Multiple industries nationwide (independent coking plants / coking divisions of 
Pilot to full-scale engineering (tens to thousands of m³/d; typically 1200–5000 Coking Wastewater: Phenol and Ammonia Undermine Each Other in the Same Tank, and Meeting COD Limits Does Not Mean It Can Be Discharged
The "difficulty" of coking wastewater is conventionally attributed entirely to phenol—it accounts for over ninety percent of the total organic matter and is both toxic and foul-smelling. But what truly cripples a project is not removing phenol alone, nor stripping ammonia alone, but the mutual sabotage that occurs once these two substances are forced into the same biological system: phenol is poison to nitrifying bacteria, while ammonia nitrification consumes alkalinity and releases acidity, and the two also compete for dissolved oxygen in the same aerobic tank. Another more insidious misreading is "COD compliance"—coking wastewater is governed by GB 16171—2012, with conventional indicators per Table 2 and special emission limits per Table 3, and each table is divided into two tiers: "direct discharge / indirect discharge"; the same COD of 150 mg/L just barely meets the standard under the indirect discharge criterion, yet exceeds it by nearly double under the direct discharge criterion.
I. First, clearly identify the three streams: their concentrations differ by an order of magnitude
"Coking wastewater" is not one kind of water, but a collective term for several streams with completely different properties. If they are not separated, all subsequent discussion about "how much phenol in the influent, how much ammonia" will lose focus. Tables 4-1 of the Ministry of Ecology and Environment's "Technical Specification for Coking Wastewater Treatment Engineering" provide the water quality and quantity of several typical discharge points; this is the most widely cited and most appropriate set of data to serve as the design starting point (Tables 4-1, Ministry of Ecology and Environment):
| Discharge Point | CODCr mg/L | Volatile Phenols mg/L | Total Cyanide mg/L | Ammonia Nitrogen mg/L | Water Volume m³/(t coke) |
|---|---|---|---|---|---|
| Residual Ammonia Liquor | 4000~9000 | 400~2600 | 30~60 | 2500~6000 | 0.10~0.23 |
| Crude Benzol Separation Water | 7000~12000 | 350~650 | 20~50 | 120~500 | 0.025~0.028 |
| Gas Final Cooling Blowdown Water | 400~700 | 100~500 | 16~25 | — | 0.09~0.11 |
| Ammonia Stripping and Recovery Process Drainage | 3000~4000 | 90~200 | 6~10 | 850~1000 | 0.25~0.34 |
| Desulfurization Waste Liquor (HPF/PDS) | 180000~420000 | 3500~5500 | — | — | — |
Two key points stand out when reading this table. First, residual ammonia liquor is the absolute main contributor, accounting for more than half of the total coking wastewater volume ("Operation and Management of the A-A/O Process in Coking Wastewater Treatment"), with ammonia nitrogen as high as 2500~6000 mg/L and volatile phenols at 400~2600 mg/L—these two figures determine the load on all subsequent process stages. Second, desulfurization waste liquor exists on an entirely different order of magnitude: COD 18 万~42 万 mg/L and phenols 3500~5500 mg/L. It is not so much "wastewater" as a diluted chemical solution, and is typically treated separately—it must never be directly mixed into the biological system.
After the residual ammonia liquor undergoes phenol removal and ammonia stripping, to what level does the water quality drop? Multiple engineering references give a consistent order of magnitude: after ammonia stripping, COD is about 3000~3800 mg/L, BOD5 600~1000 mg/L, volatile phenols 600~900 mg/L, cyanide about 10 mg/L, oil 50~70 mg/L, ammonia nitrogen about 300 mg/L or so ("Exploration of Biochemical Treatment Process and Design Key Points for Coking Wastewater"; Shanxi Fulong Coal Chemical upgrading and renovation case). This "post-ammonia-stripping" water is the true influent of the biological system. The concentration before it is tens of thousands, here it drops to several thousand; the target afterward is COD 80 or below—so naturally, all the difficulty of coking wastewater treatment is concentrated in the segment "from several thousand down to less than one hundred," and this segment happens to be exactly the one where phenols and ammonia coexist.
II. Ammonia Stripping: It Must Be Placed First, But It Should Not Be Asked to Do Everything
The purpose of ammonia stripping is to convert ammonium nitrogen into free ammonia, use steam to drag it from the liquid phase to the gas phase, and then recover it as ammonia liquor through condensation.The reason it is placed before all other processes is not that "ammonia nitrogen exceeds the standard and must be treated," but that "ammonia nitrogen is too high, and the biological system simply cannot withstand it."The biochemical influent control concentrations given in "Exploration of Biochemical Treatment Process and Design Key Points for Coking Wastewater" are very practical empirical lines: volatile phenols no higher than 300 mg/L, cyanide no higher than 40 mg/L, sulfide no higher than 30 mg/L, volatile ammonia no higher than 30 mg/L, benzene no higher than 50 mg/L; exceeding this range will significantly inhibit microbial activity. In the residual ammonia liquor stream, ammonia nitrogen is often several thousand milligrams per liter, and sending it directly to biochemical treatment is a sustained toxic shock.
