Overview of Coal Chemical Wastewater Treatment Technology

      Coal chemical industry is the process of processing coal as the main raw material through chemical means to obtain liquid, gaseous, and solid compound products. In production processes such as coal gasification, direct coal liquefaction, and coking, a huge amount of industrial wastewater is generated, including gasification wastewater, purified wastewater, domestic and laboratory wastewater, circulating wastewater, and so on. At present, coal chemical enterprises in China mainly carry out sewage treatment through pre-treatment, biochemical treatment, deep treatment, reuse treatment, and high salinity water treatment.

1.1 Pre treatment of coal chemical wastewater: Firstly, phenol, ammonia and other resources should be recovered through pre-treatment, and impurities such as oil and ash should be separated to enable subsequent treatment. At present, the main processes used for coal chemical wastewater pretreatment in China include air flotation, sedimentation, filtration, extraction, and gas extraction, among which the key technologies are extraction and air flotation. Extraction technology is mainly used for the recovery of phenol, ammonia and other resources in coal chemical wastewater, and is often used for the pretreatment of high phenol ammonia wastewater generated by coal crushing and pressure gasification projects; Air flotation technology is used to remove oil substances from coal chemical wastewater.

1.2 Biochemical treatment: After pretreatment, coal chemical wastewater will continue to undergo biochemical treatment, which involves the degradation of organic matter in the wastewater by microorganisms. At present, China's coal chemical enterprises mainly use technologies such as activated sludge and anaerobic microorganisms for the biochemical treatment of coal chemical wastewater. The advantages of activated sludge technology are low environmental requirements, significant treatment effects, mature technology, and simple operation. Its disadvantage is that it is difficult to effectively treat coal chemical wastewater with high phenol content, resulting in the production of a large amount of sludge and high management requirements. Therefore, currently this technology is mostly used for the treatment of coal chemical wastewater with low phenol concentration and poor toxicity. The advantage of anaerobic microbial technology is that it is not limited by the phenol content of coal chemical wastewater and can be used for the treatment of coal chemical wastewater with various concentrations of phenol. The main disadvantage is that anaerobic microorganisms require a certain amount of time for degradation, resulting in a longer sewage treatment cycle and being affected by carbon compounds in the sewage. Therefore, currently in China, coal chemical enterprises usually combine it with other aspects.

1.3 After pre-treatment and biochemical treatment, coal chemical wastewater still needs to be further treated to remove substances that may cause environmental damage. In the deep processing stage, the main technologies currently adopted by various coal chemical enterprises in China include advanced oxidation technology, adsorption technology, and tertiary biochemical technology. The principle of advanced oxidation technology is to generate hydroxyl radicals (- OH) through electrochemical and other means, in order to degrade organic matter in coal chemical wastewater; The principle of adsorption technology is to use porous solid materials such as activated carbon and fly ash to adsorb pollutants from coal chemical wastewater; The currently widely used tertiary biochemical technology is the biological aerated filter (BAF), which also uses microorganisms to degrade pollutants in coal chemical wastewater.

1.4 Salt containing wastewater treatment: Circulating wastewater and desalinated water station wastewater in coal chemical wastewater have the characteristics of low organic matter content and salt content below 1%. Currently, reuse water stations are commonly used for their treatment. The current treatment technology for saline wastewater is mainly based on "ultrafiltration+reverse osmosis", which can reduce the COD content in the wastewater to below 10mg/L and the ammonia nitrogen content to below 5mg/L.

1.5 When using technologies such as ultrafiltration and reverse osmosis to treat saline wastewater, high concentration saline wastewater with TDS exceeding 1% will be generated. Due to the scarcity of water resources in the northern Shaanxi region, there are no wastewater bodies that can receive pollutants. Therefore, it is necessary to treat these high concentration saline wastewater in order to achieve zero discharge of coal chemical wastewater. The current technology for treating high salinity water is to evaporate the water through evaporation ponds or recover the water through evaporation crystallization technology, while obtaining crystalline salt.