Coking Effluent and the 0.3 Biodegradability Wall: Picking Between Biofilm A/O, AOP and Zero Discharge

The stream nobody wants in the equalisation tank

Coking plants generate wastewater in three places at once: surplus ammonia liquor from coal carbonisation, gas purification wash water, and refining effluent. Add domestic sewage and you have a stream in which roughly 500 individual compounds have been identified, among them volatile phenols, polycyclic aromatic hydrocarbons and heterocycles carrying nitrogen, oxygen or sulfur. It is toxic, it is refractory, and it arrives in a volume that no operator can dilute away.

Numbers that decide the flowsheet

Published characterisations vary widely, which is itself a warning. One survey puts COD at 1,200–1,300 mg/L with ammonia nitrogen at 200–700 mg/L. Another reports COD of 2,900–4,100 mg/L, ammonia nitrogen of 100–400 mg/L and an average phenol concentration of about 483 mg/L. The BOD5/COD ratio averages about 0.3, which is poor biodegradability. Nitrogen is in surplus rather than in deficit, so the usual nutrient-balancing logic inverts: the plant has to strip nitrogen, not feed it.

Why plain activated sludge was not enough

Conventional activated sludge showed low efficiency and weak shock resistance on this stream. The improved two-stage AO arrangement, usually written A1–A2–O, paired with a biofilm composite addressed both problems by adding carriers, raising the biomass inventory and damping the shock-load response.

In a typical A1–A2–O biofilm train, wastewater first enters anaerobic acidification, where phenol, xylenol and heterocycles such as quinoline, isoquinoline, indole and pyridine are transformed or removed, lifting biodegradability above that of the raw water and supplying carbon for denitrification downstream. The anoxic stage then handles denitrification; because the indigenous carbon source is insufficient, methanol is dosed as supplement, and a portion of aerobic effluent is recycled back to supply nitrate nitrogen. The aerobic stage sees high ammonia nitrogen and low COD, so nitrification dominates and soda ash solution is added for alkalinity. Coagulation-sedimentation follows for sludge-water separation, with ferric polymer improving settleability and trimming residual COD, and PAM is dosed before dewatering to flocculate the waste sludge.

Pretreatment still governs whether any of that works. Gas condensate and turbid water from clear-turbid separation points pass through gravity oil separation and air flotation, cutting oil to below 30 mg/L so that microbial growth is not poisoned before the equalisation tank. Phosphorus nutrient salts go into that tank too, since the raw stream is phosphorus-poor.

Advanced treatment: three families, different jobs

Post-biochemical organic matter is refractory and low in biodegradability, so stable compliance or reuse needs a polishing step. Advanced oxidation covers Fenton, ozone oxidation and electrochemical routes. Fenton deals effectively with refractory organics; ozone catalytic oxidation achieves 64%–74% COD removal; an electrochemical process on biochemical effluent produces COD of about 80–100 mg/L, ammonia nitrogen less than 3 mg/L, SS less than 4 mg/L and colour below 8 times.

Membrane options run from ultrafiltration through nanofiltration to reverse osmosis, most commonly paired as the ultrafiltration + reverse osmosis dual-membrane scheme. An ultrafiltration-resin adsorption-reverse osmosis combination can meet reuse water standards, while Fenton oxidation + electrodialysis + ultrafiltration + reverse osmosis pushes recovery past 75% with water better than reclaimed-water requirements.

Zero discharge is where the cost sits. The bottleneck is concentrated brine, and the technologies are membrane concentration, falling-film evaporation, multi-effect evaporation, mechanical vapor recompression (MVR) and freezing salt separation. An MVR + multi-effect evaporation combination yields very high purity industrial sodium chloride and sodium sulfate.

Monitoring, and why the standard keeps moving

Routine indicators are COD, ammonia nitrogen (NH3-N), phenolic compounds, cyanide and thiocyanate; limits are set in the Emission Standard of Pollutants for Coking Chemical Industry (GB 16171—2012), where the direct discharge limit for COD at new enterprises is 80 mg/L and close to 100 mg/L for existing ones. In one enforcement case the COD was reduced to 108 mg/L, still above the new-enterprise limit.

Thiocyanate monitoring now has its own method. The industry standard Determination of Thiocyanate Content in Coking Wastewater (HG/T 6072-2022), effective 1 April 2023, specifies ammonium ferric sulfate spectrophotometry, in which thiocyanate reacts with ferric ions under strongly acidic conditions to form a red complex read at 456 nm, and ion chromatography with conductivity detection. Getting thiocyanate right matters because it drives COD contribution, total nitrogen control and biochemical stability.

Compliance verification mixes online instruments at the treatment station outlet with manual sampling and third-party laboratories holding CMA/CNAS qualifications. Online monitoring is also the main instrument of environmental supervision, and it is what makes data falsification and illegal discharge hard to hide.

Policy and the economics of not discharging

In May 2024 the Department of Ecology and Environment of Shanxi Province issued Jin Huan Fa [2024] No. 8, requiring zero discharge of coking wastewater through pretreatment + membrane concentration + evaporation crystallization salt separation, plus full collection and treatment of initial rainwater. Shanxi's Qingxu coking cluster responded with a jointly built central plant, Qingxu Hongbo Wastewater Treatment Co., Ltd., running biochemical treatment + reclaimed water reuse + evaporation crystallization at about 1.7×10⁴ m³/d with a reuse rate as high as 92%. Reclaimed water returns to circulating cooling; crystalline salt sells as a by-product, averaging 99.35% purity for sodium sulfate and 98.62% for sodium chloride.

Enforcement is not theoretical. Inspections at Ruzhou Tianrui Coking in Henan found no online monitoring at the station outlet and effluent COD, ammonia nitrogen and cyanide exceeding the Table 2 direct discharge limits of GB 16171—2012 by 0.5, 3.5 and 1.4 times respectively; quenching water exceeded indirect limits by 2.0, 14.2, 22.7 and 13.2 times for COD, ammonia nitrogen, volatile phenol and cyanide, and benzo[a]pyrene in the drainage ditch sludge exceeded soil environmental risk control standards. Elsewhere the Fenton reaction stage was run without reagent dosing and the ultrafiltration-reverse osmosis equipment sat idle for long periods, which reduced the control facilities to decoration.

On the technology side, BDD electrode electrolytic oxidation and the AOP endogenous carbon cracking route both target the refractory fraction, the latter breaking long-chain or polycyclic organics into small molecules, while the Fenton fluidized bed package cuts reagent cost and sludge output and removes cyanide at the same time. The O/H/O process developed by Professor Chao-Hai Wei's team in 2023, built on a mixing-reaction-separation microbial fluidized bed, has shown stable COD removal and nitrogen removal in real projects.

Where reuse actually goes

Polished effluent at industrial circulating cooling water quality can feed cooling circuits, boiler soft water make-up and plant greening, and can partially replace fresh water. Independent coking enterprises can send treated water to wet coke quenching, coal yard dust suppression, dust removal and humidification water, and circulating water make-up; steel-enterprise coking streams can be reused for batching sprinkling, slag flushing, slag quenching and circulating water make-up.

The resource story extends to solids. Dry sludge from retrofit projects has been turned into coke or catalyst carriers, worth roughly 6.57 million yuan a year in one case. The direction of travel is set: the waste stream is being converted into a product stream, and the plants that deal with the brine properly are the ones that get there.