The process conditions for ammonia stripping are highly convergent in publicly available materials and can serve as a window for design verification:adding alkali to raise pH above 10 (in engineering practice, 11~12 is often used, at which the proportion of free ammonia exceeds 90%), column temperature 95~105 ℃, steam consumption commonly 200~300 kg/m³ wastewater, single-stage ammonia nitrogen removal rate can reach above 95%, two-stage series connection can reach 99%, and ammonia nitrogen in the column bottom effluent commonly drops to 100~150 mg/L or even lower (ammonia stripping equipment materials for ammonia-containing wastewater; key points for coking wastewater treatment scheme design). The recovered ammonia liquor concentration is usually 10%~20%, and it can be reused for desulfurization and denitrification or used as a by-product.
There are also two things ammonia stripping does conveniently but that are often overlooked: first, it lowers the water temperature (coking wastewater temperature is high year-round and needs to be cooled to below 38 ℃ before entering biochemical treatment, otherwise the microbial community structure will become imbalanced; this item is mentioned in both the "Coking Wastewater" entry and operational literature from China Water Network); second, it removes part of the volatile phenols and cyanide. But itcannot remove fixed phenols, cannot remove thiocyanate, and cannot remove polycyclic aromatic hydrocarbons, and these three are precisely the sources of trouble for the subsequent biochemical and advanced treatment stages.
III. Biological System: Phenols Are Both a Carbon Source and a Toxicant, and Nitrifying Bacteria Are the Most Fragile Link in This System
Now to the main topic. The contradiction in biological treatment of coking wastewater can be summarized in one sentence:phenolic substances are both the highest-quality carbon source for this system and the most direct toxicant to nitrifying bacteria; and the nitrification process of ammonia nitrogen both consumes oxygen and consumes alkalinity, and also drags pH downward.These two things occurring in the same tank means they undermine each other.
3.1 Two Sets of Criteria for Phenols: Biodegradability 0.3, and Inhibition Begins at 20 mg/L
First, the side that "phenols are a carbon source." Phenolic substances in coking wastewater account for more than 90% of total organic matter (Ren Yuan et al., "GC/MS Analysis of Organic Components During the Treatment of Coking Wastewater by Biological Fluidized Bed A/O₂ Process," Environmental Science). Phenols are readily degradable organic matter; under aerobic conditions and the action of hydroxylases, the benzene ring is opened and gradually mineralized. Therefore, although the BOD5/COD of coking wastewater is only about 0.3 (Baidu Baike "Coking Wastewater" entry; some literature also gives 0.2~0.3), appearing to have poor biodegradability, the phenols and benzenes within it are actually the "easy-to-digest" part—what is truly difficult are nitrogen-containing heterocycles and polycyclic aromatic hydrocarbons such as quinoline, indole, carbazole, and pyridine.
Now let's discuss the other side of "phenol is toxic" — this is the side that is truly fatal in engineering terms. Multiple studies have given inhibition thresholds far lower than the "biodegradable" intuition: some studies point out that phenol at a low concentration of 30 mg/L can already inhibit the nitrification process ("Analysis of the Realization and Influencing Factors of the Nitrification Process in the Treatment of Coking Wastewater by the A/O₁/H/O₂ Process"); another review cites data showing that phenol at 20~21 mg/L achieves a biological nitrification inhibition rate of 50%, and at 34~40 mg/L reaches 75% ("Review of Research on the Treatment of Coal Chemical Coking Wastewater by the Anaerobic Ammonium Oxidation Process", Clean Coal Technology, 2025); research on a full-scale pre-denitrification process gives that phenol above 200 mg/L and p-cresol above 100 mg/L significantly inhibit nitrification ("Inhibitory Effect of Toxic Compounds on the Nitrification Process in Coking Wastewater Treatment"). These values span a considerable range, but the direction is completely consistent: the inhibition threshold is at the "tens" level, while the phenol in the influent after ammonia stripping is still at the "hundreds" level. This is why one cannot expect a single-stage aerobic tank to simultaneously eliminate both phenol and ammonia — the phenol concentration in the tank remains above the inhibition threshold for a long time, and the nitrifying bacteria are always in a suppressed state.
