The impacts of various operating conditions on submerged membrane photocatalytic reactors (SMPR) for organic pollutant separation and degradation: a review

Abstract

The rapid expansion and development of membrane based wastewater treatment in recent decades have led to the emerging technology of submerged membrane photocatalytic reactors (SMPR), which exhibit not only a lower degree of fouling but are also capable of separating and degrading organic pollutants simultaneously during the treatment process. This review intends to provide an update on the influence of several key operational parameters, i.e. photocatalyst loading (both suspended and immobilized), feed pH and concentration, wavelength and intensity of UV light, membrane module packing density and air bubble flow rate on the efficiencies of SMPR in treating degradable organic pollutants. The structure and properties of the photocatalytic membrane as well as membrane performance stability under UV irradiation are also discussed. Understanding the effect of each operational parameter is of paramount importance towards achieving optimum SMPR performance and addressing the challenges encountered in the development of SMPR. Strategies and approaches are also recommended in this review to overcome the persistent problems and facilitate the research and development of SMPR.

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C. S. Ong

Dr Ong Chi Siang obtained his B.Eng in Chemical Engineering from Universiti Tunku Abdul Rahman (UTAR) (September 2011) and a PhD in Gas Engineering from the Universiti Teknologi Malaysia (UTM) (August 2015). Currently, he is a lecturer in the Department of Chemical Engineering, Faculty of Built and Environmental Engineering at SEGi University, Kota Damansara, Malaysia. During his PhD studies, he has managed to publish 7 ISI-indexed scientific papers. Dr Ong has a very strong research interest in the development of ultrafiltration and nanofiltration for water and wastewater treatment processes. Dr Ong is also the graduate member of Institute of Engineers, Malaysia (IEM) and an Associate Member of the Institution of Chemical Engineers (IChemE).

W. J. Lau

Dr Lau Woei Jye obtained both his B.Eng. in Chemical-Gas Engineering (September 2006) and PhD in Chemical Engineering (September 2009) from the Universiti Teknologi Malaysia (UTM). Currently, he is a senior lecturer in the Department of Renewable Energy Engineering, Faculty of Chemical and Energy Engineering and a Research Fellow at the Advanced Membrane Technology Research Centre (AMTEC), UTM. Before working at the UTM, he worked as an assistant professor at the Universiti Tunku Abdul Rahman (UTAR), Kuala Lumpur. Dr Lau has a very strong research interest in the field of water and wastewater treatment processes using membrane-based technology. As of September 2015, he has published more than 60 ISI-indexed scientific papers, 1 book and 3 book chapters. Dr Lau is also a recipient of the Australian Endeavour Research Fellowship 2015 and an Associate Member of the Institution of Chemical Engineers (IChemE).

P. S. Goh

Goh Pei Sean is a Senior Lecturer in the Faculty of Chemical and Energy Engineering at Universiti Teknologi Malaysia (UTM). She holds her undergraduate and Master's Degree in Chemistry from the UTM, where she received her PhD degree in Gas Engineering in 2012. Her research interests lie in the field of fabrication of nanostructured materials for membrane-based separation processes. One of the main focuses of her research is the applications of carbon-based nanomaterials and nanocomposite membranes for acidic gas removal as well as water and waste water treatment. Owing to her years of experience and research findings in the field of material science and engineering, Pei Sean has authored and co-authored more than 35 research papers in leading high impact international journals.

B. C. Ng

Mr Ng Be Cheer holds a B.Sc. in Polymer Engineering, M.Eng in Gas Engineering from Universiti Teknologi Malaysia (UTM) and is currently doing a PhD in Gas Engineering at Universiti Teknologi Malaysia. He is working at the Advanced Membrane Technology Research Center (AMTEC), UTM, Malaysia, as a Research Officer. He has over 15 years experience in the development of membrane technology and membrane systems for reverse osmosis, nanofiltration, ultrafiltration, membrane contactor and gas separation. He has been appointed as the Project Leader for some collaboration projects with industries in membrane technology development. The development of nanofibers and carbon nanostructured materials for energy related application is also of special interest. He is the co-author of more than 30 refereed publications in international and national journals. He is also the co-inventor of 13 patent disclosures in Malaysian Patent in membrane system and design.

A. F. Ismail

Prof. Ahmad Fauzi Ismail holds a B.Sc. in Petroleum Engineering, M.Sc. in Chemical Engineering from Universiti Teknologi Malaysia (UTM) and PhD in Chemical and Process Engineering from University of Strathclyde, UK under the Association of Commonwealth Universities Scholarship. He is the Founder and Director of the Advanced Membrane Technology Research Center (AMTEC), UTM, Malaysia. Currently he is the Deputy Vice Chancellor (Research & Innovation), UTM. He has over 21 years experience in the development of membrane technology for reverse osmosis, nanofiltration, ultrafiltration, membrane contactor and gas separation. The development of nanofibers and carbon nanostructured materials for energy related applications is also of special interest. He is the author of over 400 refereed publications. He has also authored 6 books, 25 book chapters and 3 edited books. The latest outstanding awards are the Merdeka Award 2014, IChemE Malaysia Awards 2014 for Innovation and Excellence, Malaysian Toray Award 2014, and Malaysian Academic Award 2013 (Innovation & Commercialization Category).

