Optimization of Advanced Oxidation Processes for Organic Wastewater Treatment in Bangladesh: Effects of pH, Oxidant Dosage, Reaction Time, Organic Load, Degradation Kinetics, and Energy Consumption

Optimization of Advanced Oxidation Processes for Organic Wastewater Treatment in Bangladesh: Effects of pH, Oxidant Dosage, Reaction Time, Organic Load, Degradation Kinetics, and Energy Consumption


Introduction
Organic pollutants represent an important class of environmental contaminants[1]; because many of them exhibit high toxicity, persistence, bioaccumulation potential, and resistance to natural degradation[2]. They may occur in different environmental compartments, including water and air[3], and their continuous release from industrial and urban activities can create long-term risks to aquatic ecosystems and human health[4].
The rapid expansion of industrial activities has increased the generation of wastewater containing chemically stable and biologically recalcitrant organic compounds[5]. Conventional biological treatment remains an important component of wastewater management; however, its effectiveness may be limited when wastewater contains compounds with low biodegradability, high toxicity, or complex molecular structures[6]. Consequently, advanced oxidation processes (AOPs) have attracted increasing attention as complementary or alternative technologies for the treatment of difficult-to-degrade organic wastewater[7].
AOPs were initially proposed for drinking-water treatment during the 1980s and were subsequently investigated for a wide range of wastewater applications[8]. Their principal mechanism involves the in situ generation of highly reactive oxidative species, particularly hydroxyl radicals (•OH) and sulfate radicals (SO₄•−)[9]. Because these radicals generally have short lifetimes, they must be continuously generated during treatment through combinations of oxidants, irradiation, catalysts, electrochemical reactions, or other activation mechanisms[10].
Typical oxidants include hydrogen peroxide (H₂O₂), ozone (O₃), and persulfate compounds[11], whereas ultraviolet irradiation, ultrasound, electrochemical activation, and transition-metal catalysts can promote radical formation[12]. Compared with many conventional treatment processes, AOPs offer several important characteristics: rapid oxidation, broad applicability, strong degradation capability, and the potential for substantial mineralization without generating conventional secondary organic pollution[13].
The principal characteristics of AOP-based treatment can be summarized as follows[14]:
    generation of highly reactive species capable of initiating oxidation-chain reactions;
    potential mineralization of organic contaminants into simpler inorganic products;
    applicability to wastewater containing both relatively high and trace concentrations of organic pollutants; and
    compatibility with biological treatment, membrane separation, adsorption, and electrochemical technologies.
For Bangladesh, these characteristics are particularly relevant to industrial wastewater management because wastewater streams can contain complex mixtures of organic substances originating from industrial activities[15]. Rather than considering a single AOP as universally applicable, process selection should consider pollutant characteristics, initial organic load, pH, oxidant dosage, treatment time, reaction kinetics, energy demand, and integration with downstream treatment[16].
Accordingly, this study develops an optimization framework for AOP-based organic wastewater treatment in Bangladesh. Five major technology groups are considered: electrochemical catalytic oxidation, Fenton-related oxidation, photocatalytic oxidation, catalytic ozonation, and persulfate-based oxidation.
2. Conceptual Framework for AOP Optimization
The effectiveness of an AOP system can be expressed as a function of several interacting variables:
η=f(pⓜ,Hⓜ,C_Oⓜ,C_orgⓜ,tⓜ,Tⓜ,C_cat E)
where:
    η= pollutant-removal efficiency;
    pH= solution acidity/alkalinity;
    C_O= oxidant concentration;
    C_org= initial organic pollutant concentration;
    t= reaction time;
    T= temperature;
    C_cat= catalyst concentration;
    E= energy input.
For practical wastewater treatment, optimization should not maximize pollutant removal alone. A process achieving very high removal while requiring excessive oxidant consumption or electrical energy may not be economically attractive.
Therefore, the proposed framework considers a multi-objective optimization function:
F=w_1 η_COD+w_2 η_TOC+w_3 k-w_4 E_c-w_5 C_o
where:
    η_COD= COD removal;
    η_TOC= TOC removal;
    k= apparent degradation-rate constant;
    E_c= energy consumption;
    C_o= oxidant consumption;
    w_1-w_5= weighting coefficients.
Figure 1. Proposed optimization framework for AOP-based organic wastewater treatment in Bangladesh
 
