Solar Advanced Oxidation Processes Using Parabolic Trough Concentrators: A Mini-Review

Abstract

Solar advanced oxidation processes (AOPs) utilising parabolic trough concentrators (PTCs) present a promising approach for the sustainable removal of recalcitrant contaminants from wastewater. This mini-review critically evaluates 25 peer-reviewed studies employing PTC-AOP systems for the degradation of chemical pollutants and microbial pathogens. Reported applications include photolysis, photo-Fenton and photocatalysis for the treatment of synthetic dyes, contaminants of emerging concern, industrial effluents, heavy metals and pathogenic microorganisms. A performance-oriented comparison based on normalised indicators is introduced. The time required for one order-of-magnitude reduction (corresponding to 90% removal; τ₉₀) reveals a significant mineralisation setback, where parent-compound degradation outpaces total organic carbon removal. The PTC concentration ratio and photon utilisation metrics highlight substantial variability in reactor design (geometry, materials, optical performance), which directly influences the treatment kinetics. Overall, PTC-AOP systems demonstrate strong potential as a polishing step within hybrid wastewater treatment. Future research should prioritise the standardisation of performance metrics, the catalyst design suited for high-photon and -temperature operation, and the integration into scalable and climate-resilient solar wastewater treatment.

1. Introduction

The Anthropocene clearly represents socio-economic growth driven by technological progress but also unprecedented pressure on the Earth’s systems, particularly the global water cycle [1]. Human-caused climate change is the main driver of weather extremes (heatwaves, heavy precipitation, droughts, floods), which destabilise freshwater availability [2]. Also, pollution and ecosystem degradation have a harmful impact on both the quality and quantity of accessible water resources [3]. While the Sustainable Development Goals (SDGs) aim for universal access to clean water (SGD6 goal), current data indicates that out of 187 billion m³ wastewater (WW) generated annually, only 56% of domestic WW is safely treated [4]. A critical scientific problem remains: conventional secondary treatment processes are frequently unable to remove recalcitrant organic pollutants, emerging contaminants and pathogens. Their persistence presents a barrier to achieving the high-quality reclaimed water needed for industrial circularity and unrestricted irrigation.

To address this, advanced oxidation processes (AOPs) are required as tertiary or quaternary treatment [5,6]. Unlike state-of-the-art technologies such as membrane filtration [7], where pollutants are transferred to the concentrated waste stream, AOPs utilise hydroxyl radicals (^•OH) to mineralise toxic pollutants to inorganic species [8,9]. However, their practical implementation is often hindered by high energy consumption and the associated operational and maintenance costs of artificial UV irradiance [9,10].

Integration of solar energy through Concentrated Solar Power (CSP) systems offers a sustainable solution for these constraints. Recent studies highlight the potential of solar-assisted AOPs to enhance treatment efficiency while reducing energy demand [6,8,11,12,13,14,15,16]. As shown in Table 1, various reactor designs, like the Raceway Pond Reactor (RPR), Inclined Plate Collector (IPC) and Compound Parabolic Collector (CPC), are utilised [5,8,12,13,14,15,17,18,19,20,21,22,23]. However, the Parabolic Trough Collector (PTC) stands out for its superior optical performance in concentrated photochemical applications. While CPCs are limited by low concentration ratios (CR), PTC systems utilise parabolic mirrors to concentrate direct solar radiation onto a receiver tube, achieving a higher photon concentration per volume unit. Quantitatively, this feature facilitates faster ^•OH radical production rates and reaction kinetics, making PTCs more efficient for treating concentrated or highly toxic industrial streams compared to non-concentrating or stationary collectors.

Table 1. Advantages and disadvantages of CSP systems for AOP wastewater treatment.

Reactor Type	Design Principle	Advantages	Disadvantages	Key Performance Metrics
Raceway Pond Reactor (RPR)	Non-concentrating and open reactor. Usually, shallow hippodrome-shaped bathtub. Water circulation is maintained by paddle wheel system.	High capacity. Low-cost construction. Flexible design, easy to scale up.	Non-concentrating. Water loss due to evaporation. Effluent overflow in rain conditions. Limited to daylight.	CR: Non-concentrating System type: Open; possible overflow Flow regime: Laminar Irradiance: Direct and diffuse
Inclined Plate Collector (IPC)	Flat or corrugated inclined plate over which reactant fluid flows in thin film. Uses supported or immobilised photocatalysts. Uses both direct and diffuse radiation	Simple design and low cost. Large surface area for supported catalyst. Utilises both direct and diffuse radiation.	Low mass transfer rates due to laminar flow. With open and thin film of water stream, high losses of chemicals/water due to evaporation. Requires large surface area. Longer treatment times compared to other systems.	CR: 1 System type: Open Flow regime: Laminar Irradiance: Direct and diffuse
Compound Parabolic Collector (CPC)	Stationary collector with reflective parabolic surface to concentrate solar irradiance. Uses both direct and diffuse sunlight. Concentration ratio is between 1 and 15.	Utilises both direct and diffuse radiation. No tracking system, reducing complexity and cost. Homogeneous distribution of radiation onto receiver tube. High optical and quantum efficiency.	High initial investment costs. Requires large land area (for industrial implementation). Potentially excessive light irradiance and overheating. High manufacturing costs due to reflective mirror design.	CR: Low (1–1.5) System type: Closed Flow regime: Turbulent Irradiance: Direct and diffuse
Parabolic Trough Collector (PTC)	Uses parabolic reflective mirror to concentrate direct solar radiation onto receiver tube. Requires solar tracking system to keep the aperture perpendicular to the solar rays.	Receives large amount of energy per volume unit. Maintains turbulent conditions. Nearly closed system, volatile compounds do not vaporise. High UV intensity produces more hydroxyl radicals.	Uses direct irradiation, with lower efficiency on cloudy conditions. Requires tracking system. Risk of overheating. Excessive light irradiance can limit efficiency due to high electron/hole recombination.	CR: High (>15) System type: Closed Flow regime: Turbulent Irradiance: Direct
Linear Fresnel Reflectors (LFRs)	Line-focus system using array of segmented flat mirrors to track sun and reflect direct irradiation onto fixed receiver. Equipped with PTC or CPC.	Can achieve high concentration ratio with high thermal efficiency. Use small, flat mirrors for low-cost manufacture. Fixed receiver has large collection area, leading to simpler photoreactor design.	Require complex tracking mechanisms. Rely on direct sunlight radiation. Lower efficiency on cloudy days. Lower optical efficiency due to greater distance between mirrors and receiver. In sunny conditions, ultra-high illumination can limit photocatalytic activity.	CR: High (>15) System type: Closed Flow regime: Turbulent Irradiance: Direct

