Recent Progress in Solar Photovoltaic-Driven Wastewater Treatment: Effects of Photocatalytic Efficiency, Energy Recovery, and Environmental Sustainability

Recent Progress in Solar Photovoltaic-Driven Wastewater Treatment: Effects of Photocatalytic Efficiency, Energy Recovery, and Environmental Sustainability

Introduction 
Water pollution has become one of the most significant environmental challenges associated with rapid industrialization, urban expansion, agricultural intensification, and population growth. Wastewater generated from these activities may contain organic compounds, heavy metals, suspended solids, nutrients, dyes, pharmaceuticals, pesticides, and other emerging contaminants. The increasing diversity and persistence of these pollutants have reduced the effectiveness of conventional treatment approaches and increased the demand for advanced treatment technologies.
Conventional wastewater treatment generally involves physical separation, chemical oxidation, coagulation, adsorption, biological degradation, or combinations thereof. Although these processes are widely established, many of them require substantial quantities of electricity and chemical reagents. Aeration is particularly energy-intensive in biological treatment systems, while membrane-based technologies require significant hydraulic pressure. Electrochemical and advanced oxidation technologies can achieve high removal efficiencies, but their dependence on electricity may increase operating costs.
The transition toward renewable energy therefore represents an important opportunity for wastewater treatment. Among renewable energy sources, solar energy is particularly attractive because of its widespread availability, modularity, and compatibility with decentralized treatment systems. Photovoltaic technology converts solar radiation directly into electricity, allowing renewable electricity to be supplied to pumps, electrochemical reactors, aeration units, membrane systems, sensors, and control systems.
Previous research has demonstrated the feasibility of coupling photovoltaic systems with electrooxidation, reverse osmosis, electrocoagulation, aeration, electroflocculation, and Fenton-based oxidation. However, most studies have focused on individual treatment efficiencies rather than examining the broader interaction between energy conversion, treatment performance, energy recovery, and environmental sustainability.
This review consequently adopts a broader analytical perspective. Instead of considering solar energy merely as an alternative electricity source, the present study evaluates photovoltaic-driven wastewater treatment as an integrated water-energy-environment system.
Three principal variables are introduced:
 
These variables are analyzed together with treatment efficiency, energy consumption, solar irradiation, photovoltaic conversion efficiency, and system operating conditions.
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2. Conceptual Framework of Photovoltaic-Driven Wastewater Treatment
The fundamental process consists of five interconnected stages:
Solar irradiation → photovoltaic conversion → electrical energy management → wastewater treatment → water/resource recovery
The photovoltaic subsystem converts solar radiation into direct-current electricity. Depending on system configuration, the generated electricity can be supplied directly to the treatment reactor or transferred through a battery and power-management system.
The general photovoltaic conversion efficiency can be expressed as:
 
where:
     = photovoltaic conversion efficiency;
     = electrical output power;
     = incident solar irradiance;
     = photovoltaic panel area.
The available electrical energy can subsequently be distributed among pumps, electrochemical reactors, aeration systems, membrane units, sensors, and control devices.
The treatment efficiency can be expressed as:
 
where (RE) is pollutant removal efficiency, (C0) is the initial pollutant concentration, and (Ct) is the concentration after treatment.
For an integrated system, the relationship can therefore be represented conceptually as:
 
