Feasibility Study Of Continuous Flow Aerobic Granulation Based On Gravity-Driven Hydrocyclone Separation

As an improvement of activated sludge process,continuous flow aerobic granular sludge technology is a key technology for wastewater treatment plants to achieve pollution reduction and carbon reduction.More and more experiments verify that the hydrocyclone based on centrifugal separation is important for continuous flow aerobic granulation.However,the current researches focus on the traditional hydrocyclone separation needing intake pump to transport materials and vertical installation of hydrocyclone,which leads to the easy crushing of aerobic granular sludge by intake pump and poor actual separation efficiency.Therefore,in this study,we proposed a hydrocyclone separation method of continuous flow aerobic granular sludge with feed height difference and inclined hydrocyclone installation.The selective pressure based on gravity driven hydrocyclone separation was applied to plug-flow reactor,which avoided crushing of aerobic granular sludge,realized the effective separation of granular sludge,reduced energy consumption,and accelerated the granulation of continuous flow aerobic granulation.In order to explore the feasibility of continuous flow aerobic granulation based on gravitydriven hydrocyclone separation,the continuous flow aerobic sludge granulation test based on gravity-driven hydrocyclone separation was carried out through physical and chemical characteristics analysis,high-throughput sequencing and other means.The experimental results showed that under the conditions of gravity driven hydrocyclone separation,the sedimentation rate of aerobic granular sludge formed in the plug-flow reactor was 22.85 m/h,the average particle size of sludge was 1152.92 μm and the average relative hydrophobicity was 80.02%.The structure of aerobic granular sludge was compact,and the surface pore size was small.During the granulation process,species diversity decreased and the proportion of dominant bacteria increased.The content of extracellular polymeric substances and protein gradually increased,and β-polysaccharides distributed at the edge of the aerobic granular sludge played an important role in enhancing the cohesion of the aerobic granular sludge and resisting the hydrocyclone shear.The species diversity decreased and the dominant flora was increased during sludge granulation,and the relative abundance of genera related to extracellular polymeric substances secretion,such as Flavobacterium and Hydrogenophaga,was elevated.resulting in the enrichment of functional genes including nirSl,smtA,nosZ1,nosZ2,and soxY,among which the denitrification related functional genes were the main ones.Considering that selective pressures(i.e.the separation performance of the hydrocyclone driven by gravity)need to be adjusted in the process of aerobic granulation,this study conducted a test on the separation performance of gravity-driven hydrocyclone.The experimental results showed that the separation performance of the gravity-driven hydrocyclone can be markedly improved by adjusting the hydrocyclone inclination angle.When the hydrocyclone inclination angle was-45-15°,a high separation efficiency(57.01%-68.35%)and a low split ratio(39.44%-52.63%)can be obtained simultaneously,which could satisfy the operation requirements of the continuous flow aerobic granulation reactor.The operation time of hydrocyclone determines the sludge discharge amount and reflux ratio of the system.A relatively low feed height difference was only required to obtain a high separation efficiency.In the process of hydrocyclone separation strengthening aerobic sludge granulation,the sludge concentration can be controlled within a certain range to obtain an appropriate separation strength.Computational fluid dynamics simulations were carried out to study the effects of hydrocyclone inclination angle on the continuous phase flow field,particle motion,and the separation performance of the gravity-driven hydrocyclone.The simulation results showed that both the separation efficiency and split ratio are proportional to the hydrocyclone inclination angle.The angular velocity of particle revolution(277.7-633.2 rad/s except for that at the hydrocyclone inclination angle of-45°)is one order of magnitude lower than that of 0.5-mm particle self-revolution(3899-9932 rad/s)and translation(3646-8703 rad/s).The shear stress on fluid microclusters reaches its maximum near the wall,and the maximum wall shear stress is only 252 Pa.The cylindrical part of the hydrocyclone has an important strengthening effect on the regeneration process of particle pores and particle interfaces,while the particle separation mainly occurs in the conical part of the hydrocyclone.Combined with simulation and experimental analysis,it is confirmed that the interface of aerobic granular sludge is renewed under the action of swirling flow field.The effect of hydrocyclone inclination angle on the pressure distribution in the gravity-driven hydrocyclone cannot be ignored.The total static pressure drop is inversely proportional to the hydrocyclone inclination angle in general.The performance of the gravity-driven hydrocyclone can be further improved by optimizing the geometric and operating parameters.In this study,the feasibility of continuous flow aerobic granulation based on gravity-driven hydrocyclone seperation was studied experimentally and numerically,and the mechanism of gravity-driven hydrocyclone separation to enhance the continuous flow aerobic granulation was revealed,which promoted the theoretical and technical innovation in the field of continuous flow aerobic granular sludge.