Which Configuration of Photocatalytic Membrane Reactors Has a Major Potential to Be Used at an Industrial Level in Tertiary Sewage Wastewater Treatment?

Abstract

Photocatalytic membrane reactors (PMRs) have been found to be very effective in the removal of organic pollutants (particularly recalcitrant compounds) from wastewater because they allow for the mineralization of organic pollutants to innocuous by-products, thus achieving high-quality treated water. Owing to the very high volumes of water involved, treated sewage wastewater could be reused if a very efficient tertiary stage, like a PMR, can be foreseen. In this review, the two main PMR configurations (photocatalytic membranes and slurry PMRs) were analyzed as requirements of a tertiary treatment of sewage wastewater considering six design and operational parameters of such plants: (i) continuous wastewater flow rate from the secondary stage; (ii) the self-control of the photodegradation rate related to wastewater chemical–physical parameters; (iii) ability to handle variations of wastewater concentration and flow rate; (iv) the control of the quality of treated wastewater; (v) low plant footprint; and (vi) easy maintenance. In this analysis, some characteristics of photocatalysis (which involves three phases: solid (the photocatalyst), liquid (the wastewater), and gas (oxygen or air)) and those of membranes (they can be produced using different materials and configurations, different processes (pressure-driven or not pressure-driven), etc.) were considered. The obtained results show that slurry PMRs seem more suitable than photocatalytic membranes for such applications. We believe this review can trigger a shift in research from the laboratory to industry in using photocatalytic membrane reactors.

1. Introduction

Currently, wastewater treatment plays a strategic role in mitigating the potable water shortage, so it is crucial for economic and health reasons, considering the worldwide population growth [1]. Acceptable water quality is required for discharge, reuse in the original production processes, irrigation, aquifer recharge, or other uses [2]. It has been widely demonstrated that conventional sewage (municipal) wastewater treatment plants (WWTPs) are inefficient in removing many pollutants, thus contaminating the receiving ecosystems [3,4,5,6]. These pollutants are mainly emerging organic contaminants (EOCs) that can have a lethal impact on human and wildlife endocrine systems, even if present in trace quantities. They include a diverse group of thousands of chemical compounds, such as pharmaceuticals and personal care products (PPCPs), pesticides, hormones, surfactants, flame retardants, plasticizers, and industrial additives, among others. Metabolites and intermediate degradation products of parent compounds [7,8] are also present, as well as two new classes of contaminants like antibiotic resistance genes (ARGs) and microplastics [9,10]. Another new class of contaminants particularly found in both drinking water and wastewater encompasses polyfluoroalkyl substances (PFAs), which are persistent, mobile, and toxic chemicals that are hazardous to human health and the environment [11,12]. Many contaminants may undergo migration, transfer, transformation, degradation, and dissipation processes in the environment and may ultimately threat ecosystem and human health. Although WWTPs have a high pollution removal efficiency (e.g., over 95% in the case of microplastics [13]), resulting in low concentrations of these recalcitrant pollutants in the discharged water (e.g., from ng L⁻¹ to μg L⁻¹ for antibiotics [14]), the high daily flow rates (e.g., approximately 90,000 m³/day in dry weather serving an equivalent population of 378,353 people [13]) leads to a considerably high number of pollutants that reach and accumulate into the environment. So, highly efficient wastewater treatment processes are required that include an advanced tertiary treatment stage to obtain water quality suitable for discharge but better for reuse purposes, thus fulfilling the aforementioned problems of potable water needs (the Food and Agriculture Organization (2021) expected the demand of water for irrigated agriculture to account for 70% of all freshwater withdrawals in the world by 2050 [15]). The current methods employed as tertiary stages, such as adsorption on active carbon and membrane filtration, are not able to destroy contaminants but only remove them via transfer to the adsorbent and the concentrate stream, respectively [16,17]. A promising alternative is the removal of persistent organic pollutants through destruction by means of advanced oxidation processes (AOPs), consisting of the action of powerful oxidants generated in situ, as well as reactive oxidizing species (ROS), e.g., hydroxyl radical (HO^•) and superoxide radical ion (O₂^•−) [18]. Among the different AOPs, those using heterogeneous catalysts have numerous advantages over types that have homogeneous catalysts [19], and among heterogeneous AOPs, those using semiconductor photocatalysts have shown to be an eco-friendly and sustainable alternative to overcome environmental and energy challenges [20]. Thus, photocatalysis is considered as one of the most promising advanced oxidation processes due to its ability to completely degrade contaminants to safer end products or intermediate compounds, its ability to handle a wide range of contaminants, having negligible requirement of post-processing, the disposability of spent stream, and mild temperature and pressure conditions [21]. In the most studied photoreactors for wastewater treatment, photocatalysts are used in suspended mode (slurry reactor) or immobilized on a support (fixed bed reactor). A slurry photoreactor offers a high surface area for reactions, but although it is more efficient than an immobilized system [22], there are some drawbacks such as (i) the difficulty of separating the photocatalyst at the end of a batch photocatalytic process; (ii) particle aggregation and agglomeration at high photocatalyst concentration; and (iii) the difficulty of operating a continuous process. On the other side, a drawback of immobilized photocatalysts is the efficiency of light impinging on particle surfaces, which is an important prerequisite for obtaining photocatalytic activity [23]. The difficulties related to photocatalyst separation as well as continuous operation for slurry photoreactors have been overcome around 2000, when several articles concerning the introduction of photocatalytic membrane reactors (PMRs) were published [24]. In a PMR, both photocatalytic reaction and catalyst separation occur, as well as continuous operation. In addition, since the membrane (if properly chosen) can be selective at molecular level, other than retaining the photocatalyst particles, the pollutant molecules can also be retained in the reaction zone until they are completely mineralized, resulting in improvement in the overall performance of the photodegradation process [25]. PMRs have been proposed and studied over the years in two main configurations: a photocatalyst suspended in a solution (slurry) retained by the membrane or immobilized in/on the membrane (photocatalytic membrane (PM) [26]). Under these two basic configurations, numerous sub-configurations have been proposed and tested for wastewater treatment, but none of them, to the best of our knowledge, have been deeply analyzed to understand if they fit the technical key parameters of a tertiary stage of a WWTP. Indeed, these plants usually operate with very high flow rates, continuous processing (night and day), and the presence of numerous pollutants in solution. In this review, these operational characteristics, as well as those related to plant design (e.g., low footprint, robustness of operations, etc.) will be critically analyzed and related to the coupling of photocatalysis–membrane. Photocatalysis is a complex process as it not only involves three phases (solid (the photocatalyst), liquid (the wastewater), and gas (oxygen or air)) but also membrane operations (they can be carried out using different materials and configurations, different types of processes (pressure-driven or not pressure-driven), etc.). So, starting from the main characteristics of a tertiary stage of a WWTP, some main characteristics of a PMR in order to be employed as tertiary wastewater treatment will be identified and discussed. Then, some conclusions on possible PMR configurations that can be of interest for application at an industrial level will be drawn.

2. Technical Aspects of Tertiary Wastewater Treatment Technologies

Domestic and industrial wastewater systems are generally treated via the activated sludge process as the most widespread method. Although this process removes most of the organic content (which is converted into carbon dioxide and biomass), residual pollutants such as the previously mentioned emerging contaminants (ECs) will still be present, which have a significant impact on the final treated wastewater recipients [27]. Depending on the end use of the treated water, which could be its reuse, or it could be discharged into a water recipient, strict quality criteria need to be met [27]. The tertiary treatment methods that have gained significant research interest are chlorination, ultraviolet irradiation, membrane filtration, constructed wetlands, microalgae cultivation, ozonation, and photo-Fenton processes due to their low operational costs as well as their high efficiency. The most important technical efficacy parameter of these methods, namely the ability of each tertiary process to reduce pathogens and remove inorganic nutrients and ECs, was analyzed by Zagklis et al. [28], who found the photo-Fenton method to be the most technically efficient process, significantly reducing the microbial load and pharmaceutical content (by 4.9 log and 84%, respectively) of the secondary effluent. However, Lama et al. [19] provided evidence that AOPs based on the Fenton process have some drawbacks hindering their large-scale application. These include their narrow working pH range (2.5–6); the complexity of real water matrices favoring the precipitation of the transition metals present in the catalyst, leading to the formation of sludge; and undesirable by-products. Furthermore, the removal of soluble iron salts from the reacting environment requires extremely costly procedures. Some of these disadvantages related to Fenton- and photolytic-based processes can be mitigated using heterogeneous photocatalysis combined with membrane filtration [29,30,31]. The combination of these two technologies seems to be of particular interest in tertiary wastewater treatment as it has been successfully investigated in many published papers for achieving the comprehensive remediation of water streams [24,32]. The choice of the most appropriate tertiary treatment is a complex matter, since each method has certain strengths and weaknesses. Based on the requirements for the final quality of the treated water, it is possible to choose a combination in a multi-step process for the removal of target contaminants. Indeed, other than organic contaminants, nitrogen, phosphorus, and some metals could also be target components to be removed. However, in this review, we will only focus on the removal of organic contaminants in PMRs.

Tertiary treatment is performed in series after the secondary stage of a WWTP, so some of its technical characteristics are the same as those of the overall plant. In this review, we will consider the following design and operating parameters that different PMR configurations should also meet: (i) continuous wastewater flow rate from the secondary stage; (ii) the self-control of the photodegradation rate related to wastewater chemical–physical parameters; (iii) ability to handle variations in wastewater concentration and flow rate; (iv) controlling the quality of the treated wastewater; (v) low plant footprint; and (vi) easy maintenance.