3. Reactive Species and Oxidation Mechanisms
The fundamental principle of AOPs is the generation of reactive species capable of attacking chemically stable organic molecules[17].
Hydroxyl radicals are particularly important because of their high oxidation potential and broad reactivity. Sulfate radicals generated from activated persulfate provide another highly reactive pathway and can be advantageous over a broader pH range[18].
The short lifetime of these species means that their continuous generation inside the treatment reactor is essential. The efficiency of the overall system therefore depends not only on the nominal concentration of oxidants but also on the rate at which reactive species are generated and consumed.
Organic pollutants may undergo[19]:
    hydroxylation;
    dealkylation;
    aromatic-ring opening;
    bond cleavage;
    oxidation of heteroatoms;
    formation of intermediate products; and
    eventual mineralization toward CO₂ and H₂O.
Figure 2. Radical-mediated degradation pathway of organic pollutants
 
4. Electrochemical Advanced Oxidation Processes
Electrochemical oxidation is one of the important AOPs for recalcitrant wastewater[20]. In an electrochemical reactor, organic pollutants may be directly oxidized at the anode or indirectly oxidized through electrochemically generated reactive species[21].
The general mechanism can involve direct electron transfer:
Organic→Organic^++e^-
or indirect oxidation through reactive oxygen species generated at the electrode surface.
Electrochemical systems offer several advantages, including high oxidation efficiency, controllable operation, and limited chemical addition. However, their performance can be constrained by mass-transfer limitations and relatively high energy consumption[22].
A reported electrochemical system using a Ti/SnO₂+Sb₂O₃/PbO₂ anode and stainless-steel cathode achieved more than 80% organic removal from simulated isophthalonitrile wastewater with an initial concentration of 250 mg/L[23].
Another reported study using boron-doped diamond electrodes treated pesticide wastewater with an initial COD of 450 mg/L, with complete oxidation reported within 6 h[24].
Electrochemical oxidation has also been investigated for antibiotic-containing wastewater. Boron-doped diamond/stainless-steel systems were reported to degrade sulfonamide antibiotics, with hydroxyl-radical attack on S–N bonds and aromatic structures contributing to degradation.
4.1 Electrochemical–membrane integration
The combination of electrochemical advanced oxidation with membrane separation provides an important strategy for improving overall treatment performance[25].
Three configurations can be distinguished[26]:
    membrane–electrochemical anodic oxidation (M-EAO);
    membrane–electrocatalytic oxidation (M-ECO);
    membrane–photoelectrocatalytic oxidation (M-PECO).
EAO can function as membrane pretreatment, reducing organic fouling and improving membrane performance[27]. Conversely, membrane concentration can increase pollutant concentration before electrochemical treatment, potentially improving the effectiveness of the oxidation stage[28].
Carbon nanotube membranes, boron-doped diamond electrodes, carbon-based conductive membranes, and composite catalytic membranes have all been investigated as integrated systems.
Figure 3. Integrated membrane–AOP configurations
 