Selecting between PTC and CPC designs involves additional critical performance trade-offs [22,24]:

• Turbidity tolerance—CPCs are often more robust when treating turbid wastewaters or operating on cloudy days, since they capture both direct and diffuse irradiation. In contrast, PTC efficiency depends on direct irradiation, which makes them sensitive to cloud cover and water turbidity, which scatters photons.
• Mass transfer and fluid dynamics—While both systems aim for high optical efficiency, the intense energy concentration in PTC allows for a more compact reactor footprint. Furthermore, PTCs typically maintain turbulent flow within the receiver tube, ensuring mass transfer and efficient mixing of the WW, catalyst and oxidant.
• Operational complexity and cost—PTC requires precise (automated) solar tracking systems to keep the aperture perpendicular to solar rays, which increases the initial capital investment and maintenance requirements. In contrast, the stationary design of the CPC reduces mechanical complexity and costs, though it requires a larger land area to achieve comparable treatment volumes.

The feasibility of CSP systems, especially PTCs, is inherently tied to the availability of Direct Normal Irradiance (DNI) [20,25,26]. An analytical mapping of global DNI (Figure 1) shows that regions with the highest solar potential are simultaneously recognised as climate-change hotspots experiencing severe water depletion and drought, such as the Mediterranean and Balkan basins [27]. In these contexts, the integration of PTC-driven AOPs represents not only an engineering option, but a strategic approach to enhance water resilience through safe, low-energy wastewater treatment and reuse.

Figure 1. World map of direct normal irradiation (map obtained from the “Global Solar Atlas 2.12”, a free, web-based application developed and operated by the company Solargis s.r.o. on behalf of the World Bank Group, utilising Solargis data, with funding provided by the Energy Sector Management Assistance Program (ESMAP). For additional information: https://globalsolaratlas.info (accessed on 17 November 2025).

Despite the scientific progress in CSP-assisted treatments, existing knowledge on PTC-AOPs remains scattered across diverse experimental setups and geographical contexts. There is a lack of standardised performance metrics, which hinders direct comparison of treatment efficiencies and reactor designs. To address this research gap, the aim of this paper is to provide a structured, performance-oriented assessment of PTC-driven solar AOPs. This review introduces and applies normalised evaluation criteria, such as the time required for a one-order-of-magnitude reduction (τ₉₀) and the photon utilisation ratio (PU), to assess current research and identify future directions for scalable and climate-resilient solar wastewater treatment.

2. Methodology

A systematic search was conducted in the Scopus and Web of Science (WoS) databases to identify peer-reviewed research papers published between 2000 and 2025. The search strategy employed specific field tags in Scopus (article title, abstracts and keywords) and WoS (Topic) to ensure comprehensive coverage of relevant literature. The exact search strings (with Boolean operators) and the search results were as presented:

• “wastewater treatment” AND “advanced oxidation process”—this search resulted in highest number of publications, with 8306 in Scopus and 7981 in WoS;
• “wastewater treatment” AND “advanced oxidation process” AND “parabolic trough collector/concentrator”—resulted in 0 publications in Scopus and 84 in WoS;
• “wastewater treatment” AND “parabolic trough collector/concentrator”—resulted in 27 publications in Scopus and 24 in WoS.

Following the initial identification of search results, duplicates were removed, and a screening process was applied. To satisfy the requirement for explicit selection criteria, the following parameters were used during title, abstract and full-text evaluations (Table 2). After full-text evaluation, 25 studies that employed a PTC system for wastewater treatment processes were included for final review. Key variables, such as PTC configuration, AOP type, treated pollutants and process efficiencies, were extracted and further evaluated. All collected data are provided in the Supplementary Materials.

Table 2. Inclusion and exclusion criteria.

Criteria	Inclusion	Exclusion
Document type	Original peer-reviewed research articles	Non-research documents, books and book chapters, conference papers and review articles
Technology	AOPs utilising PTC reactors	AOPs utilising other CSP reactor designs
Water matrix	Both synthetic and real WW	Non-WW matrices
Light source	Solar; solar with artificial lamp	Only artificial UV systems
Scale	All experimental scales

Calculations for Performance Review

Due to incomplete data in several studies, additional variables were calculated where sufficient information was provided. The following equations were used [28,29,30]:

• PTC receiver volume (or irradiated volume; V_irr (L)), assuming cylindrical geometry, where d_r is receiver diameter (inner or outer, depending on availability; cm) and l_r is receiver length (cm):

$${\mathrm{V}}_{\mathrm{i}\mathrm{r}\mathrm{r}}={\displaystyle \frac{\mathsf{\pi }\times {\left(\frac{{\mathrm{d}}_{\mathrm{r}}}{2}\right)}^2\times {\mathrm{l}}_{\mathrm{r}}}{1000}}$$

(1)

• Recirculation number (RN), defined as the ratio between the total treated WW volume (V_total; L) and irradiated volume (V_irr; L):

$$\mathrm{R}\mathrm{N}={\displaystyle \frac{{\mathrm{V}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}}{{\mathrm{V}}_{\mathrm{i}\mathrm{r}\mathrm{r}}}}$$

(2)

• Residence time (t_res) represents the time that WW remains under irradiation in the PTC-AOP system, where Q represents flow rate (L/min):

$${\mathrm{t}}_{\mathrm{r}\mathrm{e}\mathrm{s}}={\displaystyle \frac{{\mathrm{V}}_{\mathrm{i}\mathrm{r}\mathrm{r}}}{\mathrm{Q}}}$$

(3)

• PTC concentration ratio (CR) was approximated using the PTC parabola width (w_p; cm) and receiver’s outer diameter (d_r; cm):

$$\mathrm{C}\mathrm{R}={\displaystyle \frac{{\mathrm{w}}_{\mathrm{p}}}{\mathsf{\pi }\times {\mathrm{d}}_{\mathrm{r}}}}$$