3. Analysis of the Main Parameters of Tertiary Treatment and Requirements of the Corresponding Parameters of a PMR

The previously considered parameters, as the main technical characteristics of tertiary treatment, will be reviewed and analyzed in this section to relate them with the correspondent parameters of a photocatalytic membrane reactor.

3.1. Continuous Wastewater Flow Rate from the Secondary Stage

The fact that WWTPs operate continuously imposes a continuous operating mode for photocatalytic membrane reactors. This means that photodegradation reactions in the photoreactor must be carried out continuously (night and day) at a constant designed light intensity. Many studies on the applications of photocatalysis have highlighted the use of solar light as a green energy source, but solar light is available only during the day at a variable intensity [33]. So, to take advantage of the sunlight, photovoltaics [34] or other green sources (e.g., wind) generating electrical energy could be a valuable solution to reduce the consumption of electricity from the grid while continuously powering the lamps (e.g., low-energy-requiring LED lamps) for the irradiation of photocatalysts. This means that the exposure of photocatalysts to direct sunlight, as reported in some articles [35], is not of high interest when a continuous process is required.

Generally, WWTPs treat a thousand cubic meters of wastewater per day in small-scale plants, with flow rates ranging from about 50 to 1500 m³/day, and this increases from medium- to large-scale plants [6]. In cases with significantly high flow rates, a criterion of modular designs can be followed by establishing a sub-plant size that is replicated n times until reaching the requested overall size, with these sub-plants operated in parallel. This criterion can be applied in designing a PMR after establishing its configuration.

3.2. Self-Control of the Photodegradation Rate Related to Wastewater Chemical–Physical Parameters

Common chemical–physical parameters of interest for wastewater are pollutants concentration, pH, and temperature. As reported in the Introduction, pollutants are various recalcitrant organic compounds that are still present in the effluent from the secondary stage of a WWTP. Their concentration is generally in the range of ng/L to µg/L. pH is generally in the range of 8.0–8.8, while the temperature for 75% of the world’s global wastewater was estimated to fall into the temperature range of 6.9–34.4 °C over a year [6,36]. These parameters are also the operational parameters that influence the photodegradation rate of organic compounds.

For most photocatalytic reactions in the liquid phase, the photodegradation rate follows a kinetic model that is in very good agreement with the Langmuir–Hinshelwood (LH) model used to describe heterogeneous surface reactions [37]. Assuming LH kinetics for a generic photocatalytic process of degradation of species A, an expression for the initial reaction rate, r_A, is as follows:

$${\mathrm{r}}_{\mathrm{A}}={\mathrm{k}}_{\mathrm{L}\mathrm{H}}·{\mathsf{\theta }}_{\mathrm{A}}·{\mathsf{\theta }}_{\mathrm{O}\mathrm{x}}$$

(1)

where k_LH is the second-order surface rate constant for the degradation reaction, which depends mainly upon the radiation flux, I; and C_A and θ_Ox are the fractional sites coverages by A and O₂ on the surface of semiconductor particles. The terms θ_A and θ_Ox are related to A and oxygen concentrations in a solution using the Langmuir model:

$${\mathsf{\theta }}_{\mathrm{A}}=\frac{{\mathrm{K}}_{\mathrm{A}}{\mathrm{C}}_{\mathrm{A}}}{1+{\mathrm{K}}_{\mathrm{A}}{\mathrm{C}}_{\mathrm{A}}}$$

(2)

$${\mathsf{\theta }}_{\mathrm{O}\mathrm{x}}=\frac{{\mathrm{K}}_{\mathrm{O}\mathrm{x}}{\mathrm{C}}_{\mathrm{O}\mathrm{x}}}{1+{\mathrm{K}}_{\mathrm{O}\mathrm{x}}{\mathrm{C}}_{\mathrm{O}\mathrm{x}}}$$

(3)

where K_A and K_Ox are the equilibrium adsorption constants, and C_A and C_Ox are the concentrations of A and oxygen, respectively.

The LH model states that the adsorption of the species on a catalytic surface is a rapid equilibrium process and that the slow rate-controlling surface step involves the presence of both reactants (A and O₂) in a monolayer at the solid–liquid interface. Generally, in well-mixed photocatalytic systems, oxygen or air continuously bubbles, and the concentration of dissolved oxygen can be considered constant. Thus, Equation (1) can be rewritten as follows:

$$\begin{gathered} {\mathrm{r}}_{\mathrm{A}}={\mathrm{k}}_{\mathrm{L}\mathrm{H}}^{\prime }·\frac{{\mathrm{K}}_{\mathrm{A}}{\mathrm{C}}_{\mathrm{A}}}{1+{\mathrm{K}}_{\mathrm{A}}{\mathrm{C}}_{\mathrm{A}}} \\ \mathrm{with} {\mathrm{k}}_{\mathrm{L}\mathrm{H}}^{\prime }={\mathrm{k}}_{\mathrm{L}\mathrm{H}}·\frac{{\mathrm{K}}_{\mathrm{O}\mathrm{x}}{\mathrm{C}}_{\mathrm{O}\mathrm{x}}}{1+{\mathrm{K}}_{\mathrm{O}\mathrm{x}}{\mathrm{C}}_{\mathrm{O}\mathrm{x}}} \end{gathered}$$

(4)

Tang et al. [38] studied the kinetics of the photocatalytic oxidation of five commercial dyes (Acid Blue 40, Basic Yellow 15, Direct Blue 87, Direct Blue 160, and Reactive Red 120) using TiO₂ under UV irradiation in a slurry reactor. They concluded that the different dyes follow the LH model at different pH values. At low pH, reduction using electrons in the conduction band may play a very important role in the degradation of azo dyes due to the reductive cleavage of azo bonds. In comparison, at neutral or high pH levels, hydroxyl radicals may be the predominant oxidation species. Oxygen plays an important role in the formation of hydroxyl radicals; in fact, its reaction with photoelectrons on photocatalysts’ surface disfavors electron–hole recombination, and the superoxide ions formed (O₂^•−) allow for a reaction channel for the generation of additional hydroxyl radicals through the formation of hydrogen peroxide [39,40,41], which occurs as follows:

O₂ + e⁻ → O₂^•−
O₂^•− + H⁺ → HO₂^•
HO₂^• + e⁻ → HO₂⁻
HO₂⁻ + H⁺ → H₂O₂
H₂O₂ + e⁻ → OH^• + OH⁻

Moreover, it is well known that the oxygen adsorption process on the photocatalytic surface is the rate-determining step for the kinetics of a photocatalytic reaction because it is much slower than H₂O and organic pollutants [42,43,44]. For this reason, a high oxygen concentration is used but a too-high oxygen concentration leads to competition with the reacting substrates for the adsorption sites [45]. Zhang et al. [46] used a metal-free nanotubular carbon nitride-based photocatalyst (CN NT) to activate O₂ under visible light for the degradation of organic contaminants. They observed that, when the N₂ flux increased from 0.02 to 0.1 L·min⁻¹, the degradation efficiency decreased from 71.5% to 62.3%. By contrast, when the O₂ flux increased from 0.02 to 0.1 L·min⁻¹, the degradation efficiency increased from 81.1% to 85.8%. Ma et al. [47] studied the degradation of pentachlorophenol (PCP) using TiO₂ under UV irradiation for investigating the role of generated ROS. They observed that OH^• played a dominant role in the degradation of PCP, whereas O₂^•− and H₂O₂ played a fundamental role in intermediates’ degradation (Figure 1).

The pH of aqueous media is a key parameter that highly influences the charge at the surface and thus photocatalyst activity. Indeed, the position of band edges (E_c,s) is determined using the charge at the surface, that is, the potential drop in the Helmholtz layer (U_H):

E_c,s= E⁰_c,s − qU_H

where E⁰_c,s is the position (on the energy scale) of the conduction band edge at the surface at U_H = 0, and q is the elementary charge. The Helmholtz double layer on semiconductors is typically determined through adsorption and desorption processes [48,49]. Therefore, due to pH, the charge on the semiconductor surface becomes positive or negative (Figure 2). The pH value at which the net surface charge is zero is called the isoelectric point (pH_IEP). If pH > pH_IEP, the surface is negatively charged, whereas if pH < pH_IEP, positive charges are present on the surface. This pH dependence of surface charge not only strongly influences the adsorption of charged or polarizable compounds but also determines the energetic position of the valence and conduction band, in other words, the thermodynamic feasibility of the interfacial electron transfer [37].

The influence of pH on degradation was studied by Allè et al. [50], who verified the degradation of polymethylmethacrylate (PMMA) and polystyrene (PS) nanoparticles using TiO₂-P₂₅/β-SiC as photocatalyst under UV-A radiation. They found that degradation is faster at low pH (4–6) and a low flow rate [50]. Tan et al. [51] analyzed low-density polyethylene (LDPE) degradation using Design Expert software, which allowed for the interpretation of the association between pH, temperature, and catalyst (GO-ZnO) dosage. The optimum condition for the highest microplastic mass loss (39.47%) was pH 9.66, 30 °C, and 1500 ppm after 2 h of photodegradation. Minh Viet et al. [52] used La-TiO₂ under visible light for the photodegradation of phenol. They studied different pH values (4, 6 and 8) and found that the degradation performance of phenol enhanced from 61.3% to 98.2% by improving pH from 4.0 to 8.0, respectively. Ur Rahman et al. [53] studied the degradation of cefixime as an antibiotic model, using a CuO-NiO nanocomposite photocatalyst under sunlight. They observed that the degradation of cefixime was influenced by catalyst dosage and lower pH. Doong et al. [54] studied the influence of pH on the photocatalytic degradation of 2-chlorophenol using TiO₂ and found that a higher pH value had a significant effect. Indeed, they observed less formation of aromatic intermediates, suggesting that photodegradation at high pH favors direct ring cleavage. Mirzaei et al. [55] evaluated the photocatalytic degradation of sulfamethoxazole (SMX) using ZnO in the presence of fluoride ions (F-ZnO), studying the effects of operating parameters, and they observed that the removal of SMX was more efficient under acidic conditions and at an optimum pH of 4.8 since the surface fluorination of photocatalysts can be carried out in acidic media.

