Coking Wastewater by the Numbers: What 3,800 mg/L COD Does to Process Choice

Start with the load, because the load is what rules out most process options. Coking wastewater carries COD as high as 3,000-3,800 mg/L, a phenol content of 600-900 mg/L and ammonia nitrogen of about 300 mg/L. It also contains cyanide, oils and a long list of other pollutants. Set against a national machine-coke output of 70 million tons, that adds up to roughly 45,000 tons of COD and 3,500 tons of ammonia nitrogen discharged every year if nothing is done about it.

As the Emission Standard of Pollutants for the Coking Chemical Industry has tightened, coking enterprises have found themselves under mounting pressure. Conventional processes can no longer meet the limits, and coking wastewater treatment has become a recognised hard nut to crack.

Four reasons the stream resists treatment

Concentrations are severe. The organic load is hundreds to a thousand times that of ordinary domestic sewage. At that level, treatment difficulty does not rise linearly — it rises geometrically.

Composition is complex. Phenols, ammonia nitrogen and cyanide sit alongside large amounts of emulsified oil. Phenolic compounds inhibit microorganisms strongly, which is precisely what limits biological treatment.

Emulsification is heavy. During coking production, oils mix thoroughly with surfactants to form a highly emulsified oil-water dispersion with extremely fine droplets, usually below 10 microns. Conventional oil-intercepting equipment struggles to separate that.

Cost is high. Conventional processes need large doses of flocculants, demulsifiers and pH adjusters. Those chemicals drive operating cost up and generate large volumes of sludge, which brings its own disposal problem and secondary pollution risk.

Where the conventional train breaks down

The historical combination was physicochemical plus biological: adjust pH and dose flocculant to strip some pollutants, then let microorganisms degrade the rest. Five weaknesses show up in practice — continuous chemical dosing keeps operating costs high; the chemical stage produces large volumes of heavy-metal-bearing sludge that is hard to dispose of; the biological system collapses easily when water quality swings; reagent residues and sludge can create secondary pollution; and as limits tighten, the whole arrangement is increasingly stretched to meet them.

A purely physical alternative

SINOKLE Technology Co., Ltd. approaches coking wastewater from the opposite direction: rather than dosing more chemicals, remove the oil physically and let the load drop before anything biological is attempted.

The first core unit is CDFU, the cyclonic dissolved air flotation unit, which combines cyclonic separation with dissolved air flotation to generate ultrafine bubbles of 5-30 microns that capture emulsified oil droplets. Compared with conventional flotation equipment, the bubbles are smaller and the specific surface area larger, which is what drives oil removal efficiency up.

The second is KHC, the coalescing oil separator. It exploits the coalescing effect of special materials so that tiny oil droplets dispersed in water gradually merge into large beads that separate more easily downstream. This is where highly emulsified coking acid water is dealt with most effectively.

Advantages of leaving chemistry out

• No chemical reagents. Secondary pollution is avoided at source rather than treated afterwards.
• High waste-oil recovery. Purely physical separation achieves a waste-oil recovery rate above 99%, turning a disposal cost into a product.
• Low operating cost. With no continuous chemical purchase, the long-term cost base is structurally lower.
• Enclosed operation. Waste gas leakage is prevented, which improves the working environment as well as compliance.
• Strong adaptability. The process is insensitive to water quality fluctuation and runs stably without constant tuning.

What to establish before choosing a process

Three measurements decide the design more than any others: COD and phenol concentration to establish the organic load, the emulsified droplet size distribution to judge whether conventional separation has any chance, and the acidity of the stream, since coking acid water behaves differently from a neutral effluent of the same concentration.

Where droplets sit below 10 microns, equipment selected on oil concentration alone will disappoint. A settling trial is the practical test: if a sample does not separate visibly over a reasonable standing period, the process has to supply energy and surface contact rather than rely on gravity. That single observation usually settles whether a purely physical route is viable or whether something else is required.

Recovered oil value belongs in the calculation from the start. It changes the payback period materially, and it argues for separation methods that leave the oil in usable condition rather than merely removing it from the water.

A Henan coking acid water project

A chemical enterprise in Henan produces coking wastewater that is both highly emulsified and highly acidic, and conventional processes had delivered unsatisfactory results for a long time. SINOKLE applied the purely physical process for advanced treatment. Treated effluent is clear and transparent, all indicators improved markedly, and the plant achieved genuine cleaner production. The project solved an environmental problem that had persisted for years and generated additional revenue through recovered waste oil.

For an industry facing stricter limits on a stream that resists every conventional tool, efficient treatment without chemical reagents is not a marginal improvement. It changes what compliance costs.