(4)

• PTC aperture area (A; m²), where l_p is parabola length and w_p is parabola width:

$$\mathrm{A}={\mathrm{l}}_{\mathrm{p}}\times {\mathrm{w}}_{\mathrm{p}}$$

(5)

• Photon utilisation ratio (PU; m²/L) indicates the extent of concentrated solar irradiation (A; m²) applied to the WW in the PTC receiver (V_irr; L):

$$\mathrm{P}\mathrm{U}={\displaystyle \frac{\mathrm{A}}{{\mathrm{V}}_{\mathrm{i}\mathrm{r}\mathrm{r}}}}$$

(6)

• The pseudo-first-order reaction rate constant (k_FO) was calculated, where process efficiency (eff) or mineralisation (min) and reaction time (t; min) were reported:

$${\mathrm{k}}_{\mathrm{F}\mathrm{O}}={\displaystyle \frac{\mathrm{l}\mathrm{n}\left(\frac{1}{1-\mathrm{e}\mathrm{f}\mathrm{f}/\mathrm{m}\mathrm{i}\mathrm{n}}\right)}{\mathrm{t}}}$$

(7)

• Finally, the time required to reduce the pollutant concentration by one order of magnitude (90%; τ₉₀) was determined based on the calculated k_FO for process efficiency or mineralisation:

$${\mathsf{\tau }}_{90}={\displaystyle \frac{\mathrm{l}\mathrm{n}\left(10\right)}{{\mathrm{k}}_{\mathrm{F}\mathrm{O}}}}$$

(8)

3. Overview of PTC-Driven AOPs for WW Treatment

The successful application of PTCs in advanced WW treatment requires a holistic evaluation of engineering design, process parameters, the complexity of the water matrix, and the economic and environmental feasibility of scaling up the technology. The following sections provide a detailed insight into these aspects, strictly based on the selected 25 studies.

3.1. Wastewater Treatment in PTC-AOP System

The reviewed studies demonstrate the utilisation of solar radiation for treating various pollutants, employing PTC as a prominent photochemical reactor. The benefit of the high-energy photon transfer has been exploited in photocatalysis, photo-Fenton, photolysis, and UV disinfection, across regions with high and moderate DNI. Figure 2 presents the global distribution of PTC-based applications, with the highest number of studies originating from India (8), Brazil and Egypt (3), Serbia and Iran (2), and Italy, Spain, Colombia, Japan, Mexico, Jordan, and Saudi Arabia (1).

Figure 2. Global coverage of PTC-driven solar WW treatment research and the distribution of applied processes.

The same figure also illustrates the distribution of AOP implementation across selected papers. The detailed overview of pollutants and microbial pathogens treated in PTC-AOP systems (Table 3) highlights the versatility of this technology. The reported treatment outcomes include removal efficiency and/or mineralisation, and it is important to distinguish the difference between these two metrics. For dye molecules, decolourisation represents the initial step of colour removal, where chromophore functional groups are broken. Similarly, for other pollutants, process efficiency describes a decrease from the initial concentration, whereas mineralisation is a slower process involving the (complete) conversion of intermediate byproducts into inorganic species (H₂O, CO₂, inorganic ions) [31,32,33,34].

The simplest process mechanism belongs to the photolysis, which involves either the direct or indirect (oxidant-assisted ^•OH radical production) breakdown of contaminants by UV light. Direct photolysis achieved up to 80% mineralisation of organic content in real textile effluent [35], while promising dye degradation was also reported for H₂O₂-induced photolysis [17,31]. Nevertheless, the considerable differences in initial dye concentrations across studies remain an important factor. Real textile effluents contain high loads of organic and inorganic matter, where unfixed dyes may reach concentrations of up to 300 mg/L [36]. Future pilot-scale applications of PTC-AOP systems should examine treatment of concentrated textile WW streams.

When the target contaminants of photolysis are microbial pathogens, photolysis is described as disinfection due to the reaction pathway—microorganism inactivation by damage to genetic material. The UV disinfection of surface [29] and diluted real WW [37] in PTC systems has been applied to diverse resistant and pathogenic organisms (E. coli, total coliforms, faecal coliforms, etc.). UV irradiation, together with elevated water temperatures, can induce microbial inactivation and even pasteurisation (temperatures > 60 °C). For instance, LFR coupled with PTC enabled water heating up to 92 °C, resulting in the inactivation of highly resistant Acanthamoeba cysts and Bacillus subtilis spores [38].

Table 3. Summary of water pollutants treated in PTC-driven WW treatment.

Pollutant	Concentration	Medium 1	Process 2	Ref.	Pollutant	Concentration	Medium 1	Process 2	Ref.
Dye					Tetracycline	10 mg/L	S	PC	[30]
Dyes	-	RE	P	[35]	Amoxicillin	100 mg/L	S	PC	[39]
Remazol Brilliant Blue	60 mg/L	S	P	[17]	Industrial effluents
Reactive Red 120	100 mg/L	S	P	[31]	Simulated WW	7500 mg COD/L	S	PC	[40]
			PF		Olive oil WW	-	S	PF	[41]
Methylene Blue	10 mg/L	S	PC	[33]	Municipal WW	440 mg COD/L	S	PC	[42]
Rhodamine B	15 mg/L	S	PC		Paper mill WW	490 mg COD/L	RE	PC	[43]
	2 mg/L	S	PC	[30]	Dairy processing WW	569 mg COD/L	RE	PC
Methylene Blue	5 mg/L	S	PC		Oil-field WW	80 mg DOC/L	RE	PF	[25]
Methyl Orange	5 mg/L	S	PC		Tannery WW	1428 mg COD/L	RE	PC	[44]
Magenta dye	100 mg/L	RE	PC	[45]	Gold mine WW	2948 mg CN−/L	RE	PC	[46]
Aminosilicone						50 mg CN−/L	S	PC
Silicone emulsion	653 mg COD/L	S	PF	[25]	Metal
Phenol					Cr	6.88 mg Cr/L	RE	PCR	[44]
Phenol	100 mg DOC/L	S	PF		Cr(VI)	20 mg/L	S	PCR	[47]
	550 mg DOC/L	S	PF		Pb	20 mg/L	S	PCR	[48]
	100 mg TOC/L	S	PF	[49]	Cu	20 mg/L	S	PCR
Natural organic matter					Ni	20 mg/L	S	PCR
Humic acid	50 mg/L	S	PF	[50]	Zn	20 mg/L	S	PCR
Organic acid					Microbial
Oxalic acid	900 mg/L	S	PC	[51]	E. coli log	7.55 × 102 MPN/100 mL	RSW	D	[29]
Organic alcohol					E. coli	1 × 106 CFU/mL	S	PC	[52]
Methanol	3204 mg/L	S	PC	[20]	E. coli	1.2 × 105 CFU/mL	S	PC	[30]
Pesticide					Total coliforms log	2.94 × 103 MPN/100 mL	RSW	D	[29]
Clomazone	97 mg DOC/L	S	PF	[25]	Total coliform	70,000 MPN/100 mL	DRE	D	[37]
Thiophanate-methyl	1369 mg/L	S	PF	[32]	Total heterotrophic	1.034 × 104 CFU/mL	RE	D	[29]
Isoproturon	25 mg/L	S	PC	[53]	bacterial count log
Pharmaceutical					Total bacterial counts	300,000 MPN/100 mL	DRE	D	[37]
Ciprofloxacin	10 mg/L	S	P	[17]	Sporeformer	100,000 MPN/100 mL	DRE	D