However, increasing energy input does not necessarily produce proportional improvements in treatment efficiency. This introduces the importance of energy optimization and recovery.
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3. Analytical Variables and Performance Indicators
To differentiate this review from conventional technology-by-technology reviews, the investigated systems are assessed according to a common set of variables.
Table 1. Principal variables adopted in the present review
Variable    Symbol    Description    Expected influence
Solar irradiation    (G)    Solar radiation incident on PV modules    Positive
PV conversion efficiency    (\eta_{PV})    Conversion of solar radiation to electricity    Positive
Photocatalytic efficiency    PE    Effectiveness of solar/photochemical reactions    Positive
Pollutant removal    RE    Percentage of pollutant eliminated    Positive
Specific energy consumption    SEC    Energy required per unit wastewater volume    Negative
Energy recovery    ER    Energy recovered or reutilized within the system    Positive
Water recovery    WR    Fraction of treated water recovered for reuse    Positive
Electrode stability    ES(_e)    Resistance to electrode degradation    Positive
Carbon reduction    CR    Reduction in energy-related emissions    Positive
Operating cost    OC    Economic requirement per treatment volume    Negative
Environmental sustainability    ES    Overall environmental performance    Positive
Scalability    SC    Suitability for larger treatment capacity    Positive
This multidimensional framework enables different technologies to be compared even when they treat different wastewater types.
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4. Solar Photovoltaic Technologies for Wastewater Treatment
4.1 Photovoltaic-Electrooxidation Systems
Electrooxidation is based on electrochemically generated oxidizing species that facilitate the degradation of organic pollutants. The technology is particularly attractive for contaminants that are difficult to remove through conventional biological processes.
Photovoltaic-powered electrooxidation eliminates or substantially reduces dependence on grid electricity. Earlier studies have demonstrated that photovoltaic arrays can directly supply electrochemical reactors for dye degradation and organic-carbon removal.
The effectiveness of the process depends on several parameters:
 
where:
    (J) = current density;
    (Ae) = electrode area;
    (t) = treatment time.
The use of photovoltaic electricity can improve the environmental profile of electrooxidation, but excessive current density may increase energy consumption without producing a proportional improvement in pollutant removal.
Table 2. Major factors controlling photovoltaic electrooxidation
Factor    Low condition    Intermediate condition    High condition    General effect
Solar irradiance    Limited electrical output    Stable operation    High output    Increases available power
Current density    Low oxidation rate    Improved oxidation    Possible excessive energy use    
Electrode area    Limited reaction surface    Improved contact    Higher capital requirement    
Treatment time    Incomplete degradation    High removal    Diminishing returns    
pH    May restrict oxidation    Optimum range    May reduce process efficiency    
A key research direction is therefore the development of adaptive current control according to real-time photovoltaic output.
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5. Photovoltaic Reverse Osmosis and Water Recovery
Reverse osmosis is fundamentally different from electrochemical oxidation because it relies on membrane separation rather than chemical destruction of contaminants.
The major advantage of photovoltaic-powered RO is the possibility of coupling renewable electricity with water recovery.
The water-recovery ratio can be defined as:
 
where (Vp) is permeate volume and (Vf) is feed volume.
Specific energy consumption is given by:
 
where (EPV) represents electrical energy supplied to the RO system.
Previous studies have demonstrated high contaminant removal while maintaining relatively low energy requirements under optimized operating conditions. However, membrane fouling remains one of the principal limitations.
Table 3. Comparison of major photovoltaic-RO performance indicators
Indicator    Conventional RO    PV-assisted RO    Desired development
Grid dependence    High    Reduced    Near-zero
Energy source    Electricity    Solar electricity    Renewable hybrid
Water recovery    Moderate–high    Moderate–high    Increased
Carbon emissions    Relatively high    Lower    Minimized
Membrane fouling    Present    Present    Intelligent control
Storage requirement    Low    Variable    Optimized
Remote applicability    Limited    High    Expanded
The major limitation is that large RO installations require substantial photovoltaic capacity and therefore significant physical space.
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6. Photovoltaic Electrocoagulation
Electrocoagulation combines electrochemical dissolution of sacrificial electrodes with coagulation and precipitation mechanisms.
Aluminum and iron electrodes are commonly used because they can generate hydroxide species capable of capturing suspended particles, heavy metals, dyes, and organic compounds.
The removal efficiency may be expressed as:
 