The above-described studies show that pH is an important parameter of photocatalytic reactions and that optimal pH can be different for different types of pollutants, but in a real process, only a pH value in the reacting zone can be imposed [56,57]. So, an optimal pH should be found for the overall pollutants present in wastewater. It is expected that pH is around 8.5 (about the same as that of wastewater) where hydroxyl radicals may be the predominant oxidation species for the oxidation of various pollutants. This means that no or only little pH correction is required for the wastewater to be fed for tertiary treatment.

The temperature is also an important parameter; however, owing to photonic activation, photocatalytic systems can operate at room temperature. It is reported [37] that temperatures between 20 and 80 °C weakly influence the oxidation rate of photocatalytic reactions. This behavior can be explained by the LH mechanism described before. In fact, a decrease in temperature favors adsorption, which is a spontaneous exothermic phenomenon. However, the lowering of temperature also favors the adsorption of the final reaction products, whose desorption tends to inhibit the reaction [58,59]. Ariza-Tarazon et al. studied the impact of pH and temperature on the degradation process of high-density polyethylene (HDPE) using C, N-TiO₂, and visible light [60]. They observed that a low pH value allows for hydroperoxide formation during photooxidation and confirmed that low temperatures improved the degradation favoring the formation of microfractures on the surface. For this reason, low temperatures can facilitate oxidative degradation [61]. Gao et al. [62] prepared Ag@AgCl photocatalytic material based on calcium alginate as the carrier and cetyltrimethylammonium bromide as the surfactant (CA+) for the degradation of tetracycline under visible light. They studied the influence of temperature at 35, 40, and 45 °C and found that the highest photocatalytic degradation rate was at 40 °C. Maqbool et al. [63] used La-Mn co-doped Fe₂O₃ nanoparticles for the degradation of Rhodamine B (RhB) dye under a highly efficient solar light radiation. The degradation was optimized by changing pH (from 4 to 10), catalyst dosage (from 10 to 50 mg/L), dye concentration (from 10 to 60 mg/L), and temperature (35, 45, and 50 °C). They observed that the degradation of RhB dye increased from 43.30% to 89.34% as a result of the optimization of the system.

3.3. Ability to Handle Variations in Wastewater Concentration and Flow Rate

Depending on the type of photoreactor configuration, a better or a worse ability to handle variations in wastewater concentration and wastewater flow rate is expected. The basic reactor configurations operating in a continuous flow are continuous stirred tank reactors (CSTRs) and plug flow reactors (PFRs). CSTRs are mainly used in slurry photoreactors, while a PFR using immobilized photocatalytic systems is the fixed-bed photoreactor (FBP). The slurry–CSTR has the capability to mix solids, liquids, and gases simultaneously, while agitation increases the mass transfer between the catalyst and reactants, thus providing a high surface area to be illuminated. In fixed-bed photoreactors, photocatalysts are immobilized (fixed) on a material used as support. Although fixed-bed photoreactors do not require catalyst separation, light distribution has been a problem of concern [64,65]. Some examples of immobilized photocatalysts are the tubular membrane and the packed bed [33,66,67].

An increase in pollutant concentration in wastewater influences the photodegradation rate and thus the ability to mineralize the pollutants, which can require a greater residence time. An increase in the input flow rate, instead, reduces the residence time, thus decreasing the photodegradation efficiency. In the case of CSTRs, these variations are better mitigated (a well-mixed tank reduces the concentration pulse, while a slight increase in the hydrostatic level can reduce the flow rate pulse). The same does not occur for immobilized photocatalysts; thus, for wastewater treatment, it is speculated that a CSTR performs better than FBP. In these two basic configurations are also used photocatalytic membrane reactors. The membrane permits the confinement of the photocatalyst (slurry or immobilized) in the reaction zone and some types of membranes that are able to retain pollutants also facilitate the control of the residence time of some recalcitrant pollutants in the reaction zone [24]. A summary of various reactor configurations is reported in Figure 3 [65].

In subsequent sections, an overview of some reactor configurations with or without membranes and the differences between these two systems are also described. This review of the literature can provide a significant amount of useful information to improve the design of membrane photoreactors.

3.3.1. Photocatalytic Reactor Configurations without Membrane

The performance of photocatalytic reactors can be influenced by their configuration [21]. The design of a photocatalytic reactor can include different configurations based on geometry, liquid agitation methodology (CSTR and stirrer rotation), light source (UV lamps and LED), bed dynamics (fixed bed and fluidized bed), and size (mini- and microreactors) [21,68,69].

Photocatalytic reactor design must address important parameters such as uniform light distribution, providing high illuminated catalyst surface area, and mixing inside the reactor [70]. The geometry of a photocatalytic reactor is important for maximizing the collection of the emitted light from the light source [71]. There are two main illumination configurations: external, in which the irradiation source is outside the reactor, and internal, with the lamp immersed in the liquid [71,72,73,74]. In both cases, this system can be built by using one or more lamps with different wavelengths (UV, visible, or solar light) and types of irradiation source (LED, Hg mercury lamp, etc.) [71,75,76] (Figure 4). Despite some advantages of using external illumination, such as the possibility of using direct solar irradiation as a light source and the easier reactor design, one possible limitation is the incomplete irradiation of the photocatalyst along the reactor’s thickness. Instead, a system with internal illumination has the advantage of obtaining better contact with the photocatalyst surface, thus improving the overall performance of the photoreactor [24,77].

In the design of photocatalytic reactor configurations, it is very important to obtain a uniform light distribution in order to gain maximum effectiveness of the available catalytic surface [21]. To overcome this challenge, there is great emphasis on the use of multiple lamps, leading to the evolution of tube light reactors and multi-tube reactors [75,78,79].

Advances in artificial light sources (e.g., LED) have allowed for the design of compact UV reactors also including immobilized catalysts on substrates, such as glass fibers, optical fibers, and membranes [80,81,82,83]. LEDs can reduce operating costs and decrease the heat produced by standard lamps [84]. With respect to annular photocatalytic reactors, they have flat-plate geometry (Figure 4d–f), and they are scalable and usually employed if direct solar irradiation is used as a light source [71].

A particular method of photocatalyst irradiation is the controlled periodic illumination (CPI) in semiconductor photocatalysis, which is based on a series of alternate light and dark (TON/TOFF) periods [68]. A hypothesis suggests that upon the illumination of a catalyst, there is a critical illumination time during which absorbed photons generate oxidizing species (h_vb⁺) on the surface of the photocatalyst. During an intermediate period in which the photons are not required, the generated species or their intermediates (OH^•) react with substrates. This period does not need irradiation and should be conducted in the dark.

Another factor that should be considered is that the nature of the light source (solar or artificial) strongly impacts the design of a photocatalytic reactor [70]. Solar light, with relatively low intensity, needs a large exposed surface area, unless a solar concentrator system (e.g., parabolic reflectors, Fresnel lens, and possibly optical fibers) is employed. In comparison, by using artificial light as a light source, it is important to optimize the conversion of electrical energy to useful photons in the most efficient way. Wang et al. [80] reported some designs of pilot- and full-scale photocatalytic reactors for water treatment (Figure 5).

Alpert et al. [85] reported one of the earliest pilot-scale solar photocatalytic reactors for water treatment built by Sandia National Laboratories (USA) (Figure 5a). This is a parabolic trough reactor used to treat hazardous wastewater containing trichloroethylene (PTRs), with a width of 2.1 m and length of 218 m, and a total aperture area of 465 m². To obtain maximum light absorption, this installation can control the orientation of parabolic troughs by tracking the sun’s position by means of a motor. This system has different disadvantages, such as the absence of direct solar light during cloudy days and expensive water heating when required [86]. Figure 5b shows a free-falling thin-film slurry reactor system used by Gernjak et al. (2004) and Malato Rodríguez et al. (2004), which consists of a thin film of water flowing as a catalyst slurry or over an immobilized catalyst [87,88]. This system can collect both direct and diffuse solar light and, in general, does not require a sunlight tracking system, but its performance is affected by weather conditions and can induce catalyst sedimentation in slurry photocatalytic systems.

A type of photoreactor that is installed at a pilot scale to remove some micropollutants is the compound parabolic collector (Figure 5c). The compound parabolic concentrator (CPC) photoreactor is generally composed of a number of pyrex tubes connected in series and irradiated using concentrated solar light transmitted with a parabolic round mirror, which allows for a concentration of sunlight equivalent to 25 or 70 suns, depending on the mirror area and aperture diameter [71,89]. By using compound parabolic collectors (CPCs), it is possible to capture both direct and diffuse components of solar light. They have turbulent flow regime operation and lesser dependence on weather conditions, do not overheat the water, and have a relatively small footprint [80,90]. A CPC plant with a solar collector field area of 150 m² for the removal of pesticides was installed in Spain [91].

