Semi-coke wastewater: Analysis of toxicity sources, environmental hazards, and key treatment technologies

In the energy and chemical industry, the semi-coke industry is a double-edged sword. While it offers an efficient way to utilize low-grade coal and brings economic benefits, it also produces a byproduct that can be considered an "environmental killer"—semi-coke wastewater. This dark, foul-smelling industrial wastewater, with its complex composition, high concentration, and strong toxicity, is a daunting challenge for many environmental protection professionals.

To effectively treat it, a deep understanding of its nature is essential. This article will act as an "environmental detective," delving into the core processes of semi-coke production, tracing the origins of various pollutants in the wastewater, thoroughly analyzing the environmental damage they cause, and ultimately identifying the key technological tools to break this pollution chain.

Chapter 1: Tracing the Source – Where do the "toxic substances" in semi-coke wastewater come from?

The generation of semi-coke wastewater occurs throughout the entire process of low-temperature carbonization (600-800℃) of low-rank coal (such as non-caking coal and weakly caking coal), from coal-based gas purification to steam quenching. The core pollutants are not generated out of thin air, but are the inevitable products of the "cracking-recombination-transfer" of coal, a complex organic substance, at specific temperatures.

1. The "large family" of organic pollutants: tar, phenols, and refractory organic compounds

Source: During the low-temperature carbonization of coal, the complex high-molecular-weight polymers within the coal (such as lignin and cellulose) undergo pyrolysis, but the cracking is not complete. Unlike high-temperature coking (above 1000℃), which converts most organic matter into coke and gas, the medium-low temperature conditions prevent a large amount of medium- and low-molecular-weight organic products from further condensation. Instead, they escape in liquid or gaseous form and eventually enter the wastewater during condensation and washing.

Phenolic compounds: These mainly come from the decomposition of lignin and cellulose in coal. Unit phenols (such as phenol) and polyphenols are more easily formed and remain in the water under low-temperature carbonization. This is the main source of volatile phenols (3700-5000 mg/L) and total phenols (4000-6000 mg/L) in the wastewater.

Polycyclic aromatic hydrocarbons and heterocyclic compounds: During the pyrolysis of coal, aromatic structural units undergo condensation and recombination, generating polycyclic aromatic hydrocarbons such as naphthalene, anthracene, and phenanthrene, as well as nitrogen-, oxygen-, and sulfur-containing heterocyclic compounds such as pyridine, quinoline, and furan. These substances have stable structures and constitute the main components of refractory COD (17000-30000 mg/L) and tar.

Oily substances: The "oil" in the wastewater is a mixture, including heavy tar, light oil, and emulsified oil. They are also liquid products of coal pyrolysis and are washed into the wastewater during coal-based gas washing and steam quenching, resulting in a total oil content as high as 1200-2000 mg/L.

2. The "Lethal Combination" of Inorganic Pollutants: Ammonia Nitrogen, Cyanide, and Sulfides

Source: These pollutants mainly originate from the transformation of heteroatoms (such as nitrogen and sulfur) present in coal during the pyrolysis process.

Ammonia Nitrogen: Under dry distillation conditions, some of the nitrogen elements in coal are converted into nitrogen-containing heterocyclic compounds, while others react with hydrogen to form ammonia gas. This ammonia gas is absorbed by water during subsequent coal-based gas purification and coke quenching processes, forming high concentrations of ammonia nitrogen (2000-5000 mg/L).

Cyanide: In localized high-temperature, oxygen-deficient areas of the dry distillation furnace, nitrogen and carbon in the coal can react with volatile organic compounds to produce hydrogen cyanide, which then dissolves in water to form cyanide (22-50 mg/L).

Sulfides: The sulfur content in coal produces gases such as hydrogen sulfide during dry distillation, which then dissolves in water to form sulfides.

3. Other Characteristic Pollutants: Suspended Solids, Color, and Salinity

Suspended Solids: Mainly from coke dust. During the coking process, transportation, and coke quenching, some of the coke breaks down due to mechanical wear and thermal stress, forming tiny particles that enter the wastewater.

High Color: The dark brown to soy sauce-like color of the wastewater is mainly attributed to the large amount of chromophores and auxochromes it contains. These groups are present in phenolic polymers, macromolecular polycyclic aromatic hydrocarbons, and heterocyclic compounds, resulting in a wastewater color of up to 10,000-30,000 times.

High Salinity: On the one hand, it comes from the inorganic salts contained in the coal itself, and on the other hand, it comes from the water used in the production process and the residues from processes such as phenol and ammonia recovery, resulting in total dissolved solids as high as 4500-8000 mg/L.

Summary: Coke wastewater is essentially a concentrated liquid representing the "life trajectory" of coal under low-to-medium temperature dry distillation. It concentrates and releases the toxic and harmful substances hidden in the coal in liquid form, constituting an extremely complex collection of pollutants.

Chapter Two: Analyzing the Harm – The "Triple Environmental Disaster" Caused by Semi-coke Wastewater

If this "source of pollution" is discharged without proper treatment, it will pose a multi-faceted and cascading serious threat to water bodies, soil, organisms, and even human health.

Disaster 1: A "devastating blow" to aquatic ecosystems

Instantaneous toxic effects: Cyanide in wastewater is a highly toxic substance that can directly inhibit the cellular respiration of aquatic organisms, leading to the mass death of fish and other organisms in a short period. Volatile phenols are also highly toxic and can damage gill structures and the nervous system even at low concentrations.

