In the process of chemical wastewater treatment, many enterprises produce high-salt wastewater due to process requirements. Under the expectations, scrutiny, and even requirements from both inside and outside the industry, the final treatment of high-salt wastewater is described as "zero discharge" in many companies' environmental impact assessments.

So what exactly is "zero discharge"? Does "zero discharge" mean that as long as the water is recycled and there is zero external discharge, it is considered zero discharge? How should the treatment of the final solid crystalline salt be defined in the "zero discharge" water treatment process?

The publication of the monograph "Zero Discharge—The Dream and Reality of the Post-Industrial Society," edited by Song Ruixiang, former vice-minister of the State Environmental Protection Administration, provides an authoritative interpretation of zero discharge. The literal meaning of zero discharge is zero waste in the production of industrialized products, referring to activities that infinitely reduce the amount of pollutant emissions to zero. However, in the process of treating pollutants, if the final solid crystalline salt is not treated, it undoubtedly produces new solid pollutants, and the so-called "zero discharge" becomes a false proposition after the concept is replaced.

Although many companies' environmental impact assessments describe the wastewater treatment process as "zero discharge," in practical applications, only a few companies have the conditions to truly achieve zero discharge. For example, in some industries, the salt content of wastewater is relatively simple in composition. Through membrane concentration, evaporation concentration, and evaporation crystallization technologies, high-quality solid salt is obtained, and some even have good sales. The distilled water is completely recovered and reused, and the salt is transformed into a product, thus achieving true "zero discharge of solids and liquids."

However, for most industries, due to the complex salt content of wastewater, it is very difficult to achieve "zero solid waste" discharge simply by simple evaporation. If the salt is not separated, the final solid mixed salt should basically be treated as solid hazardous waste. The current cost of treating solid hazardous waste in China is basically over 3000 yuan/ton, which is even higher than the sum of the costs of the membrane concentration, evaporation concentration, and evaporation crystallization processes. Therefore, in the design of the entire high-salt wastewater treatment process, if the final solid product can be sold as a product instead of being disposed of as hazardous waste, it will have great application value.

Here, another concept is applied, namely, "salt separation."

Salt separation refers to the process of separating salts to obtain different salt products based on the different solubilities of various solutes at the corresponding temperature and using phase diagram theory. Existing salt separation processes generally include two types: thermal salt separation and cold salt separation. Thermal salt separation is most widely used in "potassium-sodium separation (potassium chloride and sodium chloride)" and "salt lake chemical salt-nitrate separation (sodium chloride and sodium sulfate)". Cold salt separation is more widely used in "chlor-alkali electrolysis of sodium chloride solution denitrification," "AC foaming agent production process sodium carbonate removal," and some reaction solution purification. Of course, some production processes can use both thermal salt separation and cold salt separation, such as "salt-nitrate separation."

So, what are the principles for "salt separation" of the final concentrated solution? First, it must meet the needs of the salt separation process, which is the most basic requirement and purpose of salt separation; second, it must meet the economic needs of investment and return; third, the balance between the purity of the final salt and the amount of discharged mixed salt should be considered.

Meeting the needs of salt separation processes

First, in chemical wastewater, due to the quality of the raw water in the production enterprise, after the wastewater is pretreated and concentrated, the main salt content is often sodium sulfate and sodium chloride. Sometimes, there are other salts such as sodium nitrate. We will take sodium sulfate and sodium chloride as an example to discuss the salt-nitrate separation process. Thermal salt-nitrate separation is a relatively mature process and has been widely used in salt lake chemical industry for many years. Through process control, industrial-grade sodium sulfate and sodium chloride can be obtained; while cold salt-nitrate separation has been increasingly promoted with the promotion of DTRO membrane high-pressure concentration technology and NF/RO combined technology, especially the NF/RO combined technology can obtain a concentrated solution with a salt concentration as high as 16% while significantly reducing the investment and operating costs compared to the DTRO membrane technology, making cold salt separation more advantageous and valuable in terms of both investment and operating costs than thermal salt separation. In particular, the purity of the sodium chloride solution after nanofiltration is above 98.5%, and the impurities enriched in the mother liquor will not precipitate with mirabilite after the divalent ion side is frozen and crystallized, so the purity of mirabilite itself will be very high. After mirabilite is melted and recrystallized, anhydrous sodium sulfate with higher purity can be obtained. However, in thermal salt separation, the impact of impurity enrichment on evaporation is relatively greater, which will be mentioned in the third point.

Meeting the economic needs of investment and return

If the water inflow is very small or the salt concentration of the inflow is very low, it is relatively uneconomical to invest a large amount of money in salt separation. We generally compare the three-year cost of hazardous waste treatment with the investment cost of salt separation. If the former is greater, salt separation investment is suitable; if the latter is greater, direct evaporation for mixed salt hazardous waste treatment is suitable.

The balance between the purity of the final salt and the amount of discharged mixed salt

Generally speaking, under reasonable process conditions, obtaining higher purity salt means that more mixed salt mother liquor needs to be discharged. If the treatment cost of the mixed salt mother liquor and the final disposal cost of the solid mixed salt are too high, it means that simply increasing the purity of the salt product has paid a higher price for the treatment of mixed salt. Therefore, whether the salt separation process is reasonable from an economic point of view is not that the higher the purity of the salt obtained, the better, but to obtain a relatively reasonable salt purity and minimize the amount of mother liquor discharged to prevent an increase in the final total cost.

Of course, regarding the third point above, some people may argue that after the final discharged mother liquor is evaporated into mixed salt, solid hazardous waste is still obtained. This actually returns to the question of whether "zero discharge" is always reasonable, as discussed at the beginning. Relatively speaking, "zero discharge" is an idealized concept, and truly achieving zero discharge requires relatively ideal conditions. Simply pursuing "zero discharge" without considering the cost is not necessarily reasonable. Relatively speaking, maximizing "reduction of pollutant emissions" should be the mainstream process for high-salt wastewater treatment.

As an aside, a critical hidden danger in current high-salt wastewater treatment is the ultimate fate of the mixed solid salts. Due to the high cost of handling them as hazardous waste, some companies stockpile or landfill the mixed salts, which will inevitably cause significant impacts on soil, groundwater, and the environment over time. Other companies entrust the disposal of mixed salts to illegal operators, some of whom adulterate other industrial products with the salts, while others even process the waste salt into edible salt. The 'pesticide waste salt' incident, where such salt entered the food chain, is a typical example. Therefore, while strengthening wastewater treatment supervision, the country must also strengthen the management of the final solid salts from wastewater treatment, guide the adoption of salt separation processes to produce qualified industrial products from wastewater salts, and strictly regulate the ultimate fate of mixed solid salts. Otherwise, the solid salts discharged by thousands of polluting enterprises will ultimately cause serious environmental and social problems.