| Compared with conventional activated sludge process,aerobic granular sludge process has advantages of large biomass,good settlement performance,and high removal efficiency of pollutants,which has a broad application prospect.Granular sludge is aggregated and formed by activated sludge under special conditions.However,the external control condition of aerobic granulation is still unclear.The growth of activated sludge,the formation of granular sludge and the interaction of three-phase in the flow field are closely related,and especially the gas-liquid shear force is crucial to the sludge granulation.However,the interior relations of them are not clear.In this paper,CFD model describing the interaction between bubbles-liquid-sludge was established to simulate the influence of pore diameter of the diffusor,height of the reactor to its diameter ratio,superficial gas velocity,particle characteristics,substrate type and concentration,transverse agitation,on flow pattern in the bubble column reactor.The appropriate flow pattern of aerobic granulation was analyzed,which provided the hydrodynamic reference for aerobic sludge granulation in similar types of reactors.The coupled CFD-ASM simulation was established to simulate the aerobic sludge granulation,which provided ideas and methods for the rapid cultivation and determination of control conditions of aerobic granular sludge.The main results and conclusions obtained from this study are as follows:1.In the bubble column reactor,the flow pattern of a single circulation was transited to multiple vortical spiral cells with the increasing of height/diameter ratio,superficial gas velocity and particle diameter.In order to satisfy the formation conditions of aerobic granular sludge,the diameter of the diffusor’s pore should be greater than 1mm.When the simulated particle was less than 0.2mm,the flow pattern had nothing to do with particle.In this condition,the three-phase flow could be simplified to the two-phase flow.2.Simulation of air-liquid-sludge three-phase flow and air-liquid two-phase flow and experiment results suggested that,the flow pattern of transverse rotation was essential to form aerobic granules,and the corresponding SGV should be higher than1.2 cm/s;the configuration of small-size spiral vortices was necessary for the stability of aerobic granules,and the corresponding range for the SGV was 2.0 cm/s~5.0 cm/s.3.In the bubble column reactor,with higher height/diameter ratio,the flow pattern of small-scale vortices were formed more easily at a less superficial gas velocity,which was beneficial to aerobic sludge granulation.The viscosity of fluid varied with substrate type and amount,and then affected the fluid environment and hydraulic shear force.4.In experimental aerated and stirred reactor with plastic cuboid baffle agitator,the flow pattern transited from a single circulation cell to multiple vortical cells with the increasing of the rotation speed and SGV.The increasing of rotation speed had a more significant contribution to gas holdup,liquid velocity,bubble size distribution,flow pattern and shear rate than that of SGV.The case of blade speed higher than250rpm was satisfied with the formational conditions of aerobic granular sludge.5.According to the simulation results of the hydraulic control condition for aerobic sludge granulation,bubble column reactors with height/diameter ratio of 120/6,superficial gas velocity of 2.0cm/s~5.0cm/s,organic loading rate of 3.12 kg COD/(m~3d),and an aerated-stirred reactor with height/diameter ratio of 20/16.5,blade speed of 300rpm,superficial gas velocity of 1.05cm/s,organic loading rate of 3.74 kg COD/(m~3d)was established to cultivate the aerobic granules,respectively.Under the condition of fed synthesis wastewater of sodium acetate,after 35 days,the aerobic granular sludge was cultivated successfully with a mean diameter of 0.35~0.65mm and1.05mm,respectively,which verified the accuracy of the simulation.It demonstrated that the longer fluid trajectories and multi-directional shear were beneficial to the formation of aerobic granules with good characteristics,dense and smooth structure,the proper aeration was not only conducive to sludge granulation but also saves the energy. |
PDF Full Text Request