Photocatalytic ultrafiltration membrane reactors in water and wastewater treatment - A review

Highlights

• Low-pressure UF membranes are appropriate for PMR applications.

• PUMRs could be applied for micropollutant removal.

• Ceramic membranes are the best alternatives for polymeric membranes.

• Permeate flux was constant in PUMRs.

• Less membrane fouling was noticed in PUMRs.

Abstract

This paper presents an overview of the hybrid photocatalytic membrane reactors (PMRs) with ultrafiltration (UF) for the treatment of water and wastewater. The various membrane materials and the advantages of ceramic membranes over polymeric membranes are reported in detail. Membrane fouling is a major issue in slurry PMRs and it affects permeate flux. Therefore the effect of membrane fouling on permeate flux and its control measures are discussed. This review also includes the critical evaluation of potential and commercial photocatalytic ultrafiltration membrane reactor (PUMR) configurations. In the past few decades, more researches were carried out in order to investigate the photocatalyst separation efficiency of UF membranes and it reveals that PUMRs are very much efficient in separating photocatalyst particles from the water as the permeate turbidity after treatment was found to be < 0.1 NTU. Therefore, the photocatalytic separation in PUMRs is also overviewed. Micropollutants are emerging contaminants that pose a serious threat to human life even though their concentrations in the aqueous medium are in nanograms. Therefore the effects of key parameters such as; characteristics of micropollutants and membrane on the removals of MPs is critically reviewed. The economic aspects and process intensification in PUMRs is also overviewed.

Introduction

Heterogeneous photocatalytic (UV/TiO₂) systems had been extensively studied by various researchers for the degradation of organic pollutants from water. In this system, pollutants are oxidized by means of hydroxyl radicals generated by photocatalyst TiO₂ (Titanium dioxide) and ultraviolet (UV) photons. These hydroxyl radicals are capable of mineralizing hazardous organic compounds into nontoxic, simpler end-products [1,2]. The key advantages of this process are; (i) ambient operating pressure and temperature (ii) complete mineralization of parents and their intermediate compounds and (iii) less operating costs [3]. In addition to that, TiO₂ is chemically and thermally stable, inexpensive, easily available, and exhibit strong mechanical properties [4], [5], [6]. With the increase in demand for clean water availability all over the world in the last few decades, enormous researches on advanced oxidation processes (AOPs) applying TiO₂ photocatalysis have come up because of their effectiveness in degrading recalcitrant organic compounds [3,7].

Membrane separation (MS) is widely used to separate particles from the liquid medium. The role of a membrane is to identify and recover the particles from the reaction mixture [8]. Water and low molecular weight compounds permeate through a membrane while colloids and macromolecules are retained. In industry and daily life, many of the organic compounds are recalcitrant and toxic which directly impacts the health of ecosystems and poses a threat to human life. Despite their lower concentration, their endocrine-disrupting ability and genotoxicity, they can only be removed partially by conventional treatment methods. However, PMRs were found to be very effective in removing these compounds from water [9,10]. Integrated membrane separation with photocatalytic degradation technologies have recently attracted many researchers and had been implemented in water and wastewater treatment [11]. Different kinds of integrated systems were developed by many researchers with the growing interest by integrating photocatalytic reaction and membrane separation [10,[12], [13], [14], [15], [16], [17], [18], [19], [20]].

The photocatalyst in PMRs may be either suspended type (slurry reactors) - in which the photocatalyst is dispersed in the reaction mixture or fixed type (immobilized reactors) - in which the photocatalyst is embedded in a carrier material such as a membrane [11,21]. In the fixed type process, the nano-structured TiO₂ composite membranes were very effective in decomposing organic pollutants in water [22,23]. However, this process has the drawbacks of mass transfer limitations, low surface area to volume ratio, and catalyst deactivation [18]. Even though the suspended type process exhibits several advantages that include a high surface area for adsorption and reaction, high degradation rate, and no mass transfer limitations, the process is rather limited by the necessity of separating photocatalysts from the treated solution. This problem could be overcome by confining or recycling the photocatalyst in the integrated system [18,24,25]. In PMRs, the separation characteristics of the membrane also maintain the desired levels of TiO₂ in suspension. Furthermore, the low-pressure membrane processes were proved to be very effective in the separation of TiO₂ from the integrated system due to certain advantages such as; low fabrication, maintenance, and operating costs [26], [27], [28]. The low-pressure driven membrane processes utilized in PMRs may be microfiltration (MF) [14,29,30] or ultrafiltration (UF) [19,30,31].

