| In the face of the country's major demand in sludge treatment and environment improvement,the excess sludge must be treated to achieve its goal of“reduction,stabilization,harmlessness,and resource utilization”.Anaerobic digestion is widely used because of its high energy recovery efficiency,limited environmental impact,etc.In order to strengthen the performance of anaerobic digestion and subsequent sludge dewaterability and improve sludge sanitation,heat,alkali,ultrasonic and other pretreatments are often combined with anaerobic digestion to form a combined process,among which the thermal hydrolysis technology has been widely used in practical engineering.In recent years,Computational Fluid Dynamics?CFD?has become a powerful tool for predicting and optimizing the mixing effect of biological reaction systems.This paper analyzed the rheological characteristics of the activated sludge in a sewage treatment plant in Cangzhou and the thermal hydrolyzed sludge in a sludge concentration plant in Ningbo.The liquid-liquid jet and the gas-liquid jet agitation methods were used for agitation.Velocity distribution,viscosity distribution,and uniformity of stirring in the anaerobic digester were analysed and discussed.The following conclusions were drawn.?1?Comparing the experimental values?incidence flow rate,flow velocity?with the CFD simulation results shows that the CFD simulation method is valid in investigating the jet mixing characteristics of non-Newtonian fluid.In the liquid-liquid-jet stirred flow field experiment,the inlet pressure is 226043 Pa,and the mixing effect of the flow field is most obvious at this time.The analysis of mixing effect showed that there was no significant difference in the mixing effect of the activated sludge and the digested sludge inside the nozzle.Under the incident pressure,the incident flow was 314.57 m3·h-1 when the liquid-liquid nozzle pumped activated sludge,and the incident speed was 3.37 m·s-1,the incident flow rate of digested sludge was 312.05 m3·h-1,and the incident velocity was 3.20 m·s-1,and both sludges were well mixed.The mixing effect of the two kinds of sludge in the mixing tank is quite different,and the pushing distance of the active sludge body can reach 11m.The main body pushing distance of the digested sludge can reach 9m,and the flow velocity after the body pushing distance ranges from 0.000 m?s-1 to 0.002 m?s-1.?2?According to the velocity image of the cross section,the vertical velocity cloud map and the streamline diagram,the velocity of the flow area is mainly around0.60 m·s-1,and the flow velocity in the central area of the flow field is low.The streamlined graph indicates that the material in the tank is in a counter-clockwise cyclic upward state.According to the Stokes law,the sedimentation velocity of this reactor was calculated to be 0.30 m·s-1.The area where the settling velocity in the reactor less than this value is concentrated in the central area,and the volume of this part reaches 600.88 m3,and the dead zone inside the reactor accounts for 20.58%of the total volume.The hydrolyzed sludge was subjected to the combined effects of"main body convection diffusion","vortex diffusion"and"molecular diffusion"during the process of agitating.The shear strain cloud diagram of the sludge in the flow field shows that the shear rate distribution in the flow field is uneven and can be divided into jet impact area(1.50-200 s-1)and flow circulation area(0.00-1.50 s-1).?3?According to the cross-section velocity cloud map,vertical velocity cloud map,and streamline diagram,the velocity of the flow area is mainly concentrated around 0.75 m·s-1,the flow velocity in the central area of the flow field is low,and the material is in a counterclockwise cycle in the tank.The low-velocity gradient values appear on both sides of the central area of the tank,and the velocity gradient value increases near the nozzle.There is a difference between the z=2.20 m plane and the z=9.00 m plane,which indicates that the presence of gas is the cause of the velocity gradient change.The main body of the tank is in the low speed gradient range,and the low speed gradient zone?LGZ?:0?GL<3 s-1.Compared with the liquid-liquid jet,the internal mixing effect of the gas-liquid jet device is better than the liquid-liquid jet device,and the dead-zone proportion is smaller than the liquid-liquid jet stirring device. |
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