Experimental evidence summarized in the supplied literature indicates that photovoltaic-powered electrocoagulation can achieve substantial removal of turbidity and heavy metals while reducing dependence on conventional electricity.
Table 4. Principal operating variables in PV-electrocoagulation
Parameter    Effect on treatment
Current density    Controls coagulant generation
pH    Controls hydroxide formation and precipitation
Electrode material    Determines coagulation chemistry
Electrode spacing    Influences electrical resistance
Treatment time    Determines pollutant removal
Solar irradiance    Determines available electrical power
Initial pollutant concentration    Determines treatment demand
A major challenge is balancing treatment efficiency against electrode consumption. Excessive current density may improve removal but accelerate electrode degradation.
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7. Solar-Powered Aeration
Aeration represents one of the most important energy-consuming processes in biological wastewater treatment. Solar photovoltaic systems can directly drive blowers, surface aerators, or submerged aeration devices.
The principal mechanism is the increase in dissolved oxygen:
 
which enhances microbial activity and facilitates the biological oxidation of organic matter and nitrification.
Table 5. Effects of solar-powered aeration
Parameter    Expected response
Dissolved oxygen    Increase
COD removal    Increase
Ammonia removal    Increase
Microbial activity    Increase
Grid electricity consumption    Decrease
Carbon emissions    Decrease
Operating flexibility    Dependent on solar availability
The major challenge is temporal mismatch between solar availability and oxygen demand. Battery storage or intelligent intermittent aeration can address this problem.
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8. Photovoltaic Electroflocculation
Electroflocculation promotes aggregation of fine particles and dissolved contaminants through electrochemically generated coagulants.
Its principal advantages include:
    low chemical requirement;
    effective turbidity reduction;
    heavy-metal removal;
    color removal;
    compatibility with renewable electricity.
Table 6. Representative treatment characteristics reported for solar-powered EC systems
Application    Main pollutant    Important operating variable    Reported performance
Synthetic wastewater    Turbidity    Treatment time    ~72%
Nickel-containing wastewater    Ni²⁺    Current and irradiance    Up to ~100%
Domestic wastewater    COD    Current density    ~85%
Domestic wastewater    TDS    Current density    ~70%
Domestic wastewater    Turbidity    Retention time    ~87%
Lead-containing water    Pb²⁺    Current density    ~99.9%
Textile wastewater    COD    Electrolysis time    >50%
Surface water    Turbidity    Current density/time    ~97%
These values are retained as reported values from the literature supplied by the user, rather than being presented as new experimental results.
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9. Solar Photoelectro-Fenton Treatment
Photoelectro-Fenton represents one of the most promising photovoltaic-assisted advanced oxidation technologies.
The Fenton reaction can be simplified as:
 
The hydroxyl radical (\cdot OH) has a high oxidation potential and can attack persistent organic compounds.
Solar integration provides two potential benefits:
    photovoltaic electricity supplies the electrochemical process;
    solar radiation may contribute directly to photochemical regeneration and oxidation.
Therefore, the overall process can be represented as:
 
Table 7. Important parameters affecting solar photoelectro-Fenton
Variable    Increase may produce
Solar irradiance    Higher photochemical contribution
Current density    Increased electrochemical generation
Fe²⁺ concentration    Increased catalytic activity up to optimum
H₂O₂ availability    Increased radical generation
Reaction time    Greater pollutant degradation
pH    Strong influence on Fenton chemistry
NaCl concentration    Modified conductivity and oxidation pathways
The principal disadvantage is the need for careful control of chemical conditions and the potential cost associated with reagent consumption.
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10. Comparative Evaluation of the Six Solar-Driven Technologies
The six technologies considered in the original literature can be reclassified according to their principal function.
Table 8. Functional classification
Technology    Primary function    Main target    Energy demand    Main limitation
PV-EO    Oxidation    Organic pollutants    Medium–high    Electrode degradation
PV-RO    Separation    Dissolved contaminants    High    Membrane fouling
PV-EC    Coagulation    Metals/turbidity    Low–medium    Electrode consumption
PV-aeration    Biological enhancement    COD/NH₃-N    Low–medium    Solar intermittency
PV-electroflocculation    Particle separation    Suspended solids/metals    Medium    Sludge generation
PV-SPEF    Advanced oxidation    Persistent organics    Medium–high    Chemical control
This classification demonstrates that there is no universally superior photovoltaic wastewater treatment technology. Instead, technology selection should depend on wastewater composition, treatment objectives, solar resource, available land, required water-recovery rate, and energy-storage capacity.
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11. Photocatalytic Efficiency as a New Evaluation Variable
Photocatalytic efficiency is introduced in this review as a major variable because solar energy may contribute to treatment through both photovoltaic electricity and direct photochemical processes.
A simplified efficiency indicator can be expressed as:
 