3.2 Why nitrifiers are the most fragile: autotrophic, long generation time, and subject to triple inhibition
Nitrifying bacteria are obligate chemolithoautotrophs with slow growth and long generation times, so a margin of 30~60 d must be allowed in the sludge retention time (θc) (the "Biochemical Treatment Process and Design Key Points for Coking Wastewater" specifies θc=30~60 d, COD load 0.8~1.0 kg/(kgVSS·d), NH₃-N load 0.15~0.2 kg/(kgVSS·d)). They simultaneously face threefold inhibition:
| Inhibitor | Inhibition Threshold / Effect | Source |
|---|---|---|
| Phenol | 30 mg/L causes inhibition; 20~21 mg/L inhibition rate approximately 50%; 34~40 mg/L approximately 75%; >200 mg/L significant inhibition | A/O₁/H/O₂ process study; Clean Coal Technology 2025 review; PubMed 17804160 |
| p-Cresol | Above 100 mg/L significantly inhibits nitrification | PubMed 17804160 (full-scale pre-denitrification process) |
| Free Cyanide | Above 0.2 mg/L severely inhibits nitrification; ferricyanide <100 mg/L causes no inhibition | PubMed 17804160 |
| Thiocyanate SCN⁻ | >200 mg/L inhibits; at 380 mg/L ammonia oxidation inhibition rate reaches 78.1%, with nitrite accumulation occurring | PubMed 17804160; A/O₁/H/O₂ process study |
| Nitrogen-Containing Heterocycles | Half-inhibitory concentration EC₅₀: indole 9.89, pyrrole 13.46, pyridine 59.51, quinoline 112.46 mg/L | A/O₁/H/O₂ process study |
| Ammonia Nitrogen Itself | Some literature reports >150 mg/L shows slight substrate inhibition; other literature considers <350 mg/L does not constitute substrate inhibition | The two literature sources differ in their criteria; see Section 8 Fact Check |
The most easily overlooked item in this table is thiocyanate (SCN⁻). It has two characteristics that combine to form a "chain inhibition" unique to coking wastewater: first, it itself inhibits nitrification; second, its biodegradation products are mainly ammonia nitrogen—that is, SCN⁻ first "produces" a portion of ammonia nitrogen, then lets that ammonia nitrogen shock the nitrifying bacteria, while its own degradation also requires a sufficiently long hydraulic retention time ("Optimization of Cyanide Removal Process for Coking Wastewater," Wastewater Treatment Engineering Network). After upgrading, some projects found certain indicators persistently failing to meet standards; upon investigation, SCN⁻ was confirmed as the key factor, ultimately necessitating the addition of an aeration decyanation (pre-decyanation) system at the front end of the biological treatment, along with a decyanation sedimentation tank and sludge return.
3.3 Staging is the only solution repeatedly validated by engineering practice
Since phenol and ammonia cannot coexist peacefully in the same tank, the engineering solution comes down to one word: separate. Split "carbon removal and detoxification" and "nitrification" into different reactors with different retention times. The GC/MS study by Ren Yuan et al. provides a very convincing staged account: the Anaerobic stage achieves only 10%~15% COD removal, but its real contribution is decomposing large-molecule complex organics into organic acids and organic alcohols, raising the wastewater BOD/COD from 0.30 to 0.45; the first-stage Aerobic stage removes most phenol, amines, quinoline, pyridine, furan, indole, naphthalene and other components, with a COD removal rate of 65%~70%; the second-stage Aerobic stage contributes a further 40%~50% COD removal rate. Another A/O₁/H/O₂ study gives more intuitive figures: the first-stage Aerobic fluidized bed achieves removal rates of 76.8%, 99.7%, 96.9%, and 79.6% for COD, volatile phenol, SCN⁻, and CN⁻ respectively. It is precisely after this stage removes the toxicants and COD that nitrification in the second-stage Aerobic stage becomes viable, and the ammonia nitrogen in the biological system effluent can be stably reduced to below 3 mg/L.
This is the empirical evidence for "staged removal": it is not that ammonia is harder to treat and therefore placed later, but rather that the inhibitors must be removed first for nitrification to have any chance of survival. Based on this logic, the team led by Wei Chaohai at South China University of Technology developed the O/H/O (Aerobic/Hydrolysis/Aerobic) process, with the engineering conclusion that: the O1 reactor mainly performs simultaneous carbon removal and ammonification, nitrification mainly occurs in the O2 reactor, and Denitrification mainly occurs in the H reactor; under the combined bypass and recirculation operating mode, the Total Nitrogen removal efficiency reaches 88.7%.
IV. Parameter Windows: Real Values of HRT, Reflux Ratio, DO, Alkalinity, and Carbon Source
Translating the above mechanisms into design figures, the windows given by public literature and engineering data are roughly as follows. Note that the source is indicated after each column; it is normal for the same parameter to vary greatly across different projects, because the influent phenol and ammonia loads and whether dilution is applied vary greatly.