C. M. Choo

Ir. Choo Chee Ming obtained his B.Eng in Chemical Engineering from Universiti Putra Malaysia and further obtained a Master's degree in Environmental Engineering from Griffith University, Australia. Ir. Choo is currently the Head of the Chemical Engineering Discipline and Senior lecturer at SEGi University, Kota Damansara. Prior to joining the university, he worked as senior project manager at the CH2M HILL of Malaysia for the past 10 years. He has a lot of hands-on experience in the design, construction, operation and maintenance of industrial wastewater treatment systems, and soil and groundwater remediation systems in different countries. He is affiliated with the Chartered Professional Engineers Australia, APEC Engineer (Chemical), International Professional Engineers (Chemical), ASEAN Engineers (Chemical), Professional Chemical Engineers Malaysia and Board of Engineers Malaysia. He was appointed to the Engineering Accreditation Council (EAC) Evaluation Panel for the Chemical Engineering Degree Program Accreditation, Board of Engineers Malaysia. He was also the Principal Interviewer for the Professional Assessment Examination (Chemical Engineering) at the Institution of Engineers Malaysia (IEM). He is currently the Deputy Chairman of Environmental Engineering Technical Division, IEM.

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1. Introduction

Water scarcity has been increasingly recognised as a serious and growing concern, where the deteriorating quality and growing demand for clean water sources have created a significant challenge around the world. This issue has been further exacerbated by rapid population growth, industrialization and boom in commercial activities that demand large volumes of clean water resources. According to the World Health Organization (WHO), there are approximately 750 million people around the world who do not have access to improved sources of drinking water.¹ In view of this, numerous attempts have been undertaken for water and wastewater treatment to provide access to clean and safe water. With their outstanding attributes, photocatalytic membrane reactor (PMR) is highly considered as an alternative for water and wastewater treatment. In brief, PMR is a hybrid system that couples photocatalysis and a membrane process in one unit, as illustrated in Fig. 1. The photocatalysis process allows organic pollutants to be decomposed and mineralized to simple substances such as water (H₂O), carbon dioxide (CO₂) and mineral salts; moreover, the membrane enables the separation of photocatalyst from the reaction medium for further reuse. In addition, the membrane could serve as a barrier for initial compounds and by-products formed during decomposition, which prevents them from passing through to the permeate side.

Fig. 1 Schematic diagram of a laboratory-scale PMR with zinc oxide (ZnO) photocatalysts in suspension.²

There are numerous configurations of PMRs being pursued by researchers. These include pressurized and depressurized (submerged) systems that work in either batch or continuous mode. The pressurized PMR system can achieve a higher flux compared to the submerged system due to the higher driving force applied to the system. However, the major drawbacks of the pressurized PMR system are the decline of permeation rate as a function of filtration time coupled with a higher degree of surface fouling caused by remaining pollutants and/or suspended catalysts in the feed solution. The impact of suspended catalysts on membrane flux has been previously investigated by several research groups and their results have shown that suspended nanoparticles in the PMR system have a negative effect on membrane permeability.^3–5 In this regards, the depressurized PMR system, also known as the submerged membrane photocatalytic reactor (SMPR), has been generally agreed as an alternative approach to overcome the problems encountered by the pressurized system. Compared to pressurized conventional PMRs, the most notable advantages of SMPR are its (1) possibility of operating at high flux with relatively low energy consumption and fouling tendency and (2) enhanced mass transfer between UV light and targeted pollutants (for greater photodegradation efficiency) due to the generation of air bubbles in the system.¹

In PMRs, the catalyst can be either immobilized on/in a membrane or suspended in the reaction mixture. Two main configurations for PMR are generally pursued, namely, (i) a reactor with catalysts suspended in the feed solution and (ii) a reactor with catalysts immobilized in/on the membrane. The advantages and disadvantages of suspended and immobilized titanium dioxide (TiO₂) catalysts are summarized in Table 1. Although a suspended system offers a more intensive treatment, it requires an additional process to separate the catalysts from the suspended reactor. Many methods have been proposed to improve the recovery of the suspended catalysts. Some of them are (1) improving the catalysts aggregation through pH adjustment and (2) enhancing the separation of magnetic catalysts in a magnetic field. Nevertheless, these methods were only able to enhance the sedimentation and facilitate the recovery of catalyst in the batch system.^6–9

Table 1 Comparison between TiO₂ suspended and immobilized reactor

TiO2 suspended reactor	TiO2 immobilized reactor
Advantages	Advantages
1 Higher photocatalytic area to reactor volume ratio	1 No separation and recycle of the catalyst
2 Higher mass transfer and degradation efficiency	2 Less membrane fouling due to enhanced hydrophilicity of membrane surface upon nanoparticle incorporation
3 Fairly uniform catalyst distribution	3 Pollutants could be degraded either in feed or in permeate
4 Adjustable amount of nanoparticle suspension in the reactor to deal with different compositions of treated solution
Disadvantages	Disadvantages
1 Higher operating cost and requires additional treatment after degradation process	1 Degradation efficiency is lower than that of suspension mode
	2 Impossible to adjust the catalyst loading to deal with different compositions of treated solution
	3 Replacement of membrane is required when catalyst loses its activity

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The membrane photocatalytic reactor has attracted considerable attention among membrane scientists mainly due to its powerful and efficient capability for degrading and mineralizing recalcitrant compounds under vacuum pressure conditions. This great achievement was initiated by Molinari in 2002,¹⁰ and presently the SMPR has been extensively applied for the purification and disinfection of contaminated groundwater, surface water, and wastewater that contain recalcitrant, inhibitory, and toxic compounds with low biodegradability.

A comprehensive overview and detailed discussion on the development of PMR have been previously published in several review articles.^11–13 A broad range of subject matters, such as fundamental mechanism, reactor configurations, operational parameters, kinetics and modelling, and water quality analysis as well as the related life cycle assessment, have also been covered, but are mainly focused on the development between 1990 and 2010. Moreover, a brief description on the types of photocatalytic membranes, PMR configuration and potential applications could be found in several recently published books.^14–17 However, the rapid expansion and increasing demand of employing SMPR in water and wastewater treatment over the past several years have motivated us to provide an update on the recent progresses of SMPR applications. The most significant contribution of this review is to highlight and emphasize the importance of operating conditions on the performance of SMPR followed by a discussion on the existing challenges and strategies that can be implemented to heighten the current performance of SMPR.