where ∆C) represents the contaminant concentration reduction and (Esolar) represents solar energy input.
A more comprehensive indicator can combine pollutant removal and energy utilization:
 
where (RE) is removal efficiency and (SEC) is specific energy consumption.
This parameter enables comparison between systems that achieve similar removal efficiencies but require substantially different amounts of energy.
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12. Energy Recovery and Energy-Positive Treatment
Most studies evaluate energy consumption but do not adequately consider the possibility of energy recovery.
Wastewater contains chemical energy in organic matter. Consequently, future photovoltaic wastewater systems should be designed not simply to reduce electricity consumption but also to recover energy.
A generalized net-energy balance can be expressed as:
 
where:
    (EPV) = photovoltaic electricity generated;
    (Erecovered) = energy recovered from wastewater;
    (Etreatment) = electricity consumed by treatment.
When:
 
the system may theoretically operate as an energy-positive treatment platform.
Table 9. Proposed energy-recovery framework
Energy component    Source    Potential utilization
Photovoltaic electricity    Solar radiation    Direct treatment
Battery storage    Excess PV electricity    Night operation
Biogas    Organic wastewater    Electricity/heat
Hydrogen    Electrochemical treatment    Fuel
Waste heat    Equipment/processes    Thermal processes
Recovered chemical energy    Organic matter    Energy production
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13. Environmental Sustainability Assessment
Environmental sustainability should not be evaluated solely through pollutant-removal efficiency.
A system may achieve very high removal efficiency while consuming substantial electricity, generating large quantities of sludge, or requiring frequent replacement of electrodes and membranes.
Therefore, a broader sustainability index is proposed:
 
where (wi) represent weighting coefficients.
Table 10. Proposed sustainability dimensions
Dimension    Indicator    Desired direction
Water quality    Pollutant removal    ↑
Energy    Energy recovery    ↑
Energy    Specific energy consumption    ↓
Climate    Carbon emissions    ↓
Water resources    Water recovery    ↑
Materials    Electrode consumption    ↓
Economics    Operating cost    ↓
Reliability    System stability    ↑
Scalability    Treatment capacity    ↑
This multidimensional framework provides a stronger basis for comparing future systems.
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14. Interaction Between Solar Radiation and Treatment Performance
Solar radiation is not constant. It varies with:
    time of day;
    season;
    cloud cover;
    geographic location;
    panel orientation;
    temperature;
    dust accumulation.
Consequently, photovoltaic wastewater treatment should be designed around a dynamic energy supply.
Table 11. Expected effect of solar-resource conditions
Solar condition    PV output    Treatment response    Recommended strategy
Very low    Low    Reduced    Battery/grid support
Low    Limited    Partial operation    Reduced treatment load
Moderate    Stable    Normal    Standard operation
High    High    Enhanced    Maximum treatment
Highly variable    Fluctuating    Unstable    Intelligent control
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15. Integrated Solar-Wastewater Treatment Architecture
A future system should not rely on a single treatment technology.
A more effective architecture may combine:
PV → energy management → pretreatment → electrochemical treatment → membrane separation → biological polishing → water reuse
An integrated configuration could include:
    photovoltaic modules;
    maximum-power-point tracking;
    battery storage;
    electrocoagulation;
    electrooxidation;
    membrane filtration;
    solar-powered aeration;
    water-quality sensors;
    intelligent control;
    energy-recovery unit.
Table 12. Proposed integrated architecture
Stage    Technology    Primary objective
1    Screening    Remove large solids
2    PV-EC    Remove suspended solids/metals
3    PV-EO/SPEF    Degrade persistent organics
4    Biological treatment    Remove biodegradable organics/nutrients
5    PV-RO    Final separation and water recovery
6    Disinfection    Ensure reuse quality
7    Energy recovery    Recover useful energy
8    Monitoring    Real-time optimization
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16. Economic and Operational Considerations
The economic feasibility of solar wastewater treatment depends on the relationship between capital expenditure and long-term electricity savings.
The simplified payback period can be expressed as:
 