| Parameter | Value Window | Source |
|---|---|---|
| Inhibitor control in biochemical influent | Volatile phenols ≤300, cyanide ≤40, sulfide ≤30, volatile ammonia ≤30, benzene ≤50 mg/L | Biochemical Treatment Process and Design Points for Coking Wastewater |
| Anoxic zone HRT | 10~15 h (A/O² feasible technology); engineering cases have adopted 16~20 h, 23 h | Best Available Techniques for Water Pollution Control in Coking Processes; A-A/O process operation literature |
| Aerobic zone HRT | First stage 18~30 h, second stage 15~25 h; engineering cases: aerobic tank has adopted 46 h, anaerobic 8~12 h | Best Available Techniques; A/O contact oxidation engineering |
| Anoxic zone DO / Aerobic zone DO | Anoxic <0.5 mg/L (typically controlled at 0.2~0.5); aerobic 2~4 mg/L (some conditions 3~5) | Best Available Techniques; coking wastewater treatment process data |
| Temperature / pH | Aerobic 20~30 ℃, pH 6.5~8.5; anoxic 15~35 ℃, pH 7.5~8.2; optimal pH for nitrification 7.5~8.4 | Best Available Techniques; A-A/O process operation literature |
| Sludge return ratio / Mixed liquor return ratio | Sludge return 50%~200% (commonly 2~3 times); mixed liquor return 0.4~1 times, engineering cases have adopted 200%~250%, 500%~1000% | Best Available Techniques; A/O contact oxidation engineering; two-stage A/O patent examples |
| Carbon-to-nitrogen ratio | Theoretical BOD/TN > 2.86; coking A/O engineering experience requires C/N greater than 6~7 for satisfactory denitrification | A-A/O process operation literature; Biochemical Treatment Process and Design Points for Coking Wastewater |
| Alkalinity consumption and production | Nitrification: 1 g NH₄⁺-N→NO₃⁻-N consumes 7.14 g alkalinity (as CaCO₃), consumes 4.57 g oxygen; denitrification: each reduction of 1 g NO₃⁻-N provides 3.74 g alkalinity | A-A/O Process for Coking Wastewater Treatment (China Water Network / Environmental Ecology Network) |
| Sludge age and loading | θc=30~60 d; COD loading 0.8~1.0, NH₃-N loading 0.15~0.2 kg/(kgVSS·d) | Biochemical Treatment Process and Design Points for Coking Wastewater |
The item in this table most worth calculating separately is alkalinity. The nitrification reaction requires 4.57 g oxygen and consumes 7.14 g alkalinity (as CaCO₃) for every 1 g of ammonia nitrogen oxidized, while simultaneously producing acidity. Assuming the amount of ammonia nitrogen to be nitrified in the biochemical stage per day is estimated from the influent volume and concentration, if the raw water alkalinity is insufficient, alkali must be added externally (sodium carbonate or liquid caustic soda are commonly used in engineering) to maintain pH within the suitable range for nitrification. The real meaning of this calculation is: for every slight increase in ammonia nitrogen concentration, the nitrification and alkalinity costs scale up linearly—which brings us back to what was said in Section 2: every hundred milligrams of ammonia nitrogen removed by additional ammonia stripping saves oxygen and alkali for the biochemical stage.
The carbon source issue also deserves clarification. The C/N of coking wastewater often appears "sufficient" numerically, because phenol itself is an organic substance. But the problem lies in the spatiotemporal mismatch of carbon: phenol is largely consumed by heterotrophic bacteria in the aerobic zone (they are much more competitive for oxygen than nitrifying bacteria), and by the time the mixed liquor returns to the anoxic zone, the readily degradable carbon available for denitrification is already limited. Therefore, in coking A/O projects, "adding external carbon sources such as methanol/sodium acetate to the anoxic tank" is the norm rather than the exception. China Water Network's "Operation and Management of the A-A/O Process in Coking Wastewater Treatment" explicitly states that "due to the insufficient denitrification carbon source contained in coking wastewater, methanol needs to be added to the anoxic tank as a supplementary carbon source." This also explains a seemingly contradictory phenomenon: the influent organic matter concentration is very high, yet external carbon source still needs to be added—because the portion that can be utilized by denitrifying bacteria is spatially retained in the aerobic zone.
V. Advanced Treatment: What to Control and What Not to Control
The effluent from biological treatment is often not clean enough: COD is usually 150~300 mg/L, color is intense, and refractory heterocyclic and polycyclic aromatic hydrocarbons remain. The combination of the advanced stage is basically fixed as "coagulation sedimentation → advanced oxidation → adsorption/biological aerated filter polishing," with membrane added if necessary.
The gates guarded by the three advanced units can be clearly distinguished:
| Control Indicator | Coagulation Sedimentation | Fenton-type Oxidation | Catalytic Ozonation | BAF / Activated Carbon | Membrane (UF/RO) |
|---|---|---|---|---|---|
| Suspended Solids, Colloids | Strong | Weak | Weak | Medium | Strong |
| Residual Phenol, Color | Medium | Strong | Strong | Medium | Medium |
| Polycyclic Aromatic Hydrocarbons / Benzo(a)pyrene | Weak | Medium~Strong | Medium~Strong | Medium | Medium |
| COD (refractory fraction) | Weak~Medium | Medium~Strong | Medium~Strong | Medium | Strong |
| Total Nitrogen / Ammonia Nitrogen | None | None | None | Weak (only trace amounts in the filter) | Medium (retention rather than removal, returned with the concentrate) |
| Dissolved Salts | None | None | None | None | Strong (RO stage) |
| Main Cost | Sludge volume | Chemicals + iron sludge + pH adjustment | Power consumption + catalyst life | Carbon regeneration/replacement | Investment + concentrate disposal |
The most critical row in this table is "Total Nitrogen / Ammonia Nitrogen." None of the advanced oxidation methods removes nitrogen—catalytic ozonation and Fenton only mineralize or break the chains of organic matter; they do not convert ammonia nitrogen into nitrogen gas. This means that if nitrification in the biological stage has not been established, it is impossible to expect the advanced stage to remedy total nitrogen; the only things the advanced stage can remedy are COD, color, phenol, and PAHs.
