Meng Yao

The role of shear conditions on floc characteristics and membrane fouling in coagulation/ultrafiltration hybrid process – the effect of flocculation duration and slow shear force

The impact of shear conditions during coagulation on the ultrafiltration permeate flux in a coagulation–ultrafiltration (C–UF) process was investigated. Different flocculation durations and slow shear forces were used to determine the impact of shear conditions on the C–UF process. Floc characteristics, including the average size, fractal dimension and flocs size distribution under different coagulation conditions were studied. Moreover, the normalized flux and reversibility of membrane fouling were also analyzed to investigate membrane fouling. The results indicated that flocs formed at a shorter flocculation duration with small fractal dimensions and easily developed a cake layer with larger pores and a fluffy structure on the membrane surface. However, a higher frequency of small-sized flocs still remained at a flocculation time of 5 min, which was attributed to inadequate collision among particles, which could cause severe membrane fouling. As a result, slow stirring for an appropriately short time (10 min) and the reduction of low shear force (G = 8 s−1) are conducive for forming flocs with a loose and porous structure without the occurrence of more small particles in water, which seemed to effectively improve the membrane permeability.

Experimental and numerical characterization of floc morphology: role of changing hydraulic retention time under flocculation mechanisms.

The formation, breakage, and re-growth of flocs were investigated by using modified flocculation tests and numerical simulation to explore the evolution of floc morphology for different hydraulic retention times. The shorter the aggregation time was, the smaller the flocs produced for the same hydraulic conditions were. Another interesting discovery was that broken flocs that formed in shorter aggregation time had the capacity to completely recover, whereas those formed in a longer amount of time had rather worse reversibility of broken flocs. With the addition of the maximum motion step in the representative two-dimensional diffusion-limited aggregation (DLA) model, there was a transition for flocs from isotropic to anisotropic as the maximum motion step increased. The strength of flocs was mainly affected by the distribution of particles near the aggregated core rather than distant particles. A simplified breakage model, which found that broken flocs provided more chances for diffused particles to access the inner parts of flocs and to be uniformly packed around the aggregated core, was first proposed. Moreover, an important result showed that the floc fragments formed with a larger value of the maximum motion step had more growing sites than did those with a smaller msa value, which was a benefit of following the re-forming procedure.

Effect of aluminum speciation on fouling mechanisms by pre-coagulation/ultrafiltration process with different NOM fractions

Ultrafiltration is an emerging technology for drinking water production, but the membrane fouling is still a challenge. This study was carried out to investigate the effect of aluminum speciation on UF membrane fouling behavior by different NOM fractions—humic substances and proteins, as represented by humic acid (HA) and bovine serum albumin (BSA), respectively. The interesting results showed that the total fouling resistance of the mixture of HA-BSA-kaolinite solution without coagulant demonstrated a slight decrease in comparison with those of the individually filtered substances, indicating a mitigatory fouling effect. The hydrolysis of aluminum products was various as pH and membrane fouling was related to aluminum speciation. The average size of flocs dramatically increased and fractal dimension of flocs decreased with the increasing of pH value independent on water quality, which indicated that aluminum speciation had a significant impact on floc properties. For the mixture of HA-BSA-kaolinte, the slightly larger of flocs average size in comparison with the individual organic fraction after coagulation was probably attributing that BSA was encapsulated by HA to enlarge the molecular length and floc size further increased. The membrane performance also showed that coagulation effluent of HA-BSA-kaolinite mitigated membrane fouling. The strong linear relationship was observed between flocs fractal dimension and final membrane flux in this research. From the results, the control of flocs fractal dimension should be considered as a new technique for traditional hybrid coagulation/ultrafiltration system, which resulted in minimized total and irreversible fouling and has a meaningful engineering application value.

Evaluation of kaolin floc characteristics during coagulation process: a case study with a continuous flow device

In this study, we used a continuous flow device to simulate actual water treatment works and explored the impact of the coagulant dosage and shear rate combination on kaolin floc properties. The results indicated that an increase in coagulant dosage contributed to larger floc size and more compact floc structure. As coagulant dosage further increased, both floc size and fractal dimension were reduced. For different shear rate combinations, the floc size during the transitional phase increased with the decrease of fractal dimension, followed by the reduction of floc size and increase of floc density in the steady state because of the breakage and restructure of flocs. Considering that the actual coagulation process aims to generate large flocs without breakage, this indicates that the time to reach a steady state for floc size is the optimal coagulation time. Thus, the optimal shear rate combination (59–48–17 s−1) is the one which reached steady state for floc size at the end of the coagulation process. In contrast to constant shear rate, decreasing shear rate enhances the floc growth, leading to similar floc properties for different shear rate combinations. This is the first time temporal evolution of floc size and structure in a continuous flow reactor has been reported.

Characterization of re-grown floc size and structure: effect of mixing conditions during floc growth, breakage and re-growth process

The impact of mixing speed in three stages—before breakage, during breakage, and after breakage—on re-grown floc properties was investigated by using a non-intrusive optical sampling and digital image analysis technique, respectively. And then, on the basis of different influence extent of mixing speed during each stage on size and structure of re-grown flocs, coagulation performance with varying mixing speed was analyzed. The results indicated that the broken flocs could not re-grow to the size before breakage in all cases. Furthermore, increasing mixing intensity contributed to the re-formation of smaller flocs with higher degree of compactness. For slow mixing before breakage, an increase in mixing speed had less influence on re-grown floc properties due to the same breakage strength during breakage, resulting in inconspicuous variation of coagulation efficiency. For rapid mixing during breakage, larger mixing speed markedly decreased the coagulation efficiency. This could be attributed that mixing speed during breakage generated greater influence on re-grown floc size. However, as slow mixing after breakage was elevated, the coagulation efficiency presented significant rise, indicating that slow mixing after breakage had more influence on re-grown floc structure upon re-structuring and re-arrangement mechanism.