Membrane fouling by Fe-Humic cake layers in nano-scale: Effect of in-situ formed Fe(III) coagulant
Graphical abstract
Highlights
► A lower development of TMP was formed by in-situ formed Fe(III) than dosed Fe(III). ► Both Fe(III) flocs were composed by nano-scale primary particles. ► The size of primary particles formed by in-situ Fe(III) was larger. ► The in-situ Fe(III) flocs formed a thinner cake layer than dosed Fe flocs. ► In-situ Fe(III) coagulant achieved greater NOM removal than dosed Fe(III).
Introduction
Associated with the dramatic increase in the application of low-pressure membrane filtration in drinking water treatment in the past decade [1], [2] is the continuing problem of membrane fouling, causing higher energy. A number of studies have reported that humic substances, as the main component of natural organic matter (NOM), play an important role in membrane fouling [3], [4], [5]. An effective method of removing humic substances is by the application of hydrolyzing coagulants at pH values near to neutral, where dissolved organic substances can be adsorbed on to the amorphous hydroxide precipitate (‘sweep flocculation’) and then physically separated by sedimentation and/or filtration [6]. Therefore, a combined process train comprising either microfiltration (MF) or ultrafiltration (UF) and pre-coagulation, especially in-line coagulation, is particularly advantageous since it is expected to combine satisfactory operational efficiencies by controlling membrane fouling and the necessary treatment performance [7], [8], [9], [10], [11], [12]. In this arrangement both the coagulation conditions and type of coagulant are expected to influence the membrane fouling and treatment performance.
Cho et al. [13] observed that coagulation with a rapid mix followed by a slow mix gave lower specific cake resistance than that with only a rapid mix for a submerged MF membrane. The formation of loose and porous flocs and the reduction of small colloidal particles at longer flocculation time have been shown to lead to a higher membrane flux [13], [14], [15], which indicates that the inclusion of a flocculation process after rapid mixing is a preferred arrangement.
The influence of the type of coagulant used has been widely reported, where the nature of the hydrolysis products and their interaction with contaminants determines the structural characteristics of the resulting aggregates and their porosity, which in turn affects the efficiency of filtration [16], [17], [18], [19], [20]. Thus, the permeate flux has been shown to depend on the coagulant used, as illustrated by one study which found some coagulants had no influence on permeate flux, another gave a 20% increase in flux, while another coagulant gave a 50% decrease in flux [8]. Wang et al. [21] showed that MF membrane flux deteriorated much more severely when pre-coagulated by aluminum chlorohydrate (ACH) and polyaluminum chloride (PACl), than by alum. For iron coagulants, Shon et al. [22] reported that adding FeCl3 significantly improved membrane fouling in wastewater treatment, and another study considered the feasibility of applying polymeric ferric sulfate (PFS) to mitigate membrane fouling in a long-term running MBR [23]. A comparison of three conventional coagulants showed that the relative treatment performance of organic matter by coagulation-UF was in the order Al2(SO4)3>Fe2(SO4)3>FeCl3 [12], and the comparative flux decrease was Fe2(SO4)3<Al2(SO4)3<FeCl3.
In recent years, the application of combined oxidants and coagulants has been the subject of growing research interest in water treatment involving either separate chemicals, such as ozone and alum [24] or permanganate and alum/PAC [25], or dual role single chemicals, such as ferrate [26]. In this arrangement the impact of the oxidation on the contaminants and coagulants species significantly influences the nature of the resulting coagulant flocs produced. For the case of a combination of separate chemicals, the combining of an oxidant with a Fe(II) salt allows the Fe(III) coagulant species to be formed in-situ through competitive oxidation of the Fe(II) to Fe(III). This arrangement is advantageous since the cost of Fe(II) salts may be considerably lower than Fe(III) salts (as in the UK), and the action of the oxidant can achieve other water quality objectives (e.g. pre-disinfection and micro-pollutant degradation). Previous studies have indicated that in-situ formed Fe(III) was more effective than dosed Fe(III) [e.g., Fe2(SO4)3] in removing contaminants such as arsenic [27] and phosphate [28]. Most recently, in-situ formed Fe(III), produced from the oxidation of Fe(II) by KMnO4 and dissolved oxygen, was found to be a major reason for the high degree of treatment of the KMnO4–Fe(II) process with respect to the removal of Microcystis aeruginosa [29].
To-date, there has been little reported in the literature concerning the effectiveness of in-situ formed Fe(III) as a pretreatment coagulant prior to ultrafiltration in drinking water treatment. Therefore, in this paper we present some new results concerning the effectiveness of coagulation before ultrafiltration with in-situ formed Fe(III), produced by combining FeSO4 and KMnO4 (Fe/Mn-CUF), in comparison with a typical ferric salt, Fe2(SO4)3 (Fe-CUF). The impact and mechanisms involved of both pretreatment methods on membrane fouling are described with reference to the floc properties and the nature of the cake layer formed on the membrane surface. The results obtained in this study are expected to provide a better understanding of the effectiveness of in-situ formed Fe(III) for controlling membrane fouling and improving organic matter removal.
Section snippets
Synthetic raw water and coagulant
A synthetic raw water was chosen for the tests in order to simplify the study since natural surface water has difficulties associated with sample consistency and reproducibility. The raw water used in this study was a mixture of tap water (Beijing, China) and a humic acid solution at a concentration of 5 mg/L. For the latter, a mass of 5 g of humic acid, sodium salt (HA, Aldrich, Cat: H1, 675-2), was dissolved in deionized (DI) water, with pH adjusted to 7.5, and under constant mixing for 24 h.
Zeta potential
In order to understand the influence of pH on the zeta potential of flocs, measurements were made in the pH range of 5 to 9. Overall, the zeta potential of these two types of flocs decreased as the pH increased from 5 to 9, especially from 5 to 7. The zeta potential of Fe/Mn flocs was slightly lower than the Fe flocs, except those at pH 7, but the difference was very small. As shown in Table 1, the solution pH after addition of the coagulants was approximately 7.3, and at this pH the zeta
Conclusions
- 1.
The lower development of TMP with in-situ Fe(III) coagulant compared with dosed Fe(III) indicated a significantly lower membrane fouling rate that was associated with a reduced thickness and lower density of cake layer on the ultrafiltration membrane.
- 2.
The in-situ Fe(III) coagulant produced larger flocs than the pre-dosed Fe(III), which after settling in the UF tank left smaller residual flocs in the UF influent; these had a lower degree of compaction (density) than those from the dosed Fe(III)
Acknowledgments
This work was supported by National Natural Science Foundation of China (Grants 51138008 and 51108444). Also some of this work was supported by the National Science & Technology Project in Countryside (2012BAJ25B00).
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