Advanced Oxidation Processes (AOP) for Industrial ETP — Design Guide

What Are Advanced Oxidation Processes (AOPs)?

Advanced Oxidation Processes (AOPs) are treatment technologies that generate highly reactive hydroxyl radicals (•OH) — among the most powerful oxidants known (oxidation potential 2.8 V, compared to ozone at 2.07 V and chlorine at 1.36 V). Hydroxyl radicals react non-selectively with virtually all organic molecules at near-diffusion-limited rates, breaking them down to CO₂, water, and inorganic salts.

The major AOP technologies used in industrial ETP:

• Fenton oxidation: Fe²⁺ + H₂O₂ → •OH generation at pH 3–4
• Ozone (O₃): Direct ozone oxidation + •OH from ozone decomposition at alkaline pH
• Ozone/UV: UV photolysis of ozone enhances •OH generation
• UV/H₂O₂: UV photolysis of hydrogen peroxide generates •OH
• Photocatalysis (TiO₂/UV): UV-irradiated TiO₂ photocatalyst generates •OH from dissolved oxygen and water
• Electro-oxidation: Direct anodic oxidation and electrochemically generated oxidants at electrode surfaces
• Wet air oxidation (WAO): High-temperature (150–300°C) and pressure oxidation for very high-strength sludge or effluent

When to Use AOP in Industrial ETP

AOP is not suitable as a standalone primary treatment for high-volume, high-COD industrial effluents — the operating cost (chemicals, energy) would be prohibitive. AOPs are applied strategically:

• AOP as pre-treatment before biological treatment: Oxidises recalcitrant compounds (chlorinated solvents, phenols, non-biodegradable dyes) to biodegradable intermediates (acids, aldehydes) — raises the BOD/COD ratio from 0.1–0.2 (recalcitrant) to 0.4–0.5 (biodegradable), enabling effective downstream biological treatment. Used in pharmaceutical, pesticide, and chlorinated solvent ETP.
• AOP as polishing after biological treatment: Biological treatment reduces COD from 2,000–10,000 mg/L to 200–600 mg/L. AOP polishing step reduces from ~400 mg/L to ≤ 250 mg/L (CPCB inland standard) or ≤ 100 mg/L for ZLD. Used in distillery, textile, and dye intermediate ETP.
• Colour removal: Biological treatment does not remove chromophoric compounds (melanoidins in distillery effluent, azo dye fragments in textile effluent). AOP specifically targets the aromatic colour structures. A combined AOP + coagulation-flocculation step achieves colour ≤ 100 Pt-Co units.
• Micropollutant removal: Antibiotics, endocrine disruptors, and pharmaceutical active compounds that bioaccumulate in biological sludge can be destroyed by AOP — relevant for pharmaceutical and hospital ETPs.

Fenton Oxidation — Design and Application

Fenton oxidation is the most widely used AOP in Indian industrial ETPs due to its simplicity, low capital cost, and effectiveness:

• Reaction chemistry: Fe²⁺ + H₂O₂ → Fe³⁺ + •OH + OH⁻. The •OH radical attacks organic molecules; Fe³⁺ is regenerated to Fe²⁺ by further H₂O₂ reaction (photo-Fenton) or by reducing agents. Net consumption: H₂O₂ (50% technical grade) and FeSO₄·7H₂O.
• Optimal pH: 2.5–3.5. Effluent must be acidified (H₂SO₄ or HCl dosing) before Fenton reactor and re-neutralised after reaction.
• Typical dosing: H₂O₂/COD ratio = 1–3 g H₂O₂/g COD; Fe²⁺/H₂O₂ ratio = 0.05–0.10 by weight. Optimum dosing determined by jar testing at site.
• Reaction time: 30–60 minutes in a stirred Fenton reactor at pH 3–3.5.
• Post-treatment: After Fenton reaction, pH is raised to 7–8 by lime or NaOH dosing. Fe³⁺ precipitates as Fe(OH)₃ — a coagulant that adsorbs colour and residual organic fragments. Flocculator + clarifier settles the iron sludge.
• COD removal: 40–80% COD removal depending on effluent type; colour removal 70–95%.

