Wastewater

In the process of chemical wastewater treatment, many enterprises are required to produce high-salt wastewater due to the process needs. In the EIA report of many enterprises, the final treatment of high-salt wastewater has been described under the expectation, examination and even request of the industry, For "zero emissions."

So what exactly is "zero emissions"? Does "zero emissions" mean zero emissions as long as the water is recycled? How should the definition of the final solid crystalline salt be handled in "zero emissions" water treatment? What?

In 2004, the book "Zero Emissions - Dreams and Reality of Post-industrial Society", a monograph edited by Song Ruixiang, former deputy director of the State Environmental Protection Administration, made an authoritative interpretation of zero discharge. The so-called zero emissions, the literal meaning of industrial products in the production of waste zero, refers to an unlimited reduction of pollutant emissions until zero activity. In the process of dealing with pollutants, if the final solid crystalline salt is not treated, it undoubtedly produced new solid pollutants. So-called "zero emission" has become a false proposition after the concept of substitution.

Although many EIA reports are described as "zero-emission" in wastewater treatment, in actual applications, only a few enterprises have the real working conditions that will ultimately achieve zero emissions. For example, in some industries, the salty wastewater has a relatively good quality due to relatively simple components, and finally obtained good quality solid salt by membrane concentration, evaporation and concentration, evaporation and crystallization processes, and finally the distilled water is finally recovered Reuse, salt has also been transformed into products, so as to achieve a real "solid and liquid zero emissions." However, for most industries, due to the complexity of wastewater salt, it is very difficult to eventually achieve "zero solid waste" efflux simply by simple evaporation. If the salts are not salt-treated, the final solid salt The basic should be treated as solid hazardous waste disposal, and the current domestic treatment of solid hazardous waste costs in the basic 3,000 yuan / ton, the price even higher than the previous membrane concentration, evaporation and concentration, evaporation and crystallization costs of the various sections even higher. Therefore, it would be of great value to design the entire high-salt wastewater treatment process if the final solid product could be sold as a product rather than end-of-life.

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

The so-called sub-salt treatment, that is, according to the solution temperature corresponding to the different solute solubility difference, the use of phase diagram theory of salt separation resulting in different salt products process. The existing sub-salt treatment process is generally sub-salt hot and cold sub-salt two processes. Thermal salt separation process is widely used in such fields as potassium and sodium separation (potassium chloride and sodium chloride) and Salt Lake chemical salt and nitrate separation (sodium chloride and sodium sulfate) Electrolytic sodium chloride solution denitrification "," AC blowing agent production process of sodium carbonate removal "and some of the reaction liquid purification and other fields more applications, of course, some of the production process can be used both thermal method of sub-salt and salt can be used cold method, Such as "salt and nitrate separation."

So if the final concentrate "salt treatment", the treatment principle is what? The first point, of course, to meet the needs of salt separation process, which is the most basic requirements and the purpose of salt; the second point, It is to meet the economic needs of investment and earnings; the third point, we should consider the final balance of salt purity and efflux of salt.

First of all, first of all, in chemical wastewater, due to the local raw water quality of production enterprises, often after the pretreatment of wastewater, the concentrated salt concentrate showed its main salt is sodium sulfate and sodium chloride, of course, sometimes there will be Other salts such as sodium nitrate, we temporarily sodium sulfate and sodium chloride as an example for the separation process salt and nitrate. Salt and nitrate thermal separation process has been a relatively mature technology in Salt Lake chemical industry has also been widely used for many years, through the process control can eventually be industrial-grade sodium sulfate and sodium chloride; and cold salt and nitrate separation with the DTRO membrane The promotion of high-pressure concentration process and NF / RO combination process has also been increasingly promoted. In particular, the NF / RO combination process can obtain a concentration of up to 16% of salt concentration under a significantly lower investment and operating cost than the DTRO membrane process Liquid, making cold salt separation in terms of investment or from the operating costs of the hot method has a greater advantage and promotion of value, especially after the sodium chloride solution purity of more than 98.5%, while the divalent ion-side freezing Glauber's salt crystals obtained after crystallization, mother liquor-enriched impurities will not be precipitated with Glauber's salt, so Glauber's salt itself will be very high purity, and Glauber's salt by melting and recrystallization can be more pure anhydrous sodium sulfate. The use of thermal salt separation enrichment of impurities on the impact of evaporation is relatively greater, we will discuss the third point mentioned.

The second point, considering the overall economic benefits of investing. If the amount of water inflow is very small or the salinity of inflow water is very low, the economy will be relatively poor if we devote more funds to salt treatment. Generally, we will carry out the three-year hazardous waste treatment cost and salt investment cost For comparison, if the former is larger, the salt investment is suitable, and if the latter is larger, it is suitable to use direct evaporation to treat salt mixed waste.

The third point is about the balance between the purity of the final salt and the amount of salt mixed with the efflux. In general, obtaining more pure salt at reasonable processing conditions means that more of the mixed salt mother liquor is required for efflux, and if the cost of treating the mixed salt mother liquor and the cost of disposal of the final mixed salt solids is too high, then This means that simply increasing the purity of the salt product imposes a higher salt treatment cost, so it is not economically justifiable to obtain a salt that is not as pure as possible, but rather as a relatively reasonable salt purity , Minimizing the amount of effluent mother liquor to prevent the resulting increase in total combined costs.

Of course, when it comes to the third point mentioned above, some people may come up with the idea of ​​whether the discharge of the mother liquor from the effluent will eventually get solid wastes after evaporation from salt water. In fact, we are back to the question of whether the "zero emission" mentioned in the opening paragraph is a reasonable one. Relatively speaking, "zero emissions" is an idealized concept, and truly zero emission needs relatively idealized conditions to be realized. However, it is not always reasonable to chase "zero emissions" without regard to costs. In contrast, it is more feasible to achieve the maximum "emission reduction of pollutants" and the emission reductions should become the current The mainstream of high salt wastewater treatment process.

To put it in a nutshell, it is also a very serious risk that exists in the current high-salt wastewater treatment process, namely the final trend of solid mixed salt. Due to the high cost of treatment as hazardous waste, some companies accumulate mixed salt or landfill. As a result, natural conditions will have a significant impact on soil, groundwater and the environment. Some enterprises will give mixed salt to some unscrupulous personnel for disposal. The mixed salt mixed with other industrial products counterfeit use, and some even waste salt processed into edible salt, such as found in the "pesticide residue salt" into the table is a typical case. Therefore, while strengthening the regulation of sewage treatment, the state must strengthen the management of the final solid salt after sewage treatment and guide the salt water treatment to make the salt of wastewater into qualified industrial products and strictly control the final trend of solid mixed salt, otherwise, the quantity will be thousands The solid salts emitted by tens of thousands of polluting enterprises will eventually cause serious environmental and social problems.