Characteristics Of Anoxic Granules Cultivated In Sequencing Batch Reactor(SBR)

Granulation is a process of microbial self-immobilization and finally produces a form of dense biomass in a spherical shape. As compared to the sludge flocs, granules have a compact and spherical structure, good settleability, high biomass and ability to handle high strength organic and toxic wastewater. There are several factors affecting granulation, the influence of hydraulic shear force and trace metals are particularly examined to clarify the formation mechanism of denitrifying granules in sequence batch reactor (SBR). Conclusions are summarized as follows.(1) Experiment results showed that hydraulic shear force was an important factor affecting granulation. Under the stirring intensity of 150rpm, granules were formed in 53 days, while in another reactor of 80rpm stirring intensity, granules were formed in 102 days.(2) Granules cultivated under the 150rpm stirring intensity had less VSS/SS value than those cultivated under the 80rpm stirring intensity.(3) As a trace element existed in raw tap water, calcium could form infusibility sedimentations which served as the granule cores in the alkalescent solution. Calcium content of anoxic granular sludge was rather larger than anaerobic/aerobic granular sludge, up to 200⁴00mgCa/gSS. Addition of NaHCO₃ conduced to form granules, but it was inclined to cause sludge bulking, and then the sludge calcium content reduced remarkably.(4) C/N ratio and the influent COD concentration played important influence on denitrification velocity of anoxic granules, which increased with the C/N ratio. When the C/N ratio was 4:1, denitrification velocity of anoxic granules were 0.023mgNOx/mgVSS·h and 0.024mgNOx/mgVSS·h respectively, and since the COD was not sufficient, nitrite was accumulated. When the C/N ratio is 5:1, denitrification velocity was increased with the influent COD concentration, when the influent COD concentration was 300mg/L, the denitrification velocity was 0.032mgNOx/mgVSS·h and 0.031mgNOx/mgVSS·h respectively, while the initial COD concentration was450mg/L, the velocity was 0.082mgNOx/mgVSS-h and 0.063mgNOx/mgVSS-h respectively, and no nitrite was accumulated.(5)During famine period of the feast/famine regime, denitrifying bacteria metabolized to form the SMP.(6)SEM photos showed that the granular surface was not homogeneous and consecutive, but comprised of membrane patches. Most of the denitrifying bacteria existed within the outer layer, while few inside. There are much crystal of cation compounds near the core of granules.