Study Of Stability Control In The Variable Velocity Sedimentation Process Based On Particles Characteristic

Flocculation and sedimentation are the most important processes for particles and colloids in drinking water treatment,which have been widely applied in water treatment.In flocculation,particles and colloids form small flocs,and then grow into large and compact flocs.These flocs could be separated with water in sedimentation process.Flocculation and sedimentation are complicated and chaotic processes.Floc characterizations,including floc size,structure and strength,largely depend on the physicochemical conditions and the hydrodynamic environments.However,in previous studies,several flocculation experiments were conducted in a jar-test reactor,which cannot reflect the actual hydrodynamic environments in flocculation and sedimentation process.In addition,conventional sedimentat ion process presented a limit efficiency of micro particle(2~10 ?m)conrtol,which restricted the filtration process.Therefore,in this study,a continuous-flow reactor was applied to investigate and simulate the practical flocculation and sedimentation process in water utilities.Based on the analysis of flocs charactistics,this study investigated variable velocity sedimentation process in order to control the particle number.This study provided a new insight of flocs evolution in the continuous-flow reactor,which is benefit to simulate and optimize practical water purification process.This study investigated the size distribution of micro particle(2~10 ?m)in the steady state of the flocculation and sedimentation process.The experimental results indicated the number of small size particles(less than 10?m)gradually decrease in the flocculation process,with the increase of particles size bigger than 5 ?m and the decrease of particles size smaller than 5?m.Because of the micro particles of size less than 5 ?m presented a direct relationship with the efficiency of filtration,the numbers of less than 5 ?m micro particle could be considered as an evaluation index of sedimentation efficiency.Besides,there is a remarkable correlation between the fluctuation of amount of micro particles and the filtration efficiency.Therefore,both the variable coefficient(CV)and the stability coefficient(K)of the amount of micro particles could be considered as the evaluation index for the operational stability in flocculation and sedimentation process.In order to control the number micro particles(less than 5 ?m),this study established a variable velocity sedimentation method.Kaolin-humic acid suspension was used as the synthetic water sample and polyaluminium chloride was chose as the coagulant.In order to investigate the effect of flocs charac teristic on the variable velocity sedimentation,an in-situ recognition technology and an on-line particle counter were applied to measure floc size,boundary fractal dimension,eccentricity rates,particle numbers,and turbidity.The experimental results indicated the increase of PAC dosage could promote the formation of superposed layer,thereby improving the removal efficiency of micro particles.At PAC dosage of 5 mg/L,the numbers of particle size less than 5?m were 2175/m L by the lamella sedimentation and 480/m L by the variable velocity sedimentation.Moreover,the stability coefficient(K)decreased from 3.789 of lamella sedimentation to 2.641 of the variable velocity sedimentation.In addition,the increase of initial particles concentration could reduce the average floc size in the end of flocculation stage and reduce boundary fractal dimension,resulting in the deterioration of floc compactness.But,the increase of initial particles concentration was benefit for the formation of suspended layer.In addition,the effect of flocs characteristic on the formation of suspended layer was significant.The suspended layer tended to formation at large flocs size diversity.With the increase of suspended layer height,This study discussed the effects of floc breakage and reaggregationon the aggregate structures.The changes of stirring strength lead to the variety of floc breakage and reaggregationprocess.Based on the study of flocs evolution,this study obtained the relation between stirring strength and aggregate structures.At relative low stirring strength,flocs surface erosion result a poor flocs reaggregation process.However,at relative high stirring strength,large-scale fragmentation was the main breakage pathway,and the flocs reaggregation efficiency based on the degree of breakage.Furthermore,the density of reaggregationflocs affected the sedimentation efficiency.A gentel breakage and reaggregationprocess could improve the micro particle removal.The quantity of micro particles decreased from 1351/m L without breakage and reaggregationprocess to 971/m L under breakage and reaggregationprocess with the shear rate of 69 s-1.This study further investigated the hydraulic breakage and reaggregation process.Perforated clapboards with different pore size were installed between the flocculation units,in order to adjust the water flow rate and optimize the flocs structure.The clapboard changed the hydraulics conditions of flocculation process,thereby strengthening "breakage and rreaggregation" process and improving of the removal efficiency of variable velocity sedimentation.With the P1-P1 clapboard,the removal efficiency of micro particles by variable velocity sedimentation was 2 fold of that by lamella sedimentation,and the turdibity decreased from 3.3 NTU of lamella sedimentation to 2.69 NTU of variable velocity sedimentation.Moreover,this study monitored the characteristics of flocs morphology,structure and size distribution.A dynamic flocs broken and reaggregationmodel was established based on flocs structural evolution.This model was used to analyz e the mothod for improving the suspended layer efficiency.