Dynamic analysis on the variation of particle size distribution (PSD) and the fractal characteristics of PSD (Df) were investigated to better understand the continuous procedure of the floc growth and optimize the control of flocculation process. It was found that the flocculation process could be divided into three stages, i.e., the micro-flocculation stage, the growth stage and the steady (or breakage) stage. As the stage which is crucial to the morphology of micro-flocs (the building blocks of large flocs), the micro-flocculation stage plays an important role on flocculation/sedimentation process. The results showed that an increase in shear rate (11s-1<G<30s-1) during the micro-flocculation stage contributed to micro-flocs with larger size and more compact structure. As shear rate further increased (30s-1<G<55s-1), the micro-floc average size gently decreased from 13.61μm to 10.91μm, whereas two-dimension fractal dimension of micro-flocs gradually increased from 1.85 to 1.89. This indicated that further increase of shear rate during the micro-flocculation was incline to the formation of smaller micro-flocs with more compact structure. According to the results of final floc properties, the moderate shear rate (G=30s-1) benefited to the micro-floc formation to form final flocs with desired properties, further improved the treatment efficiency in the whole process. Based on the kinetics in the micro-flocculation stage, a conceptual model was proposed to describe the micro-floc growth under different shear rates, further revealed the reason for the different properties of final flocs under various shear rate during the micro-flocculation stage. Combining the results with model, it was concluded that shear rate during the micro-flocculation stage mainly affected final flocs by the domination of micro-floc structure. This research gives indications both for theoretical and actual works to improve the efficiency in the solid/liquid process.

Effect of additional coagulant on floc breakage and re-growth process was investigated regarding coagulation performance and floc properties. Polyaluminum chloride (PACl) and cationic polyacrylamide (PAM) were used as additional coagulants and dosed at half way of the first breakage stage. The results indicated that additional coagulant was conductive to floc re-growth after first breakage regardless of coagulant type. Moreover, additional PACl still facilitated floc re-growth after second breakage. Compared to PACl, additional PAM couldn't enhance floc re-growth after second breakage. Results of different additional dosages showed that floc recoverability after first breakage was elevated with increasing additional PACT dosage, but presented initially increasing and then decreasing trend when higher PAM dosage was added. In addition, floc recoverability after second breakage was elevated at higher dosages of additional PACl, whereas floc recoverability after second breakage with additional PAM was nearly same as that without additional PAM. It seems likely that the inactive surface points and coverage extent of broken flocs were dominant parameters for PACT addition and PAM addition, respectively. (C) 2016 Elsevier B.V. All rights reserved.

This research focused on the influence of molecular weight (MW) fraction of humic acid (HA) on ultrafiltration membrane (MW cut off of 100kDa) fouling and further investigated the impact of Al species of polyaluminum chlorides (PACls) on coagulation-ultrafiltration process. Results indicated that MW fraction of 30–50kDa generated the most severe membrane fouling. On the basis of experiment results in this research and other literatures, we proposed that reversible fouling and irreversible fouling was largely associated with floc properties and residual HA in coagulated water, respectively. This could fully explain the influence of different PACls, PACla (with high monomeric species), PAClb (with high medium polymer species) and PAClc (with slightly higher colloidal or solid species), on membrane fouling. After floc breakage and re-growth, floc size followed the order of PACla>PAClc>PAClb and the sequence of floc fractal dimension was as follows: PAClb>PACla>PAClc. Reversible fouling of PACls was consistent with floc fractal dimension, which was directly correlated with cake layer resistance. In coagulated water, HA with MW more than 50kDa had similar removal for PACls, whereas HA with MW less than 50kDa achieved the highest removal efficiency for PAClb, followed by PAClc and PACla.

This work investigates the influence of decreasing shear rate on temporal evolution of floc properties in continuous flow device.

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.