¹ Medium abbreviations: RE—real effluent; S—synthetic matrix; RSW—real surface water; DRE—diluted real effluent. ² Process abbreviations: D—disinfection; P—photolysis; PC—photocatalysis; PF—photo-Fenton; PCR—photocatalytic reduction.

Table 3. Summary of water pollutants treated in PTC-driven WW treatment.

Pollutant	Concentration	Medium 1	Process 2	Ref.	Pollutant	Concentration	Medium 1	Process 2	Ref.
Dye					Tetracycline	10 mg/L	S	PC	[30]
Dyes	-	RE	P	[35]	Amoxicillin	100 mg/L	S	PC	[39]
Remazol Brilliant Blue	60 mg/L	S	P	[17]	Industrial effluents
Reactive Red 120	100 mg/L	S	P	[31]	Simulated WW	7500 mg COD/L	S	PC	[40]
			PF		Olive oil WW	-	S	PF	[41]
Methylene Blue	10 mg/L	S	PC	[33]	Municipal WW	440 mg COD/L	S	PC	[42]
Rhodamine B	15 mg/L	S	PC		Paper mill WW	490 mg COD/L	RE	PC	[43]
	2 mg/L	S	PC	[30]	Dairy processing WW	569 mg COD/L	RE	PC
Methylene Blue	5 mg/L	S	PC		Oil-field WW	80 mg DOC/L	RE	PF	[25]
Methyl Orange	5 mg/L	S	PC		Tannery WW	1428 mg COD/L	RE	PC	[44]
Magenta dye	100 mg/L	RE	PC	[45]	Gold mine WW	2948 mg CN−/L	RE	PC	[46]
Aminosilicone						50 mg CN−/L	S	PC
Silicone emulsion	653 mg COD/L	S	PF	[25]	Metal
Phenol					Cr	6.88 mg Cr/L	RE	PCR	[44]
Phenol	100 mg DOC/L	S	PF		Cr(VI)	20 mg/L	S	PCR	[47]
	550 mg DOC/L	S	PF		Pb	20 mg/L	S	PCR	[48]
	100 mg TOC/L	S	PF	[49]	Cu	20 mg/L	S	PCR
Natural organic matter					Ni	20 mg/L	S	PCR
Humic acid	50 mg/L	S	PF	[50]	Zn	20 mg/L	S	PCR
Organic acid					Microbial
Oxalic acid	900 mg/L	S	PC	[51]	E. coli log	7.55 × 102 MPN/100 mL	RSW	D	[29]
Organic alcohol					E. coli	1 × 106 CFU/mL	S	PC	[52]
Methanol	3204 mg/L	S	PC	[20]	E. coli	1.2 × 105 CFU/mL	S	PC	[30]
Pesticide					Total coliforms log	2.94 × 103 MPN/100 mL	RSW	D	[29]
Clomazone	97 mg DOC/L	S	PF	[25]	Total coliform	70,000 MPN/100 mL	DRE	D	[37]
Thiophanate-methyl	1369 mg/L	S	PF	[32]	Total heterotrophic	1.034 × 104 CFU/mL	RE	D	[29]
Isoproturon	25 mg/L	S	PC	[53]	bacterial count log
Pharmaceutical					Total bacterial counts	300,000 MPN/100 mL	DRE	D	[37]
Ciprofloxacin	10 mg/L	S	P	[17]	Sporeformer	100,000 MPN/100 mL	DRE	D

¹ Medium abbreviations: RE—real effluent; S—synthetic matrix; RSW—real surface water; DRE—diluted real effluent. ² Process abbreviations: D—disinfection; P—photolysis; PC—photocatalysis; PF—photo-Fenton; PCR—photocatalytic reduction.

Solar photocatalysis is by far the most widely applied process, accounting for 48% of studies. TiO₂ was used as the semiconductor catalyst, either in slurry or supported within the PTC receiver tube. Fixed-bed catalysts used in photocatalysis were TiO₂ immobilised cement beads [53], TiO₂-coated cement balls [39] and TiO₂-coated silica gel beads [30]. The general reaction mechanism starts with photon excitation—TiO₂ absorbs energy to promote an electron (e⁻) from the valence band to the conduction band, leaving a positive hole (h⁺). These charge carriers react in a way to produce ^•OH and superoxide O₂^•− radicals [19,34,42,54,55,56,57]. A schematic representation of solar irradiation concentration within the PTC system, together with the TiO₂ photocatalytic reaction mechanism, is presented in Figure 3. Solar photo-Fenton is second-in-line (22%). In general, the combination of Fenton’s reagent, either homogeneous (FeSO₄/H₂O₂) or heterogeneous (Fe ions supported on various materials/H₂O₂), with concentrated solar UV-vis light rapidly increases the ^•OH radicals’ production [25,31,32,34,41,49,50].