2. Factors affecting SMPR performance

In this section, a brief review on the performance of SMPR with different operating parameters for water and wastewater treatment is presented. The influences of various operating parameters on the photodegradation efficiency and membrane separation performance of SMPR are highlighted. Several operating parameters, such as catalyst loading, UV light wavelength and its intensity, feed concentration and pH, module packing density, and air bubble flow rate (ABFR), are discussed. Table 2 summarizes several important findings on SMPRs that were previously reported for treating different types of organic pollutants. The photocatalytic activities of the system are found to be closely associated with the operating parameters, as mentioned before. Thus, it is of great significance to study the correlation between these parameters to fully understand their impacts on the SMPR performance.

Table 2 The influence of various SMPR operating conditions on photodegradation performances in the SMPR system

Type of photocatalyst/max loading	Membrane configuration/polymer^a	Operating conditions	System/reactor volume	Targeted pollutant(s)^b	Initial concentration/range of concentration	Type of UV/power intensity	Time required to degrade at least 50% of pollutants (h)	Ref.
a PTFE – polytetrafluoroethylene, PVDF – polyvinylidene fluoride, PEI – polyethylenimine and PP – polypropylene.b RB5 – reactive black 5, HA – humic acid, CBZ – carbamazepine, AO7 – acid orange 7, DFC – diclofenac, TrOC – trace organic compound, AR1 – acid red 1 azo dye.
Degussa P25 TiO2/0.5 g L^−1	Flat sheet/PTFE	Submerged membrane with TiO2 catalyst suspension ABFR: 1.5 L min^−1, pH 6.4–6.9	3 L	RB5 dye	125 ppm	UVC (15 W, intensity: N/A)	∼120 min (with 0.5 g L^−1 TiO2 at 100 ppm)	Damodar et al.^25
Degussa P25 TiO2/1 g L^−1	Hollow fiber/N/A	Submerged membrane with TiO2 catalyst suspension. ABFR: null; Pohang seawater: pH 8.1; Masan seawater: pH 8.2 Mooncheon lake water: pH 7.98	800 mL	Two seawater sources (from city of Pohang and city of Masan) and Mooncheon lake water	Pohang seawater: 0.198 ppm (TOC), Masan seawater: 2.03 ppm (TOC) Mooncheon lake water: 4.91 ppm (TOC)	UVA (8 W, intensity: N/A)	Mooncheon lake water: ∼90 min (with 1 g L^−1 TiO2 at 4.91 ppm); no significant TOC reduction for Pohang and Masan seawater	Kim et al.^26
Degussa P25 TiO2/0.6 g L^−1	Hollow fiber/polypropylene (PP) or PVDF	Submerged membrane with TiO2 catalyst suspension. ABFR: 5 L min^−1, pH: N/A	4 L	HA	TOC: 10 ppm	UVA (8 W, intensity: N/A)	∼15 min (with 0.6 g L^−1 TiO2 at 10 ppm)	Halim et al.^27
Degussa P25 TiO2/0.5 g L^−1	Flat sheet/PVDF	Flat submerged membrane with TiO2 suspension. ABFR: 4 L min^−1, pH 7	8 L	HA	DOC: 10 ppm	UVC (16 W, 1.17 mW cm^−2)	∼30 min (with 0.5 g L^−1 TiO2 at 10 ppm)	Yong et al.^28
Degussa P25 TiO2/1.5 g L^−1	Hollow fiber/PVDF	Submerged membrane with TiO2 catalyst suspension. ABFR: 20 L min^−1, pH: N/A	9 L	Polysaccharides	TOC: 2.5 ppm	Three UV-A (30 W, 8.3 mW cm^−2)	<360 min (with 0.5 g L^−1 TiO2 at 2.09 ppm)	Sarasidis et al.^29
Degussa P25 TiO2/1 g L^−1	Hollow fiber/PVDF	Submerged hollow fiber membrane with P25 TiO2 suspension, pH: N/A	1 L	Carbamazepine (CBZ)	5 ppm	240 units of vis-LED (<0.5 W m^−2)	∼120 min (with 1 g L^−1 TiO2 at 5 ppm)	Wang et al.^30
Degussa P25 TiO2/18 wt%	Hollow fiber/polyethylenimine (PEI)	Submerged membrane embedded with TiO2 catalyst; ABFR and pH: N/A	25 mL	Acid orange 7 (AO7)	20 ppm	Four UVA (8 W, intensity: N/A)	<60 min (with 18 wt% TiO2 at 20 ppm)	Zhang et al.^31
Degussa P25 TiO2/0.75 g L^−1	Hollow fiber/PVDF	Submerged membrane with TiO2 catalyst suspension ABFR: 5 L min^−1, pH 6.8	3 L	Diclofenac (DFC)	2.5 ppm	Four UVA (24 W, 14.4 mW cm^−2)	<300 min (with 0.5 g L^−1 TiO2 at 2.5 ppm)	Sarasidis et al.^23
Degussa P25 TiO2/0.5 g L^−1	Hollow fiber/PVDF	Submerged membrane with TiO2 catalyst suspension ABFR: 1.2 L min^−1, pH 3, 8	4 L	Trace organic compound (TrOC)	0.5 ppm	Seven UVA (8 W, intensity: N/A)	<240 min (with 0.5 g L^−1 TiO2 at 0.5 ppm)	Fernández et al.^32
Anatase TiO2/2.0 g L^−1	Hollow fiber/polypropylene (PP)	Submerged membrane with TiO2 catalyst suspension ABFR: 3 L min^−1, pH 3, 7 and 10	5 L	Acid red 1 azo dye (AR1)	15–75 ppm	UVC (8 W, 62.91 mW cm^−2)	∼80 min (with 0.5 g L^−1 TiO2 at 15 ppm)	Kertèsz et al.^33
Degussa P25 TiO2/4 wt%	Hollow fiber/PVDF	Submerged membrane embedded with TiO2 catalyst ABFR: 1–5 L min^−1, pH 7	14 L	Oil molecules	250–10 000 ppm	UVA (8 W/0.333 mW cm^−2)	∼60 min (with 2 wt% TiO2 at 1000 ppm)	Ong et al.^34
TiO2–ZrO2/0.15 g L^−1	Hollow fiber/PVDF	Submerged membrane with TiO2 catalyst suspension. ABFR: 4 L min^−1, pH 4	2 L	HA	50 ppm	UVC (4 W, intensity: N/A)	<240 min (with 0.15 g L^−1 TiO2–ZrO2 at 50 ppm)	Khan et al.^35

