| With the continuous improvement of sewage treatment rate and effluent standard in China,the traditional sedimentation tank is difficult to meet the numerous demands of current sewage treatment due to its low hydraulic surface loading,large footprints and low treatment efficiency.Ballasted flocculation is a fast flocculation sedimentation process that uses micro-sand or other ballasted agent to improve the floc settling performance.Due to high treatment efficiency,good effluent effect,rapid response and flexible application,it has been widely used in many projects such as combined sewer overflows,initial rainwater,river water and urban sewage upgrading and transformation.However,there are few researches on the mechanism of ballasted flocculation in China,and the property requirements of ballasted agent are not clear.Therefore,this study focuses on the performance of the ballasted agent and the performance evaluation of the micro-sand circulation system in the micro-sand ballasted flocculation precipitation.A series of bench-scale tests were carried out with ceramics,quartz sand,glass,river sand and volcanic rock as the ballasted agent,the influence of PAC dosage,PAM dosage,stirring speed in flocculation stage,gravity and particle size of ballasted agent on the flocculation effect was explored.Taking quartz sand as the loading medium,the engineering experiment of the advanced treatment of urban sewage was carried out,and the removal ability of the flocculation and sedimentation process loaded with microsand on total phosphorus and turbidity was explored.The computer fluid dynamics method was used to simulate the internal flow field of hydrocyclone in the micro-sand circulation system,and the key control parameters of the system were analyzed,in order to lay a foundation for the evaluation of the performance of the micro-sand circulation system.The main research contents and conclusions are as follows:(1)The turbidity of the supernatant liquid and the characteristics of ballasted flocs are the main evaluation indicators.A series of jar-test was carried out to explore the influence of PAC dosage,PAM dosage,flocculation stirring speed,specific gravity and particle size of ballasted agent on the flocculation effect.The test results show that the turbidity of the supernatant after precipitation decreases with the increase of the dosage of PAC and PAM,first decreases and then increases with the increase of the stirring speed.The theoretical settling rate of ballasted flocs is positively correlated with PAM dosage,specific gravity and particle size of ballasted agent,the average diameter of ballasted flocs is positively correlated with PAM dosage and ballasted agent particle size,and negatively correlated with the specific gravity of the ballasted agent,The average specific gravity of ballasted flocs is positively correlated with the specific gravity and particle size of the ballasted agent,and negatively correlated with the dosage of PAM.(2)Taking quartz sand as ballasted agent,engineering test was carried out in the upgrading project of a sewage plant,and the results of jar-test were verified.The results show that the removal rate of total phosphorus in the micro-sand ballasted flocculation sedimentation process is 65%～78%,which is mainly affected by the dosage of PAC.The turbidity removal rate reached 86%～95%,which was mainly affected by the dosage of PAM.Under a certain flocculation velocity gradient,the quartz sand with an equivalent diameter of more than 180 μm has little effect on the system.(3)Based on the computer fluid dynamics model,the formation process of the air core in the hydrocyclone and the distribution law of the static pressure and tangential velocity were analyzed,the total separation efficiency of the hydrocyclone is 95.4%,the overflow ratio is 89.6%,and the pressure drop is 5.85 k Pa under the preset boundary conditions.Combined with the operating conditions of the micro-sand circulation system,the micro-sand loss is estimated to be about 7.7mg/L. |
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