Figure 3. Scheme of PTC-driven solar radiation concentration and TiO₂ photocatalytic reaction mechanism.

Both processes were implemented to treat contaminants of emerging concern (CECs). Although pharmaceuticals and pesticides were effectively removed in selected PTC-AOP systems, it is important to note that all matrices were synthetic. Reported CEC concentrations in surface waters typically range from ng/L to µg/L globally [58]. In contrast, Table 3 shows significantly higher initial concentrations used in PTC experiments, from 10 mg/L [17,30] to over 1000 mg/L [32]. Photocatalysis and photo-Fenton demonstrated strong potential for mineralising antibiotics and pesticides [25,32,39,53]. Interestingly, H₂O₂-photolysis of the Ciprofloxacin achieved 88% degradation but only 8% mineralisation. This discrepancy is likely related to structural changes that eliminate UV-vis absorption peaks, suggesting molecular transformation and inactivation rather than complete mineralisation [17]. In the reviewed studies, photo-Fenton systems generally operated under acidic conditions (around pH 3), whereas TiO₂-photocatalysis was performed between pH 5 and 10, implying the need for an additional neutralisation step.

Photocatalytic reduction was examined in three studies. Typically, an organic additive such as citric acid is added as a hole (h⁺) scavenger to facilitate the reductive pathway [47,48]. Treating mixed WW streams using simultaneous oxidative–reductive photocatalysis represents an innovative approach in which organic pollutants act as hole scavengers (being oxidised), while metals are reduced by photogenerated electrons [44]. However, metal deposition onto the catalyst surface remains a major limitation, making regeneration necessary to improve catalyst reusability.

In summary, this section presented the broad application base of PTC-AOP systems for diverse WW challenges. However, the initial pollutant concentration remains a critical factor governing reaction kinetics. High pollutant loads can saturate catalyst active sites and absorb photons, thus limiting catalyst activation. In such cases, adding an oxidant can help maintain radical generation and process efficiency. Catalyst dosage and configuration also play important roles. Immobilised catalysts eliminate the need for post-treatment filtration, although they usually require longer reaction times compared with slurry systems. WW pH is another crucial process parameter. The following section provides insights into how PTC configuration influences overall treatment performance.

3.2. PTC Design and Optical Performance

The engineering design of PTC systems reported in the selected 25 studies shows considerable variability, with 20 PTC configurations identified. Four of these reactors were presented in more than one publication, while the remaining designs were unique. Several adaptations were observed: (1) collectors connected in series to increase the effective irradiated volume [25,32,33,35,46,49], (2) integrated supplementary UV light sources to enhance photochemical activation under low-irradiance conditions [20,30], (3) hybrid configurations such as a sunlight-adjustable parabola [30] and rotary PTC [46], and (4) PTC-coupled systems with pretreatment filters [43] and a moving-bed biofilm reactor (MBBR) [42]. Diversity among PTC systems reflects ongoing efforts to optimise operational and technological objectives for various water matrices.

3.2.1. PTC Receiver Design

Receiver tubes are a critical component influencing optical performance, thermal stability and overall treatment efficiency. Their characteristics (materials, dimensions and volume) are summarised in Table 4. Among the twenty PTC systems, borosilicate (seven reactors) and ordinary glass tubes (six reactors) were the most employed receiver materials, due to their optical transparency, chemical resistance and relative affordability. Quartz receivers were used less frequently due to their high cost, despite offering superior UV transmittance. Pyrex glass tubes were used in a limited number of systems, and one study employed PET bottles as an extremely low-cost alternative [50]. Two PTC systems were equipped with multiple receivers [30,42], enabling larger irradiated volumes without enlarging the parabola aperture area and overall footprint. Reported tube diameters generally ranged from 2 to 6 cm, thus allowing efficient photon absorption while minimising “dark zones”, especially for slurry photocatalysis, where turbidity can restrict light transmission. Receiver length varied substantially (38–380 cm), which, together with diameter, contributed to the relatively low irradiated volumes (0.11–4.50 L). PTC systems with irradiated volumes below 1 L can be regarded as laboratory-scale, while those above are closer to pilot-scale operation.

Table 4. PTC system characteristics—receiver.

Refs.	PTC System	Material	Outer Diameter (cm)	Inner Diameter (cm)	Receiver Length (cm)	Volume Irradiated (L)	Volume Treated (L)	Flow Rate (L/min)	Recirculation Number 1
[17]	1	Quartz	6.00	5.5	120	2.20	5	0.17	2
[32]	2	Quartz	- 2	1.6	122	0.25 1	6	6.20	24
[29]	3	Borosilicate	5.80	-	170	5.00	5	Batch	1
[25,49]	4	Borosilicate	-	1.10	120	0.11	2 5	1.20 30.0	18 45
[39,53]	5	Borosilicate	2.54	-	125	0.63	6 10	1.00	10 16
[40,47,48]	6	Borosilicate	-	3.80	180	1.00	5	0.75	5
[20]	7	Borosilicate	2.80	2.60	38.0	0.20	2	12.3	10
[46]	8	Borosilicate	2.54	2.24	50.0	0.55	20	20.0	36
[44]	9	Borosilicate	-	2.5	96.0	0.47	5	15.0	11
[37]	10	Pyrex	2.54	-	200	1.01	100	4.5	99
[31,45]	11	Pyrex	1.86	1.26	130	0.18	1	0.05	6
[43]	12	Pyrex	10.7	-	-	-	-	-	/ 3
[35]	13	Glass	-	-	150	4.50	8	1.95	2
[41]	14	Glass	-	-	200	-	-	1.03	/
[33]	15	Glass	2.00	-	50.0	0.94	7	-	7
[51]	16	Glass	2.54	-	-	-	10	-	/
[52]	17	Glass	3.40	3.15	-	-	20	1.50	/
[42]	18	Glass × 2	-	3.80	380	4.31	20	0.10	5
[50]	19	PET	-	-	-	1.00	1	Batch	1
[30]	20	Non specified × 4	2	-	-	-	1	-	/

¹ Values written in italics are calculated. ² Data were not reported in the original study. ³ The value could not be calculated due to missing data.