Another type of configuration is the rotating disk photocatalytic reactor shown in Figure 5d [92]. Mehling et al. [93] recently described the use of a photocatalytic immersion rotary body reactor and a sub-stream ozonation as a fourth cleaning stage for its application in real wastewater. This photocatalytic immersion rotary body reactor with a 36 cm disk diameter, covered with stainless steel grids coated with titanium dioxide, was irradiated using UV-A light-emitting diodes. In this system, the concentrations of the generated radicals can be influenced by photon flux and ozone dosage, and the main limitation for the pollutant degradation rate is the layer thickness of water on the cyclically wetted rotating disks. Therefore, a further limitation of reaction rates should be expected through further scale-up processes for large-scale applications. Figure 5e shows a double-skin sheet reactor, which is simple in design and fabricated from a plexiglass double-skin sheet [21]. It provides better degradation efficiency than other reactors for a variety of contaminants and photocatalysts. A flat-plate reactor is a laminar flow reactor, which has less degradation efficacy, owing to limited solar radiation penetration at higher flow rates. Nevertheless, it produces better results than immobilized types in slurry-type reactors.

Another type of configuration is the fountain photocatalytic reactor (Figure 5f), which is a slurry-type reactor in which a thin film of polluted water containing dispersed photocatalyst is continuously generated by pumping water through a specially designed nozzle [94,95].

A configuration that can operate during both day and night with UV lamps inserted in the annular tube is the multi-tubular photoreactor (Figure 5g) [14,96]. Vaquez et al. reported the degradation of 14 antibiotics by using a tubular membrane photoreactor, operated in continuous mode and low residence times (from 3.9 to 12.2 s), in the treatment of demineralized water (DW) and secondary-treated urban wastewater (UWW) by using hydrogen peroxide (H₂O₂), persulfate (S₂O₈²⁻), or ozone (O₃) as oxidant and titanium dioxide (TiO₂) or silver molybdate (Ag₂MoO₄) as photocatalysts [14]. In the UWW, by using O₃ as an oxidant, the removal of 12 antibiotics was higher than 90%; in contrast, by using S₂O₈²⁻ with Ag₂MoO₄ as photocatalyst under UVC, the removal was higher than 60%.

Figure 5h shows a UV photocatalytic water treatment (PWT) unit that is an annular thin-film reactor, where slurry suspensions of TiO₂ are pumped to a liquid distribution system and falling freely along a vertically mounted hollow column with a UV lamp suspended in the middle of the hollow column through a quartz or borosilicate protective sleeve [80,97]. This configuration has the advantage of capturing almost all photons emitted by the light source; in addition, this typology of photoreactor is very efficient because the footprint is greatly reduced [98,99].

Another type of configuration (Figure 5i) is the serpentine-like geometry [100]. Mesa et al. [101] developed a UV PWT reactor with serpentine-like geometry that was tested in the treatment of effluents taken from handicraft factories by using a UV/H₂O₂ process and TiO₂-based photocatalysis. They obtained 70.80% of dye discoloration with 0.07 M of H₂O₂ and TiO₂ as photocatalysts.

3.3.2. Photocatalytic Membrane Reactors

Scientists started to realize the advantage of coupling heterogeneous photocatalysis with membrane processes in water and wastewater treatment around 2000, with the first studies on PMRs [102,103].

The main purpose of membrane use in PMRs is retaining photocatalyst particles and pollutant molecules in the reaction zone until they are mineralized, resulting in an improvement in the overall performance of the photodegradation process. PMRs are commonly classified on the basis of photocatalyst arrangement in two main configurations [25]: (1) slurry PMRs, where the photocatalyst is suspended in the reaction mixture, and (2) PMRs with photocatalytic membranes (PMs), where the photocatalyst is immobilized in/on a substrate acting as a membrane [32,74,104]. Despite the advantage of immobilized photocatalysts over slurry systems, many studies have reported that slurry photocatalysts perform better than immobilized photocatalysts because they have larger active surface areas exposed to light [105]. Furthermore, Molinari et al. [106] reported that the use of a configuration with an immobilized photocatalyst on a membrane can cause the photodegradation of the polymeric membrane due to the presence of reactive oxygen species (ROS) in the oxidizing environment. Indeed, in this case, the membrane must be directly irradiated because pollutant degradation occurs inside the pores of the membrane or on its surface. For example, a PVDF membrane was used in various studies for immobilizing the photocatalyst [107,108], but in a study by Dzinun et al. [109], it was concluded that UV irradiation has some effect on the stability of the TiO₂/PVDF membrane within 30 days of exposure. Therefore, the use of polymeric membranes is limited for their low photostability and resistance against oxidative species, so a membrane material (e.g., inorganic membranes) with adequate stability against oxidative species under light irradiation should be used. Instead, the use of ceramic membranes can be preferred for their higher stability. Another disadvantage of PMRs with PMs is the lower rate of pollutant degradation with respect to slurry systems due to a lower mass transfer rate on the immobilized photocatalyst because the presence of support can limit the contact between the substrate and the photocatalyst. The first configurations of slurry PMRs, reported by Molinari et al. [110], were integrated systems based on the continuous recirculation of the slurry between the photoreactor (irradiated using an internal or external lamp) and the membrane cell that were operated in batch (recycling both the retentate and the permeate in the photoreactor) or in continuous mode by feeding the water into the photoreactor for treatment and withdrawing the permeate. A similar configuration with a hollow fiber ultrafiltration membrane (Figure 6) was investigated by Zhang et al. [111] in a photocatalysis–ultrafiltration process for dying wastewater treatment. They found that the rejection of dye in the presence of TiO₂ markedly decreased due to the deposition of TiO₂ particles on the membrane interface. So, the interactions between pollutants and photocatalysts and their effect on the performance of the membrane should be taken into consideration in this type of configuration.

Slurry PMRs can also be classified into two subcategories: (1) integrative-type PMRs, in which the photocatalytic reaction and membrane separation (MS) take place in the same vessel, where the inorganic or organic (polymeric) membrane is submerged in the slurry photocatalytic reactor; (2) split-type PMRs, where the photocatalytic reaction and membrane separation take place in a photoreactor and membrane module that are appropriately coupled units [112,113,114,115].

PMRs can also be classified based on the placement of the light source: (1) PMRs with the light source above/inside the feed tank; (2) PMRs with the light source above/inside the membrane unit; and (3) PMRs with the light source above/inside an additional vessel placed between the feed tank and the membrane unit [25].

Although slurry PMRs achieve higher efficiency than that of PMRs with immobilized photocatalysts, there are two factors limiting their performance: (1) light scattering by the suspended photocatalyst nanoparticles (NPs); and (2) membrane fouling, with a consequent flux decline, caused by photocatalyst deposition on the membrane surface [78,102]. This last problem can be limited by using a membrane module with the correct selection of hydraulic conditions [116]. In general, membrane fouling can be reduced by using PMRs with PMs compared with conventional membranes because the photocatalyst generally makes the membrane more hydrophilic. To solve this problem, Molinari et al. [75] designed and built a new PMR configuration consisting of a slurry photocatalytic membrane reactor based on a vertical filter, working with gravity, and an external membrane that was tested in the photodegradation of a model pollutant in water. Figure 7 shows the configuration scheme of this PMR, in which the vertical filter is able to retain the photocatalyst, and the external nanofiltration (NF) membrane is able to retain pollutants in the photoreaction zone.

The photocatalytic reactor shown in Figure 7 consists of a cylindrical plexiglass tank as the main unit with an external diameter of 24 cm and a height of 40 cm, with a maximum useful volume of 13 L inserted inside a cylindrical filter and four UV LED lamps [75]. Fixed at the bottom of the reactor, in the annulus area, there is a fine bubble air ventilation system (Figure 7c) consisting of a tubular polypropylene membrane circularly distributed over the entire surface to allow for setting appropriate mixing conditions and to provide the required oxygen for the photocatalytic oxidation of contaminants. The liquid was recirculated from the bottom of the well of the cylindrical filter to the external annulus (Figure 7a) using a tube connected to a pump that was used to feed the external membrane cell (Figure 7b). Some advantages of this type of configuration are (i) very reduced fouling via TiO₂ deposition in the external membrane cell because the vertical filter retains the photocatalyst, ensuring greater flux than a submerged membrane (200 vs. 40 L m⁻² h⁻¹ average values); (ii) the easier replacement of the photocatalyst with a new one in comparison to a photocatalytic membrane; (iii) the external membrane (which is not exposed to light irradiation) facilitates recycling of non-photodegraded pollutants in the photoreactor, leading to higher quality of the treated water (permeate) than a submerged membrane. The membrane location is far from the strong oxidizing zone, thus preserving it from photodegradation and providing the possibility to use commercial spiral wound membranes with lower capital costs. The particles confined in the reaction zone and noncirculating in the membrane device, the very simple regeneration of the vertical filter, the use of low-energy LED-immersed lamps operating night and day using electrical energy (e.g., obtained via photovoltaic conversion of sunlight or other sources) are further advantages of this novel configuration. Researchers have reported 50% to 90% retention by changing the pH value, as well as 80% gemfibrozil degradation.

