I. Coking Industry
Coking chemical industry is an important part of coal chemical industry. The main processing methods of coal include high temperature coking (950 - 1050 degrees Celsius), medium temperature coking and low temperature coking. The metallurgical industry generally uses high temperature coking to obtain coke and recover chemical products. The product coke can be used as fuel for blast furnace smelting, casting, non-ferrous metal smelting, water gas manufacturing, producer gas for producing synthetic ammonia, and calcium carbide to obtain raw materials for organic synthesis industry. The chemical products produced in the coking process can be recovered, processed and extracted, such as tar, ammonia, naphthalene, hydrogen sulfide, crude benzene and so on. The net coke oven gas and coal tar can be obtained. After the refinement and deep processing of crude benzene, benzene, toluene, xylene and carbon disulfide can be produced. These products are widely used in chemical industry and medicine. Industry, refractory industry and national defense industry. Net coke oven gas can be used for civil use and industrial fuel. Ammonia in gas can be used to produce ammonium sulfate, concentrated ammonia water, anhydrous ammonia, etc. There are hundreds of products in Coking Chemical industry. China's coking chemical industry has been able to produce more than 100 chemical products from coke oven gas, tar and crude benzene, which is of great significance to China's national economic development.
1. Water quality characteristics of coking wastewater
Coking wastewater is produced in the process of coke oven gas primary cooling and coking. Its composition is complex. It contains a large number of organic pollutants such as phenols, biphenyls, pyridines, indoles and quinolines. It also contains toxic and harmful substances such as cyanide, inorganic fluoride ions and ammonia nitrogen. The color of pollutants is high and it is harmful to the environment.
The main components of coal coking wastewater are volatile phenol, mineral oil, cyanide, phenol and benzene compounds, ammonia nitrogen, etc. It is one of industrial wastewater with high concentration of pollutants and complex pollutant components, which is difficult to treat. The key to its treatment lies in:
(1) High phenol content
The average content of phenol in coking wastewater is 1500-2000 mg/L, which is directly reflected in the COD value of wastewater. The COD content of coal coking wastewater without phenol removal is as high as 3000-5500 mg/L. The biodegradability of phenol is poor. Before entering the biochemical treatment system, coking wastewater should be removed by ammonia evaporation system. After ammonia evaporation, the phenol content in wastewater is generally 450-850 mg/L. Such phenol content can be completely degraded by biological method, and it is more economical and applicable for biological treatment.
(2) High ammonia nitrogen content
The ammonia content in ammonia distillation wastewater is high, the average content is 4500 mg/L. In this way, high concentration ammonia can not be removed by biochemical method, and it has a serious toxic effect on the biochemical treatment unit, which can kill activated sludge and destroy the whole biological treatment system. Therefore, the pretreatment process of ammonia removal should be set up before the wastewater containing high ammonia nitrogen enters the wastewater treatment station.
The concentration of ammonia nitrogen in wastewater pretreated by deamination is about 80-200 mg/L, and the average concentration is generally less than 200 mg/L. After treatment by this process, it can completely meet the standard of less than 10 mg/L. The removal of ammonia nitrogen is the first problem to be considered in the selection of this kind of wastewater treatment process.
(3) High content of refractory organic matter
Coal coking wastewater contains a large number of refractory organic compounds such as benzene, naphthalene and heterocyclic compounds, which are difficult to be directly treated by aerobic activated sludge process. Therefore, before aerobic method, it is necessary to improve its biodegradability and BOD: COD value.
2. Biochemical Treatment of Coking Wastewater
A2/O process is to add an anaerobic section before the traditional A/O process in order to further improve the removal rate of organic matter and the biodegradability of wastewater.
The wastewater first flows into the anaerobic tank. Under the action of facultative anaerobic bacteria and specific anaerobic bacteria, the organic matter in the wastewater is decomposed into biogas and absorbed into the body of microorganisms, which can be removed in the form of sludge. In addition, NH3-N was partially removed due to cell synthesis, which reduced the concentration of NH3-N in sewage, but the content of NO3-N remained unchanged. Moreover, anaerobic process can greatly improve the biodegradability of benzene and anthraquinone organic compounds which are difficult to be degraded directly by aerobic biochemical method in wastewater, improve the treatment efficiency of subsequent biological oxidation method, and reduce energy consumption of subsequent biochemical process. The main purpose of the anaerobic section is to improve the biodegradability of wastewater and remove some organic matter.
The wastewater from anaerobic reactions enters the anoxic tank. At the same time, part of the nitrate (supernatant) is returned to the anoxic tank through aerobic treatment, and denitrification is carried out in the anoxic tank. Denitrifying bacteria capture nitrite and oxygen-oxidizing organic matter in nitrate in reflux nitrification solution and remove nitrite and nitrate nitrogen by reducing them to nitrogen.
Denitrification is a process in which heterotrophic denitrifying bacteria reduce NO3-N to N2 in wastewater under anoxic conditions. Its biochemical reaction formula is as follows:
N2 is insoluble in water and can be blown off by gas.
In the aerobic tank, organic matter is biodegraded by microorganisms, and the removal rate is higher. At the same time, ammonia nitrogen in wastewater was oxidized to nitrite and nitrate by nitrifying bacteria. Nitrification mixtures were separated by solid-liquid separation in sedimentation tank, and most of the separated nitrates were returned to anoxic tank for denitrification and denitrification. Another single treatment water was discharged after further treatment, and all the sludge was returned to aerobic tank.
Under aerobic conditions, the oxidation of NH3 to NO3-N by autotrophic nitrifying bacteria and nitrifying bacteria is the first step of biological denitrification. Its biochemical reaction formula is as follows: