Cross-flow microfiltration of aggregated submicron particles

Modeling the Transition between Localized and Extended Deposition in Flow Networks through Packings of Glass Beads

We use a theoretical model to explore how fluid dynamics, in particular, the pressure gradient and wall shear stress in a channel, affect the deposition of particles flowing in a microfluidic network. Experiments on transport of colloidal particles in pressure-driven systems of packed beads have shown that at lower pressure drop, particles deposit locally at the inlet, while at higher pressure drop, they deposit uniformly along the direction of flow. We develop a mathematical model and use agent-based simulations to capture these essential qualitative features observed in experiments. We explore the deposition profile over a two-dimensional phase diagram defined in terms of the pressure and shear stress threshold, and show that two distinct phases exist. We explain this apparent phase transition by drawing an analogy to simple one-dimensional mass-aggregation models in which the phase transition is calculated analytically.

Impacts of composite flocculant in coagulation/ultrafiltration hybrid process for treatment of humic acid water: the role of basicity

The effects of the composite flocculant, polyaluminium chloride and poly dimethyldiallylammonium chloride (PACl-PDMDAAC) in comparison with PACl on coagulation efficiencies and membrane fouling in coagulation-ultrafiltration (C-UF) process were analysed, which was conducted in the conditions of different basicity (B) values and the presence of Mg²⁺. Results showed that PACl-PDMDAAC enhanced the ability of charge neutralization and absorption bridging, and improved the coagulation efficiency. When B value was 1.5, the flocculant hydrolyzed to form more Al_b morphology and effectively removed HA molecules. The presence of Mg²⁺ could improve the coagulation performance through bridging ability. The results of the ultrafiltration test showed that the flux reduction for PACl was 70%, while the flux reduction for PACl-PDMDAAC was 60% in C-UF process. PACl-PDMDAAC could effectively reduce membrane fouling mainly by reducing strongly attached cake/gel layer. When B value was 1.5, the Al_b content of the flocculant was higher and the ability of adsorption charge neutralization was strong, resulting in forming a stable cake layer. Therefore, the membrane fouling was the lightest. In addition, the presence of Mg²⁺ in raw water reduced the membrane fouling.

Effect of the dosage ratio and the viscosity of PAC/PDMDAAC on coagulation performance and membrane fouling in a hybrid coagulation-ultrafiltration process

This study systematically determined the optimal dosage ratio and the viscosity (η) of co-coagulants, polyaluminum chloride (PAC) and poly dimethyldiallylammonium chloride (PDMDAAC), on coagulation performance and membrane fouling in a hybrid coagulation-ultrafiltration (C-UF) process for natural organic matter (NOM) removal. Floc characteristics-including floc size, fractal dimension, strength and re-growth ability-were studied with respect to coagulant-dosing operations. Membrane fouling was evaluated in association with assessment of NOM removal performance by the hybrid process. The best coagulation performance was achieved when PAC and PDMDAAC were dosed with 1.0 mg/L and 0.1 mg/L, respectively. The addition of PDMDAAC could enhance the NOM removal efficiency, especially at low PAC dosages. Co-coagulants PAC/PDMDAAC (η_PDMDAAC = 2.18 dL/g) resulted in formation of the largest flocs with the smallest D_f under all shear conditions, while the flocs formed by PAC/PDMDAAC (η_PDMDAAC = 1.86 dL/g) had higher recovery abilities. The results from ultrafiltration experiments indicated that coagulation using PAC/PDMDAAC with a viscosity range from 0.99 dL/g to 1.86 dL/g can significantly reduce membrane fouling, leading to increasing water fluxes from 0.1170 to 0.4906 in the ultrafiltration process.

Effects of cake collapse caused by deposition of fractal aggregates on pressure drop during ceramic filtration

A cake collapse model was developed by taking the combined effects of fractal dimension, relaxation ratio, coordination number, and aggregate diameter into consideration. The cake porosity including intraaggregate and interaggregate porosities was modeled successively by three typical coordination numbers (n = 6, 8, and 12). Accordingly, an inversion method made it possible to deduce the coordination number using the measured cake porosities, and the reverse-calculated value with minimum error and the corresponding relaxation ratios were applied as the parameters for the model. As a result, the profiles of intraaggregate and interaggregate porosities and cake porosity were respectively predicted in contrast to the integrated variation of the relaxation ratio and the fractal dimension. Furthermore, a comparison between the model predictions of the cake pressure drop gradients with and without aggregate compression was conducted to validate the presence of cake collapse. The results show that the predictions based on the proposed collapse model are in agreement with the experiments, and the coordination number is one of the key factors that must be incorporated into the cake collapse models.

Use of chemical coagulants to control fouling potential for wastewater membrane bioreactor processes

Chemical coagulation with ferric chloride, alum, and an organic polymer were used to control the fouling potential of mixed liquors for submerged membrane bioreactor (MBR) processes in treating municipal wastewater. Their filterability was evaluated using a submerged hollow fiber ultrafiltration apparatus operated in constant permeate flux mode. The collected transmembrane pressures over filtration time were used to calculate the membrane fouling rates. The results showed that coagulation pretreatment can reduce fouling rates when MBRs were operated above the critical flux. Even though coagulation with the organic polymer formed larger mixed liquor suspended solids particles and had shorter time-to-filtration than those with ferric chloride and alum, the filterability for membrane filtration were similar, indicating that the membrane fouling in MBR systems was mainly controlled by the concentration of smaller colloidal particles.

Permeability of collapsed cakes formed by deposition of fractal aggregates upon membrane filtration

We have investigated, theoretically, the physical properties of cake layers formed from aggregates to obtain a better understanding of membrane systems used in conjunction with coagulation/flocculation pretreatment. We developed a model based on fractal theory and incorporated a cake collapse effect to predict the porosity and permeability of the cake layers. The floc size, fractal dimension, and transmembrane pressure were main parameters that we used in these model calculations. We performed experiments using a batch cell device and a confocal laser-scanning microscope to verify the predicted specific cake resistances and porosities under various conditions. Based on the results of the model, the reduction in inter-aggregate porosity is more important than that in intra-aggregate porosity during the cake collapsing process. The specific cake resistance decreases upon increasing the aggregate size and decreasing the fractal dimensions. The modeled porosities and specific cake resistances of the collapsed cake layer agreed reasonably well with those obtained experimentally.

In-line coagulation with low-pressure membrane filtration

The objectives were to investigate the effects of in-line coagulation on removal of natural organic matter (NOM) and turbidity and on fouling of membranes during ultrafiltration (UF). Coagulants were added prior to UF, without intermediate use of conventional processes for removal of particles. Synthetic and natural raw waters were examined. Charge neutralization, sweep floc, and under-dose conditions (with respect to conventional treatment) were all effective for removal of NOM and turbidity by UF. Most coagulation conditions resulted in decreased resistance to filtration and improved hydraulic removal of filter cake, including conditions that would be ineffective prior to conventional settling and rapid-filtration. Flocs that were produced under charge neutralization conditions and acidic under-dose conditions produced lower hydraulic resistance and were less compressible than for sweep floc conditions.