ReviewA review of reverse osmosis membrane fouling and control strategies
Graphical abstract
Introduction
Water shortage is one of major challenges in many places around the world (Adeniji-Oloukoi et al., 2013, Avrin et al., 2015, Garcia-cuerva et al., 2016, Hibbs et al., 2016). It is exacerbated by water pollution from agricultural residues, sewage as well as industrial waste (Yao et al., 2016). In order to meet the rising demand for fresh water, strategies like water reuse and seawater desalination have already been applied (Bartman et al., 2011, Gu et al., 2013). Membrane technology is one of the most promising ways to produce high quality water (Lin et al., 2016, Ochando-Pulido et al., 2016, Tang et al., 2016b).
The common membrane technologies for water treatment include but are not limited to microfiltration (MF) (He et al., 2016), ultrafiltration (UF) (Sun et al., 2015), nanofiltration (NF) (Ribera et al., 2014), reverse osmosis (RO) (Yang et al., 2017), forward osmosis (FO) (Boo et al., 2012), membrane distillation (MD) (Bush et al., 2016), electrodialysis (ED) (Zhang et al., 2015c) and pervaporation (PV) (Subramani and Jacangelo, 2015). RO membrane technology is widely used in seawater desalination, drinking water production, brackish water treatment and wastewater treatment. RO is currently the most energy-efficient technology for desalination, with energy cost about 1.8 kWh/m3, which is much lower than that of other technologies (Xu et al., 2013b). Also, RO membrane has the advantages of high water permeability and salt rejection, fulfillment of the most rigorous rules for public health, environmental protection and separation process (López-Ramírez et al., 2006).
However, RO membrane fouling is a main challenge to reliable membrane performance. Fouling is a complicated phenomenon which involves different mechanisms under different circumstances (Khan et al., 2014). For example, a lot of RO projects reusing wastewater with high levels of phosphate are in operation worldwide (Chesters, 2009). In these plants, calcium phosphate scaling on membrane surfaces is a big problem, resulting in poor plant operation and high cleaning and maintenance cost (Chesters, 2009). Membrane fouling could significantly reduce productivity and permeate quality while increasing operation cost due to increased energy demand, additional pretreatment, foulants removal and membrane cleaning, maintenance, as well as reduction in membrane lifetime (Al-Amoudi, 2010, Eric et al., 2001, Kochkodan et al., 2014, Tang et al., 2011). In order to control membrane fouling, a variety of methods such as pretreatment, membrane monitoring, membrane cleaning, surface modification, as well as developing novel RO membranes have been studied (Al-Juboori and Yusaf, 2012, Brehant et al., 2002, Henthorne and Boysen, 2015, Nguyen et al., 2012, Robinson et al., 2016). The application of different methods could result in different control effects and therefore, in practice these techniques are usually applied together to reduce RO membrane fouling.
Statistical analysis revealed that in the last 25 years, over 3000 papers were published to address the issue of RO membrane fouling (shown in Fig. 1, see Supplementary Information for more details on statistical analysis method), indicating researchers' great interest in this area. Specifically, the number of SCI papers published in 2016 increased by around 20 times compared to papers published in 1992 and was around twice as the papers published 5 years ago (i.e., year 2011). A polynomial model was derived to describe the cumulative number of publications from 1992 to 2016, with the equation P = 0.3735 ∗ Y3 − 6.881 ∗ Y2 + 67.139 ∗ Y − 83.109 (R2 > 0.999), where P is the cumulative number of publications and Y denotes the number of years since 1992. Based on this model, and assuming that no revolutionary breakthroughs in RO membrane technology and alternative technologies as well will be made in the next ten years, then it can be predicted that by the year 2022, the cumulative number of papers published will possibly be about twice that of 2016. Although the research trend may not be predicted precisely simply by this model, it can at least give us an indication that research interest in this field will continue to bloom.
Therefore, it is necessary to provide an up-to-date review of RO fouling and its control. This paper reviews membrane fouling and fouling control strategies, with a focus on the latest advances. The first objective of this paper is to elucidate the types of fouling. The second objective is to discuss state of the art strategies for fouling mitigation, including pretreatment, monitoring, cleaning, surface modification as well as novel membrane materials and synthesis process. Especially, statistical analysis (bibliometric method) is adopted where appropriate in this review paper to reveal researchers' interest in related sub-fields. This comprehensive review may provide an avenue for future research work related to RO membrane fouling.
Section snippets
Membrane fouling
Generally fouling is the accumulation of undesired deposits on the membrane surface or inside the membrane pores, causing decrease of permeation flux and salt rejection (Malaeb and Ayoub, 2011). Since water is the operating environment for most RO applications, it is important to understand the behaviors of water as well as ion transport through RO membrane, which could indicate how RO fouling occurs. Water permeation through membrane could take place in the form of Brownian diffusion, flush
Membrane fouling control strategies
A lot of efforts have been done to address the problem of fouling. For example, through improving hydrodynamics of the filtration process, membrane fouling could be reduced. A detailed discussion on hydrodynamics can be found in another review paper (She et al., 2016) and is not the focus of this paper. Generally, the difficulty to mitigate fouling is different depending on fouling types. Inorganic scaling could be easily reduced through chemical and physical methods. In contrast, organic
Concluding remarks
RO membrane technology is one of the best technologies for wastewater treatment and desalination. Membrane fouling seems to be an inborn and inevitable problem of membrane technology. Depending on feed water qualities, operation conditions and membrane characteristics, one or several types of fouling could occur, such as biofouling, organic fouling, inorganic scaling and colloidal fouling. Although different types of foulants may have different forming processes, sometimes there are no distinct
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