To ensure optimal mass transfer in heterogeneous treatment systems, the flow rate is a decisive operational parameter that dictates the fluid regime inside the receiver tube. Generally, increasing the flow rate raises the Reynolds number (turbulence), which enhances the mixing between the WW, catalyst and/or oxidant, while eliminating possible sedimentation [35,46]. Also, higher flow rates can help moderate the temperature rise generated by the concentrating effect of PTC. On the other hand, excessive flow rates may influence low absorption of solar irradiance by the catalyst, resulting in insufficient generation of ^•OH radicals [39]. Additionally, to explore the optical behaviour and hydrodynamics of PTC systems, the recirculation number (defined as the ratio of the total treated volume to irradiated volume) was determined. This parameter illustrates how many times the WW must pass through the irradiated zone. Most PTC systems treated relatively small influent volumes (<5 L), resulting in moderate recirculation. In contrast, PTC system 10 [37], required notably higher recirculation due to its much larger treated volume (100 L). The presented differences highlight the receiver design and system scale on mass transfer, photon distribution and consequently treatment efficiency.

3.2.2. PTC Parabola Design

The second essential component of the PTC system is the parabola, which defines the energy intensity and determines the system’s ability to operate under varying solar conditions (Table 5). Aluminium variants (polished or anodised) were the predominant reflective materials used across the reviewed studies, offering a good compromise between high reflectivity and resistance to oxidation [59]. Also, they are low-weight and cost-effective. Several studies employed steel sheets or galvanised steel (PTC systems 3, 5, 11 and 17), which provide a cheaper and more structurally robust alternative to high-grade aluminium. Newly applied glass mirrors [43] and flexible mirrors [30] highlight the attempts to reduce manufacturing costs and maximise reflectivity.

Table 5. PTC system characteristics—parabola.

Refs.	PTC System	Material	Length (cm)	Width (cm)	Aperture Area (m2)	Concentration Ratio	Photon Utilisation (m2/L)	Tracking Mechanism	Solar Intensity (W/m2)
[17]	1	Anodised aluminium	110	95.0	1.05	6.00	0.48 1	Yes, 2 axes	700 2
[32]	2	Polished aluminium	-	-	-	-	-	No, fixed	900 2
[29]	3	Galvanised steel	170	18.0	0.34	1.11	0.07	No, fixed	650 2
[25,49]	4	Polished aluminium	-	-	0.26	-	2.28	No, fixed	-
[39,53]	5	Stainless steel	125	80.0	1.00	19.9	1.59	Yes	860 2
[40,47,48]	6	-	172	57.8	0.99	4.84	0.99	Yes, 1 axis	862 2
[20]	7	Aluminium foil	38.0	8.17	0.04	3.10	0.18	Yes, 1 axis	-
[46]	8	Aluminium sheet	50.0	30.9	-	3.87	-	Yes	55 2
[44]	9	Aluminium chrome-plated	104	84.0	0.87	10.7	1.85	-	985 2
[37]	10	Aluminium foil	200	27.2	0.54	3.41	0.54	No, fixed	-
[31,45]	11	Stainless steel sheet	130	113	1.47	19.3	8.37	Yes	1000 2
[43]	12	Glass mirror	300	100	3.00	3.13	-	-	-
[35]	13	Aluminium sheet	200	-	-	-	-	Manual	850 2
[41]	14	Reflective material	200	86.0	1.72	-	-	-	1220 2
[33]	15	-	140	50.0	0.70	7.96	0.74	No, fixed	-
[51]	16	Aluminium sheet	-	-	0.72	13.0	-	Yes, 2 axes	-
[52]	17	Steel sheet	-	-	-	-	-	Yes	750 2
[42]	18	Polished aluminium	172	57.8	0.99	4.84	-	Manual	700 2
[50]	19	-	30.0	100	3.00	-	3.00	No	2600 3
[30]	20	Flexible mirror	-	5.40–39.6	-	0.86– 6.30	-	Yes	1500 4

¹ Values written in italics are calculated. Solar intensity presented by various type of irradiated data: ² Global solar irradiance, ³ UV irradiance, ⁴ concentrated flux.

Where available, parabola dimensions (aperture length and width) were extracted to calculate PTC aperture area and concentration ratio. These parameters are essential for optical performance evaluation. Once again, great variability is detected across PTC systems. The CR varies from 1.1 (PTC 3) to nearly 20 (PTC 5 and 11). When compared to non-concentrating systems, PTC with CR < 5 can capture more irradiance without high temperature generation, while systems with CR > 10 focus intense energy onto the receiver, which may be beneficial for thermal-synergistic processes (like microbial pathogens pasteurisation), but the risk of overheating must be managed [59]. To further complement reported data, photon utilisation (PU) was calculated based on the ratio of aperture area and irradiated volume of WW. In contrast, the commonly used accumulated UV dose (Q_UV), applied mainly in stationary systems such as CPC reactors, does not account for the CR and, therefore, neglects the effect of optical concentration. Consequently, PU represents a normalised metric that reflects the efficiency of solar-irradiance delivery to the reaction medium. PTC system 11 [31,45] showed the highest value (8.37), indicating a very large parabola aperture area for a small irradiated volume. This set-up can likely ensure rapid (single-pass) treatment, but with low throughput, or it may be particularly beneficial in regions with lower DNI.

Reported solar intensities significantly vary across the studies, probably due to inconsistencies in reporting standards rather than regional differences. Most PTC systems operated under standard DNI values between 600 and 1000 W/m². One study reported a notably lower value (55 W/m²), which likely corresponds to UV irradiance rather than DNI [46], while another reports 2600 W/m² [50], possibly representing concentrated UV flux that reaches the receiver. Although high radiation intensity creates a significant photon supply for radical formation, highly concentrated flux may elevate the temperature and negatively influence structural changes in the oxidant and catalyst. In this context, the recently developed sunlight-adjustable parabola [30] may offer a good solution for regions with variable DNI. To ensure effective application, a tracking mechanism is a necessary part of the PTC system. Almost half of the PTC systems employed automatic tracking, with one or two axes, whereas a significant number still rely on manual tracking or no tracking at all. The absence of a sun-tracking mechanism can greatly reduce manufacturing and operational costs, but it limits the use of PTCs to a narrow operational window (hours with the highest irradiation).

