Physicochemical treatment methods for tar wastewater

Coal tar is one of the important products in the coking industry. Its composition is extremely complex. In most cases, it is separated, purified, and utilized by specialized companies in the coal tar industry.

The fine processing of coal tar can yield various chemical products, but the coal tar processing generates a large amount of toxic wastewater.This type of wastewater contains high concentrations of organic matter, cyanide, and other highly toxic substances. It is highly toxic and complex in composition. The organic pollutants are mainly monocyclic or polycyclic aromatic compounds and heterocyclic compounds containing nitrogen, sulfur, and oxygen, such as high concentrations of phenol, naphthalene, aniline, pyridine, quinoline, and benzo[a]pyrene. Phenolic compounds are toxic to all organisms, as they can inactivate cells and coagulate proteins; polycyclic aromatic hydrocarbons can cause cancer in humans and are generally difficult to biodegrade.

Currently, both domestically and globally, significant research is being conducted on the treatment of coking wastewater, with little in-depth study on coal tar wastewater. Coal tar processing wastewater shares similarities and significant differences with traditional coking wastewater, also known as phenol-cyanide wastewater. In addition to containing high levels of cyanide and ammonia nitrogen, coal tar processing wastewater has much higher concentrations of volatile phenols, indole, benzo[a]pyrene, naphthalene, indene, and oils compared to traditional coking wastewater.

Based on the characteristics of the tar processing production process, coal tar wastewater mainly originates from: ① water separated from tar settling in the large tar tank, which is collected separately; ② water separated from the first and second stage tar evaporators, and water separated from the industrial naphthalene oil-water separator; ③ water separated from the large tar tank and wastewater from various tar processing separators sent to the company's wastewater tank; ④ wastewater from washing and decomposition of Na2SO4 and wastewater from the refined phenol unit. The refined phenol high-concentration wastewater has a volatile phenol content of 3% to 10% and is returned to the washing and decomposition alkali addition tank for volatile phenol recovery, while the washing and decomposition wastewater is stored and treated separately; ⑤ wastewater generated from pipeline cleaning, surface water, and domestic sewage. Currently, most tar wastewater in China is not thoroughly treated, leading to severe pollution of the water environment and posing a threat to human health.

The treatment methods for tar wastewater are generally similar to those for coking wastewater. Through general pretreatment and biological denitrification secondary treatment, it is difficult to meet the final standards for COD, ammonia nitrogen, and other indicators. This paper reviews the treatment methods for tar wastewater in recent years, both domestically and internationally, analyzes the existing problems, and proposes the development trends in tar wastewater treatment technology.

1 Current Status of Treatment for Difficult-to-Degrade Organic Matter in Tar Wastewater

1.1 Physicochemical Treatment Methods

1.1.1 Coagulation Method

The key to the coagulation method lies in the coagulant. Common coagulants include aluminum salts, iron salts, and polyaluminum chloride. Yan Jiabao et al. prepared a novel coagulant, polyferric silicate, using sodium silicate and ferric sulfate and investigated the effects of the molar ratio of Fe to Si, pH, and dosage on its coagulation performance. They found that when the molar ratio n(Fe):n(Si) was 1.00:1.00, the water sample pH was 6.52, and the dosage was 20 mg/L, the oil removal rate reached 90.2%, and the COD removal rate was approximately 62.5%. The outstanding performance of this flocculant is attributed to the addition of active silicic acid during its preparation, which improved the morphology and structure of the polymer. The development of novel coagulants with low cost and high efficiency is conducive to the efficient treatment of wastewater. However, exploring the performance of polyferric silicate through only three factors is still insufficient; other factors such as temperature should also be considered.

1.1.2 Supercritical Oxidation Method

Supercritical water oxidation (SCWO) technology is a novel oxidation technology proposed in the mid-1980s that can thoroughly destroy organic matter structure. It involves heating and pressurizing water to a supercritical state (Tc ≥ 374.3℃, Pc ≥ 22.1MPa) for oxygenation reaction. In this state, the solubility of organic matter in water increases significantly, allowing for sufficient contact and reaction with oxidants. Almost all organic matter can be oxidized and decomposed into CO2 and H2O, achieving high decomposition efficiency.

Under optimal reaction conditions of 420℃ temperature, 25MPa pressure, 30min reaction time, and 2 times the amount of hydrogen peroxide as an oxidant, Quan Kui et al. treated high-phenol wastewater from tar using an intermittent supercritical water oxidation device. The COD removal rate reached 99.1%, with the effluent COD concentration at 152mg/L. Except for ammonia nitrogen, the effluent basically met the national Class II discharge standard. The amount of oxidant added is crucial for the reaction; too much or too little will affect the effluent water quality. This method is effective in treating high-concentration organic wastewater, and it is recommended to increase the industrial application of supercritical devices.