where:
     = photovoltaic investment;
     = storage investment;
     = annual energy-cost savings.
Table 13. Economic factors affecting system feasibility
Factor    Economic effect
PV module price    Determines initial investment
Battery price    Strong effect on off-grid systems
Electricity price    Determines savings
Solar irradiation    Determines annual energy generation
Maintenance    Influences long-term cost
Electrode replacement    Important for EC/EO
Membrane replacement    Important for RO
System lifetime    Determines return on investment
Water reuse value    Can improve economic feasibility
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17. Major Research Gaps
The reviewed literature reveals several unresolved issues.
Table 14. Research gaps and proposed solutions
Research gap    Consequence    Proposed solution
Solar intermittency    Variable treatment capacity    Predictive energy management
Low storage efficiency    Night-time limitations    Advanced batteries
Electrode degradation    Increased operating cost    Durable electrode materials
Membrane fouling    Reduced RO efficiency    Electrochemical pretreatment
Limited energy recovery    Energy loss    Integrated recovery units
Large PV footprint    Land requirement    High-efficiency PV and floating PV
Lack of unified indicators    Difficult comparison    Standardized performance index
Limited pilot-scale studies    Poor scalability    Long-term field demonstrations
Weather dependence    Operational instability    Hybrid PV-storage systems
Insufficient life-cycle assessment    Uncertain environmental benefit    Full LCA analysis
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18. Future Research Directions
Future studies should move toward integrated photovoltaic-water-energy systems rather than isolated photovoltaic-powered treatment units.
Particular attention should be given to five directions.
18.1 Intelligent energy management
Artificial intelligence and predictive control can estimate photovoltaic output and dynamically adjust current density, aeration intensity, membrane pressure, and treatment time.
18.2 Energy recovery
Photovoltaic generation should be integrated with biogas, hydrogen, and other energy-recovery technologies.
18.3 Resource recovery
Wastewater should increasingly be viewed as a resource rather than merely a waste stream. Nitrogen, phosphorus, metals, hydrogen, and organic energy can potentially be recovered.
18.4 Hybrid treatment
Combining PV-electrocoagulation, PV-electrooxidation, biological treatment, and membrane processes may reduce the energy requirements of individual technologies.
18.5 Life-cycle sustainability
Future evaluations should include:
    embodied carbon of PV modules;
    battery manufacturing;
    electrode production;
    membrane replacement;
    sludge management;
    transportation;
    end-of-life disposal.
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19. Proposed Comprehensive Performance Index
To facilitate future comparison, a normalized Solar Wastewater Sustainability Index (SWSI) can be proposed:
 where:
    (RE) = pollutant-removal efficiency;
    (PE) = photocatalytic efficiency;
    (ER) = energy recovery;
    (WR) = water recovery;
    (CR) = carbon reduction;
    (SEC) = specific energy consumption;
    (OC) = operating cost.
This index is proposed as a conceptual framework for future research, not as an experimentally validated index.
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20. Overall Comparative Matrix
Table 15. Overall assessment of photovoltaic-driven wastewater technologies
Technology    Pollutant removal    Energy efficiency    Water recovery    Sustainability    Scalability
PV-EO    High    Moderate    Low    High    Moderate
PV-RO    Very high    Moderate    Very high    High    High
PV-EC    High    High    Moderate    High    High
PV-aeration    Moderate    High    Moderate    Very high    Very high
PV-electroflocculation    High    Moderate    Moderate    High    High
PV-SPEF    Very high    Moderate    Low    High    Moderate
Integrated hybrid system    Very high    High potential    Very high    Very high    High potential
