Fouling in reverse osmosis membranes: monitoring, characterization, mitigation strategies and future directions

Abstract

Water scarcity has been a global challenge for many countries over the past decades, and as a result, reverse osmosis (RO) has emerged as a promising and cost-effective tool for water desalination and wastewater remediation. Currently, RO accounts for >65% of the worldwide desalination capacity; however, membrane fouling is a major issue in RO processes. Fouling reduces the membrane's lifespan and permeability, while also increases the operating pressure and chemical cleaning frequency. Overall, fouling reduces the quality and quantity of desalinated water, and thus hinders the sustainable application of RO membranes by disturbing its efficacy and economic aspects. Fouling arises from various physicochemical interactions between water pollutants and membrane materials leading to foulants' accumulation onto the membrane surfaces and/or inside the membrane pores. The current review illustrates the main types of particulates, organic, inorganic and biological foulants, along with the major factors affecting its formation and development. Moreover, the currently used monitoring methods, characterization techniques and the potential mitigation strategies of membrane fouling are reviewed. Further, the still-faced challenges and the future research on RO membrane fouling are addressed.

Keywords: Characterization of foulants; Fouling monitoring and mitigation; Membrane modification; RO-Membranes.

Fig. 1

Projected water scarcity in 2025 [4].

Fig. 2

Global desalination capacity (a) according to the used technology, (b) according to feed water source [239], and (c) Improvement in power consumption for reverse osmosis seawater desalination over the past decades [240].

Fig. 3

Number of publications during the last 22 years using SciFinder data, (search words used: fouling and reverse osmosis).

Fig. 4

Factors affecting RO membrane fouling [241].

Fig. 5

Possible mechanisms of fouling; (a) Pore-clogging, (b) partial pore-clogging, (c) internal pore-clogging, and (d) cake formation [172].

Fig. 6

Mechanism of biofouling development [147].

Fig. 7

Schematic illustration of essential steps in the scales formation [242].

Fig. 8

(a–c)_SEM images of cross-sections of three membranes [41], (d, e) SEM images illustrating the presence of abrasion marks on membrane surfaces [89].

Fig. 9

AFM data of (a) virgin, (b) physically, and (c) chemically damaged membranes [102].

Fig. 10

ATR-FTIR spectra of two fouled membranes and the foulant materials [88]. Membrane A is mainly fouled by aluminosilicates and silica; membrane B is fouled by biogenic materials of proteins, aminosugars, DNA/RNA/free phosphate/phospholipid, sugars and lipids.

Fig. 11

(a) Location of EEM peaks (symbols) for five EEM regions [120], (b–d) 3D FEEM of DOM in: (b) feed water, (c) DAF effluent, (d) UF effluent, and (e) RO effluent [91].

Fig. 12

A schematic diagram of RO unit with conventional and UF pretreatments [243].

Fig. 13

SDI paper (a)before and (b) after using coagulant [137].

Fig. 14

Pollutants that can be eliminated by membrane processes [244].

Fig. 15

Flow direction in (a) forward flushing [178], (b) reverse flushing [178], and (c) backwashing [245].

Fig. 16

Mechanism of salt cleaning [196].

Fig. 17

Schematic illustration of the synthesis nanosilver - thin film nanocomposite membrane [246].