Coking Wastewater Treatment: Phenol, Cyanide, Thiocyanate and Ammonia Removal

Coking wastewater is among the most complex industrial effluents in existence, simultaneously carrying phenol, cyanide, thiocyanate, ammonia, sulphide and polynuclear aromatic hydrocarbons. No single treatment technology can address this cocktail. Successful plants rely on carefully sequenced physico-chemical and biological stages, each targeted at a specific interfering contaminant, with sequencing chosen so that an inhibitor is removed before the biology that would be harmed by it.

The effluent originates in coke oven gas cooling, coal tar separation, and by-product recovery in coke plants serving steel mills and standalone coke producers. Typical raw concentrations are severe: phenol 600 to 2,000 mg/L, ammonia nitrogen 300 to 1,200 mg/L, cyanide 5 to 40 mg/L, thiocyanate 200 to 800 mg/L, sulphide 50 to 300 mg/L and COD 2,000 to 6,000 mg/L. Comparable combined loads of phenolics and sulphides appear in refinery wastewater treatment for oil, phenolics and sulphide, which faces a similar chemistry with a different ratio of components.

Contaminant Interactions and Sequencing Logic

The order of treatment stages is dictated by toxicity and by mutual interference.

• Cyanide is acutely toxic to nitrifying bacteria and inhibits phenol degradation at concentrations above 2 mg/L

• Free ammonia above 100 mg/L inhibits phenol-oxidising biomass

• Thiocyanate requires a dedicated Thiobacillus population that grows only after cyanide has been substantially removed

• Phenol above 500 mg/L exerts solvent toxicity on activated sludge and can destabilise the floc

• Sulphide consumes oxygen and precipitates heavy metals, affecting trace nutrient availability

The consistent conclusion is that cyanide and ammonia should be dealt with before biological treatment, phenol should be partly recovered or oxidised if very high, and thiocyanate degradation should be sequenced after cyanide destruction within the biological stage.

Phenol Recovery and Oxidation

At concentrations above 1,000 mg/L, phenol recovery is economically justified, both because phenol has resale value and because removing it reduces the load on downstream biology by a large margin.

Solvent Extraction

Counter-current extraction with an organic solvent removes 90 to 95 percent of phenol, producing an extract that is stripped to recover a concentrated phenol stream returned to the by-product recovery plant. Common solvents include methyl isobutyl ketone, diisopropyl ether and tributyl phosphate dissolved in kerosene. The raffinate carries 100 to 200 mg/L phenol, which is then handled biologically.

Adsorption

Coal-based activated carbon adsorbs phenol effectively, and spent carbon can be regenerated by thermal desorption, recovering the phenol and restoring capacity. Adsorption is economic where the carbon is already being regenerated on site for other purposes.

Advanced Oxidation

Where recovery is not viable, oxidation reduces phenol to biodegradable intermediates. The hydroxyl radical chemistry that underpins Fenton oxidation pretreatment for refractory COD applies directly to phenol-laden coking effluent, with iron dosing at a molar ratio of 3 to 10 relative to hydrogen peroxide.

Cyanide Destruction

Cyanide must be reduced to below 1 mg/L before biological treatment, and preferably below 0.3 mg/L.

Alkaline Chlorination

Two-stage alkaline chlorination is the most widely used approach. The first stage, at pH 10.5 to 11.0, oxidises cyanide to cyanate using chlorine or sodium hypochlorite. The second stage, at pH 8.0 to 8.5, oxidises cyanate to nitrogen and carbon dioxide. The stoichiometric demand is roughly 3.5 kilograms of chlorine per kilogram of cyanide for complete destruction, but actual dosing runs 1.5 to 2 times theoretical because of competing reducing agents.

Biological Cyanide Degradation

Where concentrations are below 20 mg/L, cyanide can be degraded biologically by specialised bacteria at a rate of 0.2 to 0.5 kilograms per cubic metre per day. The advantage is the absence of chemical cost and the elimination of chlorinated by-products. The disadvantage is sensitivity to shock loads. Many coking plants operate a combined biological cyanide and thiocyanate stage after partial chemical destruction.

Peroxide and Ozone Oxidation

Hydrogen peroxide with a copper catalyst, or ozone at pH 10 to 11, both oxidise cyanide without producing chlorinated organics. Ozone consumption is 4 to 5 kilograms of ozone per kilogram of cyanide, which makes it expensive for high concentrations but attractive as a polishing step. The same oxidation chemistry underpins the reduction of hexavalent chromium in electroplating wastewater cyanide destruction.

