Characterization and influence of floc under different coagulation systems on ultrafiltration membrane fouling

doi:10.1016/j.chemosphere.2019.124659

Chemosphere

Volume 238, January 2020, 124659

https://doi.org/10.1016/j.chemosphere.2019.124659 Get rights and content

Highlights

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Coagulation pretreatment was effective in alleviating UF membrane fouling.
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High initial NOM concentration limited the performance of PAC and PAC-PolyDMDAAC.
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PAC + PolyDMDAAC dual coagulation system was the best in mitigating membrane fouling.
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Floc properties were closely associated with membrane fouling due to cake layer.
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Flocs formed by sweeping flocculation had large size and high strength.

Abstract

A hybrid system was developed in this study consisting of different coagulation systems and ultrafiltration (UF). Property and effect of flocs formed in different coagulation systems on ultrafiltration membrane fouling control were investigated. All three coagulation systems, as pretreatment of UF, were effective in improving membrane flux and reducing membrane resistance within an appropriate range of natural organic matters (NOM) concentration. At high initial NOM concentration, the performance of polyaluminum chloride (PAC) on NOM removal and fouling control was severely limited. For PAC-poly dimethyl diallyl ammonium chloride (PAC-PolyDMDAAC) coagulation system, the limitation of initial NOM concentration on removing NOM and alleviating membrane fouling was slightly weakened, indicating composite flocculant PAC-PolyDMDAAC produced larger flocs through combined action of charge neutralization and adsorption bridging. In PAC + PolyDMDAAC dual coagulation system, the combined action of adsorption-bridging effect, sweeping effect, and charge neutralization were the mechanisms under both low and high initial NOM concentration. Although the flocs formed in PAC + PolyDMDAAC dual coagulation system had poor recovery ability compared with those formed in PAC and PAC-PolyDMDAAC coagulation system, flocs formed through adsorption-bridging and sweeping had large size and higher ability to resist shear force, resulting in the formation of cake layer with porous and fluffy structure and less blockage in membrane pore in PAC + PolyDMDAAC dual coagulation system. These results demonstrated that dual coagulation system combined PAC coagulation and PolyDMDAAC flocculation as a pretreatment of UF process can improve the characteristics of flocs and structure of cake layer for improving NOM removal and controlling membrane fouling.

Graphical abstract

Introduction

At present, ultrafiltration (UF) is one of the most promising and reliable technologies in surface water treatment because it can effectively remove suspended particles, bacteria, viruses and dissolved organic matter from water (Rasouli et al., 2017; Liu et al., 2019). However, membrane fouling is still the main impediment limiting the wide application of UF technology because it reduces membrane flux and increases the costs for backwashing and membrane materials (Shao et al., 2014). Natural organic matters (NOM), as the main concern in the surface water, including humic substances, proteins and polysaccharides, is a heterogeneous mixture exiting at various concentrations, which have been considered as the most detrimental foulants causing membrane fouling (Wang et al., 2017). Humic acids (HA), bovine serum albumin (BSA) and sodium alginate (SA) are considered as the models of humic substances, proteins and polysaccharides in various studies. Researchers have employed various pretreatments to remove fouling materials and alleviate membrane fouling in UF process, including coagulation, adsorption and oxidation, etc (Tang et al., 2018; Cheng et al., 2019).

Coagulation, as an effective pretreatment, has been widely applied couple with UF to improve NOM removal and alleviate membrane fouling, being referred to as coagulation-ultrafiltration (C-UF) (Liu et al., 2017). The formation of cake layer in membrane surface and blockage in membrane pores were the main fouling mechanisms in UF process as a result of concentration polarization and pore plugging (Shao et al., 2016). The cake layer causes reversible fouling because it can be removed by tangential flow while pore blockage leads to irreversible fouling. The structure of cake layer and pore blockage are related to flocs characteristics. Therefore, the floc characteristics, including floc size, fractal dimension and ability to resist shear force, affect membrane fouling. Then, the floc characteristics are determined by coagulant type because of the different coagulation mechanisms including charge neutralization, adsorption-bridging, and sweep flocculation (Ly et al., 2018; Ma et al., 2018; Du et al., 2019).