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2.1 Catalyst loading

Catalyst loading is one of the key operating parameters that could affect the photocatalytic oxidation rate. The amount of photocatalyst used in the process is directly proportional to the reaction rate.¹¹ The principal mechanism of photocatalytic degradation is described as follows. When a photocatalytic surface is exposed to a radiation of energy equal to or greater than the bandgap energy, it will create a positively charged hole in the valance band and negatively charged electron in the conduction band by exciting the electrons in the valance band to the conduction band. The conduction band electron reduces oxygen into O₂⁻, which can be adsorbed by the photocatalyst surface, whereas the positively charged hole oxidizes either organic pollutants directly or indirectly using water to produce hydroxyl free radicals (HO˙). These generated species act as a powerful oxidizer to disintegrate harmful organic pollutants in wastewater and convert them into CO₂ and H₂O. When the photocatalyst loading is increased, there is an increase in the number of active surface sites available for adsorption and degradation. However, the excessive use of photocatalyst would increase solution opacity (in the case of the photocatalyst suspension), which reduces UV light penetration in the reactor.^18–20 Moreover, the loss in surface area by catalyst agglomeration at high photocatalyst loading (for both the catalyst suspension and immobilization case) is likely to cause UV light scattering and deterioration of the overall photocatalytic performance.^21–23 Therefore, any chosen photoreactor should be operated below the saturation level of the photocatalyst to ensure efficient photon absorption. With respect to the membrane flux performance, it is generally reported that an increase of photocatalyst loading tends to negatively affect permeate flux, mainly because of the catalyst agglomeration on the membrane surface, which creates additional transport resistance to water molecules.^15,24

Furthermore, the performance of SMPR at an optimum catalyst loading can also be influenced by the dimension of the photoreactor. This is because different reactor designs tend to have different water flow hydrodynamics and photon absorption rates.¹¹ A large reactor volume usually has a lower saturated catalyst loading and lower efficiency compared to a small reactor. A considerable amount of studies have reported the effect of TiO₂ loading on the process efficiency, as summarized in Table 3.^18–21 The observed discrepancies in photodegradation and membrane performance can be attributed to the differences in the reactor configurations, light sources and contaminant properties as well as the interaction between operating conditions employed. To avoid an excessive use of photocatalysts and to ensure the highest photodegradation efficiency, it is very important to determine the optimum catalyst loading based on the characteristics of wastewater.

Table 3 Effect of TiO₂ catalyst loading on SMPR photodegradation performance

Targeted pollutant	Photocatalyst	Range of catalyst loadings	Optimum catalyst loading	Light source, power and intensity	Ref.
Fulvic acid	Suspended	0–0.6 g L^−1	0.5 g L^−1	UVC 11 W, 0.75 mW cm^−2	18
Bisphenol-A	Suspended	0.2–2 g L^−1	0.5 g L^−1	UVA 8 W, intensity: N/A	19
Biologically treated sewage effluent	Suspended	0.5–1.0 g L^−1	1 g L^−1	UVA 10 W, 46.15–276.96 mW cm^−2	21
Polysaccharide	Suspended	0.25–1.5 g L^−1	0.5 g L^−1	Three UV-A 30 W, 8.3 mW cm^−2	23
Acid red 1	Suspended	0.01–2 g L^−1	0.5 g L^−1	UVC 8 W, 62.91 mW cm^−2	33
Oily wastewater	Immobilized	0–4 wt%	2 wt%	UVA 8 W, 0.333 mW cm^−2	34
Carbamazepine	Suspended	0.3–1 g L^−1	1 g L^−1	240 units of vis-LED with intensity <0.5 W m^−2	30

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2.2 Feed concentration

As reported in the literature, the degradation rate of targeted pollutants is mainly influenced by the initial concentration of pollutants. The Langmuir–Hinshelwood model as expressed in eqn (1) describes the relationship between organic compound concentration and its photodegradation rate.where k_r is the intrinsic rate constant (mg L⁻¹ min⁻¹) and K_ad is the adsorption equilibrium constant (L mg⁻¹). When the adsorption is relatively weak and/or the concentration of organic compound is low, eqn (1) can be simplified to first-order kinetics with an apparent rate constant, k_app (min⁻¹), as shown in eqn (2).C₀ is the initial concentration of organic pollutant (mg L⁻¹), C is the final concentration of the pollutant after time t of the photocatalytic decomposition (mg L⁻¹), k_app is the apparent rate constant of a pseudo first order reaction (min⁻¹) and t is the time of photocatalysis (min). According to eqn (2), the reaction rate is expected to increase with irradiation time due to the decreasing amount of contaminants.¹¹