The cost basis for the advanced treatment stage is also worth laying out. For heterogeneous catalytic ozonation treating coking secondary biochemical effluent, the literature gives a COD removal rate of 64%~74%, with an operating cost (power consumption) of 1~3 元 per ton of water; at a certain plant, the biochemical effluent COD is 150~220 mg/L, and after heterogeneous catalytic ozonation it is stably below 80 mg/L, with a removal rate of 62%~72% ("Research on New Advanced Treatment Technologies for Coking Wastewater and Their Respective Coupling Characteristics," Coal Science and Technology, 2021). In comparison with Fenton, a pilot test for advanced treatment of coking wastewater gives: under Fenton conditions of retention time 2 h, pH=3, H₂O₂ 40 mmol/L, Fe²⁺ 8 mmol/L, the influent COD is 280~309 mg/L, the removal rate is 72.1%~77.4%, the effluent is 68.2~83.6 mg/L, and the chemical cost is about 6.45 元/ton (excluding labor and sludge disposal); for the same water sample using catalytic ozonation, with contact time 2.5 h and ozone concentration 100 mg/L, the removal rate is 74.1%~79.2%, the effluent is 61~74 mg/L, and the electricity cost is about 2.45 元/ton. The study further provides an engineering budget estimate for a 1500 t/d scale: total investment of 2002.31 万 yuan, and a treatment cost of 4.74 元 per ton of water.
VI. Real Ledger: Influent/Effluent and Process Chains of 11 Public Engineering Cases
The table below is entirely from public engineering cases and literature reports. Wherever the original text gives only the "applicable standard" without "measured effluent," this table marks it as "applicable/design" according to the original wording; please do not confuse the two.
| Project / Scale | Influent Phenol-Cyanide-Ammonia Nitrogen | Process Chain | Effluent and Compliance Status | Source |
|---|---|---|---|---|
| Hebei Yuzhou Coal Chemical, 60 万 t coke/year, 1440 m³/d | Specific values not given in the original text (only states "high ammonia nitrogen concentration") [To be supplemented] | Oil separation + Micro-electrolysis + Air flotation + A²O² + Catalytic Ozonation + Biological fluidized bed + Coagulation sedimentation + Filtration | Design/compliance with GB 16171 Table 3 special emission limits: COD<80, ammonia nitrogen<10, volatile phenol<0.1, cyanide<0.2 mg/L | Zhongke Langbo Environmental Engineering Case |
| Hebei Lima Gas, 80 万 t, 1200 m³/d | Specific values not given in the original text [To be supplemented] | Oil separation + Air flotation + A²O + Advanced oxidation + Coagulation sedimentation | Design/compliance with Table 2 indirect discharge: COD<100, ammonia nitrogen<15 mg/L (another source records COD≤150, ammonia nitrogen≤25) | Zhongke Langbo Environmental Engineering Case |
| Handan Cixian Xinsheng Coal Chemical, 108 万 t, 1680 m³/d | Specific values not given in the original text [To be supplemented] | O/A/O + In-situ adsorption + Targeted oxidation + Coagulation sedimentation | Design/compliance with Table 2 indirect discharge and owner requirements: COD≤100, BOD≤20, ammonia nitrogen≤10 mg/L | Zhongke Langbo Environmental Engineering Case |
| Shanxi Fulong Coal Chemical Upgrading and Retrofitting, 1400 t/d (2017 年) | COD 3000~3800, phenol 600~900, cyanide 10, oil 50~70, ammonia nitrogen approx. 300, TDS 3000~6000 mg/L | Oil removal + Anaerobic-Anoxic-Aerobic + Coagulation sedimentation + Photocatalytic oxidation (pH≈3, hydrogen peroxide + ferrous sulfate) | Effluent COD 80, ammonia nitrogen 10 mg/L, stably meeting GB 16171 special emission limits | Wastewater Treatment Engineering Network Engineering Case (2022) |
| Shougang Shuicheng Iron and Steel Coking, 80 m³/h | Influent values not given item by item in the original text [To be supplemented] | O/AO/AO + Electromagnetic strong oxidation advanced treatment (in-situ retrofit, footprint 362 m²) | Biochemical coagulation sedimentation effluent COD stably <150; after advanced treatment COD average 22.7, Total Nitrogen average 7.4, ammonia nitrogen <2 mg/L, meeting Table 3 direct discharge (COD≤40, Total Nitrogen≤10, ammonia nitrogen≤5) | Shougang Shuicheng Iron and Steel (Group), Wastewater Treatment Engineering Network (2023) |
| A Certain Large Coking Enterprise Advanced Treatment and Reuse, 1200 m³/d | COD approx. 3500, ammonia nitrogen approx. 300, volatile phenol approx. 800 mg/L | Pretreatment + A/O/O three-stage biochemical (total HRT>120 h) + Coagulation sedimentation + Ozonation + Biological Activated Carbon + UF + RO | Effluent COD stably <50, ammonia nitrogen <5, volatile phenol <0.5 mg/L; RO water recovery approx. 65%, product water conductivity <100 μS/cm; concentrate used for coke quenching | Qianshui Environment Project Report |
| A Certain Coking Plant Retrofitting in North China, 200 万 t coke/year, 5000 m³/d, investment 4800 万 yuan | Volatile phenol 1200~1800, fixed phenol 300~500, cyanide 80~150, ammonia nitrogen 2000~3000 mg/L | Ammonia stripping and phenol removal + Diisopropyl ether three-stage countercurrent extraction (total phenol <50) + Short-cut Nitrification SBR (35 ℃) + Anammox (TN loading 1.2 kgN/(m³·d)) + Catalytic Ozonation (TiO₂/Activated Carbon) | Effluent volatile phenol ≤0.1, total cyanide ≤0.2, COD ≤50, ammonia nitrogen ≤5 mg/L; per-ton water cost reduced by 28% compared to original process | Weilin Environmental Public Case |
