Flocculation properties of cohesive fine-grained sediment in the Three Gorges Reservoir under variable turbulent shear

Sediment flocculation is a key process for the deposition of fine-grained sediments in the Three Gorges Reservoir(TGR)of China.Sediment flocculation influences the evolution of the river regime,but also hampers the smooth navigation in the long term.However,the flocculation process and its controlling factors are poorly understood.We experimentally determined the flocculation properties of cohesive sediment of samples from the TGR(predominantly a mixture of clay and silt)over a range of turbulent shear rates and sediment concentrations.The experiments were conducted in an almost isotropic turbulence field,which was simulated by an array of horizontal oscillating grids in a water tank.Sediment flocculation was recorded by a camera and investigated by image analysis.Our new data indicate that flocculation is generally a response in equilibrium median floc size(d_(f,50))to the increase of the shear rate G.The peak value is attained at G=16.5 s^(-1),where d_(f,50) is 81.3μm(for the suspended sediment concentration(ssc)=0.4 g/L)and 107μm(for ssc=0.7 g/L),respectively.At low shear rates(G<16.5 s^(-1)),the equilibrium floc sizes d_(f,50) increase with rising shear rate G and isinversely related to the Kolmogorov micro length scale η.We attribute this variability to an insufficient deposition time of the sediment flocs in the water tank.Settling velocities,calculated from our experimental data of the floc sizes,are almost consistent with in-situ measured settling velocities,and are ten times larger than the terminal settling velocity of primary particles as calculated from Stokes'law.

Coagulation behavior and floc properties of compound bioflocculant–polyaluminum chloride dual-coagulants and polymeric aluminum in low temperature surface water treatment

This study was intended to compare coagulation behavior and floc properties of two dualcoagulants polyaluminum chloride–compound bioflocculant（PAC–CBF）（PAC dose first） and compound bioflocculant–polyaluminum chloride（CBF–PAC）（CBF dose first） with those of PAC alone in low temperature drinking water treatment. Results showed that dualcoagulants could improve DOC removal efficiency from 30% up to 34%. Moreover, CBF contributed to the increase of floc size and growth rate, especially those of PAC–CBF were almost twice bigger than those of PAC. However, dual-coagulants formed looser and weaker flocs with lower breakage factors in which fractal dimension of PAC–CBF flocs was low which indicates a looser floc structure. The floc recovery ability was in the following order：PAC–CBF 〉 PAC alone 〉 CBF–PAC. The flocculation mechanism of PAC was charge neutralization and enmeshment, meanwhile the negatively charged CBF added absorption and bridging effect.

New insights into membrane fouling in coagulation-ultrafiltration by cake characteristics analysis:A case study with PACl-Al_(13)and PACl

Membrane fouling is a bottleneck issue that hindered the further application of ultrafiltration technology.To alleviate membrane fouling,coagulation-ultrafiltration(C-UF)process using polyaluminum chloride(PACl)and PACl-Al_(13)with high proportion of Al_(13)O_(4)(OH)_(24)^(7+)as coagulants,respectively,were investigated at various pH conditions.Results indicated that an increase in solution pH contributed to larger floc size and looser floc structure for both PACl and PACl-Al_(13).It was conducive to the formation of more porous cake,as evidenced by mean pore area and pore area distribution of cake,leading to lower reversible fouling.Furthermore,humic acid(HA)removal presented a trend of first increasing and then decreasing with the increase of pH.The optimal HA removal was achieved at pH 6 regardless of coagulant type,suggesting that the slightest irreversible fouling should be occurred at this point.Interestingly,the irreversible fouling with PACl coagulant achieved a minimum value at pH 9,while the minimal irreversible fouling with PACl-Al_(13)was observed at pH 6.We speculated that the cake formed by PACl could further intercept HA prior to UF process at alkaline pH.Furthermore,compared with PACl,PACl-Al_(13)had a stronger charge neutralization ability,thus contributing to more compact floc structure and higher HA removal at various pH conditions.By UF fractionation measurement,higher HA removal for PACl-Al_(13)was due to higher removal of HA with molecular weight less than 50 kDa.

Surface water treatment benefits from the presence of algae:Influence of algae on the coagulation behavior of polytitanium chloride

Titanium-based coagulation has proved to be effective for algae-laden micro-polluted water purification processes.However,the influence of algae inclusion in surface water treatment by titanium coagulation is barely reported.This study reports the influence of both Microcystis aeruginosa and Microcystis wesenbergii in surface water during polytitanium coagulation.Jar tests were performed to evaluate coagulation performance using both algae-free(controlled)and algae-laden water samples,and floc properties were studied using a laser diffraction particle size analyzer for online monitoring.Results show that polytitanium coagulation can be highly effective in algae separation,removing up to 98%from surface water.Additionally,the presence of algae enhanced organic matter removal by up to 30%compared to controlled water containing only organic matter.Polytitanium coagulation achieved significant removal of fluorescent organic materials and organic matter with a wide range of molecular weight distribution(693–4945 Da)even in the presence of algae species in surface water.The presence of algae cells and/or algal organic matter is likely to function as an additional coagulant or flocculation aid,assisting polytitanium coagulation through adsorption and bridging effects.Although the dominant coagulation mechanisms with polytitanium coagulant were influenced by the coagulant dosage and initial solution pH,algae species in surface water could enhance the charge neutralization capability of the polytitanium coagulant.Algae-rich flocs were also more prone to breakage with strength factors approximately 10%lower than those of algae-free flocs.Loose structure of the flocs will require careful handling of the flocs during coagulation-sedimentation-filtration processes.

Polytitanium sulfate(PTS):Coagulation application and Ti species detection

Interest in the development of inorganic polymerized coagulants is growing; however, there are only limited studies on the synthesis of polytitanium coagulants, which are expected to exhibit improved coagulation efficiency with better floc properties. This study presents the synthesis of polytitanium sulfate（PTS） for potential application in water purification,followed by characterization of PTS flocs and titanium species detection. Stable PTS solutions were successfully synthesized and standard jar tests were conducted to evaluate their coagulation efficiency. Electrospray ionization time-of-flight mass spectrometry（ESI-TOF-MS） speciation analysis revealed that a variety of mononuclear and polynuclear complexes were formed in PTS solution, indicating the polymeric nature of the synthesized coagulant. Floc characteristics were studied through on-line monitoring of floc size using a laser diffraction particle size analyzer. Results showed that PTS had a comparable or in some cases even higher organic matter and particulate removal efficiency than Ti（SO4）2.The effluent p H after PTS coagulation significantly improved toward desirable values closer to neutral p H. Properties of flocs formed by PTS were significantly improved in terms of floc size, growth rate and structure. This study showed that PTS could be an efficient and promising coagulant for water purification, with the additional benefit that its coagulated sludge can be used to recover valuable TiO2 nanoparticles for various commercial applications.