The Effect And Mechanism Of Variation Of Superficial Gas Upflow Velocity On Sludge Granulation In SBRs

Compared with the traditional activated sludge,aerobic granular sludge has the advantages of good settling performance,high biomass,better resistance to shock loads,simultaneous nitrogen and phosphorus removal,so it has been much concerned by scholars of both domestic and foreign.It is one of the most promising technologies for the new generation of wastewater biological treatment in the twenty-first century.Sludge granulation and its characteristics are affected by many factors because of complex formation mechanism.Hydraulic shear force is one of the most important factors affecting the formation of aerobic granules.When hydraulic shear is too larger,particles structure is broken into fragments,reducing the particle size and specific gravity,serious of sludge settling performance resulting in the loss of sludge in the reactor.While hydraulic shear is too small,the zoogloea of relatively poor settling performance is easy to disintegrate resulting in granular sludge startup difficulty.And it is proportional to the gas upflow velocity.Therefore,it is necessary to reveal the effect of the change of superficial gas upflow velocity on the granulation in SBR reactors.Two identical sequencing batch reactors named R1 and R2,in which superficial gas upflow velocity was kept constant at 3.0 cm/s(R1)and gradually decreased from 3.0 cm/s to 1.0 cm/s(R2),were adopted to study the effects of variation of the superficial gas upflow velocity on coagulation of floc and sludge granulation.The aim was to provide theoretical basis for revealing the mechanism of sludge granulation and speeding up the process.1.The results showed that granular sludge could be cultivated successfully after 35 days in R1 with the mean diameter of 564.59 ?m,while it took only 24 days in R2 with the mean diameter of 978.71 ?m.The gas upflow velocity decline model was better than the constant mode,which can accelerate the granulation process.And the particle size growth rate was faster.Meanwhile,it can save energy consumption with certain operating economy.2.The relationships between the variation of superficial gas upflow velocity and the particle size distribution were in accordance with the change of the eddy scale.The rheological properties of mixed liquor changed as the superficial gas upflow velocity reduced.In R2,the eddy scale was greater than in R1.It was suggested that the more fraction of the sludge was distributed in dissipation range of the eddy scale in R2,the faster it accelerated the sludge granulation via enhancing shear cohesion of the floc in the corresponding dissipative region.When there were particles,the particle size distribution was mainly concentrated in the eddy scale of 3 to 12 times.3.While the superficial gas upflow velocity decreased gradually from 3.0 cm/s to 2.0 cm/s in R2,fractal dimension(D2)of the sludge increased continuously.As the superficial gas upflow velocity decreased from 2.0 cm/s to 1.6 cm/s,the D2 decreased firstly and then increased subsequently.The specific expanding ratio of diameter of the sludge was calculated out as about 0.16 ± 0.01.The biomass-shear-loss rate of the sludge varied from 1.35×10-2 mg VSS/cm2·d before adjustment,then undergoing a slightly decline,and finally increased to 1.88×10-2 mg VSS/cm2·d which affected by the increase of sludge size.Meanwhile,the particles structure of the sludge became more compact.When the superficial gas velocity was kept less than 1.6 cm/s,the force of eddy became weak and the D2 decreased.4.The Vmax,Ks and Kd of aerobic granular in the R2 were greater than R1,indicated that the microbial activity was higher than R1;YH of aerobic granular were 0.15?0.17 kg MLVSS/kg COD in the R1 and R2,respectively.With the decrease of superficial gas velocity,the production of excess sludge was slightly more than R1,but it was far less than conventional activated sludge system.