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21. Conclusions
Solar photovoltaic technology provides an increasingly attractive pathway for reducing the energy and environmental burdens associated with wastewater treatment. Unlike conventional treatment systems that depend predominantly on grid electricity, photovoltaic-driven systems can utilize locally available renewable energy to power electrochemical reactors, membrane units, aeration systems, and advanced oxidation processes.
The present review extends conventional assessments by introducing three interconnected variables: photocatalytic efficiency, energy recovery, and environmental sustainability. These variables provide a broader framework for evaluating the performance of solar wastewater-treatment systems beyond conventional pollutant-removal efficiency.
Photovoltaic-electrooxidation systems are particularly promising for persistent organic contaminants, whereas photovoltaic reverse osmosis offers substantial potential for water recovery. Photovoltaic electrocoagulation and electroflocculation are attractive for heavy metals, turbidity, and suspended pollutants. Solar-powered aeration can substantially improve biological treatment while reducing electricity requirements. Photoelectro-Fenton systems provide strong oxidation capacity for difficult-to-degrade organic contaminants.
Nevertheless, the widespread implementation of these systems remains constrained by solar intermittency, energy-storage limitations, electrode degradation, membrane fouling, photovoltaic land requirements, and differences between laboratory and full-scale operating conditions.
Future development should therefore focus on hybrid photovoltaic-water-energy systems capable of simultaneously achieving pollutant removal, water recovery, energy recovery, and carbon reduction. Intelligent control systems, high-efficiency photovoltaic materials, durable electrodes, advanced membranes, and integrated energy-storage technologies will be critical to achieving reliable operation.
Ultimately, the transition from conventional wastewater treatment toward solar-powered circular water systems requires a shift from evaluating individual treatment technologies to evaluating the entire water-energy-environment nexus. Such an approach can improve the technical efficiency, economic feasibility, and environmental sustainability of wastewater treatment, particularly in remote and energy-constrained regions with abundant solar resources.
Ethical Considerations
Not applicable. This study did not require ethical approval because it does not include human or animal subjects and does not involve any personal or sensitive data.


Acknowledgment: 
The author would like to express their sincere gratitude to The International Journal of Applied Sciences - Noor Al-Ilm for Publishing and Distribution for their generous support in waiving all publication fees and facilitating the publication of this manuscript free of charge. Their commitment to promoting scientific research and supporting researchers is highly appreciated.








Author Contribution: 
All authors contributed equally to the main contributor to this paper. All authors read and approved the final paper.

Declaration of generative AI and AI-assisted technologies in the writing process
The authors hereby declare that no generative artificial intelligence or AI-assisted technologies were used at any stage during the preparation of this manuscript, including language editing, proofreading, or content development. The authors take full responsibility for the originality and integrity of the work presented in this publication.


Funding: 
This research received no external financial funding. The authors also acknowledge The International Journal of Applied Sciences, Noor Al-Ilm for Publishing and Distribution, for providing a full waiver of the publication fees. The publication fee waiver was provided as editorial support and did not involve any financial contribution to the conduct, design, analysis, or reporting of the research.


Conflicts of Interest: 
“The authors declare no conflict of interest.”