Table 4 summarizes the findings from previous studies on feed concentration, in which optimum PMR performance can be achieved. According to Kertèsz et al.,³³ as the dye concentration increases, the color of the irradiated solution becomes more intensive and concentrated, which in turn affects the penetration depth of UV light. The dye molecules also tend to absorb part of the light photons (UV-screening effect of the dye itself), which leads to insufficient photon energy for hydroxyl radical generation.^33,36 Furthermore, the thick fouling layer formed at a high feed concentration would adversely affect photocatalytic degradation due to less active surface sites for UV irradiation. This is further supported by a recent study, where the degradation of oil under UV irradiation was highly efficient at low concentrations (see Fig. 2).²⁴

Table 4 Effect of feed concentration on the photodegradation performance of SMPR that used TiO₂ as the photocatalyst

Targeted pollutants	Range of feed concentration (ppm)	Feed concentration at which optimum performance was achieved (ppm)	Light source, power and intensity	Ref.
a The concentration of the organic components was determined based on total organic carbon (TOC).
Bisphenol-A	10–50	10	Four UVA 8 W, intensity: N/A	37
Humic acid	1–10	1	UVA 8 W, intensity: N/A	38
Biologically treated sewage effluent	0–100	50	UVA 10 W, 46.15–276.96 mW cm^−2	21
Acid red 1	15–75	15	UVC 8 W, 62.91 mW cm^−2	33
Synthetic oily wastewater	250–10 000	250	UVA 8 W, 0.333 mW cm^−2	34
Carbamazepine with humic acid	1.5–14.5^a	5	240 units of vis-LED 15–60 W, <0.5 W m^−2	30
Humic acid	5–50^a	10	UVA 8 W, intensity: N/A	27

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Fig. 2 Effect of feed concentration on TOC degradation of PVDF–TiO₂ composite membrane in the SMPR system (operating conditions: temperature = 25 °C, membrane type: PVDF with 2 wt% TiO₂, module packing density = 35.3%, vacuum pump flow rate = 15 mL min⁻¹ and pH = 7).²⁴

On the contrary, Halim et al.²⁷ reported that photocatalytic efficiency increased with increasing initial total organic carbon (TOC) concentration of HA from 5 to 50 ppm, due to the larger amount of reactants anticipated in the reaction mixture, which led to more frequent collisions between the organic molecules and the catalyst particles and higher adsorption rate. As implied in the abovementioned studies, PMR membranes achieve optimum performance depending on the characteristics of the targeted pollutants.

2.3 Module packing density

Very few studies have been devoted to investigate the impacts of module packing density on permeate flux. The voids among the fibers not only act as a water flowing pathway but also facilitate mass transfer between the feed and membrane surface.^39–43 Although a high packing density of small-diameter hollow fibers can contribute to a high filtration surface area, at the same time it promotes severe inter-fiber fouling due to the unfavorable hydrodynamic conditions within the fibers.^44,45

One recent study has reported that permeate flux increased with increasing the module packing density from 17.6% to 35.3%, due to the enhanced mass transfer between the water molecules and membrane surface (see Fig. 3). However, when the module packing density was further increased to 52.9%, the fibers were likely to attach to each other and this resulted in limited spaces available between adjacent fibers, thus reducing the active surface sites for UV irradiation. This as a consequence, deteriorated both photodegradation and membrane flux. This is in agreement with the experimental study conducted by Kiat et al.,⁴⁶ where severe fouling tended to occur when the packing density of the module exceeded a critical value, i.e. 30.8%. Under an optimized packing density, Yeo et al.⁴¹ reported that promising membrane permeability could be achieved. However, it must be pointed out that a high density of fiber packing could cause foulant accumulation inside the fiber bundle and adversely decrease permeability. Contradictory results were reported by Gunther et al.,⁴⁴ where the water flux increased by 25% when the fiber packing density was decreased from 80% to 40%. This enhancement was attributed to the suppression of cake layer formation when a low packing density of hollow fiber membranes was used.

Fig. 3 Effect of module packing density on (a) permeate flux and oil rejection and (b) TOC degradation of PVDF–TiO₂ composite membrane in the SMPR system (operating conditions: temperature = 25 °C, membrane type: PVDF with 2 wt% TiO₂, vacuum pump flow rate = 15 mL min⁻¹ and pH = 7).²⁴

Wu and Chen⁴⁷ investigated the effect of flow distribution on shell-side mass transfer performance in randomly packed hollow fiber modules. They observed that the mass transfer coefficient was rapidly decreased with increasing packing density until 50% of the total volume fraction, and a further increase in packing density tended to increase the mass transfer coefficient. They attributed the improved mass transfer coefficient to the better orientation of the hollow fibers, which facilitated the water flowing through the adjacent fibers. As can be seen, the optimum packing density varies depending on the feed solution properties, module dimension and membrane material. Extensive investigations on this subject are certainly needed to provide a better understanding on how the module packing density governs the efficiency of photodegradation.

2.4 Air bubble flow rate (ABFR)

Air bubble flow is able to alleviate the fouling problem based on the generation of circulation flow, which enhances the mixing of pollutants and restricts their attachment on the membrane surface. When air bubbling scours the membrane surface, it can detach the deposited cake layer on the membrane surface and thus increase membrane water flux. With respect to photocatalytic activity, higher photocatalytic degradation can be achieved when a higher ABFR is applied in the SMPR system. Briefly, the detailed mechanism can be explained by the following pathways. OH˙ radicals are generated from an abundance of oxygen bubbles as a result of higher ABFR (eqn (3) & (4)) and also from the dissociation of water molecules upon UV illumination (eqn (5)). These OH˙ radicals mineralize the hydrocarbon groups in the organic-based wastewater to become CO₂ and H₂O (eqn (6)). Fig. 4 illustrates the mechanism of the photocatalytic reaction in the presence of air bubbles.