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5. Fenton and Electro-Fenton Oxidation
Fenton oxidation is based primarily on the reaction between Fe²⁺ and H₂O₂, producing highly reactive hydroxyl radicals[29]:
Fe^(2+)+H_2 O_2→Fe^(3+)+⋅OH+OH^-
The generated hydroxyl radicals can rapidly attack organic pollutants.
Zero-valent iron (ZVI) has also been investigated as an alternative approach. The source data report the following reactions:
Fe^0+O_2+2H^+→Fe^(2+)+H_2 O_2
Fe^0+H_2 O_2+2H^+→Fe^(2+)+2H_2 O
Fe^(2+)+H_2 O_2→Fe^(3+)+⋅OH+OH^-
A reported ZVI-assisted Fenton system achieved complete degradation of 100 mg/L phenol, while under optimized conditions approximately 75% TOC removal occurred within 3 min.
The introduction of UV irradiation can improve Fenton oxidation by promoting H₂O₂ decomposition and reducing the effective Fe²⁺ requirement. For azo-dye wastewater at an initial concentration of 400 mg/L, UV-Fenton treatment achieved more than 95% decolorization under optimized conditions.
However, UV-Fenton systems may require additional equipment and energy, and UV utilization becomes a practical consideration during scale-up.
Electro-Fenton technology combines electrochemical and Fenton reactions. Hydroxyl radicals may be generated through cathodic processes involving Fe²⁺ and H₂O₂, while direct or mediated anodic oxidation may provide additional degradation pathways.
 6. Photocatalytic Oxidation
Photocatalytic oxidation uses semiconductor materials capable of generating electron–hole pairs under irradiation[30].
For TiO₂:
TiO_2+hν→e^-+h^+
The photogenerated holes can react with water:
TiO_2 (h^+ )+H_2 O→TiO_2+⋅OH+H^+
and with hydroxide:
TiO_2 (h^+ )+OH^-→TiO_2+⋅OH
The resulting reactive species attack organic pollutants and can ultimately convert them into smaller molecules and mineralization products[31].
The principal parameters affecting photocatalytic efficiency include[32]:
    light-response range;
    quantum yield;
    electron–hole recombination;
    catalyst stability;
    catalyst surface area;
    pollutant concentration;
    irradiation intensity; and
    reaction time.
TiO₂ remains one of the most extensively studied photocatalysts. However, its practical performance can be restricted by limited light utilization and electron–hole recombination.
Recent research has therefore explored doped and composite photocatalysts, including carbon nitride, graphene-derived materials, carbon nanotubes, and metal-based nanostructures.
One reported study involving Ag/MnO₂ porous hollow microspheres achieved complete E. coli inactivation within 10 min under optimized conditions[33]. Other investigations have demonstrated the potential of solar photocatalysis for persistent chlorinated pollutants and industrial wastewater streams, including brewery, paper, dye, and oil-production wastewater[34].
A particularly relevant approach involves coupling graphitic carbon nitride with biochar. Such composites can simultaneously exploit adsorption and photocatalytic degradation, allowing organic pollutants to be concentrated on the carbonaceous surface and subsequently degraded under irradiation[35].
Figure 5.  Photocatalytic degradation mechanism
 
7. Catalytic Ozonation
Ozone is a powerful oxidant that can react directly with organic compounds or decompose to produce secondary reactive species[36].
The degradation pathway is influenced strongly by solution pH[37]. Important reactions include:
OH^-+O_3→O_2+HO_2^-
HO_2^-+O_3→HO_2^∙+O_3^(∙-)
O_3^(∙-)+H^+→HO_3^∙
HO_3^∙→⋅OH+O_2
An increase in pH can promote ozone decomposition and hydroxyl-radical formation. Addition of H₂O₂ may further accelerate reactive-species generation[38].
However, ozone has relatively limited solubility in water, and incomplete mineralization may occur. Catalysts can therefore improve ozone utilization and promote the formation of more reactive species[39].
Catalytic ozonation can be classified into homogeneous and heterogeneous systems. Homogeneous catalysts often involve dissolved metal ions, whereas heterogeneous catalysts include activated carbon, composite metal oxides, and mineral-based catalysts[40].
In a reported phenolic wastewater study, catalytic ozonation using a Pt/Al₂O₃ catalyst achieved 93% phenolic removal and 47% COD removal after 120 min at pH 7[41].
Another UV/O₃ treatment of textile wastewater achieved more than 64% COD removal, with the final COD reported as 79 mg/L, while color removal exceeded 99%, with final turbidity reported at approximately 1 NTU under optimized conditions[42].
These results highlight the importance of distinguishing pollutant disappearance from complete mineralization. High color removal, for example, does not necessarily indicate equivalent COD or TOC removal.
Figure 6. pH-dependent ozone activation
 