3.4. Control of the Quality of Treated Wastewater

The required final quality of the treated wastewater depends on its final use (discharge in water bodies, irrigation, aquifer recharge, etc.), but regardless, it needs to reduce the concentration of recalcitrant pollutants at the exit of the tertiary stage. Because different pollutants require different residence times for their mineralization, the presence of a selective membrane permits such a type of control acting at a molecular level. UF and MF membranes used to build photocatalytic membranes do not have such ability, so they will likely not result in good-quality treated water because not all pollutants will be mineralized with high efficiency in a continuous flow operation since, in this case, the residence time can not be controlled for all pollutants. Nanofiltration (NF) and reverse osmosis (RO) membranes, because of their ability to retain many organic pollutants, seem the most suitable to control the treated water’s quality [117,118]. An RO membrane was tested in series to a submerged membrane to increase the quality of the treated water [119]. In some works, membrane distillation coupled with photocatalysis has also been found to result in high-quality treated water [120]. However, membrane distillation commercialization has been hampered by low flux performance (average flux of commercial membranes ~16 kg/m² h), while new carbon-incorporated membranes have higher flux performance (average flux of 47 kg/m² h), which shows the great potential of this membrane design [121]. However, the high energetic demand of MD represents a significant cost compared with pressure-driven PMRs [122].

3.5. Low Plant Footprint

When designing a plant, one criterion is the minimization of the consumption of soil, which means a low footprint. This objective can be reached by a proper choice of both operating conditions and plant components. For example, a low reactor volume is possible if the design permits a high reaction rate, which, in photoreactors, also depends on the irradiated photocatalyst surface. If direct solar light is used to irradiate the photocatalyst, a flat photoreactor allows for the maximization of the exposed surface, but this design has a higher footprint than a slurry photoreactor (aside from the problem of non-constant light intensity and light availability only during the day). With a simple calculation, this aspect can be evident. If we consider a slurry photoreactor of 1 m³, the footprint can be, for example, 1 m² (if 1 m high is chosen) by irradiating the suspension with an appropriate number of immersed lamps. For flat photoreactors, if we hypothesize a light penetration depth of 0.1 m into the slurry, the 1 m³ suspension volume is contained in a surface of 10 m², which means a 10 times higher footprint. Obviously, these values give only one idea of the differences between these two types of configurations. Indeed, the light penetration depth depends on the characteristics of aqueous media. In general, light intensity transmitted through a medium follows a logarithm decay according to the Beer–Lambert law. Light attenuation increases with the concentration of solute, the penetration depth, and the wavelength. In the case of pure water, negligible absorption has been observed in the wavelength range of 300–700 nm, while outside this range, beam attenuation is observed [123]. However, for aqueous media, dissolved salts, organic substances, and suspended particulates result in beam attenuation, which is the sum of absorption and scattering phenomena. These considerations mean that we can only foresee a lower slurry reactor footprint than a flat photoreactor, but the extent of the difference can be determined with measurements on related pilot plants in a real environment.

Other parameters that influence the footprint of membrane photoreactors are (i) the reaction rate of immobilized and suspended photocatalysts; (ii) light source, light intensity, and wavelength; and (iii) membrane configuration.

3.5.1. Reaction Rate of Immobilized and Suspended Photocatalysts

As already reported, two basic photoreactor systems, namely the suspended configuration and the supported photocatalyst configuration, have been studied. A suspended photocatalyst system is generally more efficient than a supported system due to its larger surface area. It is crucial that all photocatalyst particles are illuminated, but this does not always happen in suspended systems because the particles further away from the light source are hidden from the radiation by the particles near the light source (although good mixing reduces this drawback, as will be discussed later). A solution to this problem could be the use of an immobilized system where the photocatalyst is placed as a layer, and all particles are illuminated. However, the mass transfer is limited, only a part of the photocatalyst is in contact with the solution, and illumination is always achieved on the same side of the particle surface, meaning that if this is fouled, the other part of the surface cannot be used [124,125]. Slurry systems do not have these drawbacks. Manassero et al. [126] tested three different reactor configurations: a slurry reactor, a fixed-film reactor with TiO₂ immobilized onto the reactor window; and a fixed-bed reactor filled with TiO₂-coated glass rings. Their performance was assessed considering two parameters: (1) the photonic efficiency, which is the ratio of the reaction rate to the rate of incident photons; and (2) the quantum efficiency, which is the ratio of the reaction rate to the photon absorption rate (Figure 8). The results show that the slurry reactor was the most efficient configuration, and the fixed-bed reactor had a value of quantum efficiency only one-third lower than the suspended system.

The support (carrier) materials used for immobilizing the photocatalyst must meet certain requirements, which can be distinguished into two groups. The first group includes stability, reusability, environmental aspects, and the cost of preparation. The second group includes the influence on photoelectrochemical reactions, considering better light harvesting, less charge loss, and more contact between the adsorbed pollutants’ molecules and the photogenerated ROS [127]. An example of an immobilized photocatalyst is presented in the research of Uheida et al. [128], in which a new approach was used for the degradation of microplastics (MPs) like spherical particles of polypropylene (PP) suspended in water via the visible light irradiation of zinc oxide nanorods (ZnO NRs) immobilized onto glass fibers. After two weeks under visible-light-led irradiation, they obtained a reduction in the average particle volume by 65%. Shen et al. [129] used a Bi₂WO₆ photocatalyst coated on silica sands for the degradation of sodium isobutyl xanthate (SIBX) under visible light irradiation, in a continuous fixed-bed photoreactor. The maximum degradation percentage reached 95.40% after 70 min of visible light irradiation under optimal conditions. Ljubas et al. [130] degraded trimethoprim (TMP) using a nanostructured TiO₂ film on a glass ring at the bottom of a reactor under a solar radiation lamp, and they observed the complete degradation of TMP in ultrapure water after 3 h.

As regards slurry photoreactors, Molinari et al. designed a submerged slurry photocatalytic membrane reactor able to retain pollutants (gemfibrozil) in the photoreaction zone, where the TiO₂ photocatalyst is present under UV irradiation [75]. They observed an 80% reduction in initial concentration after 4 h. In another study [131], using a slurry reactor with 1200 mg L⁻¹ of H₂O₂ (solution 25 wt%) under UVC irradiance of 31.8 mW cm⁻², Easton et al. obtained a 26.6% of mass loss of microplastic polyethylene terephthalate after 9 h of irradiation. García-Muñoz et al. [132] studied the degradation of different sizes of polystyrene (PS) nanoplastics in water by using both slurry and immobilized reaction modes and TiO₂ as a photocatalyst. From an analytical point of view, they observed that an immobilized photocatalyst poses fewer analytical problems, owing to the absence of photocatalyst microparticles in the samples compared with a slurry reactor.

Aside from the above results on immobilized and suspended photocatalysts, an important point for application is their kinetic difference because this has an impact on the plant footprint. Indeed, the higher the reaction rate, the lower the reactor footprint. In

Table 1. Comparison of the reaction rate of the immobilized and suspended (slurry) photocatalyst.

Photocatalyst	Pollutant	Slurry	Fixed-Bed	Results	Kinetic Constant (*)	Ref.
GCN-T	Metoprolol and venlafaxine	1 g L−1 of GCN-T	GCN-T immobilized in the form of a film (GCN-T/PVDF) of 30 cm2	100% pollution removal after 180–240 min using the GCN-T/PVDF film and after 30–60 min using the GCN-T powder	kiapp were 0.766 min−1 and 0.879 min−1 for 0.90 µM and 0.065 min−1 and 0.056 min−1 for 90.1 µM of metoprolol and venlafaxine, respectively	[133]
TiO2	50 mg L−1 phenol	bubbling air, 300 mg L−1 TiO2	The glass plates (4.5 cm × 4.5 cm × 3 mm) were prepared with 0.10 ± 0.01 g of catalyst	Significant catalyst deactivation was observed in the immobilized-catalyst systems	ksapp = 0.0013 min−1 kiapp = 0.0010 min−1 After 24 h of reaction kiapp was 60% of its initial values	[134]
TiO2	20 mg L−1 of clofibric acid (CA)	0.5 g L−1 of TiO2 in a solution saturated with pure oxygen	TiO2 supported on the illuminated reactor window (fixed film reactor, FFR) with a mass of 36.5 mg, and TiO2 immobilized over the surface of glass rings used to fill the reactor (fixed bed reactor, FBR) with a mass of 21.2 mg	The slurry reactor was the most efficient configuration, the fixed-bed reactor gave a value of quantum efficiency only one-third lower than the suspended system	ksapp = 3.26 × 103 min−1 kiapp = 1.12 × 103 min−1 (fixed bed reactor) kiapp = 1.28 × 103 min−1 (fixed-film reactor)	[126]
TiO2	100 µg L−1 of 15 emerging contaminants (ECs)	5 mg L−1 of TiO2	The photoactive layer of TiO2 was deposited on glass spheres using the sol–gel dip-coating technique. Around 0.5 mg of TiO2/sphere	The removal percentage was between 100% in most of the compounds and 70% of the sulfamethoxazole and atrazine. The degradation rate in the TiO2 slurry was similar at low concentrations	Ofloxacin: kiapp = 1.577 min−1 and k5iapp = 0.228 min−1, Flumequine: ksapp = 0.141 min−1 and kiapp = 0.46 min−1), Hydroxybiphenyl: ksapp = 0.025 min−1, kiapp = 1.577 min−1 and k5iapp = 0.047 min−1	[135]
TiO2	200 mg L−1 of trichloroethylene (TCE)	0.2% wt/wt TiO2	72 g of TiO2 films containing 1.4 wt% of TiO2 added to reactor	The operation of the solar light/TiO2 slurry and immobilized systems showed 100% (TiO2 slurry system), 80% (TiO2 immobilized system) degradation of the TCE after 6 h	kiapp = 0.0052 min−1 ksapp = 0.0224 min−1 (with H2O2) kiapp = 0.0083 min−1 (with H2O2) ksapp = 0.0304 min−1 (with S2O82−) kiapp = 0.0207 min−1 (with S2O82−)	[136]
ZnO	Carbamazepine (CBZ)	0.5 g L−1 of ZnO nanoparticle suspensions and 10 mgL−1 of CBZ	10 μmol L−1 of CBZ and 0.5 g L−1 of ZnO (nanoparticles grafting onto inert foam substrates)	The photocatalytic degradation of CBZ was 67% after 4 h using the foams	ksapp = 3.3 × 10−3 min−1 kiapp = 3.0 ± 0.4 × 10−3 min−1 (foams sintering at 900 °C for 6 h) kiapp = 3.2 ± 0.2 × 10−3 min−1 (foams sintering at 1000 °C for 12 h)	[137]
TiO2	10 or 40 mg L−1 of 4-nitrophenol (4-NP)	0.5 g L −1 of suspended TiO2	Amounts of immobilized TiO2 were 0.76, 2.04, 4.08, and 6.12 mg/cm2 of polymeric membrane	51% w/w of 4-NP was removed after 5–6 h of operation in the immobilized system and 80% w/w of 4-NP degradation was obtained in 5–6 h of operation in the suspended system	ksapp = 0.27 h−1	[106]
TiO2 (P25)	10−3 M of 3,5-dichlorophenol (3,5-DCP)	1625 ppm of P25	TiO2 nanocrystalline thin films were developed on glass substrates by applying dip coating and doctor-blade deposition techniques with a surface area of 0.8 cm2	Complete degradation of 3,5-DCP occurs in less than 100 min using the slurry system, with the doctor-blade TiO2 films achieved after about 360 min of illumination and for the sol–gel films after about 1600 min	kiapp = 0.0079 min−1 and 0.0023 min−1 for Optical Fiber/TiO2 Coating and Optical Fiber/TiO2 sol–gel-coated, respectively	[138]

