Enhanced Fine Inorganic Particle Removal Based On The Swirl Grit Removal Technology

In order to ensure the carbon for phosphorus and nitrogen removal many municipal wastewater treatment plants have canceled primary sedimentation tank in the design or conduct of the primary settling tank beyond, leading the fine inorganic particles(particle size less than 200 ?m) that should be removed in the primary settling tank into the biochemical unit. Conventional grit removal chamber generally set relative density of 2.65 and a particle size greater than 200?m as design targets. But in the actual operation the removal efficency of grit chamber is different due to different range size of particles. And with the influence of the attachment of organic matter in sewage the equivalent proportion characteristic of particles will exhibit differences due to the different diameters. And such differences behave more prominent in fine particles, which will directly affect their movement and settlement characteristics. It can enhance the removal of fine particles through vortex principle of swirl grit chamber by fully considering the characteristics of particle size gradation and specific gravity of inorganic particles in sewage and determining the target for the particle size and proportion reasonablely and optimizing the design and operation parameters of swirl grit chamber. To this end, the paper first conducted the survey of tracking and monitoring the size and concentration of inorganic particulate matter in and out of the swirl grit chamber and the concentration and MLVSS/MLSS ratio of biochemical activated sludge, and analysied swirl grit removal efficiency of fine inorganic particles combined with the vortex principle, and conducted settling column experiments to study the proportion properties of inorganic fine particles after coated with grease. Then the paper determined the appropriate target size and proportion to strengthen the removal of fine particles. The study on enhancing fine inorganic particle removal based on the swirl grit removal technology through particle movement theory has finally been made, the results showed that:(1) The average particle diameter of inorganic particle into the swirl grit chamber was 43.95~74.75?m in dry season which fine particulate matter accounted for 98.26%. The average rainfall peaks diameter was 107.46~205.37?m which fine particulate matter accounted for 71.60%. The concentration of inorganic particles into the swirl grit chamber was 117~187mg/L in dry season. The peak concentration of inorganic particles into the swirl grit chamber was about 3h after rainfall occurs, and the peak concentration was 872~3981mg/L. During the study period of 2 months there were about 34.8% of the total inorganic particles accounted for rainfall. The amount of inorganic particles was increaseing by the increase of days before rain W(W=2.2D+27.7, R2=0.925) in average rainfall intensity I?6.3mm case. The amount of inorganic particles was increaseing by the increase of rainfall intensity I(W=4.7I+15.9, R2=0.911) in days before rain D?7d situation.(2) Removal efficiency of swirl grit chamber for particle diameter ?200?m was 71.23%~92.95% while the fine inorganic particle removal efficiency was 8.87%~10.90%. The total removal efficiency of grit chamber was about 12%~27%. The size of studied wastewater treatment plant is 50000 m3/d. By accounting the balance of inorganic particulate material in biochemical pool the cumulative total amount of the inorganic particles was 410.25 t in biochemical pool during the study of 2 months, which 399.45 t deposited on the bottom, and the rest suspended in the activated sludge mixture. The concentration of activated sludge MLSS was maintained at 2200~2800mg/L level during the study, and the MLVSS/MLSS of activated sludge was from 0.52 down to 0.40, and the MLVSS of activated sludge decreased from 1410mg/L down to 930mg/L. By analyzing the size distribution curve of inorganic particles and combining with the suspension characteristic of fine inorganic particles in the biochemical pool the grit chamber can set particle diameter of ?73?m as design goal to ensure that the 50%~80% of the particles in particle size distribution curve can be effectively removed during rains and 15%~45% of the particles during dry days.(3) The settling column experiments for inorganic particles coated with oil showed that the oil-coated inorganic particles settling with population had the proportion of 3.74% ~39.01% and the oil-coated inorganic particles with suspended state separately had the proportion of 3.74% ~39.01%. Inorganic particles coated with oil individually settled as similarly freedom settling, and the smaller the particle size d is, the smaller the equivalent proportion E of inorganic particles coated with oil(E=1.362+0.0035 d, R2=0.889). By analysizing the drop of equivalent proportion of fine inorganic particles coated with oil the design proportion of fine inorganic particles in grit removal systems needed to be optimized. Swirl grit chamber can be designed with relative density of 1.55 and particle size above 73?m to enhance the removal of the fine inorganic particles.(4) The main factors affecting the settling of grit in swirl grit chamber include the flow rate of inlet channel, swirl velocity in component selection, effective depth and hydraulic retention time. Theoretical calculation showed that in order to remove the particles with relative density of 1.55 and size above 73?m the flow rate in inlet channel of grit chamber was preferably 0.26~0.29m/s, and the effective depth of sorting area should be less than 0.90 m, and swirl velocity was preferably 0.25~0.29m/s, and the minimum hydraulic retention time was preferably 21~33s. The application effect of the pilot model test based on enhancing the removal of fine inorganic particles with swirling grit technology showed that the removal efficiency of grit chamber for sand disrtibuton in rainfall was 64.86%, which increaseed efficiency of 38.28% compared with the actual swirl grit chamber in wastewater treatment plant; and the removal efficiency of grit chamber for sand disrtibuton in dry days was 35.96%, which increased efficiency of 23.62% compared with the actual swirl grit chamber in wastewater treatment plant.