8. Persulfate-Based Advanced Oxidation
Persulfate compounds provide another important pathway for generating reactive species[43].
The standard oxidation potential of persulfate is approximately:
E^0=2.01V
After activation, persulfate can generate sulfate radicals:
S_2 O_8^(2-)→2SO_4^(∙-)
with a sulfate-radical oxidation potential reported at approximately:
E^0=2.6V
Sulfate radicals exhibit high reactivity and can participate in electron-transfer reactions with saturated and aromatic compounds. They also have a broader effective pH range and a reported half-life of approximately 30–40 μs[44].
Activation can be achieved through:
    thermal activation;
    UV irradiation;
    transition-metal catalysts;
    carbon-based catalysts;
    alkaline conditions.
Temperature has a major influence on radical generation, but higher temperature does not necessarily produce better pollutant removal. In the reported thermal activation of persulfate for perfluorooctanoic acid degradation, the removal at 80°C exceeded that obtained at 150°C, illustrating the importance of radical scavenging and excessive thermal input[45].
Carbon-based activators such as graphene oxide, activated carbon, and carbon nanotubes have attracted considerable interest because of their high surface areas and chemical stability. Mn-doped g-C₃N₄/PMS systems have also been reported to promote pollutant degradation through singlet oxygen and superoxide-radical pathways[46].

9. Effects of Key Optimization Variables
9.1 Effect of pH
pH affects almost every major AOP mechanism. It influences[47]:
    radical formation;
    oxidant stability;
    catalyst speciation;
    pollutant ionization;
    ozone decomposition;
    iron cycling;
    membrane behavior.
The effect is especially pronounced in Fenton and ozonation processes.
For Fenton treatment, acidic conditions generally favor Fe²⁺ availability and H₂O₂ activation. For ozone systems, increasing alkalinity can accelerate ozone decomposition and hydroxyl-radical generation. [48] 
Thus, pH should be treated as a primary optimization variable rather than as a simple operational parameter.
9.2 Oxidant Dosage
The amount of H₂O₂, O₃, or persulfate strongly influences treatment efficiency[49].
Insufficient oxidant dosage can limit radical generation, whereas excessive dosage can increase operating costs and, in some systems, promote radical scavenging[50].
Consequently, the optimum oxidant dose should be determined jointly with initial pollutant concentration and reaction time.
9.3 Reaction Time
Reaction time controls the extent of pollutant degradation and mineralization[51].
However, prolonged reaction times may produce diminishing returns. For example, the reported electrochemical treatment of pesticide wastewater achieved complete oxidation within 6 h, whereas the ZVI-Fenton system achieved complete degradation of 100 mg/L phenol under optimized conditions and approximately 75% TOC removal within 3 min.
These results demonstrate that apparent pollutant removal and mineralization kinetics can differ substantially.
9.4 Initial Organic Load
Initial organic concentration affects radical demand and mass-transfer behavior.
At high organic loading, the available radicals are consumed rapidly, potentially reducing the fraction of pollutants removed per unit oxidant. At very low pollutant concentrations, however, some electrochemical systems may become less economically attractive because large wastewater volumes must be processed relative to the contaminant mass[52].
Therefore, initial COD/TOC and specific pollutant concentration should be incorporated into process optimization.
10. Degradation Kinetics
For many AOP systems, degradation can be approximated using a pseudo-first-order model:
ln⁡(C_0/C_t )=k_obs t
where:
    C_0= initial pollutant concentration;
    C_t= concentration at time t;
    k_obs= observed degradation-rate constant.
The corresponding half-life is:
t_(1/2)=ln⁡2/k_obs 
For practical optimization, kinetic analysis should be performed alongside COD and TOC measurements because disappearance of the parent compound does not necessarily represent complete mineralization.
Figure 7. Proposed kinetic model for AOP treatment
 