With air flow (O₂ = ∼78%) under 365 nm irradiation:⁴⁸

Ozone (O₃) → O (¹D) + H₂O → 2OH˙ (formation of hydroxyl radicals) (4)

Oil + 3OH˙ → xCO₂ + yH₂O (mineralization) (6)

Fig. 4 Schematic diagram of photocatalytic reaction in the presence of photocatalyst TiO₂ and air bubbles.⁴⁹

The ABFR plays an important role in the photocatalytic reaction because the supplied oxygen could provide sufficient electron scavengers to trap excited conduction band electrons from recombination.¹¹ The flux improvement at a higher ABFR is likely due to the generation of circulation flow in the SMPR, which limits the adsorption of pollutants onto the membrane surface and further reduces the membrane fouling tendency. The large airflow might enhance the mass transfer inside the treated wastewater and more OH⁻ radicals would be produced in the feed solution, which in turn results in a higher photodegradation efficiency and membrane water flux. However, when excessive air bubbles are present in the system, the adsorption reaction of targeted pollutants onto the suspended catalyst is greatly reduced, which may result in a lower degradation rate because photocatalytic oxidation is a surface-oriented reaction. Chin et al.¹⁹ investigated the degradation of a TiO₂ suspended submerged membrane reactor by varying the ABFR from 0.2 to 4 L min⁻¹. The photocatalytic reaction rate increased with an increase in bubbling rate and reached a maximum value at a bubbling rate of 0.5 L min⁻¹. Bubbling can not only increase the liquid film mass transfer coefficient around the aggregates, but may also provide a bubble cloud that can attenuate UV light transmission in the photoreactor. The balance of the competing effects between mass transfer and light attenuation might lead to an optimal bubbling rate, which can achieve the highest photodegradation efficiency.³⁰

2.5 Feed pH

In SMPR, the initial pH of the solution is of importance because it dictates the surface charge properties of the particles that affect the sorption of targeted compounds on the catalyst surface. Many studies have discovered that pH affects the interaction between targeted compounds and membrane surface. As mentioned in the review article published in 2010,¹² the effect of pH on the photodegradation of organic pollutants is associated with (1) the ionization state of the photocatalyst surface, (2) position of the valence and conduction bands of the photocatalyst, (3) agglomeration of photocatalyst particles and (4) formation of hydroxyl radicals. Wang et al.³⁰ investigated the degradation of CBZ at different pH using synthesized C–N–S tridoped TiO₂ nanoparticles. Their experimental results indicated that a higher CBZ degradation rate could be achieved under alkaline conditions as compared to that under acidic conditions. This was attributed to the formation of more OH˙ radicals at higher pH, leading to higher CBZ photocatalytic degradation. It is commonly found that in an alkaline solution, OH˙ can be generated more easily by the oxidation of more hydroxide ions on the TiO₂ surface.^19,30,50 Fig. 5 illustrates how the presence of OH⁻ ions could improve the efficiency of this process.

Fig. 5 Proposed mechanism of CBZ degradation with HA in a TiO₂ suspension under vis-LED irradiation.³⁰

It is worth mentioning that the concentration of OH˙ might increase with the increasing concentration of H⁺ in the acidic condition.^18,51,52 This is in agreement with the study conducted by Chin et al.,³⁷ where BPA degradation at a low pH was remarkably higher than that at a high pH. As pH was increased, the TiO₂ surface became progressively more negative and this led to the development of greater repulsive forces between the TiO₂ surface and BPA compounds, thus retarding the total degradation efficiency. Similarly, Khan et al.⁵³ found that HA degradation was two times higher at a low pH compared to that obtained at a high pH (see Fig. 6). This is most likely due to the stronger electrostatic attractions between the HA and photocatalyst TiO₂–ZrO₂ in an acidic environment. Table 5 summarizes the effects of pH on the photodegradation of various pollutants using the SMPR treatment process. It is revealed that organic molecules tend to have different photocatalytic reactivities in different pH environments, depending on the nature of the respective pollutants.

Fig. 6 Effects of pH solution on UV₂₅₄ removal efficiency [HA] = 50 mg L⁻¹, temperature = 28 °C using TiO₂–ZrO₂ particles.⁵³

Table 5 Influence of pH on the photocatalytic degradation of PMR using TiO₂ as photocatalyst

Targeted pollutants	Tested pH value	Optimum pH	Light source and its intensity	Ref.
FA	3.4, 6.5, 8.2 and 10.3	3.4	UVC 11 W, 0.75 mW cm^−2	18
BPA	4, 7 and 10	4	UVA 8 W, intensity: N/A	37
AR1	3, 7 and 11	11	UVC 8 W, 62.91 mW cm^−2	33
HA	4, 7 and 10	4	UVC 4 W, intensity: N/A	53
CBZ	3, 6, 9 and 12	12	240 units of visible LED with intensity <0.5 W m^−2	30

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2.6 Light wavelength and intensity

UV wavelength has a significant effect on photocatalytic reactivity. For UV irradiation, its corresponding electromagnetic spectrum can be classified into UV-A (315–400 nm; 3.10–3.94 eV), UV-B (280–315 nm; 3.94–4.43 eV) and UV-C (100–280 nm; 4.43–12.4 eV).¹¹ According to a study, only 5% of the total irradiated natural sunlight has sufficient energy to initiate effective photodegradation.⁵⁴ The need for continuous illumination for an efficient photocatalytic process has diverted solar utilization to an artificial UV lamp-driven process.

In particular, light intensity is one of the few parameters that affect the degree of photocatalytic reaction on organic substrates. A relatively abundant light intensity is required to adequately provide the catalyst surface active sites with sufficient photons energy. The photoactivity of catalysts in the presence of UV wavelength (<400 nm) in many studies obeys the linear proportional correlation to the incident radiant flux and becomes steady at excessive radiant flux in the photoreactor. This phenomenon was observed by Ho and his co-workers,²¹ where the organic matters from biologically treated sewage effluent (BTSE) was treated using an SMPR system. As shown in Fig. 7, a similar tendency of photocatalytic degradation was observed irrespective of light intensity, where the degradation rate dropped significantly for the first 30 min of operation followed by an almost constant rate when all the particles absorbed photons and produced electron–hole pairs. Considering the energy consumption, the minimum intensity of 46.61 mW cm⁻² was determined as the optimum intensity.