Polyaluminum chloride (PAC), an Al-based polymer, has been the most extensively used coagulant due to the cost and low temperature sensibility (Zhang et al., 2018). In order to further improve the treatment performance of PAC, researchers use organic polymers as coagulant aids. In previous studies, poly dimethyl diallyl ammonium chloride (PolyDMDAAC), a positively charged organic polymer with low toxicity and high stability, has been found to achieve high NOM removal rate, large floc size and light membrane fouling when using combined with PAC at low initial concentration of NOM (Shen et al., 2017). However, some researchers found that the application of cationic polymer would cause a negative effect on membrane performance. Therefore, the impact of different coagulation systems on membrane fouling control was still uncertain. Moreover, the initial NOM concentration was also an important factor influencing treatment performance under different coagulation systems.

In the present work, three coagulation systems, PAC coagulation, PAC-PolyDMDAAC coagulation and PAC + PolyDMDAAC dual coagulation system, were employed to control UF membrane fouling under various initial NOM concentration. Firstly, the NOM removal efficiency of three coagulation systems under various initial NOM concentration were investigated. Then, to understand the mechanism of alleviating membrane fouling by three coagulation systems more comprehensively, the fouling alleviation were evaluated by membrane flux and membrane resistances, and the mechanism of membrane fouling control were analyzed by characteristics of flocs and cake layer. This work will give deep understanding into the effect of flocs on membrane fouling control and guide the application of coagulation-ultrafiltration in surface water treatment under various NOM concentration.

Section snippets

Raw water and flocculant preparation

Synthetic water was prepared to simulated typical surface water. HA, BSA and SA were purchased from Sigma-Aldrich (USA) and added to local (Qingdao, China) tap water in a mass ratio of 10:3:3. To more realistically simulate surface water, Kaolin clay (Sinopharm Chemical Reagent Co., China) was added at a concentration of 10 mg/L as the model of suspension. The characteristics of the synthetic raw water with different NOM concentrations (as HA concentration) of 0, 5, 10, 20, 50 and 100 mg/L are

NOM removal

NOM removal at different initial NOM concentrations were determined in terms of DOC removal efficiency and zeta potential. Coagulant PAC was applied in this section and the dosage of 6.0 mg/L was used according to the preliminary experimental results discussed in Supplementary material (Fig. S1). Fig. 1 (a) showed that turbidity removal exceeded 80% at the NOM concentration of 0–20 mg/L, and then decreased with the increase of NOM concentration, even became negative at a NOM concentration of

Discussion

The properties of flocs play a key role in controlling membrane fouling because membrane fouling was induced by cake layer and pore blockage which were determined by the properties of flocs (Jermann et al., 2007; Wang and Tarabara, 2008). To understand the mechanism of membrane fouling control by three coagulation systems, the characteristics of cake layer and the properties of flocs, including size, structure, breakage and reformation, were investigated. Fig. 5 showed the SEM images of the

Conclusion

This study focused on the effects of floc characteristics on the mitigation of UF membrane fouling by various coagulation systems at different initial NOM concentration. The key findings can be summarized as follows:

(1)
Application of coagulation as a pretreatment effectively mitigated UF membrane fouling, especially PAC + PolyDMDAAC dual coagulation system performed better in NOM removal and membrane fouling control than PAC and PAC-PolyDMDAAC coagulation system at the same initial NOM

Declarations of interest

The authors declare that they have no conflict of interests.

Acknowledgments

This study was supported by the National Natural Science Foundation of China (No. 51678349) and the Taishan Scholar Foundation of Shandong Province (No. ts201511003). The thoughtful suggestions from the anonymous reviewers are gratefully acknowledged.

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