Research On Fouling Mechanism And Efficiency Of Cross-flow Filtration In Pipeline Of Slurry With Micron And Submicron Solid Particles

As the development of economic globalization and incessant improvement of urbanization,rivers and lakes in lots of districts in our country are seriously polluted and water environment deteriorated.The biggest pollution source is sediment deposited at the bottom of rivers and lakes.Therefore,environmental sludge dredging becomes an effective method to manage river and lake pollution.The management of dredging sludge needs to follow some rules-volume decrease,stabilization,harm reduction and reclamation.Among these rules,volume decrease is the precondition of others.In environmental dredging,volume decrease is realized by sludge concentration and dehydration.And sludge concentration is the most important step.Though it is so important,the technique of sludge concentration didn’t acquire qualitative improvement.Nowadays,sludge concentration becomes the bottleneck of sludge management.Filtration is a normal method for solid-liquid separation.And cross-flow filtration is a special form of filtration.During the cross-flow filtration,the flow direction of slurry is parallel to the surface of filter and shear stress is generated to hinder the deposition of solid particles to filter surface and infinitely development of filter cake.Hence,the filtration rate can be kept at rather high level.Normally,a pipeline with length from hundreds of meters to a few kilometers is needed to transport dredging sludge from dredging boat to concentration basin.This thesis combined cross-flow filtration and dredging sludge pipeline,took pipeline as not only a transportation carrier,but also a filter module with shell of pipe as filter,aiming at concentrating dredging sludge in the pipeline,to provide appropriate sludge condition for sludge dehydration.To apply cross-flow filtration to dredging sludge concentration,this thesis researched contents as follows:The first problem met by the research of cross-flow filtration is to identify the blockage mechanism.Firstly,this thesis analyzed existing blockage mechanisms.Based on this,we considered the nonuniform distribution of solid particles in slurry,took solid particle stability theory as guidance,derived new blockage models for cross-flow filtration.Then we carried out curve fitting between experimental results of cross-flow filtration and formulas of blockage mechanisms.Correlation coefficient R-square was used to evaluate the fitting results,by which the blockage mechanism suited for our research scope was identified.The most important content of cross-flow research is to predict the filtration process,which include the prediction of filtration rate and particle size distribution of filter cake.Taken solid particle nonuniformity into consideration,this thesis regarded the sum of characteristic velocity of Brownian diffusion,shear induced diffusion and inertial lift as the backward velocity of solid particles.By comparing the relative magnitude of cross-flow filtration rate and solid particles backward velocity,whether and how many solid particles deposited on the filter are calculated.By combining Darcy’s law and Kozeny-Carman equation,the calculation model was obtained and the equilibrium state of cross-flow filtration was pointed out.Then experimental results were compared to the computed results to verify derived model.Also,guided by this new model,this thesis researched the effect of slurry flow rate,solid volume fraction and filter resistance on cross-flow filtration.The most important purpose of cross-flow filtration research is to find ways to improve average filtration velocity.Guided by the derived model,the effect of flocculation and back wash on cross-flow filtration velocity was researched.Experimental results were used to verify the effect of flocculation and back wash on cross-flow filtration.Based on this research,we understood the process of drudging sludge cross-flow filtration more deeply,identified the blockage mechanism of drudging sludge crossflow filtration,realized relative accurate prediction of cross-flow filtration,found ways to improve cross-flow filtration velocity.Research results show that the future of application of cross-flow filtration on drudging sludge concentration is bright.