Among the pressure-driven processes, UF had been proved to be one of the best alternatives replacing conventional drinking water treatment technologies. The UF membrane process is very much efficient in separating suspended solids, colloids, emulsion, natural organic matters, macromolecules, bacteria, and viruses. It has been applied in various industries that include dairy, beverage processing, food, and pharmaceuticals. UF process is used to recover, purify, and concentrate products and by-products of waste. It is also being used in municipal water and wastewater treatment [32].

Ultrafiltration (UF) is a pressure-driven membrane technique in which molecular weight cut-off (MWCO) is the most widely used parameter for characterization. PMRs with UF had been found very effective for the removal of organic compounds from water because; (i) It can be carried out at an ambient temperature, (ii) there is no phase change, and (iii) permits the removal of compounds up to 90%. UF also was found to be more effective for oily wastewater treatment since no necessity for chemical additives and less energy cost [33]. Furthermore, the UF membrane of MWCO between 100,000 and 200,000 Da, rejected 96% of total hydrocarbon concentration and 54% benzene, toluene, and xylene (BTX) in oily wastewater. This efficiency was not observed in MF membranes [34]. In addition, for heavy metals like Cu and Zn, the removal efficiencies obtained from UF membranes were about 95% [34,35].

PMRs have been developed rapidly during the last few years and many researchers recently reviewed the literature on PMRs starting from Mozia [11] in which different configurations and applications of PMRs with different emphases are available. Ganiyu et al. [36] reviewed the integration of membrane processes with various advanced oxidation processes (AOPs). Wang et al. [37] reviewed both bench-scale and pilot-scale PMR applications for the separation of photocatalysts and removal of pollutants. Rajca [38] reviewed the effectiveness of natural organic matter (NOM) removal from natural water using PMR-UF and PMR-MF systems in which the authors stated that the PMR-UF system was more advantageous as no catalyst losses took place. Iglesias et al. [39] critically reviewed integrated photocatalytic membrane processes for developing PMRs, identified the intensification indices and explored its future perspectives. Another review carried out by Zhang et al. [40], focussed on the membrane fouling in PMRs. The authors discussed the relationship between photocatalysis and membrane fouling and reviewed various fouling control strategies. Molinari et al. [41] discussed the recent progress of PMRs in water treatment and in the synthesis of organic compounds. Another review reported by Zheng et al. [42] discussed the different PMR configurations; both slurry and immobilized and the various influencing factors such as; photocatalyst, light source, water quality, membrane, and aeration on the performance of PMRs. Janssens et al. [43] reviewed slurry PMR technology for the removal of pharmaceutical compounds especially cytostatic drug elimination in wastewater. Argurio et al. [44] critically reviewed the recent progress and the challenging perspectives in the development of photocatalytic membranes (PMs) in PMRs for long-term applications. Molinari et al. [45] overviewed PMRs in organic synthesis including photocatalytic conversion of CO₂, synthesis of KA oil, conversion of acetophenone to phenylethonol, synthesis of vanillin and phenols as well as hydrogen production. Horovitz et al. [46] critically reviewed the applications of ceramic membranes in PMRs. In the recent review, Kumari et al. [47] discussed (both the split and integrated type PMRs) the construction, techniques, and properties of photocatalytic membranes. They also have explored the major challenges and identified the gaps in the development and upscaling of PMR technology in field applications. Among the available reviews, no one study had entirely focused on the implementations of low-pressure UF membranes in PMRs.

Therefore, this paper mainly focuses on the utility of UF membranes in PMRs for water and wastewater treatment. The key constituents of PMRs such as; photocatalyst and membrane (material) are reviewed in detail. Membrane fouling, which is a severe operating problem in slurry type reactors is discussed in detail with reduction techniques. The effects of membrane fouling on permeate flux and photocatalyst separation efficiency of UF membranes in PUMR are also overviewed.