11. Energy Consumption and Process Sustainability
Energy demand is one of the most important parameters when transferring AOPs from laboratory studies to industrial applications.
For electrically driven systems, a general specific-energy expression can be represented as:
E_sp=(P×t)/V
where:
    P= electrical power;
    t= treatment duration;
    V= wastewater volume.
For electrochemical systems, high energy requirements may become a major limitation, particularly for large wastewater volumes and low pollutant concentrations.
Photocatalytic systems can reduce chemical demand but require suitable irradiation. UV-based systems may have higher electrical requirements than solar-driven systems.
Ozonation also requires energy for ozone generation and gas transfer.
Therefore, the optimization objective should not simply be:
maximum removal efficiency
but rather:
high removal + high mineralization + acceptable reaction time + low oxidant consumption + acceptable energy demand.
12. Comparative Evaluation of AOP Technologies
AOP    Principal reactive species    Main controlling variables    Major advantage    Main limitation
Electrochemical oxidation    •OH and surface-bound oxidants    Current density, electrode material, electrolyte, time    Strong controllability    Energy demand
Fenton    •OH    pH, Fe²⁺, H₂O₂, time    Rapid oxidation    Iron sludge and chemical consumption
Electro-Fenton    •OH    Current, Fe²⁺, H₂O₂, pH    Enhanced oxidation    Electrical energy
Photocatalysis    •OH, h⁺, O₂•−    Light intensity, catalyst, pH, time    Solar potential    Electron–hole recombination
Catalytic ozonation    •OH + O₃    pH, ozone dose, catalyst    Strong oxidation and disinfection    Ozone utilization
Persulfate activation    SO₄•−, •OH    PMS/PDS dose, activator, pH, temperature    Broad reaction conditions    Activation and chemical cost
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13. Proposed AOP Strategy for Organic Wastewater in Bangladesh
For the Bangladesh context, AOP selection should be based on wastewater characteristics rather than a single universally preferred technology.
A practical treatment framework is:
Wastewater characterization → COD/TOC/color/pollutant analysis → biodegradability assessment → selection of AOP → pH optimization → oxidant/catalyst optimization → kinetic assessment → energy assessment → membrane/biological polishing → treated effluent
For wastewater with high color and aromatic organic compounds, ozonation, photocatalysis, and Fenton-based processes may be considered.
For concentrated recalcitrant wastewater, electrochemical or electro-Fenton systems may provide strong oxidation but should be evaluated carefully against electrical consumption.
For wastewater requiring final polishing, an AOP–membrane or AOP–biological hybrid configuration can provide complementary treatment mechanisms.
Figure 8. Proposed treatment train for Bangladesh
 
14. Integration with Membrane and Biological Processes
AOPs do not necessarily need to operate as stand-alone treatment systems. Their integration with conventional processes can improve overall treatment efficiency[53].
An AOP can serve as[54]:
    pretreatment, increasing biodegradability before biological treatment;
    membrane pretreatment, reducing organic fouling;
    membrane post-treatment, degrading residual dissolved organics;
    polishing treatment, removing persistent contaminants after biological treatment.
This approach is particularly important because excessive oxidation of high-organic-load wastewater can increase chemical and energy requirements.
Figure 9. Hybrid AOP–biological–membrane treatment concept
 