(*) kⁱ_app is the apparent kinetic constant of the immobilized photocatalyst; k^s_app is the apparent kinetic constant of the slurry photocatalyst; k⁵ⁱ_app is the apparent kinetic constant of the immobilized photocatalyst after five cycles. In all three cases, pseudo-first-order kinetics was considered.

, a comparison between the reaction rate of the same photocatalyst in immobilized and suspended forms is reported.

3.5.2. Light Source, Light Intensity, and Wavelength

The minimum radiation wavelength required to promote an electron from the valence to the conducting band of a semiconductor depends upon the bandgap energy of the photocatalyst. When a photon of a certain wavelength impinges on the photocatalyst surface, if the energy is equal to or greater than the bandgap value, electrons are moved from the valance band and transferred to the conducting band. This leaves positive holes in the valence band, which, in the presence of O₂ and H₂O, generate OH^• radicals, the most potent oxidizing agents, leading to the destruction of pollutants [139]. The species formed are named ROS, which in addition to hydroxyl radical (OH^•), also include superoxide radical anion (O₂^•−) and H₂O₂. The most dominant ROS is the hydroxyl radical, which, despite its rather short lifetime (approximately 10 μs in natural water), is the most oxidant species [47]. When a semiconductor is present as a powder in suspension, each particle acts as a small photocell, and in 100 mg of powder, consisting, for example, of 0.1 mm diameter particles, more than 10¹¹ independent particles are present, thus making the system very effective [37]. The incident light that initiates the photocatalytic reaction is in a wavelength region that is absorbed by the photocatalyst [140]. The minimum wavelength required to promote an electron depends upon the physicochemical properties of the photocatalyst [141]. Many synthesized photocatalysts have different bandgap energy values, and for this reason, the optimization of the light source is an important parameter. Generally, for UV irradiation, UV lamps are used, but the use of this type of lamp has some disadvantages like fragility, toxicity (due to mercury), short lifetime, high-energy consumption, etc. [142,143]. Alternatively, over the last few years, light-emitting diodes (LEDs) have opened a new path for their low energy consumption [144]. However, the use of UV-LED is still under debate, with researchers discussing whether ultraviolet light is enough for photocatalysts and questioning the proper frequency range to be used during the oxidation process. Nevertheless, there are some advantages to the use of LEDs such as reduced costs, compact size, lightweight, lower operating temperature, long lifetime, the possibility of operating night and day, and choice of the number and position of the lamps inside the photoreactor to optimize the reaction rate and thus the reactor’s footprint. With the development of UV-LED technology, it is possible to pulse UV-LEDs thus conducting controlled periodic illumination (CPI) [145], as already described in Section 3.3.1. CPI is characterized by a period of light-on and light-off. CPI application allows us to have an optimal number of photons a photocatalyst can utilize in a given turnover number (T_ON) period or under continuous illumination before saturation. Pulsed LED light allows for the enhancement of the efficiency of a photocatalytic reaction, thus facilitating the determination of the specific light intensity required for improvement in the photonic efficiency [68]. In particular, it is known that, at low light intensity (0–20 mW/cm²), the reaction rate increases linearly with light intensity (first order); in contrast, at intermediate light intensities (25 mW/cm²), the rate depends on the square root of the light intensity [146]. The increase in light intensity allows for electron–hole formation as well as their recombination, while at a low light intensity, electron–hole formation is predominant, and recombination does not occur. When the light intensity is increased, electron–hole for degradation will compete with electron–hole recombination [147]. Liang et al. [148] used pulsed UV-LED with modulation for TiO₂ irradiation in an advanced oxidation process for the degradation of pharmaceutical micropollutants. They observed that when TiO₂ is immobilized on a substrate and irradiated using UV-LEDs, the use of CPI is determining. Sannino et al. [149] studied the impact of light modulation on the decolorization of Acid Orange 7 (AO7), using a fixed-bed photocatalytic reactor containing a N-TiO₂ photocatalyst under irradiation by visible light generated using 240 white LEDs. The system was optimized with a current between 50 and 100 mA, a period of 10 s, and a sinusoidal modulation.

3.5.3. Membrane Configuration

The low footprint of photocatalytic membrane reactors is related also to membrane configuration. If a membrane reactor is based on photocatalytic membranes (which means immobilized photocatalysts in various configurations), the irradiation of the photocatalytic layer requires maximizing the exposed surface area, which means a higher footprint than a CSTR–slurry photoreactor, as estimated at the beginning of this section. For CSTR–slurry photoreactors, the most studied configuration is that with a submerged membrane where the permeate is sucked with a pump. In applications different from photocatalysis, a recent approach that does not use a pump to withdraw the permeate is the use of a gravity-driven membrane, which is subject to the phenomenon of flux stabilization and requires less energy consumption [150,151]. A gravity-driven membrane that could be of interest for photoreactors at the industrial level is the submerged flat plate that was used in a membrane bioreactor (MBR) in a case study by Toray. The membrane had pores of a nominal size of 0.08 μm (between ultrafiltration and microfiltration) and the average flux was 25.7 LMH (L m⁻² h⁻¹); the filtration method was via gravity; the total membrane area per module was 560 m² in a double-deck arrangement, with 10 modules and 144 m³/h total capacity; and system footprint required 12 m of length and 4 m of width for the membrane tank [152]. However, as already reported by Molinari et al. [75], ultrafiltration and microfiltration membranes do not have a high capacity to retain many pollutant molecules, so, if a high quality of treated water is required, nanofiltration (NF) or reverse osmosis (RO) membranes must be used to treat the permeate from MF or UF. In that work, an NF membrane was used, and instead of a submerged membrane, a vertical filter working with gravity was used. In this configuration, the plant footprint, aside from the photoreactor, is related to NF or RO membrane configuration. The spiral wound module is the most used for NF and RO operations in plants with a water processing capacity of thousands of cubic meters per day, due to its high packing density (membrane surface per unit volume), high-level of salt rejection and water permeation, and flexibility of scale-up (ranging from a small size to large size) [153,154]. In choosing the type of spiral wound module, the feed spacer is of particular interest because it influences the performance of the module [155]. It is foreseen that these types of modules (NF and RO), which are commercially available with different characteristics (membrane material, rejection, flux, feed spacer, etc.), can contribute to improving the water quality obtained from a PMR as well as a reduction in plant footprint.

The use of a suitable selective membrane also contributes to reducing the plant footprint by controlling the residence time in the slurry–CSTR to achieve the maximum mineralization of the various pollutants. Indeed, the rejection ability of the membrane to retain many pollutants in the reaction ambient until they are mineralized allows for the design of the photoreactor volume by considering only the minimum retention time (RT = V/Q, where V is the reactor volume, and Q is the incoming flow rate). Since different pollutants need different retention times to be mineralized, the slurry–CSTR system combined with RO or NF has a significant advantage over conventional photoreactors and photocatalytic membranes (these usually operate in a flow-through mode). In these last two cases, the size of the reactor volume must be determined using the highest retention time, meaning a high volume (and thus a high footprint) if Q is fixed.