Chronic suffocation and eutrophication: Extremely high concentrations of ammonia nitrogen and COD entering the water body will rapidly consume dissolved oxygen. The water body falls into an anaerobic state, leading to the death of aerobic organisms and triggering anaerobic fermentation, resulting in black and foul-smelling conditions. At the same time, ammonia nitrogen is an excellent nutrient for algae growth, which may cause severe water eutrophication, forming "algal blooms" or "red tides," further disrupting the ecological balance.

Long-term toxicity and bioaccumulation: Many members of polycyclic aromatic hydrocarbons and heterocyclic compounds have been proven to have "three-carcinogenic" effects (carcinogenic, teratogenic, and mutagenic). They are chemically stable and difficult to be naturally degraded, and will remain in the bottom sediment of water bodies for a long time, accumulating through the food chain, ultimately threatening humans at the top of the food chain.

Disaster 2: A "fatal inhibition" of treatment systems

"Blocking" and "poisoning" effects of oily substances: Heavy oil and emulsified oil in wastewater will coat the surface of microorganisms, hindering their respiration and nutrient absorption. More seriously, they will adhere to fillers and membrane components, causing equipment blockage and significantly increasing cleaning frequency and operating costs. Oily substances are also important contributors to COD, directly increasing the treatment load.

"Mass extinction" effect of biological toxicity: High concentrations of phenols and cyanides have a fatal killing effect on the microbial community in activated sludge. They can denature proteins and inhibit enzyme activity, leading to the collapse of the biochemical system. This is the fundamental reason why tar wastewater has "extremely poor biodegradability." Due to its high toxicity and extremely poor biodegradability, treating semi-coke wastewater directly with conventional biological methods is highly inefficient and often fails to meet discharge standards. The "double pressure" of ammonia nitrogen: High concentrations of ammonia nitrogen inhibit nitrifying bacteria and simultaneously place a huge burden on biological denitrification systems, requiring extremely long hydraulic retention times and enormous aeration energy consumption, making it highly uneconomical and technically unfeasible.

Disaster 3: "Invisible erosion" of soil and crops

When semi-coke wastewater seeps into the soil or is used for irrigation, the salts and phenolic substances in it alter the physical structure and chemical properties of the soil, leading to soil compaction, salinization, and decreased fertility.

Toxic organic compounds and heavy metals (although not listed in the table, they may be present in the actual wastewater) accumulate in the soil and are absorbed by crops, not only affecting crop growth but also entering the human body through agricultural products, posing a health hazard.

Chapter 3: Breaking the Cycle – Disrupting the Pollution Chain, Starting with Efficient Oil Removal and Pre-treatment

Faced with such a severe challenge, what is the path to effective treatment? The answer is: we must adopt a smart strategy of "differentiated pretreatment, staged reduction, and resource recovery." In this strategy, oil removal pretreatment is the crucial and indispensable first link in the entire treatment chain.

Why is oil removal so critical?

1. Protection of subsequent processes: Efficient removal of oils, especially emulsified oils, effectively prevents contamination and blockage of subsequent ammonia stripping towers, phenol extraction towers, biochemical pools, and membrane systems, ensuring the stable operation of the main process.

2. Resource recovery and load reduction: The recovered tar is an economically valuable byproduct. At the same time, removing oils significantly reduces the COD in the wastewater, greatly reducing the pressure on subsequent biochemical treatment.

3. Elimination of biological inhibition: Reducing the encapsulation and toxic inhibitory effects of oils on microorganisms creates the necessary conditions for improving the biodegradability of wastewater.

Qingdao Brator Environmental Protection Technology Co., Ltd.: A technology service provider specializing in oil removal and pretreatment of coal gasification wastewater.

Addressing the challenges of tar wastewater, characterized by its complex composition, severe emulsification, and difficulty in separation, Qingdao Bailida Environmental Protection Technology Co., Ltd., leveraging its extensive industry experience and technological innovation, has launched a "three-stage cascading coupled oil removal" system specifically designed to overcome these persistent problems.

First Stage: GZ-type "Tank-within-Tank" System – Acting as the vanguard, this system integrates regulation, cyclone, and gravity sedimentation, efficiently removing most of the free oil and coarsely dispersed oil, while simultaneously reducing suspended solids, clearing the way for subsequent precision treatment.

Second Stage: WXFL-type "Micro-bubble Cyclonic Separator" – As the key problem solver, it combines cyclone and micro-bubble technologies to effectively remove finely dispersed oil and deeply purify suspended solids, significantly improving the appearance and quality of the treated water.

Third Stage: ZJYS-type "High-Precision Oil-Water Separator" – As the ultimate solution, its core utilizes a patented demulsification and coalescence filter element unit jointly developed with the Chinese Academy of Sciences. It precisely tackles the problem of emulsified oil, separating micron-sized oil droplets through a physical demulsification-coalescence process, achieving deep oil removal.

Our Commitment:

Efficient and Stable: The oil content in the treated water is stably reduced to ≤500mg/L, meeting the stringent requirements of subsequent processes.

Economical and Environmentally Friendly: The entire process primarily uses physical methods, eliminating the need for chemical additives and preventing sludge increase and secondary pollution at the source.

Reliable Technology: We possess multiple invention patents, and our equipment is skid-mounted and highly automated, helping your company achieve stable compliance and green development.

Conclusion

Semi-coke wastewater is a serious environmental challenge left behind by the coal conversion process. Only by tracing the source and understanding the nature of its harm can we use precise technological solutions to break the chain of pollution. Qingdao Brator Environmental Protection Technology Co., Ltd. is willing to be your reliable partner on your treatment journey with our professional oil removal pretreatment solutions, jointly protecting our green mountains and clear waters, and promoting a clean, efficient, and sustainable future for the coke oven industry.