivities of partial nitrifying granules were severely inhabited when influent phenol was200mg/L. And the inhibitory effects were more significant to ammonium-oxidizing bacteria (AOB) than to NOB, and then NOB took advantage in the competition with AOB. FISH analysis also showed that the proportion of NOB increased largely while that of AOB decreased. The quantitative changes of AOB and NOB transformed partial nitrification into full nitrification in Sequencing Batch Airlift Reactor (SBAR). Scanning electron microscope (SEM) revealed that the dominant bacteria were transformed from cocci bacteria into filamentous bacteria under the influence of phenol. This resulted into the sharp increases of the mean diameter, deterioration of structures and setting properties of partial nitrifying granules. While the structures of granules became loose and the dominant bacteria was changed from cocci bacteria to filamentous fungus and bacillus bacteria. Phenol can also stimulate partial nitrifying granules to secrete EPS and there were significant increases of EPS content to withstand the toxicity of phenol. The congestion of partial nitrifying granules became more severe and a mount of granules were washed out from reactor when phenol was raised to300mg/L, then the performance of reactor became worse. Results above indicated that partial nitrifying granules cannot withstand high concentrations phenol, and then it is necessary to make some efforts to remove or reduce phenol in the wastewater to avoid the influences of phenol on partial nitrifying granules.