Ammonia Removal

After phenol and cyanide reduction, ammonia becomes the dominant nitrogen load and must be addressed before biological treatment of the remaining organics.

Steam stripping at pH 11 removes 90 to 95 percent of ammonia nitrogen, producing an ammonia-laden vapour that is either scrubbed with sulphuric acid to ammonium sulphate or destroyed thermally. The packed tower design, pH control and air-to-liquid ratio follow the same parameters as ammonia nitrogen removal by stripping and biological nitrification. Some plants avoid dedicated stripping and rely instead on high-recycle biological nitrification, which is viable where the influent ammonia is below 300 mg/L.

Biological Treatment Configuration

Coking wastewater requires two-stage biological treatment because the inhibitory load exceeds what a single stage can absorb.

Anaerobic-Aerobic-Aerobic (A1/A2/O) Configuration

The first anaerobic stage hydrolyses complex organics and improves biodegradability. The first aerobic stage, operated at a sludge age of 25 to 40 days, degrades thiocyanate and continues phenol oxidation. The second aerobic stage, with a longer hydraulic retention time, polishes remaining COD and nitrifies residual ammonia. Total hydraulic retention time is typically 60 to 100 hours, which is two to three times that of conventional domestic sewage treatment.

Denitrification

An anoxic zone ahead of the aerobic stages allows denitrification using the phenol and residual COD as the carbon source, recovering the alkalinity consumed by nitrification and reducing the need for external carbon dosing. Recycle ratios of 200 to 400 percent from the aerobic zone back to the anoxic zone are typical.

Biofilm Hybrids

Adding suspended biofilm carriers to the aerobic stages raises the biomass inventory and protects slow-growing nitrifying and thiocyanate-degrading organisms from washout and shock loading. This is the same rationale that drives biofilm hybrid systems in membrane biological treatment of high-strength nitrogenous streams.

Polishing and Zero Liquid Discharge

After two-stage biological treatment, coking effluent typically still contains 150 to 300 mg/L COD dominated by colour bodies and refractory organics, together with residual total nitrogen.

• Coagulation and sedimentation: ferric or PACl dosing with polymer, removing colour and residual suspended solids

• Ozone or Fenton polishing: destroys residual colour and trace aromatics

• Activated carbon filtration: removes residual organics and odour, 0.5 to 1.0 kilograms of carbon per cubic metre treated

• Membrane concentration: for plants subject to zero liquid discharge, reverse osmosis concentrates the remaining salts, and an evaporator and crystalliser finish the concentrate

Where zero liquid discharge is required, the concentrate disposal train is substantially the same as in other industries, and the design method described for brine concentrator and crystallizer selection applies without major modification.

Conclusion

Coking wastewater treatment is a sequencing problem before it is a technology problem. Cyanide and excess ammonia must be removed before biological treatment can function, phenol should be recovered where its concentration justifies the investment, and thiocyanate degradation requires time and a stable population that only a long-sludge-age system can sustain. Plants that respect this hierarchy achieve consistent compliance; plants that attempt to treat coking effluent in a single biological stage rarely do.

Frequently Asked Questions

What is the correct treatment sequence for coking wastewater?

Primary oil and tar removal, then phenol recovery or oxidation, then cyanide destruction, then ammonia stripping, then two-stage biological treatment with denitrification, then physico-chemical polishing. Each stage exists to remove an inhibitor that would compromise a later stage, which is why the order cannot be rearranged without consequences.

How long does biological treatment of coking wastewater take?

Total hydraulic retention time of 60 to 100 hours across all biological stages, with a sludge age of 25 to 40 days. This compares with 12 to 24 hours for domestic sewage and reflects both the low growth rate of thiocyanate-degrading organisms and the need to dilute inhibitory compounds. Similar extended retention is characteristic of biological treatment of other highly inhibitory industrial streams.

Can coking wastewater be discharged to a municipal sewer?

Only after full pretreatment to meet the local trade effluent limits, which typically requires COD below 500 mg/L, ammonia nitrogen below 45 mg/L, phenol below 0.5 mg/L and cyanide below 0.5 mg/L. In practice, most coking plants treat to full direct discharge standards on site because municipal treatment works cannot reliably polish the residual thiocyanate and complex aromatics. Where discharge standards are very stringent, the plant may need to consider evaporation and crystallisation to eliminate the liquid discharge entirely.