For many years, the development of China's environmental protection market has mainly focused on water treatment, neglecting solid waste. According to incomplete statistics, the disposal rate of solid waste in China is less than 40% (the accuracy of the data needs further verification). The current approach to handling solid waste generated during wastewater treatment mainly focuses on reduction and harmlessness, resulting in a very low resource utilization rate. However, resource utilization is the ultimate development direction for solid waste treatment.

In recent years, many zero-discharge wastewater projects have been implemented across various regions and industries. A major challenge arising from these projects is the disposal of the resulting salt. Sending it to solid waste disposal sites is not only expensive (>3000 yuan/ton) but also carries the risk of secondary environmental pollution. Different industries produce different types of salt. This article briefly introduces the resource utilization of sodium chloride and sodium sulfate generated during wastewater treatment. The following summary is based on the consulted materials and personal understanding:

1) Resource utilization direction of sodium sulfate

2) Resource utilization direction of sodium chloride

For zero-discharge projects that have already been implemented and produce byproduct salts according to standards such as "Coal Chemical Industry By-product Industrial Sodium Chloride" (T/CCT002-2019) or "Industrial Salt" (GB/T5462-2015), and "Coal Chemical Industry By-product Industrial Sodium Sulfate" (T/CCT001-2019), the resource utilization direction can be determined according to the table above.

For future new zero-discharge wastewater projects, the focus of research will be on achieving salt resource utilization during wastewater treatment, as shown in the figure below:

Salt Resource Utilization Process Route in Zero-Discharge Wastewater Treatment

1) For sodium chloride solution produced after nanofiltration salt separation, with a concentration in the range of 3%-5%, sodium hypochlorite can be directly produced using a sodium hypochlorite generator (concentration 0.5%-0.8%). The scale of the sodium hypochlorite generator can be calculated based on the amount of sodium hypochlorite used in the entire zero-discharge wastewater system (salt consumption 4.5kg salt/kg sodium hypochlorite), achieving self-sufficiency of sodium hypochlorite in the system;

2) For sodium chloride and sodium sulfate solutions produced by nanofiltration salt separation (total divalent and above divalent cations <1ppm, which is more difficult), after concentration to a salt solution concentration of 10%-15%, bipolar membranes are used to produce acids and bases. The concentration of the produced acid is approximately 10%-15%. The treatment scale of the bipolar membrane can be determined according to the consumption of acids and bases in the entire water treatment system. The energy consumption per ton of water is between 120-180 kWh, and the investment per ton of water is estimated to be between 3-5 million yuan.

3) The above two methods are difficult to scale up to completely achieve resource utilization of all the salt produced by the system. The way to achieve complete resource utilization of all the salt produced by the system is to produce sodium carbonate and ammonium sulfate based on the principles of Solvay and joint soda ash production. The market price of sodium carbonate is above 3500 yuan/ton, and that of ammonium sulfate is around 1000 yuan/ton. There are already operating cases of this method in China, which greatly increases the added value of byproduct salt. However, this method is based on the availability of carbon dioxide resources around the project, such as flue gas from industrial kilns (carbon dioxide concentration between 15%-35%). Otherwise, the source of carbon dioxide will significantly affect the investment and operating costs of the project.

The significance of resource utilization of industrial waste salt is substantial, mainly in two aspects: it can solve the problem of land occupation caused by landfill and avoid secondary pollution; and it can achieve resource utilization of salt, reducing the mining of mineral resources. With the continuous improvement and perfection of industrial waste salt resource utilization technology and equipment, the future resource utilization of industrial waste salt will become increasingly mature and stable.