Coking wastewater has always posed a challenge in industrial wastewater treatment
because of its complex composition and many refractory substances. Recently,
the market for zero discharge of coking wastewater has emerged. The industrial
integration and upgrading of the coking industry are bringing a large number of
wastewater-related new construction and renovation project opportunities.

Image by Daniel from Pixabay

Coking is a coal conversion process in which coking coal is finally made into coke through processes such as drying, pyrolysis, melting, bonding, solidification, and shrinkage while obtaining gas, coal tar, and recycling other chemical products. Its downstream industries are mainly iron, steel, and coal chemical industry.

There are more than 500 coking production enterprises nationwide, mainly in Shanxi, Hebei, Shandong and Inner Mongolia. Among them, Shanxi Province has the largest production capacity, exceeding 100 million tons. According to the 14th Five-Year Plan for the coking industry, the overall production capacity of the industry is expected to remain unchanged (222.17 million tons of projects planned and under construction in 2020). However, the coking wastewater discharge treatment policy has become stricter as a result of increased industrial pollution and water scarcity, and enterprises are encouraged to carry out zero-discharge coked wastewater treatment regulations.

In the past five years, the coking industry has been increasingly involved in environmental matters as a result of the stricter regulations and environmental initiatives carried out by the government. Since then, the coking industry has introduced the process of desulfurization, denitrification, special emission limit modification, coke quenching wastewater treatment, unorganized treatment, and VOCs treatment. Furthermore, the goals of the 14th Five-Year Plan for the industry encourage the control of the salinity of discharged wastewater, which has the potential to further promote the process of zero discharge of coking wastewater. The FYP for the coking industry further requires that by 2025, the production of coking wastewater be reduced by 30%, and the production of nitrogen oxides and sulfur dioxide by 20%.

The structure of China's coking industry is dominated by independent coking plants that account for 2/3 of the country's coke production. Different from the coke dry quenching + system of steel coke enterprises, the production wastewater of traditional coking plants is usually used for wet coke quenching after pretreatment to reach the standard. The Shanxi Provincial Government also issued a series of control measures in 2018, clearly requiring coking enterprises to support dry quenching and formulating zero discharge measures for coking production wastewater.

On January 1, 2021, the Shanxi local standard "Integrated Wastewater Discharge Standard" came into effect, clearly requiring the salt content to be less than 1000mg/L. Strict salt discharge restrictions will directly promote a new round of upgrading of high-salt wastewater including the coking industry, including near-zero discharge or zero-discharge treatment.

In 2018, the General Office of the Shanxi People's Government issued a notice on the implementation plan for the transformation and upgrading of the coking industry in Shanxi Province. The implementation plan mentioned the need to develop zero discharge measures for coking production wastewater in the improvement of the standards for new coking projects.

In November 2020, the three national ministries and commissions jointly issued a notice on further strengthening the management of environmental impact assessment of coal resource development, clearly stating that the salt content should not exceed 1000 mg/L. At the beginning of this year, Shanxi Province's local standard "Comprehensive Wastewater Discharge Standard" has been implemented.

Coking wastewater zero discharge technology usually combines membrane method and thermal method to finally achieve salt crystallization. At present, high-salt wastewater such as coking wastewater is generally treated with a pretreatment→concentration→crystallization system to achieve zero or near-zero discharge, and produce salt solids for disposal or recycling. The zero-discharge process mainly includes the membrane method and thermal process: the thermal process is a traditional zero-discharge process, energy consumption increases with the increase of wastewater concentration, high cost, and high energy consumption. As an economical and energy-saving pre-treatment process for crystallization, the membrane process has been favored and extensively explored in the zero-emission concentration stage, and has been rapidly developed in China, as China is already at the world's leading level in the field of the membrane process.