Fig. 7 Effect of UV light intensity on the photooxidation process (operating conditions: initial TOC = 12.47 mg L⁻¹; TiO₂ concentration = 1.0 g L⁻¹).²¹

Recently, Wang et al.³⁰ also reported the enhancement of CBZ photocatalytic degradation efficiency with an increase in visible-light intensity. Compared to 68% removal of CBZ under high intensity UV irradiation, only 28% of CBZ was degraded at low intensity UV irradiation. Furthermore, Kertèsz et al.³³ compared the decolorization of an aqueous acid red 1 (AR1) solution under two different UV light wavelengths (254 nm and 366 nm). As can be seen from Fig. 8, no decolorization of the aqueous AR1 solution occurred in the absence of a catalyst using a lower UV wavelength. Complete decolorization of AR1 could only be achieved with the simultaneous presence of a catalyst and UV irradiation. Faster initial degradation was observed at 254 nm compared to 366 nm, but at the end of the irradiation experiments (at 90 min), the two decolorizations were almost equal. Fujishima et al.,⁵⁵ however, indicated that the initiation of photocatalysis reaction rates is not highly dependent on light intensity because very few photons of energy can sufficiently induce the surface reaction. Based on these findings, it can be concluded that the impact of different operating parameters on PMR performance is very complicated and an optimum condition for a specified application should be selected on the basis of several preliminary studies with similar operational parameters.

Fig. 8 Influence of UV irradiation wavelength on the decolorization process (operating conditions: C₀ = 15 mg L⁻¹, C_TiO2 = 0.5 g L⁻¹, pH = 7, T = 25 °C, n = 400 rpm, I_UV = 62.9 mW cm⁻²).³³

2.7 Structure and properties of photocatalyst

Apart from the operating parameters, it is also important to understand the photoactivity of selected catalysts, which is dependent on the surface and structural properties of the photocatalyst such as crystal composition, surface area, particle size distribution, porosity and band gap energy.⁵⁶ Among these properties, band gap energy is the main criteria for a catalyst to be selected. Several studies have compared the photocatalytic activities of different semiconductors in the process of degrading aqueous pollutants. For instance, Miyauchi et al.⁵⁷ studied the effect of different oxides (e.g. TiO₂, SnO₂, ZnO, WO₃, SrTiO₃, V₂O₅, CeO₂, CuO, MoO₃, Fe₂O₃, Cr₂O₃ and In₂O₃) on the degradation rate of methylene blue (MB) adsorbed on a thin film surface. Among these, TiO₂, SrTiO₃ and ZnO exhibited the highest photodegradation of MB under UV illumination followed by SnO₂, which showed relatively low photoactivity. The rest of oxides were found to be inactive for MB degradation. They attributed the results to the different band gap energies of the photocatalysts, as shown in Fig. 9. When a photocatalyst possesses a higher band gap energy, more photon energy is required to promote the electron from valence band to conduction band, thus reducing the photodegradation efficiency. On the other hand, Khalil and co-workers⁵⁸ evaluated the efficiency of TiO₂, ZnO and WO₃ over the photodegradation of aqueous Cr(VI) and found that the photodegradation of Cr(VI) followed the pattern of TiO₂ > ZnO > WO₃. These findings were in agreement with the study of Miyauchi et al.⁵⁷

Fig. 9 Redox potentials of the valence and conduction bands as well as band-gap energies for various metal oxides at pH 7. The redox potential positions of H⁺/H₂ and OH˙/OH⁻ at pH 7 are also illustrated (*NHE: Normal Hydrogen Electrode).⁵⁷

Furthermore, Luisa et al.⁵⁹ compared the photodegradation efficiency of diphenhydramine (DP) and methylene orange (MO) by incorporating three different photocatalysts, i.e. self-synthesized TiO₂, graphene oxide combined with TiO₂ (GO–TiO₂) and commercial P25–TiO₂ into a flat sheet membrane. The results indicated that the membranes prepared with the GO–TiO₂ composite exhibited the highest photocatalytic activity, due to the lowest band gap energy coupled with highest surface area, as compared to the TiO₂ catalyst. The current research priority of catalyst development is focused on reducing the band gap energy of particles, in addition to increasing the surface area, with the aim to achieve a faster degradation rate.

2.8 Membrane performance stability

In general, there is a possible deterioration of membrane performance by the formation of hydroxyl radicals and/or byproducts from the partial degradation of pollutants under UV irradiation. The impact is greater particularly for membranes made of polymeric materials. Mozia et al.⁶⁰ previously evaluated the influence of process conditions and photocatalyst type on the stability of four commercial ultrafiltration membranes, which were made of polyethersulfone, with respect to flux and dextran removal rate. They found that the effect of reactive oxygen species on the stability of the membrane was not significant as compared to the photocatalyst particle itself (for the suspension case). For the photocatalyst suspension case, the membrane surface was possibly damaged by the suspended photocatalysts in the feed when a large air flow was applied in the reactor. It was suggested that focus should be placed on the enhancement of membrane stability rather than on the abrasion caused by the photocatalyst. Furthermore, transmembrane pressure was found to have significant influence on the membrane stability. Cross-flow velocity, meanwhile, has very little impact on membrane damage throughout the treatment process. Chin et al.⁶¹ and Molinari et al.⁶² also reported that suspended TiO₂ in the SMPR system reduced the lifespan of the photocatalytic membrane. With respect to pH, Mendret et al.⁶³ showed that AO7 organic compounds were greatly adsorbed on the membrane surface under acidic conditions due to the electrostatic attraction, which caused rapid fouling on the membrane surface and a reduced water flux.

