Fenton Oxidation Process Design and Operation: Reading HJ 1095-2020 Clause by Clause

1. Status and Applicable Wastewaters

HJ 1095-2020, Technical Specifications of Fenton Oxidation Process for Wastewater Treatment, was approved by the Ministry of Ecology and Environment on 13 January 2020 and took effect on 14 January 2020. It is a guidance standard and its first edition. Fenton oxidation may be used either as pretreatment ahead of biological treatment or as advanced treatment after it, and is mainly applied to wastewater containing refractory organics: papermaking, dyeing and finishing, coal chemical, petrochemical, fine chemical and fermentation wastewater, landfill leachate and centralised industrial park effluent.

2. Influent Restrictions (clause 4.3.2)

• Pollutants that readily generate toxic or hazardous gases under acidic conditions, such as sulphide and cyanide, must not enter the Fenton unit.
• Suspended solids in the influent should be kept below 200 mg/L.
• Chloride, dihydrogen phosphate, bicarbonate, oil and other inorganic ions or pollutants that interfere with Fenton oxidation must be controlled, with limiting concentrations fixed by testing.
• Where conditions are not met, pretreatment is required: coarse and fine screens, grit chambers, sedimentation or coagulation-sedimentation when used as biological pretreatment; coagulation-sedimentation and/or filtration when used for advanced treatment; hydrated lime dosing where dissolved phosphate is high; oil separation where oil is present; chemical precipitation or chemical oxidation for sulphide, and chemical oxidation for cyanide.

3. Acid Dosing (clause 6.2)

• Concentrated or dilute sulphuric acid is dosed to bring pH into the range of 3.0 to 4.0.
• The acid dosing tank is mixed hydraulically, mechanically or with air, with a mixing time of not less than 2 minutes.
• Acid is dosed by metering pump, with automatic control from an online pH controller.

4. Catalyst Mixing (clause 6.3)

• Ferrous sulphate is the usual catalyst, mixed in a dedicated catalyst mixing tank.
• Mixing time not less than 2 minutes.
• Ferrous sulphate solution concentration preferably below 30% by mass, dosed by metering pump.

5. Oxidation (clause 6.4)

• The oxidation tank may be completely mixed or plug flow; a completely mixed tank should have no fewer than 2 stages connected by overflow or baffle wall.
• Effective depth for a completely mixed tank: 2.5 to 6.0 m.
• Hydraulic retention time: 2.0 to 8.0 h when used as pretreatment; 2.0 to 6.0 h when used for advanced treatment.
• Reagent ratios where no test data exist: the ratio of hydrogen peroxide concentration in mg/L to COD in mg/L is preferably 1:1 to 2:1; the ratio of hydrogen peroxide concentration to ferrous ion concentration is preferably 1:1 to 10:1. Testing should supersede these defaults.
• Tower reactors: velocity gradient G of at least 500 per second in the mixing zone; distribution orifice exit velocity 1.0 to 1.5 m/s; recirculation ratio not below 100%; height to diameter ratio 1.0 to 5.0; height not above 15 m; 316L stainless steel shell with glass flake lining.

6. Neutralisation and Degassing (clause 6.5)

• Alkali is dosed in the neutralisation tank to bring pH to neutral; sodium hydroxide or sodium carbonate solution is preferred and calcium hydroxide solution is not recommended.
• For direct discharge, pH must satisfy both solid-liquid separation and discharge requirements; where effluent continues to further treatment, pH must suit the downstream process.
• Neutralisation mixing time not less than 2 minutes.
• Where air mixing is not used in oxidation and neutralisation, an air-mixed degassing tank is required, with hydraulic retention time of not less than 15 minutes and an air to water ratio of not less than 5:1.

7. Solid-Liquid Separation (clause 6.6)

• Sedimentation or dissolved air flotation is used; coagulant or flocculant may be added where separation is poor, following HJ 2006 and HJ 2007.
• Coagulant: polyaluminium chloride, dosed at 100 to 200 mg/L.
• Flocculant: polyacrylamide, dosed at 3 to 5 mg/L.

8. Common Misapplications and Checkout Points

• Treating Fenton as universal pretreatment. Above 200 mg/L suspended solids, reagent is consumed by particulates rather than dissolved organics, so coagulation-sedimentation or flotation must come first.
• Ignoring residual hydrogen peroxide. When Fenton precedes biological treatment, provide a degassing tank, buffer tank or reducing agent to remove residual peroxide, otherwise downstream biology becomes unstable.
• Neutralising with lime. The standard explicitly discourages calcium hydroxide solution; it increases iron sludge volume and causes scaling.
• Letting pH drift. Above pH 4, iron precipitates as hydroxide and the catalyst is passivated, cutting hydroxyl radical yield; well below about pH 2.5, hydrogen peroxide stabilises and radical formation is likewise suppressed.
• Misclassifying iron sludge. Sludge must be assessed against the National Hazardous Waste Catalogue and the identification standards GB 5085.7 and HJ/T 298; it cannot be assumed to be general industrial solid waste.
• Redundancy. Oxidation tanks, neutralisation tanks and separation facilities should each be built with no fewer than two parallel cells; oxidation and neutralisation tanks should have water spray or foam-breaking spray, and separation units should have skimmers.

References

• HJ 1095-2020, Technical Specifications of Fenton Oxidation Process for Wastewater Treatment (MEE release copy, in Chinese)
• HJ 2010-2011, Technical Specifications for Membrane Biological Processes (basis for MBR downstream of Fenton)