Moderate KMnO4-Fe(II) pre-oxidation for alleviating ultrafiltration membrane fouling by algae during drinking water treatment
Graphical abstract
Introduction
The worldwide occurrence of algal blooms in water has caused great concern among the public. The penetration of algal cells and release of odorous and toxic substances frequently cause water quality to deteriorate (Li et al., 2012; Wert et al., 2014). Algal organic matter (AOM) serves as a precursor for the formation of disinfection by-products (DBPs) during disinfection (Wei et al., 2011; Zhou et al., 2014). As a result, the presence of algae poses a substantial risk to aquatic health, and therefore, it is vital that algae is removed effectively during water treatment.
Due to the negative surface charge, high motility, diverse morphology, and low specific density of algae, traditional coagulation treatment cannot achieve a satisfactory removal efficiency (Ma et al., 2007; Takaara et al., 2010). Retained algae can be nourished in treatment reactors, and sedimentation and filtration tanks, and microorganisms can be nourished by the algal cells that have penetrated filtration tanks through sparge pipes and secondary water supply facilities, such as service pipes (Plummer and Edzwald, 2001). As a result, the residual chlorine at the end of the pipe network is gradually depleted, further nourishing the microorganisms and deteriorating water quality (Ma et al., 2012).
To prevent these adverse effects, various pretreatment methods to enhance the removal efficiency of algae during water treatment have been investigated, including increasing the coagulant dosage, electrocoagulation, air flotation, pre-chlorination/oxidation, and ultrafiltration (UF). Although increasing the coagulant dosage can improve the algal removal efficiency, the dosage usually requires a dual fold increase (Ma et al., 2012). For electrocoagulation, the frequent changing polarity and electrode passivation limit its application on a wide scale (Mollah et al., 2004). Most water treatment plants lack the appropriate equipment for air floatation. Thus, they require expensive reconstruction (Ma et al., 2012). Pre-chlorination/oxidation (i.e., Cl2, O3) has a considerable advantage as it inactivates algae, however, algal cells can easily be damaged by excessive oxidant dosages, resulting in the release of intracellular organic matter (IOM) and increased effluent water risk (Garzon-Sanabria et al., 2013).
To overcome these disadvantages, we developed a novel moderate pre-oxidation method (KMnO4-Fe(II) process) that achieves a balance between the release of IOM and a high algal removal efficiency (Ma et al., 2012, 2014; Qi et al., 2016a, 2016b). We demonstrated that algae (Microcystis aeruginosa) are moderately inactivated by the sequential introduction of KMnO4 and Fe(II). The subsequent dose of Fe(II) not only effectively halts the KMnO4 reaction, but also further transforms into in situ Fe(III), which has a larger reactive surface area than traditional preformed Fe(III). A lower concentration of Fe results in the effluent as a by-product of the KMnO4-Fe(II) process than as a by-product of Fe(II) coagulation (Ma et al., 2012). Based on cell integrity and residual manganese, the optimized molar ratio of KMnO4 to Fe(II) was 1:3, and the appropriate dosing interval time was 5 min. In addition, the suspension ability of the flocs formed during the KMnO4-Fe(II) process is high, resulting in low deposition and rendering the process suitable for long-term transportation pipelines (Qi et al., 2016b).
The use of UF membranes in drinking water treatment is a promising technology and has continued to expand owing the excellent resultant water quality and small-scale land use (Huang et al., 2009; Drioli et al., 2011; Miao et al., 2017). These membranes can completely remove micro algae during filtration as their pores have small diameters (Mouchet and Bonnelye, 1998; Liang et al., 2008; Slade and Bauen, 2013; Miao et al., 2014). However, the deposition of algal cells and extracellular organic matter (EOM) on UF membrane surfaces causes serious fouling, resulting in the formation of a dense cake layer. This is independent of the pore diameter and surface hydrophilicity/hydrophobicity of the membrane (Liang et al., 2008; Li et al., 2014; Qu et al., 2014).
To date, three different processes have been investigated to alleviate membrane fouling, including pre-adsorption, direct-filtration and integrated filtration (Ma et al., 2017). The integrated filtration process is considered particularly advantageous in water treatment as it requires little land and exhibits a high removal efficiency during operation (Zhang et al., 2016; Ma et al., 2017). To better resolve algae-related water problems, our novel KMnO4-Fe(II) pretreatment process was integrated with UF membranes; KMnO4 and Fe(II) were sequentially injected into the membrane tank, and FeCl3·6H2O was also directly injected for comparison.
To fully investigate the practicality of this integrated process, raw water was collected from the Miyun Reservoir (N:40°29′; E:116°49′), which is the main drinking water source for Beijing, China. Owing to the reduced water quality and high water temperature during summer, the reservoir faces the risk of algal pollution (Su et al., 2015, 2017). In this study, we aimed to: 1) investigate the UF membrane fouling behavior during the KMnO4-Fe(II) process in the presence of algae-laden water; 2) compare the performance of the UF membrane between the KMnO4-Fe(II) process and FeCl3·6H2O coagulation; and 3) evaluate the practicality of the integrated UF process after long-term operation based on effluent water quality parameters, including its turbidity, chromaticity, and Mn and Fe concentrations.
Section snippets
Characteristics of raw water
The raw water was used immediately after collection from the Miyun Reservoir, and its specific properties are presented in Table 1. The concentration of algae was measured by a flow cytometer (Accrui C6, BD Biosciences, USA), and ranged from 1.99 × 107 to 2.67 × 107 cells/L. Table 2 shows the proportions of the different algal species, and Microcystis was found to be predominant.
KMnO4-Fe(II) process optimization tests
All chemical reagents were of analytical grade (Sinopharm Chemical Regent Beijing Co., Ltd, China), and included KMnO4
Dosage optimization of Fe(II)
According to previous studies (Ma et al., 2012, 2014; Qi et al., 2016a, 2016b), the optimized molar ratio of KMnO4 to Fe(II) to maintain cell integrity and control residual manganese in effluent is 1:3. Thus, different dosages of Fe(II) were initially used to investigate the UF membrane fouling at a RKMnO4:Fe(II) of 1:3. As shown in Fig. 1, severe UF membrane fouling occurred with raw water alone, and J/J0 was significantly reduced to 0.42 after 500 s of operation. However, membrane fouling was
Discussion
Previous studies have demonstrated that pore constriction and pore blocking, and cake layer formation are the main causes of UF membrane fouling (Wang and Tarabara, 2008). To investigate the membrane performance without coagulation, the particle size distribution of raw water was investigated (Fig. 6), and ranged from 0.15 to 2.5 μm. The particle size of raw water was much larger than the average membrane pore diameter (30 nm), indicating that there was little chance for pore blocking. However,
Conclusions
To help resolve algae-related water problems, a novel KMnO4-Fe(II) pretreatment method was integrated with a UF membrane, and compared to Fe(III) coagulation. We found that the UF membrane was seriously fouled without pretreatment, while fouling was alleviated on coagulation, especially with moderate pre-oxidation.
The algae exhibited a strong adsorption ability to the membrane's surface; however, the cake layer formed by the flocs was loose, resulting in many of the original pores remaining
Acknowledgements
This study was supported by the National Key R&D Program of China (2016YFC0400802), National Natural Science Foundation for Young Scientists of China (51608514), and Special Fund from the Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (Project No. 17Z03KLDWST).
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