| A Certain Large Coking Enterprise, 200 万 t coke/year, 1200 t/d, investment approx. 3800 万 yuan | Phenol approx. 1500, COD>5000, ammonia nitrogen 800 mg/L | Steam stripping for phenol removal (removal rate approx. 95%) + A²/O (retention 72 h) + Catalytic Ozonation + Activated Carbon | Effluent phenol 0.3, COD <50, ammonia nitrogen 5 mg/L, meeting Table 3 special emission limits | Weilin Environmental Public Case |
| A Certain Coking Plant in Inner Mongolia (Heterogeneous Catalytic Ozonation Advanced Treatment) | Influent is secondary biochemical effluent, COD 150~220 mg/L | Heterogeneous Catalytic Ozonation (biochemical effluent fed directly) | Effluent COD stably <80 mg/L, removal rate 62%~72%, per-ton water operating cost (electricity) 1~3 元 | Coal Science and Technology (2021) Review |
| Catalytic Ozonation Advanced Treatment Pilot / Design 1500 t/d | Influent COD 280~309 mg/L | Fenton (2 h, pH 3, H₂O₂ 40 mmol/L, Fe²⁺ 8 mmol/L) compared with Catalytic Ozonation (2.5 h, ozone 100 mg/L) | Fenton removal rate 72.1%~77.4%, effluent 68.2~83.6, chemical cost approx. 6.45 元/ton; Catalytic Ozonation removal rate 74.1%~79.2%, effluent 61~74, electricity cost approx. 2.45 元/ton; total project investment 2002.31 万 yuan, per-ton water 4.74 元 | "Study on Catalytic Ozonation Treatment of Reverse Osmosis Concentrate" (Thesis) |
| Coking Wastewater Zero Liquid Discharge Integrated Process (Wei Chaohai Team Engineering Case) | Influent values not given item by item in the original text [To be supplemented] | Dual physicochemical A/O/H/O/A + Ozonation + Softening and defluoridation + Membrane unit + Terminal treatment | Achieves zero pollutant discharge and over 98% water reuse, whole-process operating cost below 18 元/m³ | South China University of Technology Team Achievement Report (2023) |
Reading these ledgers horizontally, several patterns emerge. First, the cases that can stably meet Table 3 direct discharge limits (COD≤40, ammonia nitrogen≤5, Total Nitrogen≤10) are without exception combinations of "well-executed staged biochemical treatment + advanced oxidation as the final safeguard" — none rely on a single advanced treatment unit alone. Second, the cost magnitude of the advanced treatment stage is 1~7 元/ton (single oxidation unit), while the cost of the whole process (including membranes and Zero Liquid Discharge) can reach over 18 元/ton — these are not the same concept, and the basis must be clarified when quoting. Third, whenever the influent phenol rises to the 1200~1800 mg/L level (without ammonia stripping or with incomplete ammonia stripping), the process chain invariably includes a "phenol removal extraction" step, indicating that for high-phenol influent, "removing the phenol first" is more economical than "letting the biology consume the phenol."
VII. Process Comparison: A/O, A²/O, O/A/O, Enhanced A/O + Advanced Oxidation
| Process | Structural Key Points | Special Constraints in the Coking Context | Applicability Assessment |
|---|---|---|---|
| A/O (Anoxic/Aerobic) | Anoxic Denitrification + Aerobic Nitrification, mixed liquor recirculation | Phenol is preferentially degraded in the aerobic zone; the anoxic zone lacks carbon and requires external carbon source; phenol concentration in a single-stage aerobic tank often exceeds the nitrification inhibition threshold | Applicable when influent phenol and ammonia loading is moderate and effluent requirements are relatively lenient (indirect discharge); inhibitors in the biological influent must be controlled |
| A²/O (Anaerobic/Anoxic/Aerobic) | An anaerobic (Hydrolysis Acidification) stage is added before A/O | The phosphorus removal function has limited significance for coking wastewater (coking wastewater is phosphorus-deficient and actually requires phosphorus nutrient supplementation); its value lies mainly in Hydrolysis Acidification improving B/C (0.30→0.45 order of magnitude) and providing effective carbon source for Denitrification | Mainstream in China for over twenty years, with more than 800 engineering cases (Wei et al., 2020); still the preferred solution for low-load groups |
| O/A/O (Aerobic/Anoxic/Aerobic) | Front-end Aerobic first removes carbon and detoxifies, then Anoxic Denitrification, then post Aerobic Nitrification | Front-end Aerobic converts organic nitrogen to ammonia and removes toxins such as phenol first, creating conditions for subsequent Nitrification; however, front-end Aerobic consumes carbon source | Stronger adaptability to high C/N and highly toxic influent; 1200 m³/d engineering cases already exist |
| O/H/O (Aerobic/Hydrolysis/Aerobic) | Dual Aerobic combined with Hydrolysis for Denitrification, AMPhIX fluidized bed structure | O1 mainly removes carbon and performs ammonification, O2 mainly performs Nitrification, H mainly performs Denitrification; Total Nitrogen removal rate of 88.7% with combined bypass and recirculation mode | Ranked optimal in the high-load group in AHP evaluation (Wei et al., 2020); can be extended to pathways such as partial nitritation-Anammox |