3.6. Easy Maintenance

Each industrial plant requires accurate engineering actions during the life cycle, allowing for restoring the system to a state of functionality and, eventually, improving its functionality if the upgraded components of the plant will be available. These components include, for example, a new photocatalyst with improved performance, new types of lamps, new membrane modules, etc. If the plant has been designed with a criterion that allows for easy acquisition of new functionalities, this could be very relevant. For membrane photoreactors, a component that could require adjustment during the plant life cycle is the photocatalyst. Depending on the confinement mode of the photocatalyst (slurry confined by the membrane or immobilized in/on a membrane), the difficulty of adjustment can be significantly different. This could concern (i) the amount and type of photocatalyst, and (ii) photocatalyst deactivation and regeneration, as described in the following section.

3.6.1. Amount and Type of Photocatalyst

The photocatalytic effect is related to the bandgap value of the photocatalyst, which enables the chemical reaction [156]. The initial step for heterogeneous photocatalyst systems is photonic excitation on the catalyst surface. Therefore, the reaction rate depends on the amount and type of photocatalyst. In fact, it is known that, after a certain value of photocatalyst mass, the reaction rate levels off and becomes independent of mass. This limit depends on parameters such as geometry, the working conditions of the photoreactor, and the mechanism of the photocatalytic reaction that promotes the formation of OH^• radicals and substrate adsorption on the catalyst surface. High quantities of photocatalysts cause a screening effect, which masks part of the photosensitive surface, and thus leads to a reduction in OH^• radical generation, which reduces photocatalytic activity [58,75,157]. However, some scattering from directly irradiated particles to some indirectly irradiated particles can occur, thus reducing the masking effect. This phenomenon mainly occurs in slurry photocatalysts rather than in immobilized photocatalysts.

It is also important to define the correct type of photocatalysts for water pollution degradation. Cao et al. compared different catalysts for polystyrene (PS) degradation such as TiO₂, ZnO, ZnS, and g-C₃N₄ [158]. From this study, they observed that TiO₂ is the worst for polystyrene degradation. In comparison, for different antibiotics, Habibi et al. highlighted the impressive activity of a CeO₂-x/AgFeO₂/Ag photocatalyst [159]. Bismuth sillenite crystals were studied by Baaloudj et al. for water treatment [160]. Carbon material in various forms (nanoporous carbon, nanotubes, etc.) as a dopant on TiO₂ was reviewed by Mestre and Carvalho for the photodegradation of various recalcitrant pharmaceuticals in wastewater [161]. Photocatalytic adsorbents like Fe/Mn-SiO₂ nanocomposites and Fe-AC (activated carbon) were studied by Baek et al. [162] in the photocatalytic degradation of pharmaceuticals and perfluorinated compounds in an aqueous environment. Qanbarzadeh et al. [163] used a boron nitride photocatalyst to efficiently degrade poly-/perfluoroalkyl substances in complex water matrices. These examples show that research on new and more active photocatalysts is of broad interest. Furthermore, bare photocatalysts can be limited by their wide bandgap, which requires higher energy of incident light, resulting in a response region only in the ultraviolet light. For this reason, it is of interest to consider some approaches that promote charge separation and light utilization like support presence, metal loading, doping nonmetallic element, and decorating with functional group [164]. Packialakshmi et al. [165] synthesized ZnO/SnO₂/reduced graphene oxide nanocomposites (ZnO/SnO₂/rGO NCs) for the photocatalytic degradation of organic dye pollutions. The use of ternary nanocomposites (NCs) provides different properties, such as crystallinity, low recombination of photogenerated charge carriers, energy gap, and surface morphologies. They compared photocatalysts like ZnO, ZnO/rGO, SnO₂/rGO samples, and ZnO/SnO₂/rGO NCs. They found that nanocomposites had great photocatalytic effectiveness for the destruction of orange II (99.8%) and reactive red 120 dye (97.02%), after 120 min of sunlight irradiation. Gordanshekan et al. [166] degraded Cefixime (CFX) using Bi₂WO₆/g-C₃N₄ (BC mass ratio of 60%) and Bi₂WO₆/TiO₂ (BT mass ratio of 12.5%) under UV–visible light irradiation. After 180 min of reaction, CFX and the by-products were entirely mineralized. The LC-MS and TOC tests revealed that most toxic products were not present after 135 min of reactions. Leelavathi et al. [167] synthesized a ternary g-C₃N₄/Co/ZnO heterojunction photocatalyst for methylene blue (MB), crystal violet (CV), and rhodamine B (RhB) degradation, and degradation rates of 96.3%, 74.5% and 75.14%, were observed, respectively. RhB was irradiated under visible and sunlight, and the degradation efficiency increased to 91.6% under sunlight. Maulana et al. [168] used Ag/TiO₂ for the degradation of polyethylene (PE); in particular, they studied different microplastic range sizes, namely 100–125, 125–150, and 150–200 μm. Their results showed that, at an initial concentration of 100 ppm, the best degradation percentage at particle size 125–150 μm was 100%, which was achieved at 90 min irradiation. Denisse et al. [169] synthesized C,N-TiO₂ and tested the purification of 50 mL solution with a 2 wt/vol% aqueous dispersion of polystyrene (PS) and 0.01 wt/vol% aqueous dispersion of PS in a batch-type glass reactor. They studied the calcination treatment of the photocatalyst at different temperatures and found that the TiO₂-based photocatalyst can be a good candidate for polystyrene degradation but not enough to achieve high degradation efficiency.

The choice of the type of photocatalyst for tertiary wastewater treatment is also related to the pH range where the photocatalytic activity is high. Indeed, as already reported, the pH of the wastewater that feeds the tertiary stage (PMR in our case) is around 8.5, and it is not conceivable to change it because of the very high wastewater flow rate. This prompts the use of photocatalysts with maximum activity in a wide pH range with an average value of 8.5. For example, in the study of Despotovic et al., using ZnO nanoparticles, the apparent kinetic constant was higher in the pH range 7–10 for the drug amitriptyline, while for the pesticides clomazone and sulcotrione, it was independent of pH [170]. In another work, the photocatalytic degradation of sulfamethoxazole was carried out at pH 4.7 on ZnO and TiO₂ (P25) photocatalysts [55]. Despite the interesting results from a scientific point of view, these last data are of no interest for sewage WWT because the working pH (4.7) is outside of that of interest (8.5 as the average value).

A further aspect that can limit the development of photocatalytic reactors in sewage WWT, and PMRs in particular, is the availability of massive quantities of photocatalysts as well as the availably of photocatalyst mixtures. Indeed, it can be foreseen that a mix of photocatalysts could perform better than a single photocatalyst because, from the existing literature, it has been widely observed that various pollutants are photodegraded with different rates on the same photocatalyst. This means that research should be conducted by testing real wastewater (or mixtures of many pollutants) and different mixtures of photocatalysts.

3.6.2. Photocatalyst Deactivation and Regeneration

Many studies have highlighted the potential of photocatalysts such as ZrO₂, SnO₂, Fe₂O₃, WO₃, CeO₂, ZnS, TiO₂, etc., for their excellent photocatalytic efficiency under UV light and energy band gap (Figure 9), but an important parameter to consider is photocatalyst deactivation [171].

This phenomenon depends on the reaction ambient; indeed, during the photocatalytic reaction, the photocatalyst will be constantly exposed to light and mixtures of fresh and treated solutions containing a variety of pollutants (target and non-target). In this way, the accumulation of various organics, including the parental organics, intermediates, and non-target pollutants, will likely occur (via adsorption) on the surface of the photocatalyst. This is a problem because it causes a dramatic decrease in photocatalytic activity (degradation of organics) [173,174]. Liqiang et al. [175] examined the lifetimes of ZnO and TiO₂ nanoparticles in the gas-phase photocatalytic oxidation of n-C₇H₁₆ or SO₂. The obtained results showed that ZnO could be deactivated in the gas-phase photocatalytic oxidation of n-C₇H₁₆, while TiO₂ could nearly retain its activity. In the gas-phase photocatalytic oxidation of SO₂, ZnO, and TiO₂ could both be deactivated. They justified the deactivation after photocatalytic reaction via the adsorption of the oxidation products such as H₂O, CO₂, and SO₃ on the semiconductor photocatalyst surface. For these reasons, generally, after some cycles of reaction, the regeneration of the photocatalyst is necessary. In a review by Yan Tang et al. [176], various aspects such as lifetime, deactivation mechanism, and regeneration efficiency/characterization of deactivated photocatalysts were discussed. The deactivation of the photocatalyst in the photocatalytic degradation of methyl orange (MO) was studied by Yan Li et al. [177], who observed that it was not due to pore blocking by the reactant (MO) or intermediate products but due to the surface adsorption of MO and reaction intermediates. The spent photocatalysts were regenerated after washing with methanol and hydrogen peroxide (H₂O₂) and then treated with heat. H₂O₂ treatment generated the highest regeneration rate because H₂O₂ is a strong oxidizing agent that oxidized the deposited species on the surface of the photocatalyst. In other studies, regeneration was accomplished with different methods such as the use of basifying agents [178], thermal process [179], use of UV irradiation, and oxidation with H₂O₂ [180,181].

The problem of photocatalyst deactivation described in this section allows us to anticipate the easy maintenance of PMR plants because, among other components, photocatalysts may also need a regeneration program.