3.3. PTC-AOP System Performance

In this section, more focus is given to AOP (normalised) performance in the degradation of various contaminants. The collected data on process efficiency and mineralisation are summarised in Table 6 and grouped according to the applied treatment process (photolysis, photo-Fenton and photocatalysis). Reaction kinetics were reported in several studies, and, where possible, pseudo-first-order rate constants were calculated. While k_FO provides a convenient metric for pollutant degradation, it often fails to capture the spatial non-uniformity of the radiation field within concentrating reactors. Consequently, relying solely on these values can lead to misleading comparisons across different pollutants and water matrices unless they are complemented by energy-normalised indicators. To enable comparison across different treatment processes and PTC configurations, the time required for a one-order-of-magnitude reduction (τ₉₀) was determined. This parameter is useful for engineering design and residence time calculations. For example, electrical energy per order (E_EO), which is the industry standard for artificial UV systems, is not well-suited to solar-driven AOPs, as electrical energy consumption is limited to pumping and is therefore negligible. This makes E_EO an ineffective metric for evaluating PTC efficiency.

Table 6. Performance of PTC-AOP systems applied in photolysis, photo-Fenton and photocatalysis.

Ref.	PTC System	Pollutant		Catalyst		H2O2			Efficiency		Mineralisation
		Name	Conc. (mg/L)	Name	Conc. (g/L)	Conc. (mM)	pH	Time (min)	PFO 1 (min−1)	τ90 2 (min)	PFO (min−1)	τ90 (min)
Photolysis
[35]	13	Dyes	RE	-	-	-	8.2	360	0.0040	574.1	0.0043	531.1
[31]	11	Reactive Red 120	100	-	-	5.00	7	120	0.0137	167.5
[17]	1	Remazol Brilliant Blue	60	-	-	100	-	180	0.0045	516.3	0.0069	334.8
		Ciprofloxacin	10	-	-	10.0	-	180	0.0097	238.6	0.0005	4971
Photo-Fenton
[31]	11	Reactive Red 120	100	CuOFeB	0.050	5.00	7	120	0.0051	451.5	0.0058	398.6
				AlFeB	0.100	5.00	4	120	0.0145	158.8	0.0058	398.6
[25]	4	Silicone emulsion	653	FeSO4	0.279	163	3	300			0.0074	313.0
		Phenol	100	FeSO4	0.056	100	3	240			0.0096	240.0
			550	FeSO4	0.056	100	3	300			0.0020	1155
[49]	4	Phenol	100	FeSO4	0.028	7.00	3	180			0.0128	180.0
[50]	19	Humic acid	50	FeSO4	0.004	0.59	4	30			0.1304	17.66
[25]	4	Clomazone	97	FeSO4	0.056	13.5	3	180			0.0256	90.00
[32]	2	Thiophanate-methyl	1369	FeSO4	0.034	0.04	3	120			0.0134	171.7
[41]	14	Olive oil WW	-	Fe2+ salt	0.194	19.9	3	40			0.0262	87.73
[25]	4	Oil-field WW	80	FeSO4	0.056	200	3	270			0.0089	258.2
Photocatalysis
[33]	15	Methylene Blue	10	g-C3N4/TiO2	0.020	-	10	80	0.0231	99.68
[30]	20	Methylene Blue	5	TiO2-SGB	-	-	-	-	0.0190	121.2
[33]	15	Rhodamine B	15	g-C3N4/TiO2	0.020	-	10	80	0.0191	120.6
[30]	20	Rhodamine B	2	TiO2-SGB 4	-	-	-	-	0.0338	68.12
		Methyl Orange	5	TiO2-SGB	-	-	-	-	0.0094	245.0
[53]	5	Isoproturon	25	TiO2-ICB 5	0.750	14.7	5	240	0.0080	287.8	0.0079	291.3
[30]	20	Tetracycline	10	TiO2-SGB	-	-	-	-	0.0201	114.6
[39]	5	Amoxicillin trihydrate	100	TiO2-CCB 3	1.000	3.53	5.8	240	0.0105	218.8	0.0074	311.9
[40]	6	Simulated organic WW	7500	TiO2	1.000	353	6.8	180			0.0109	210.8
[42]	18	Municipal WW	440	TiO2	1.000	-	7.6	240			0.0049	471.8
[44]	9	Tannery WW	1428	TiO2-NP 6	1.000	-	8.2	300			0.0318	72.43
[46]	8	Cyanide gold mine WW	2948	TiO2	0.550	-	9.5	240	0.0048	475.8
		Cyanide	50	TiO2	0.300	-	9.5	180	0.0115	200.1

¹ PFO—The pseudo-first-order reaction rate constant; Values written in italics are calculated. ² τ₉₀—The time required for one order of magnitude; Values written in italics are calculated. ³ TiO₂-CCB—TiO2-coated cement balls. ⁴ TiO₂-SGB—TiO2-coated silica gel beads. ⁵ TiO₂-ICB—TiO₂ immobilised cement beads. ⁶ TiO₂-NP—TiO₂ nanoparticles.

Photolysis was found to be the least efficient treatment option. High τ₉₀ values indicate slow reaction kinetics. The case of Ciprofloxacin [17] mineralisation clearly shows a significant presence of the inevitable intermediates, whose degradation is mandatory due to possible negative effects on the environment. This critical secondary effect indicates that, although optically powerful, PTC systems are more feasible in hybrid set-ups, where WW undergoes pretreatment (such as a series of filters [43] or biological treatment [42]).

When comparing the degradation of synthetic dyes, the influence of the PTC concentration ratio becomes evident (Table 5). PTC system 11 exhibited a substantially higher CR (19.3) than PTC system 1 (6.00), enabling faster ^•OH radical generation and improved decolourisation efficacy. Nevertheless, promising results were obtained for the real WW matrix, the textile dying effluent, where direct photolysis provided a low-cost but time-consuming option for both decolourisation and mineralisation [35]. To enhance overall photolysis efficiency, it can be suitable to implement PTC systems with high photon utilisation, such as PTC 11. Additional suitable enhancements are the optimisation of the WW matrix pH, since it may lead to oxidant scavenging reactions [60], and flow rate, to enhance WW turbulence in the PTC receiver [35].