3. Challenges in SMPR development

The excitement and great benefits harnessed from the SMPR development have spurred great interest in the wastewater industry sectors. To ensure the constant and steady development of these innovative and sustainable technologies, several key technical constraints, which range from photocatalytic membrane properties to process operating conditions, should be carefully addressed. The current photocatalytic membranes are still facing challenges where the contact area between the photocatalyst, targeted pollutant and UV light is lower than that in the catalyst suspension case. More effort needs to be devoted to improve the photocatalytic degradation of the catalyst immobilized membrane. Moreover, the mechanical strength and durability of materials remain as a challenge, which affects photodegradation performance and limit their wide-scale application.

From energy point of view, utilization of renewable solar energy is attractive in water industry. Although several visible light induced photocatalysts have been developed,^64–67 most of these photocatalytic membranes need to be initiated under UV irradiation. The effectiveness of catalysts within the membrane matrix is jeopardized by lower photocatalytic degradation and wide band gap energy. It is necessary to develop a novel catalyst that possesses a small band gap energy, high surface area, and excellent resistance against thermal shock. On the other hand, the transformation of organic pollutants might cause a variety of organic intermediates that can be toxic and more persistent than the original pollutants themselves.^68,69 Therefore, attention also needs to be paid to understand the adverse effects caused by the harmful by-products generated from the partial degradation of organic pollutants. Besides the targeted pollutants, inorganic impurities that cannot be degraded by photocatalytic membranes might accumulate on the membrane surface and unfavourably reduce the photocatalytic activity.

With respect to UV light, the key factor limiting the feasibility of the process on the real scale is the short life of UV sources, which must be periodically replaced. In addition, the operation of UV lamps consumes a lot of energy, which is estimated to account for approximately 80% of operation costs.⁷⁰ Although researchers have proposed the use of ultraviolet light emitting diodes (UV-LED), which have a longer life span and do not contain hazardous mercury, their purchase cost is still higher than UV lamps at the same electrical energy conversion. Solar photocatalytic oxidation is a very promising process; however, it is limited by the large working area required for solar light irradiation. The studies on the long term stability of novel photocatalysts with wavelengths in the visible or solar light range are still insufficient.

In terms of safety, SMPR should be properly covered to prevent direct exposure of UV light to the human body. A proper cooling device is also required as high temperatures may occur and cause overheating of the UV lamp in the long run. In addition, the destructive effect of UV light or hydroxyl radicals on polymeric membranes is another key issue when photocatalysis is involved in the separation. This is because the immobilized photocatalysts might absorb UV light energy, which causes membrane ageing and further alters its surface morphology and separation performance. It is in urgent need to find appropriate polymeric materials that are highly resistant towards UV irradiation and can be readily dissolved in a wide range of solvents.

4. Recommendations and conclusion

The development of SMPR has undoubtedly contributed to innovative and sustainable water treatment technologies. In this review article, the recent progress of SMPR is reviewed with respect to operating conditions during the treatment process. The main intention of this contribution is to render further insights into the impacts of each key operational parameter, i.e. catalyst loadings, light wavelength and intensity, feed concentration and pH, module packing density and ABFR on the performance of SMPR. The understanding in this aspect is expected to provide evidence on the operating conditions that render optimum photocatalytic degradation efficiency for SMPR application. Numerous studies have shown that the critical operational parameters might vary depending on the feed properties, catalyst type and the interaction between operating conditions. Therefore, the photocatalytic degradation of organic pollutants based on all the aforementioned parameters must be given due consideration.

Despite their assuring applications and significant achievement in recent decades, there are still some persistent problems that are encountered in operating SMPR systems. Some of these are reduced effectiveness of the catalysts embedded within the membrane matrix, polymer degradation under UV exposure and partial transformation of organic pollutants into hazardous by-products. Severe membrane fouling in the long term operation and its consequences on plant maintenance and operating cost have limited the viability of SMPR in the wastewater treatment industry. Gratefully, dedicated scientific investigations in recent years have offered several innovative approaches; for instance, the fabrication of dual-layer hollow fiber membranes with the catalyst immobilized in the outer surface layer. Dual-layer hollow fiber membranes have the advantages of maximizing membrane performance using an extremely high-performance or functional membrane material as the selective layer, while employing a low-cost material as the supporting layer. This approach significantly reduces the overall membrane material cost without compromising filtration and photocatalytic performance.

More research in this area is still required to resolve the aforementioned issues as well as to enhance the performance efficiency, reliability and stability of SMPR for industrial implementation. With the rapid progress made in materials science and engineering, it is expected that newly developed catalysts could outperform existing ones under visible light or solar illumination. In terms of membrane materials, polymers that can withstand UV irradiation and can be easily dissolved in common solvents, such as N-methyl-2-pyrrolidone and dimethylacetamide, are highly desired. The use of ceramic membranes that possess excellent thermal and chemical resistance and resistance to abrasion by suspended photocatalysts may be another promising option. With respect to the system configuration, the photocatalytic membrane reactor design should be further improved based on the total irradiated surface area of catalyst per unit volume, UV light power and its intensity as well as the light distribution within the reactor.

Last but not least, it is recommended that solar heterogeneous photocatalytic oxidation should be combined with photovoltaics to reduce the energy consumption of the process. To reduce fouling problems in SMPR systems, another photocatalyst removal pretreatment stage, such as sedimentation or precoat filtration, prior to membrane filtration might be advantageous, especially when solar photocatalytic oxidation is operated in batch mode. Although it might take years to resolve the remaining challenges in this field, it appears certain that SMPR will become universal and effective in dealing with a large variety of industrial wastewater applications in the future.

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