Ozone and Ozone/UV Treatment

Ozone treatment provides oxidation without adding iron or chemical sludge:

• Ozone generation: On-site via corona discharge ozone generators fed with dry air or pure oxygen. Ozone concentration: 6–14% by weight from air; 10–16% from oxygen. Generation efficiency: 8–15 kWh per kg O₃ from air.
• Contact systems: Fine bubble diffusion in packed towers or bubble columns (ozone half-life 20–30 minutes at pH 7, 20°C); or venturi injectors with dissolution efficiency 80–95%.
• Typical dosing: 0.5–2.0 g O₃/g COD removed for polishing; 5–15 g O₃/g colour for colour removal from distillery/textile effluent.
• Off-gas destruction: Unreacted ozone in off-gas must be destroyed by thermal (300°C) or catalytic (MnO₂) ozone destruct unit — CPCB mandates ozone destruct for occupational safety (ozone TLV: 0.1 ppm).
• Ozone/UV: UV irradiation (253.7 nm wavelength, low pressure Hg lamps) accelerates ozone decomposition to •OH — increasing •OH yield 3–5× over ozone alone. Effective for micropollutant destruction at low ozone doses.

UV/H₂O₂ and Photocatalysis

UV/H₂O₂ and photocatalysis are used for lower-flow, higher-value applications:

• UV/H₂O₂: H₂O₂ absorbs UV light at 253.7 nm and splits into 2 •OH radicals. Effective for clear or light-coloured effluents (turbidity and colour absorb UV, reducing penetration depth). UV dose: 1,000–5,000 mJ/cm²; H₂O₂ dose: 10–50 mg/L. Common in pharmaceutical and groundwater remediation applications.
• Photocatalysis (TiO₂/UV): TiO₂ semiconductor photocatalyst in slurry or immobilised form generates •OH from UV-irradiated surface reactions with water and dissolved oxygen. Effective for low-concentration micropollutant removal. Challenges: TiO₂ slurry recovery (requires membrane filtration), UV attenuation in coloured effluents. Primarily used at pilot/research scale for antibiotics and pharmaceutical compounds.

Electro-Oxidation

Electro-oxidation is gaining adoption in Indian industrial ETPs for small-flow applications:

• Working principle: DC current applied between dimensionally stable anodes (DSA — titanium coated with RuO₂/IrO₂ or PbO₂) generates •OH, Cl₂ (in chloride-containing effluent), and ozone directly in the effluent.
• Applications: Electroplating rinse water (cyanide destruction, heavy metal oxidation), textile CMC effluent, landfill leachate, and small pharmaceutical unit ETPs. Flow rates 1–50 m³/hour typical.
• Energy consumption: 5–20 kWh/m³ for moderate COD removal — significantly higher than biological treatment but acceptable for small volumes or ZLD polishing applications.
• No chemical addition: Unlike Fenton (H₂O₂, FeSO₄) or ozone (electricity), electro-oxidation avoids most chemical inputs — relevant for remote plants with chemical supply difficulties.

AOP Cost Comparison

Compliance and Safety Considerations

Compliance and safety requirements for AOP installations:

• H₂O₂ storage: 50% H₂O₂ is a strong oxidiser — storage tanks must be HDPE or SS316 in bunded areas; segregated from organic solvents and reducing agents; PESO clearance required for large storage volumes.
• Ozone safety: Ozone TLV is 0.1 ppm (8-hour TWA). Ozone generators, contactors, and off-gas lines must be in enclosed or well-ventilated areas with continuous ozone gas detectors. Off-gas destruction mandatory. Ozone piping: SS316 or PVDF — never rubber or PVC (ozone degrades organics rapidly).
• Iron sludge from Fenton: Fenton sludge (Fe(OH)₃ with adsorbed organics) may be classified as hazardous waste under HWM Rules if the effluent contains heavy metals or organic priority pollutants. Characterise and dispose to authorised TSDF or dewater for reuse as pigment.
• Effluent monitoring: AOP-treated effluent must be tested for residual H₂O₂ and ozone before discharge — these should not exceed 1 mg/L to avoid harm to receiving water ecology. Toxicity testing (fish bioassay) recommended for pharmaceutical and dye intermediate ETPs using AOP.

Frequently Asked Questions