| The effective removal of organic matter and colloidal particles is the primary challenges for drinking water treatment industry today.The dissolved air flotation?DAF?process has advantages in the removal of small-sized and low-density flocs formed by organic matter and colloidal particles during coagulation.Compared with the rapid development of DAF in the world,the research and application of DAF in domestic drinking water treatment field are laid behind.Previous flotation researches were mostly focused on the particles control,while the study on the removal of natural organic matters?NOM?and organic micro-pollutants was rarely involved.This study was based on the previous basic research work of our group on the removal of organic matter by micro-bubbles.The efficiency and mechanism of pollutants removal using multiphase pump DAF in combination with sedimentation,a hollow-fiber ultrafiltration membrane?UFM?and an up-flow membrane biofilter?MBF?were investigated respectively.The feasibility of bubble surface modification for NOM removal was proved.The enhanced removal of NOM and organic micro-pollutants was studied with chitosan used as a coagulant aid and a surface modifier respectively.The interaction mechanism between micro-bubbles and various NOM fractions was evaluated via several methods,such as organic matter fractionation,EEM and FTIR.In this paper,the effect of organic matters and particles removal by the combined process of sedimentation-DAF was studied with raw water at heavy-and micro-pollution periods.Microbubble concentration of the EDUR DAF pump flotation unit was measured.The flotation efficiency of flocs particles was analyzed using the "white water bubble blanket" model.The removal of odor-causing compounds were also investigated.Experimental results showed that the average size of microbubbles in the mixing zone was about 20.1 ?m and the thickness of the bubble bed in the separation zone was in the range of 59±8 cm.The sedimentationDAF process can effectively remove the particles in raw water.Turbidity of the floated water was maintained at 1.7 NTU and the removal percentage of 2-200 ?m solid particles was about 85%.EEM measurements revealed that the macromolecular humic matters was preferentially removed by sedimentation-DAF process,while the removal of small molecular NOM components and proteins was limited.The turbidity of sedimentation and DAF effluent had linear relationship with their total Al concentration.The Al added in the coagulation process can be effectively removed by sedimentation-DAF.Prior to flotation,the second coagulation for sedimentation effluent could improve the separation of small sized particles?2-3?m?,but had no obvious effect on NOM removal.The influence of the variation of operational parameters on flotation efficiency was properly represented using the "white water bubble blanket" model.Both the model analysis and testing data proved that particles with a diameter less than 100-200 ?m was recommended to be separated using DAF.The primary odor-causing compounds,such as geosmin,2-methylisoborneol,pyridines,pyrazine,sulphides,aldehydes and ketones were effectively removed by the demonstration water treatment project.The removal of small molecular NOM,NH4+-N and small sized particles?<5?m?by sedimentation-DAF were not as well as expected.In order to solve this problem,DAF was combined with UFM and MBF respectively.A constrastive research on pollutants removal in DAF-UFM and DAF-MBF processes was conducted in this paper.The influence of DAF on the development of membrane fouling was analyzed.The removal of organic micro-pollutants in DAF-MBF processes was also examined.Experimental results showed that DAF and ultra-filtration membrane were strongly complementary to each other in the removal of different sized particles.Removal of micro-level particles by DAF effectively lowered the particles load for the subsequent membrane modules.Turbidity of both UFM and MBF membrane permeates were close to 0.1 NTU and the particles concentration were lower than 200 per milliliter.Likewise,there was nearly no pathogen detected in the membrane permeates.In MBF,the cooperation between chemical adsorption of NH4+-N in the lower zeolite layer and biological nitrification in the upper biological activated carbon?BAC?layer guaranteed the efficient removal of NH4+-N.The NH4+-N concentration in MBF effluent was kept at 0.1mg/L.The high dissolved oxygen concentration in DAF effluent could promoted the adsorption and biodegradation of organic matters by BAC.EEM measurements revealed that the residual small molecular NOM and proteins in floated water were effectively removed by MBF.In addition,the organic micro-pollutants were also effectively removed in the DAF-MBF process.Compared with UFM,the curtain membrane in MBF exhibited obviously slower fouling development.In order to enhance the organic pollutants removal in the multiphase pump DAF process,cationic polyelectrolytes were selected for bubble surface modification.The feasibility of bubble surface modification for NOM removal was evaluated.Then based on the conventional coagulation-DAF process?conv.DAF?with PACl used as coagulant,the removal of different NOM fractions and also their THMFP and HAAFP were examined with chitosan used as either a coagulant aid or a bubble surface modifier.Experimental results showed that a novel,functionalized bubble surface can be obtained by dosing hexadecyl trimethyl ammonium bromide?CTAB?,poly-dimethyl diallyl ammonium chloride?PDADMAC?and chitosan in the recycle water of DAF pump system.The zeta potentials of floated water and theFTIR analysis of float scum indicated that most of the chitosan molecules were adsorbed onto the bubble surface.Under the action of hydrophobic attraction,microbubble surface had a preferential affinity for the hydrophobic and high molecular weight NOM fractions in conv.DAF and duco.DAF processes,while the removal of hydrophilic NOM was limited.However,in posi.DAF after the p H of recycled water was adjusted from 7.0 to lower that the p Ka of chitosan molecules?i.e.,p H4.0 and p H5.5?,the increase of the positive charge density of chitosan in bubble surface enhanced the removal of hydrophilic NOM fractions.Both electrostatic attraction and the formation of hydrogen bonding networks may be involved in this process.In addition,the removal of THMFP and HAAFP were also improved with different degrees. |
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