1.1.3 Ozonation Method

Ozone has strong oxidizing properties and can rapidly react with most organic matter and microorganisms in wastewater. It can remove phenol and cyanide from wastewater, reduce COD, and simultaneously achieve decolorization, deodorization, and disinfection.

Chang et al. treated wastewater using the ozonation method. Color and thiocyanate were almost completely removed. When the ozone consumption rate was reduced to 0.2, the TOC removal rate increased to 30%, indicating that easily degradable pollutants were almost completely degraded. However, due to disadvantages such as high investment and high power consumption, this method is generally used for the deep treatment of wastewater.

1.1.4 Ultrasonic Method

Research on ultrasound in chemistry began in 1927 when Richards and Loomis discovered that ultrasound could accelerate conventional reactions and redox reactions. In recent years, ultrasound has been used to solve problems related to water pollution, especially in removing toxic and refractory organic matter from wastewater.

Ning et al. conducted comparative experiments. One group used single activated sludge, and the other group used activated sludge treated with ultrasound. After treating wastewater with both for 240min, the COD removal rate of the latter increased from 48.29% to 80.54%.

1.1.5 Fenton Oxidation Method

Traditional Fenton reaction generates highly active hydroxyl radicals through hydrogen peroxide and divalent iron salts under acidic conditions, which can oxidize organic compounds. However, traditional Fenton method produces Fe3+, causing troublesome sludge problems. In recent years, various studies have been conducted to enhance the traditional Fenton oxidation process.

Chu et al. improved the Fenton oxidation method by replacing divalent iron salts with iron powder, forming a novel Fenton reagent with hydrogen peroxide. Experiments found that no Fe3+ was produced, and at pH 6.5 and hydrogen peroxide concentration of 0.3 mol/L, the COD removal rate reached 44%-50% and the total phenol removal rate approached 95% after 1 hour of reaction. Most organic substances, including bicyclic furan, quinoline, resorcinol, and furfural, were completely removed. The Fenton reagent method has great potential in treating toxic, harmful, and difficult-to-biodegrade organic wastewater such as tar wastewater. However, the treatment cost of this method is high, and it is only suitable for low-concentration, small-volume wastewater treatment.

1.1.6 Chlorine Dioxide Method

Chlorine dioxide has strong reactivity and oxidizing ability, and can react with many organic compounds under water treatment conditions. Dioxide reacts with phenolic compounds, polycyclic aromatic hydrocarbons such as anthracene, phenanthrene, benzo(a)anthracene, and benz(a)anthracene, and organic sulfides (such as methanethiol, sulfides, and disulfides) under water treatment conditions or specific conditions. Chlorine dioxide does not react with aliphatic and aromatic hydrocarbons, carboxylic acids RCOOH (where R is a saturated alkyl group), alcohols, some unsaturated carboxylic acids, N-heterocyclic compounds, and organochlorine pesticides.

Zuo Jinlong studied the removal effect of chlorine dioxide on wastewater containing coal tar using a specific factory's actual project. The results showed that under the conditions of pH 7, temperature 45℃, and reaction time 1h, the maximum removal rate of coal tar was 42% when treating water samples with a mass concentration of 0.845mg/L. This indicates that coal tar contains a large amount of refractory substances, especially asphalt components. The treatment of wastewater containing coal tar requires further in-depth research.

1.1.7 Incineration Method

Yang Yuanlin et al., through research on incineration treatment of wastewater, believe that incineration treatment technology is a feasible method for treating high-concentration wastewater generated by coking plants and gas plants, especially suitable for cold regions in the north. Moreover, the incineration process can also produce steam for production and domestic use, which can greatly reduce operating costs. Although incineration has high efficiency and does not cause secondary pollution, its treatment cost is expensive. It is widely used abroad but rarely used in China.

1.1.8 Plasma Treatment Technology

Research on the technology of treating organic wastewater using high-voltage nanosecond pulsed discharge plasma began in the 1990s. Under the action of nanosecond high-voltage pulses, discharge plasma is generated in the gas gap. There are a large number of high-energy electrons in the discharge plasma. These high-energy electrons act on water molecules, producing a large number of hydrated electrons and other strong oxidizing groups to oxidize organic matter in the water, thereby achieving the purpose of degrading organic matter.

Jiang Bairu et al. used discharge plasma to degrade cyanide, ammonia nitrogen, COD, and other organic matter in wastewater. The removal effect of ammonia nitrogen and polycyclic aromatic hydrocarbons on COD in wastewater has a significant impact, with the COD concentration showing a trend of decreasing, increasing, decreasing, increasing, and decreasing. The removal effect of cyanide and ammonia nitrogen is better. Multiple discharges can reduce the inhibitory effect of cyanide and ammonia nitrogen on microorganisms during biological treatment and improve biodegradability. However, the cost of this treatment equipment is high and requires further development to reduce costs.