Both heterogeneous and homogeneous photo-Fenton processes were further examined. The influence of metal active sites on ^•OH radical production supported faster mineralisation kinetics when compared to photolysis. Among investigated PTC-AOP systems, PTC 11 exhibited the highest CR (19.3) and photon utilisation (8.37 m²/L), but the applied clay-based catalysts showed relatively slow reaction kinetics for both dye decolourisation and mineralisation [31]. With a high CR comes the generation of heat, which may influence the self-decomposition of H₂O₂ into molecular O₂ and H₂O (>50 °C) [61], leading to lower process efficiency. Also, it is possible that the presented catalysts block some of the focused solar irradiance, leading to a longer reaction time [60]. In this case, the flow rate was the lowest among all systems (0.05 L/min; Table 2); thus, a low mass transfer rate can also be a detrimental factor for heterogeneous Fenton reaction performance. Since this PTC-AOP system has the highest photon utilisation per irradiated WW volume, its implementation in photolytic treatment (as a polishing step) could be more profitable.

Further, most photo-Fenton applications relied on homogeneous Fenton reagents (FeSO₄ and H₂O₂) under acidic conditions (pH 3). The fastest mineralisation among the analysed studies was achieved for humic acid (~18 min) [50]. Phenol degradation was investigated in PTC system 4 [25,49], where higher pollutant loads required significantly prolonged reaction time and further optimisation of Fenton reagent ratios. Comparison between real and synthetic oil–WW matrices [25,41] indicates the need for increased oxidant dosages to effectively decompose high organic loads. Overall, τ₉₀ values provide valuable insight into treatment performance in PTC-AOP systems and highlight the need for further optimisation of operating parameters to achieve rapid and extensive mineralisation. The (necessary) final stage of Fenton treatment is neutralisation, a step where pH is increased, residual Fe ions are precipitated, and Fe-sludge is removed. Accordingly, optimisation and neutralisation are particularly important for sustaining pilot-scale applications for treating WW volumes of 100 L or more.

Photocatalytic processes were investigated using various TiO₂-based catalysts. Immobilised-TiO₂-coated silica gel beads (TiO₂-SGB; [30]) within PTC receiver and S-scheme graphitic carbon nitride/TiO₂ composite (g-C₃N₄/TiO₂; [33]) in slurry mode were applied for the decolourisation of Methylene Blue and Rhodamine B. Initial dye concentrations were significantly lower than those used in photolysis, the photo-Fenton process, and real textile effluents [36], resulting in lower τ₉₀ values. From an industrial-scale perspective, it is crucial to consider competitive adsorption and reaction pathways when dye molecules are present in excess, as they compete for catalyst active sites [34]. An imbalanced ratio between pollutants, catalysts and oxidants can significantly reduce process performance.

In contrast, immobilised photocatalysts used for the degradation of CECs [30,39,53] generally exhibit slower kinetics, possibly due to mass transfer limitations. Slurry systems, on the other hand, often operate under turbulent conditions [40,53], leading to lower τ₉₀ values, while ensuring effective pollutant–catalyst and catalyst–photon contact (highly concentrated near receiver wall). When TiO₂-based photocatalysis was applied to complex WW matrices, faster reaction kinetics were observed for green-synthetised TiO₂ [44] and when TiO₂ was combined with H₂O₂ [40]. Smaller catalyst particle sizes provide a higher density of active sites, leading to improved process performance.

Although not shown in the table, PTC-driven WW disinfection can be supported by PTC-AOP systems with high potential in heat generation (CR > 10). The synergistic effect of solar irradiation and water temperature can lead to solar pasteurisation of highly resistant microorganisms [38]. This optical–thermal synergy enables chemical-free disinfection, highlighting a critical functional advantage of PTCs over CPC-based systems.

4. Future Perspectives

The transition of PTC-driven AOPs from laboratory-scale to industrial and/or municipal applications requires addressing several critical technical and economic barriers. Future research should prioritise the following, in order to strengthen the viability of CSP technologies:

• Sludge management (homogeneous Fenton process) and catalyst recovery (heterogeneous Fenton process and photocatalysis)—The generation of Fe-rich sludge after Fenton reactions remains a significant operational challenge. Future studies should incorporate sludge management strategies, such as the development of heterogeneous catalysts to minimise secondary-waste generation. Also, greater attention should be given to catalyst recovery, reusability and regeneration.
• Life Cycle Assessment (LCA) and Techno-Economic Analysis (TEA)—Currently, both LCA and TEA are underrepresented in the PTC-AOP literature. The feasibility of PTC systems depends on balancing the high initial capital cost (precision sun-tracking mechanisms and reflective parabola materials) and land footprint requirements with long-term reductions in operational cost through renewable solar energy incorporation. These assessments should quantify the environmental trade-offs between chemical savings and the energy-intensive manufacturing of concentrator components.
• Scaling up of PTC-AOP technology—Most of the existing literature relies on laboratory or pilot conditions; thus, future work should address industrial-scale implementation. Hybrid configurations, where PTC can excel in polishing of recalcitrant pollutants and serve as a tertiary/quaternary stage, could be integrated into existing infrastructure (municipal WW treatment plants) or developed as standalone systems with automated control systems (adjustment of incident angle, flow rate, catalyst/oxidant dosage) to ensure consistent effluent treatment in real time.

5. Conclusions

This mini-review synthesised the performance of PTC-driven AOPs for WW treatment, leading to the following key findings:

• Superior optical performance—PTC systems demonstrate a clear advantage over stationary and non-concentrating reactors for the treatment of toxic and persistent pollutants and concentrated effluents. The high concentration ratio can facilitate improved and accelerated reaction kinetics with rapid ^•OH radical production, leading to a shorter treatment time.
• Necessity of standardised metrics—The systematic analysis revealed that the use of normalised performance indicators, like the time required for a one-order-of-magnitude reduction (τ₉₀; min) and photon utilisation (PU; m²/L), is necessary for assessing treatment efficiency across diverse geographical locations and reactor designs.
• Climate and water resilience—Analytical mapping confirms that PTC-driven technologies are most viable in DNI hotspots. As climate change expands areas affected by severe water scarcity and elevated solar potential, like the Mediterranean and Balkan regions, PTC-AOPs may present a solution for safe WW treatment and reuse, contributing to the circularity and SDG6 targets.