15. Discussion
The effectiveness of advanced oxidation processes (AOPs) is strongly influenced by the characteristics of the target pollutant and the specific treatment configuration employed. In the reported electrochemical system, wastewater containing 250 mg/L isophthalonitrile showed a removal efficiency exceeding 80%. A higher degree of treatment was observed with boron-doped diamond electrodes, which achieved complete oxidation of wastewater characterized by an initial chemical oxygen demand (COD) of 450 mg/L after 6 h of treatment . Fenton-based approaches demonstrated substantially faster reaction kinetics under optimized operating conditions. In particular, the ZVI-assisted Fenton system completely degraded 100 mg/L phenol while attaining approximately 75% total organic carbon (TOC) removal within only 3 min.
The UV-Fenton results further demonstrate that high apparent pollutant removal can be achieved for concentrated dye wastewater, with more than 95% decolorization at 400 mg/L azo-dye concentration . Nevertheless, color removal should not automatically be interpreted as complete mineralization because oxidation may generate intermediate compounds.
Catalytic ozonation also demonstrates the importance of pH. The reported 93% phenolic removal but only 47% COD removal after 120 min at pH 7 indicates that parent-compound disappearance and overall organic-carbon removal may occur at different rates. This distinction supports the inclusion of COD and TOC alongside specific pollutant concentrations in the proposed Bangladesh optimization framework.
Persulfate activation provides an additional route for treating recalcitrant pollutants. However, the reported thermal-activation behavior demonstrates that increasing temperature does not necessarily improve treatment proportionally, emphasizing the need to consider radical-scavenging reactions and energy consumption.
Collectively, these findings indicate that the optimization of AOP systems requires simultaneous consideration of chemical efficiency, reaction kinetics, pollutant loading, operating conditions, and energy requirements.
16. Research Gaps and Future Development
Several research priorities emerge from the combined analysis.
First, more attention should be given to the relationship between initial organic loading and oxidant demand. Second, kinetic models should be integrated with energy-consumption analysis rather than evaluating removal efficiency alone.
Third, catalyst stability and reuse remain important considerations. Although heterogeneous catalysts can reduce separation requirements, their long-term activity and potential leaching must be assessed.
Fourth, membrane–AOP systems require optimization of mass transfer, membrane fouling, catalyst recovery, and energy demand.
Finally, the development of hybrid systems involving AOPs, biological treatment, membranes, adsorption, electrochemical processes, and solar irradiation could provide more sustainable treatment configurations for complex industrial wastewater.
For Bangladesh, future experimental studies should obtain wastewater-specific data from representative industrial sectors and evaluate AOP performance under locally relevant conditions. Such experiments should systematically vary pH, oxidant dosage, reaction time, organic loading, catalyst concentration, and energy input.
17. Conclusions
Advanced oxidation processes provide powerful tools for treating recalcitrant organic wastewater because of their ability to generate highly reactive species such as hydroxyl and sulfate radicals. Electrochemical oxidation, Fenton and electro-Fenton processes, photocatalysis, catalytic ozonation, and persulfate activation each provide distinct oxidation mechanisms and operating characteristics.
The retained benchmark data demonstrate substantial treatment potential, including more than 80% organic removal for 250 mg/L isophthalonitrile wastewater, complete oxidation of 450 mg/L initial-COD pesticide wastewater within 6 h, complete degradation of 100 mg/L phenol using ZVI-assisted Fenton oxidation, more than 95% decolorization of 400 mg/L azo-dye wastewater through UV-Fenton treatment, and 93% phenolic removal during catalytic ozonation at pH 7.
However, high pollutant removal alone is insufficient for evaluating an AOP system. The proposed Bangladesh-oriented framework therefore incorporates pH, oxidant dosage, reaction time, initial organic load, degradation kinetics, and energy consumption as interconnected optimization variables.
The integration of AOPs with membrane separation, biological treatment, and other physicochemical processes provides an important pathway toward improved treatment performance and reduced operational limitations. Future work should validate the proposed framework using wastewater collected from representative Bangladeshi industrial sources and should prioritize simultaneous optimization of removal efficiency, mineralization, reaction kinetics, chemical consumption, energy demand, and long-term catalyst stability.


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.

List of Abbrevations:
(AOPs): Advanced oxidation processes; (SO₄•−): sulfate radicals ; (H₂O₂): hydrogen peroxide ,  (O₃): ozone, (COD): chemical oxygen demand ); (TOC): total organic carbon;

Acknowledgment: 
The authors gratefully acknowledge the Department of Chemical Engineering, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh, for providing academic and institutional support for this research under Grant No. BUET.DHAKA.KK49855.D2025.GG8. The authors also sincerely thank The International Journal of Engineering Sciences, Noor Al-Ilm for Publishing and Distribution, for granting a full waiver of the article publication fees, thereby enabling the manuscript to be published without publication charges. The authors greatly appreciate the institutional, academic, and editorial support that contributed to facilitating the completion and dissemination of this research.

Author Contribution: 
B.M.A., P.K.M., S.M.C., T.P.M., S.S.N., and K.M.N. conducted the experiments. M.A.A. contributed to writing the initial draft of the manuscript and language editing. All authors read and approved the final manuscript.

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 was financially supported by a research grant from the Department of Chemical Engineering, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh, under Grant No. BUET.DHAKA.KK49855.D2025.GG8. The authors also gratefully acknowledge The International Journal of Engineering Sciences, Noor Al-Ilm for Publishing and Distribution, for granting a full waiver of the article publication fees. This waiver was provided exclusively as editorial support and did not constitute financial support for, or involvement in, the research design, study implementation, data analysis, interpretation of findings, or preparation and reporting of the manuscript.

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