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The Influence of Floc Morphology Character of Micro-flocculation Stage on Filtration Efficiency

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Tutor: NanJun
School: Harbin Institute of Technology
Course: Municipal Engineering
Keywords: micro-flocculation stage,floc morphology,filtration efficiency,lowtemperature an
CLC: TU991.2
Type: Master's thesis
Year:  2012
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Abstract:
Filtration is a indispensable link in water treatment, its efficiency has direct impacton safety and stability of drinking water. It has certain limitations to strengthen fromfiltration itself, coagulation which is a basic part of water treatment has remarkableeffect on the efficiency of follow-up processing. Micro-flocculation stage is the initialstage of coagulation, in whicn stage paricles change rapidly and its floc morphologicalcharacteristics have great effect on coagulation and follow-up processing.There was significant correlation between the floc morphological characteristics offiltration inflow and filtration efficiency through the study of direct-filtration processing(contact-filtration and micro-flocculation-filtration). Based on this, studied the impact offloc morphological characteristics of micro-flocculation stage on filtration efficiency incoagulation-precipitation-filtration processing, introducing diatomaceous earth andcalcium carbonate to change initial particle size distribution and made the conclusion ofexperiment confirmed sufficiently, also found that the both had significantreinforcement on treatment efficiency. Apply the conclusion of the first two cract to thetreatment of low temperature and low turbidity water by strengthening themicro-flocculation stage to improve filtration processing.Results showed that: as for direct-filtration processing, the greater floc fractaldimension of filter inflow was, the less number of filter effluent particles and morestable of changes were, the smaller the initial pressure and the slower increasing rate ofwater head of filter layer were. Remaining particles of filtration and the pressuredifference of filter layer had linear negative correlation with floc fractal dimension,the correlation coefficient was above0.7.In conventional process,9.2min of coagulation met the characteristics of micro-flocculation stage which particle size changed rapidly, therefore set0~9.2min ofcoagulation as micro-flocculation stage in this experiment conditions. Under thecondition of changing the dosage of PAC¡¢diatomaceous earth and calcium carbonate,precipitation residual particles¡¢ precipitation residual turbidity¡¢ filtration residualparticles¡¢the sum of the particles which affect filtration remarkably and pressuredifference of filter layer had negative correlation with the changing rate of floc fractal dimension in micro-flocculation stage, the correlation coefficient was above0.7. Thedosing of diatomaceous earth or calcium carbonate can improve the water quality offiltration significantly and slow down the pressure changing rate above filter layer. Thenumber of residual particles after filtration could be controlled below200int/mL and100int/mL when the dosage of diatomaceous earth or calcium carbonate was20mg/L.The influence of low temperature and low turbidity on water treatment was that therate of destabilization particles forming larger particles was low and terminated inmedium-size flocs at the end of coggulation, the changing rate of fractal dimension inmicro-flocculation stage was lower and lead to residual particles after filtration andpressure above filter layer was higher. Put forward three measures to deal with lowtemperature and low turbidity water effectively from the the perspective ofstrengthening floc morpHology in micro-flocculation stage, they were adoptingappropriate hydraulic conditions¡¢using reasonable flocculant and changing the initialparticles. The conclusion was: under the circumstances of fast stirring time1min, faststirring velocity400rpm,12mg/L dosage of ferric chloride,10mg/L dosage ofdiatomaceous earth, the particle size changed more rapidly, the fractal dimension beganto change earlier and changed more quickly.12mg/L dosage of ferric chloride and10mg/L dosage of diatomaceous earth was the best congditions under this experiment todeal with low temperature and low turbidity water, residual particle number could becontrolled under100int/mL after5minutes of filtration and its pressure was lower.
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