| Enhanced A/O + Advanced Oxidation (with MBBR/fluidized bed media) | Media added to the biological stage to increase biomass; advanced stage uses Catalytic Ozonation/Fenton + BAF/Activated Carbon as polishing | Media improves shock resistance and is the most direct engineering measure for coping with phenol and ammonia fluctuations; the advanced stage only polishes COD/color/PAHs/phenol, without Denitrification | Currently the most widespread compliance upgrade route; whether to add membranes depends on reuse requirements |
Here it is necessary to clarify the question of "whether A²/O is optimal." A²/O has dominated China's coking wastewater sector for over twenty years, with cumulative engineering cases exceeding 800, but Wei et al. pointed out in Science of the Total Environment (2020, DOI: 10.1016/j.scitotenv.2020.140400) that the reason A²/O has long occupied the mainstream is partly because the Total Nitrogen requirements for effluent were relatively lenient at the time, and "diluting influent—diluting discharge" had not yet been fully prohibited; in A²/O, the sludge activity in the anaerobic and anoxic tanks is strongly inhibited by toxins, leading to elevated organic loading rates and decreased Nitrification rates in the aerobic tank. The Analytic Hierarchy Process (AHP) evaluation results of that study show: the low-load group prefers A/A/O (A²/O), while the high-load group prefers O/H/O. The direct implication of this conclusion for process selection is—do not assume "A²/O is universally applicable," and do not assume "the newer the better"; it depends on which tier the influent loading falls into.
VIII. Four Calibers That Require Fact-Checking
IX. Six Engineering Truths
- Ammonia stripping is not a process for "bringing ammonia nitrogen to compliance." Its mission is to reduce ammonia nitrogen from 2500~6000 mg/L down to the 100~150 mg/L range, to reduce the load on the biological stage and relieve toxicity; insisting on pushing ammonia nitrogen down to single digits in the ammonia stripping stage will drag down the economics of the entire process chain through steam costs and equipment investment. Saving steam by keeping the column bottom above 300 mg/L will instead spend that money back on oxygen consumption in the aerobic tank and on liquid caustic soda.
- "Carbon removal and nitrogen removal in a single tank" basically does not hold in coking wastewater. The inhibition threshold of phenol (tens of mg/L) is far below the influent phenol concentration (hundreds of mg/L), and nitrifying bacteria remain suppressed until phenol has been degraded. Staging (anaerobic/hydrolysis acidification → anoxic → aerobic, or O/H/O) is not a process preference but a structural requirement forced by this type of water.
- The alkalinity account must be balanced at the system level, not just by looking at pH in the aerobic tank. Nitrification consumes 7.14 g of alkalinity per gram of ammonia nitrogen (as CaCO₃) and 4.57 g of oxygen, while denitrification provides 3.74 g of alkalinity per 1 g of nitrate nitrogen reduced. Consumption and recovery occur in different tanks, and judging "whether to add alkalinity" from the pH reading at the end of the aerobic tank is often already too late.
- Thiocyanate (SCN⁻) is the item most easily left out of the calculation. It both inhibits nitrification and degrades into ammonia nitrogen, amounting to "first producing ammonia, then removing ammonia, with inhibition along the way." Some upgrading projects failed to meet standards for a long time after commissioning precisely because the original design did not account for SCN⁻, forcing the addition of a pre-decyanation system. It must be included in Total Nitrogen when calculating the biological load.
- Advanced oxidation cannot safeguard Total Nitrogen. Catalytic Ozonation, Fenton Oxidation, BAF, Activated Carbon, membranes—none of them can convert ammonia nitrogen into nitrogen gas; all they can be relied on for is COD, color, residual phenol, and PAHs. If nitrification in the biological stage is not established, no amount of money spent in the advanced stage can rescue the Total Nitrogen target. Conversely, RO will "retain" ammonia nitrogen into the concentrate, and when the concentrate is reused for coke quenching or evaporated, this nitrogen does not disappear.
- "Compliance" must be judged by destination, not by habit. For the same set of effluent, reuse for coke quenching/coal washing/blast furnace slag flushing is assessed against indirect discharge limits, discharge into the environment is assessed against direct discharge limits, and local implementation of special emission limits is yet another tier. In addition, coke dry quenching is rapidly becoming widespread (reported water consumption for wet coke quenching is 0.37~1.2 t per ton of coke, while coke dry quenching drops to about 0.19~0.6 t or even essentially no water consumption), so "using coke quenching as a disposal outlet" is itself narrowing—this must be factored into the judgment when preparing a reuse plan, otherwise the process is designed for indirect discharge and may be required to operate under direct discharge a few years later.
10. References
- GB 16171—2012 "Emission Standard of Pollutants for Coking Chemical Industry", Ministry of Ecology and Environment, implemented on 2012-10-01 (Table 1/Table 2/Table 3 water pollutant emission concentration limits and benchmark wastewater discharge per unit product).