4. Analysis of the Main PMR Configurations Related to the Main Parameters of a Tertiary Stage

As described in the previous sections, the basic configurations of PMRs are the photocatalytic membranes (using organic or inorganic membranes), where the photocatalyst is immobilized in/on the membrane, which is used as support, and the slurry–PMRs, where the photocatalyst is suspended in the reaction media, and the membrane is able to confine the photocatalyst in the photoreaction zone. In these two configurations, numerous sub-configurations have been proposed, and some of them have been described before. However, here, a gross comparison between these two basic configurations is provided to identify which one seems more suitable for a tertiary sewage WWT. The fact that sewage WWTPs operate continuously requires maintaining optimal conditions constantly. This means that solar light (available only during the day) for direct photocatalyst irradiation must be excluded, but it can be considered as a photovoltaic electrical energy source (e.g., accumulation system) for the integration of the electrical energy required to power the lamps. Furthermore, it is known that the intensity of solar radiation is variable during the day, so a variable photodegradation rate will be obtained during the day. The composition of wastewater from the secondary stage could be variable during this time, so a slurry–PMR will mitigate these oscillations better than a photocatalytic membrane, resulting in less oscillation of the photodegradation rate. The quality of the water from the tertiary stage could be controlled with various types of membranes (NF or RO), mainly the spiral-wound type widely used in many large-scale plants. The plant footprint is strongly dependent on the type of photocatalyst confinement. The immobilized photocatalyst is present as a layer to maximize the exposed surface to the light, but this also gives a high plant footprint compared with a slurry–PMR, where the same photocatalyst amount is suspended in a CSTR photoreactor with a smaller footprint (see Section 3.5). Furthermore, the irradiation of the photocatalytic layer requires (in continuous operation) external frontal lamps. Instead, cylindrical bulb lamps, immersed in the slurry, can irradiate at 360° with increased efficiency of the required electrical energy. With the use of external lamps, the photocatalytic layer in/on the photocatalytic membrane can also be placed in vertical mode to reduce the plant footprint. However, a drawback of this configuration still exists. Indeed, all immobilized photocatalyst particles are always illuminated on the same side and can be subject to a shorter life cycle than a slurry–CSTR. If we consider a photocatalyst particle having a spherical shape, this particle could be covered in some part of the surface by adsorbed species that inactivate that part to photonic excitation. If particles are suspended, they can move and rotate in all directions and can be subject to excitation on the other parts of the spherical surface. Indeed, the loss of active photocatalyst surface for the immobilized photocatalysts is higher than for suspended photocatalysts (the initial available active surface is that of all spherical surfaces for suspended particles, while it is a spherical cup for immobilized particles). This last aspect requires a more frequent regeneration in the case of immobilized photocatalysts. The regeneration in the case of slurry–CSTR systems is conceptually simple: the cessation of aeration allows for the sedimentation of particles that can be removed and substituted by a fresh photocatalyst. In the case of immobilized photocatalysts, depending on the type of immobilization, this operation can be quite difficult and could require the substitution of the overall photocatalytic membrane with a significant increase in maintenance costs. The final assessment of the six parameters discussed in Section 3 is reported in

Table 2. Suitability or unsuitability of each of the six parameters to fit the requirement of a tertiary stage of wastewater treatment for the main configurations of PMRs.

Configuration of PMR	Continuous Wastewater Flow Rate (Section 3.1)	Self-Control of the Photodegradation Rate (Section 3.2)	Ability to Handle Variations of Concentrations and Flow Rate (Section 3.3)	Control of the Quality of the Treated Wastewater (Section 3.4)	Low Footprint (Section 3.5)	Easy Maintenance (Section 3.6)
Photocatalytic membrane	Yes	No	No	No	No	No
Slurry–CSTR PMR	Yes	Yes	Yes	Yes	Yes	Yes

, which provides a quick overview of which configuration of PMR could be more suitable in a tertiary sewage WWT. It should be noted that all the described features have been considered for a specific type of wastewater, but for other types of wastewater, other specific parameters should be found and analyzed.

As shown in Table 2, the slurry–CSTR PMR seems the most suitable configuration. In particular, the configuration employing a gravity filter to keep the photocatalyst in the reaction in the reaction zone and an NF or RO membrane to control the residence time of pollutants in the photoreactor seems, at this stage of knowledge, the specific configuration on that the research efforts should be concentrated. Furthermore, as widely described in the review of Rueda-Marquez et al. [182], extensive studies on real wastewater detoxification using photocatalysts are also needed.

The conclusion reached in this analysis (slurry/suspended photocatalysts perform better than immobilized photocatalysts) is focused on the comparison between the two basic PMR configurations in a particular application (tertiary sewage treatment), considering the possibility of obtaining volumes of good quality water available for reuse. However, other than this first type of comparison, to decide if slurry–PMR systems are advantageous or not at an industrial scale, a comparison with other technologies in tertiary wastewater treatment, by means of sustainability assessment, including the associated economics, must also be carried out.

Multi-criteria analysis was carried out by Plakas et al. [183] to assess four tertiary wastewater treatment (WWT) technologies regarding their sustainability performance to obtain useful information for designers and decision makers. The technologies were powdered activated carbon adsorption coupled with ultrafiltration membrane separation (PAC-UF), reverse osmosis, ozone/ultraviolet-light oxidation, and heterogeneous photocatalysis coupled with low-pressure membrane separation (PMR). Although the PAC-UF technology appeared to be the most appropriate, PMR was highlighted as the technology configuration. In particular, the configuration employing a gravity filter to keep the photocatalyst in the reaction under ambient conditions and an NF or RO membrane to control the residence with the least variability in its performance. For carrying out an assessment for the comparison of similar water treatment processes, it is necessary to use appropriate performance indicators to take into account economic issues like equipment capital expenses, operating expenses, and sustainability assessment, considering the fact that, for specific applications, the selection of the appropriate water treatment technology should be made on the basis of “triple bottom line” (TBL), i.e., by using environmental, economic and social criteria [184].

Concerning the cost estimation of a PMR, Samhaber and Nguyen [185] evaluated the costs separately because of the specific different nature of the photocatalytic part and the membrane part. The cost items are energy, membrane, chemicals, capital, UV–visible lamp replacement, personnel, etc., which are converted into unit cost per m³ of treated water. The cost estimation [185] among various AOPs like H₂O₂/medium-pressure UV, H₂O₂/O₃, hydrodynamic cavitation with H₂O₂, and TiO₂ photocatalysis under UV (without membrane) showed that, for the first three technologies, the costs were in the range of 0.71–1.13 USD/1000 gallons treated (0.19–0.30 USD/m³) for a TOC in the range 0–8–8 mg/L, while for photocatalysis–UV, they increased from 1.68 USD/1000 gallons to 3.08 USD/1000 gallons (0.44–0.81 USD/m³) with the increase in TOC. This happens because the treatment of higher TOC concentration effluent will surely lead to a shorter photocatalyst lifetime and more often reactor and lamp cleaning. Another cost estimation was carried out by Rani and Karthikeyan [186] on the degradation of a mixture of polycyclic aromatic hydrocarbons (PAHs) in a UV slurry photocatalytic membrane reactor. They compared photocatalysis (UV-TiO₂), membrane separation (MS), and integrated photocatalytic membrane (UV-TiO₂ + MS) and found that UV-TiO₂ + MS could be successful based upon the operating time and operational costs, as the operating time was twice lesser than the conventional UV-TiO₂ process. They estimated a treatment cost per m³ of effluent containing PAHs in PMR to be USD 10.4 to USD 3.6.

The results of cost estimation are generally expressed as cost/m³, but an important aspect to be considered is the source of the used data. They can come from a laboratory plant or plants of different sizes, but the same scale factor (e.g., same treated water flow rates) should be considered when different technologies are compared. Indeed, for the same technology, it is also well known that the costs/m³ for plants of larger sizes are generally lower than that of smaller sizes. These factors should also be considered in research, and this means that more data are needed on pilot plants of suitable size for PMRs to obtain reliable information for decision makers.

5. Conclusions

The research on photocatalytic membrane reactors (PMRs) has shown their great potential in wastewater treatment, and some studies are at bench scale and pilot plant level. The field of sewage wastewater treatment involves very high flow rates, so high volumes of good-quality treated water could be reused for various purposes provided an efficient tertiary treatment is available. PMRs could mineralize many recalcitrant pollutants in the tertiary stage to reach the requested quality also due to the use of selective NF or RO membranes. The analysis of the two main configurations of PMRs (photocatalytic membranes and slurry–PMRs) was carried out using six design and operational parameters, concerning the sewage wastewater treatment: (i) a continuous wastewater flow rate from the secondary stage; (ii) the self-control of the photodegradation rate related to wastewater chemical–physical parameters; (iii) the ability to handle variations in concentration and wastewater flow rate; (iv) the control of the quality of treated wastewater; (v) low plant footprint; and (vi) easy maintenance. Important findings that emerged are the continuous flow of real industrial effluents and the very high flow rates that prompt, as future research, the use of PMR configurations with continuous illumination (e.g., low-energy-consuming LED lamps), a photodegradation rate with small oscillation, membranes with high packing density able to work at high flux and high rejection rates versus recalcitrant pollutants, low footprint design, and the easy substitution of the photocatalyst when it is deactivated. All these points seem fit well, at the current state of knowledge, with the choice of slurry–PMRs compared with photocatalytic membranes. Furthermore, research on mixtures of different photocatalysts is needed to optimize the photodegradation rate on a wide range of pollutants as well as the use of real wastewater (or mixtures of various pollutants) to obtain information on the robustness of PMR technology. Tests on pilot PMR plants of suitable size are required to obtain reliable data for performing a sustainability assessment of the slurry–PMR process compared with other processes already used in tertiary wastewater treatment. With this study, it is envisioned that researchers could acquire a good direction for the development of future research in the effort to bring photocatalysis combined with membranes to practical application in the reuse of sewage wastewater.