- HJ 2022—2012 "Technical Specification for Coking Wastewater Treatment Engineering", Ministry of Ecology and Environment (Table 4-1 typical coking wastewater quality and quantity indicators).
- "Best Available Techniques Guidelines for Pollution Prevention and Control in the Iron and Steel Industry — Coking Process", Ministry of Ecology and Environment (Table 2.3-5 water quality at each wastewater discharge point of the coking system).
- Wei C., Wei J., Kong Q., et al. Selection of optimum biological treatment for coking wastewater using analytic hierarchy process. Science of the Total Environment, 2020, DOI: 10.1016/j.scitotenv.2020.140400.
- Professor Wei Chaohai's team: Principles and case application achievements of the innovative O/H/O process series for industrial wastewater treatment, South China University of Technology, 2023.
- "Realization and Influencing Factor Analysis of the Nitrification Process in A/O₁/H/O₂ Process for Coking Wastewater Treatment" (including optimal nitrification parameters and pollutant inhibition EC₅₀).
- "Inhibitory Effects of Toxic Compounds on the Nitrification Process of Coking Wastewater Treatment", PubMed 17804160 (threshold values of phenol, p-cresol, free cyanide, ferricyanide, SCN⁻).
- "Review of Research on Anaerobic Ammonium Oxidation Process for Treating Coal Chemical Coking Wastewater", Clean Coal Technology, 2025 (phenol 20~21 mg/L inhibition rate 50%, 34~40 mg/L inhibition rate 75%).
- Fan Dan, Liao Jianbo, Wei Cong, et al. Unit analytical model of operational energy consumption for coking wastewater treatment engineering — taking the OHO fluidized bed process as an example. Acta Scientiae Circumstantiae, 2015.
- Ren Yuan, et al. GC/MS analysis of organic components during coking wastewater treatment by biological fluidized bed A/O₂ process. Environmental Science (phenols account for more than 90% of organic matter; B/C 0.30→0.45).
- "Research on New Advanced Treatment Technologies for Coking Wastewater and Their Respective Coupling Characteristics", Coal Science and Technology, 2021 (engineering data and costs of heterogeneous catalytic ozonation).
- "Research on Catalytic Ozonation Treatment of Reverse Osmosis Concentrate" (dissertation) (comparison of Fenton and catalytic ozonation, cost per ton of water and engineering budget estimate).
- "Operation and Management of A-A/O Process in Coking Wastewater Treatment", China Water Network / Environmental Ecology Network / Wastewater Treatment Engineering Network (oxygen and alkali consumption by nitrification, alkalinity recovery by denitrification, BOD/TN>2.86, external methanol carbon source).
- "Biochemical Treatment Process and Design Points of Coking Wastewater", Civil Engineering Online (control concentrations of inhibitors in biochemical influent, C/N>6~7, reflux ratio 4~6, sludge age and load).
- "Best Available Techniques and Main Technical Indicators for Water Pollution Control in Coking Process" (feasible technical parameters of A/O² and pretreatment + A/O²).
- "Optimization of Cyanide Removal Process for Coking Wastewater", Wastewater Treatment Engineering Network (SCN⁻ as the key influencing factor for upgrading and reconstruction, reconstruction of pre-decyanidation system).
- Upgrading and reconstruction case of coking wastewater treatment at Shanxi Fulong Coal Chemical Co., Ltd., Wastewater Treatment Engineering Network, 2022.
- Shougang Shuicheng Iron and Steel (Group) Co., Ltd.: Upgrading and reconstruction of coking wastewater treatment process (O/AO/AO + electromagnetic strong oxidation), Wastewater Treatment Engineering Network, 2023.
- Public engineering cases of Zhongke Langbo Environmental Protection: Hebei Yuzhou Coal Chemical 1440 m³/d, Hebei Lima Gas 1200 m³/d, Handan Cixian Xinsheng Coal Chemical 1680 m³/d.
- Qianshui Environment: Report on advanced treatment and reuse project of coking wastewater of a large coking enterprise (1200 m³/d).
- Public case of Weilin Environmental Protection: Treatment of phenolic industrial wastewater (200 万 t coke/year, 1200 t/d; 5000 m³/d reconstruction).
- Wuhai Media Center: Report on the coke dry quenching and waste heat power generation project of Inner Mongolia Meifang Coal Coking, 2026-01 (comparison of water consumption per ton of coke between wet quenching and dry quenching).
- Ningxia News Network: Report on the coke dry quenching project of Xinsheng Coking, 2020-08 (water consumption of wet quenching is about 0.53 t per ton of red coke).
- China Chemical Industry News: Report on Shenmu Tianyuan Chemical's conversion from traditional soaking method to dry quenching [year to be supplemented] (water consumption per ton of coke by traditional soaking method 0.373 t, about 0.19 t after conversion).
- HG/T 6072—2022 "Determination of Thiocyanate Content in Coking Wastewater", implemented on 2023-04-01.
- Baidu Baike entry "Coking Wastewater" (average BOD₅/COD about 0.3; more than 300~500 compounds have been identified in coking wastewater; 2022 年 national coke output about 4.73 亿 t and wastewater volume estimation).
