Summary of Key Technologies and Methods for Zero Discharge of High-Salt Wastewater

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What are the methods for treating high-salinity wastewater? At present, there are dozens of methods for treating high-salinity wastewater, including thermal methods, membrane methods, ion exchange methods, hydrate formation methods, solvent extraction methods, and freezing methods. Among them, thermal and membrane desalination technologies are the main methods currently used for large-scale industrial applications. Thermal methods can mainly be divided into multi-stage flash (MSF), multi-effect evaporation (MED), and vacuum distillation (VC). In the 1990s, the primary technology for seawater desalination was multi-stage flash distillation, especially in the Middle East**; however, MSF later faced significant competition from multi-effect evaporation and membrane technologies. Membrane desalination and water purification technologies, represented by RO technology, do not require large amounts of thermal energy, making them suitable for saltwater desalination on a large, medium, or small scale. For the zero-discharge treatment of high-salt wastewater, direct evaporation crystallization can achieve zero discharge, but it is extremely costly and energy-intensive, as well as wasteful of resources. Membrane technology can be used to further concentrate high-salinity wastewater into ultra-high-salinity wastewater; the fresh water component can be reused directly, while the concentrated ultra-high-salinity wastewater can be subjected to evaporation and crystallization to achieve zero discharge. This approach significantly reduces energy consumption while making rational use of some of the water resources. However, membrane technology has certain requirements regarding the quality of the incoming water. Therefore, high-salt wastewater must undergo pretreatment (such as chemical softening, filtration, ion exchange, etc.), which effectively reduces membrane fouling and improves both the service life of the membranes and the quality of the treated water. Key technologies for zero discharge of high-salt wastewater. Based on the analysis above, the key technologies for zero discharge of high-salt wastewater can be divided into three stages: pretreatment stage, membrane treatment stage, and evaporation stage. 1. Pretreatment: Hardness is divided into total hardness, temporary hardness, and permanent hardness. Among them, total hardness refers to the total amount of Ca2+ and Mg2+ in water. Temporary hardness, also known as carbonate hardness, has Ca(HCO3)2 and Mg(HCO3)2 as its main chemical components. Since these salts decompose into precipitates and are removed from water upon heating, it is called temporary hardness. Permanent hardness, also known as non-carbonate hardness, mainly refers to salts in water such as CaSO4, MgSO4, CaCl2, MgCl2, Ca(NO3)2, and Mg(NO3)2. This type of hardness cannot be removed by heating, which is why it is called permanent hardness. Hardness is an important indicator of water quality, and the removal of hardness from water is known as water softening. Currently, water softening mainly includes methods such as precipitation softening, enhanced crystallization technology, as well as adsorption and ion exchange methods. Chemical softening methods mainly include traditional chemical softening methods and the method of producing carbonate precipitates through the biodegradation of urea. Traditional chemical softening methods are further divided into lime softening, lime-gypsum softening, and lime-soda softening, among others. The disadvantage of such methods is that they may cause secondary pollution, and the cost of the chemicals is high, resulting in increased expenses. The method of producing carbonate precipitates from biodegradable urea relies on a series of biochemical reactions in which biological enzymes break down urea, resulting in the formation of carbonate precipitates, which are then removed through filtration. The disadvantage of this method is that the concentration of ammonium ions generated during the reaction is high, which in turn increases the costs associated with subsequent processing. The use of fluidized bed technology to remove hardness from water in enhanced crystallization processes first emerged in the 1990s. The basic principle of a fluidized bed is to use gas or liquid to keep solid particles in a suspended state. A researcher used aeration of the wastewater to raise its pH value in order to enhance crystallization, resulting in removal rates of 65%, 51%, and 34% for phosphates, Mg2+, and Ca2+, respectively. Today, solid particles such as granular calcite (CaCO3) and quartz sand are primarily added to fluidized bed reactors. The advantage of this is that it not only enables the effective removal of calcium and magnesium ions but also allows for the recycling of the resulting precipitates containing calcium and magnesium. Adsorption and ion exchange methods: Ion exchange for hardness removal is mainly used prior to membrane treatment, to completely or partially remove Mg2+ and Ca2+ from water. Since the 20th century, the study of low-cost, renewable adsorbents has been a key focus in the fields of adsorption and ion exchange. Abroad, people have achieved good results using alginates for the adsorption of Mg2+ and Ca2+ ions in water, and this approach has been widely adopted. This non-toxic polysaccharide alginate is extracted from brown algae. At the same time, some people have also used chemically modified sugarcane honey and mercerized cellulose to remove Mg2+ and Ca2+ from water, achieving relatively significant removal effects. Ion exchange resin is another material used for water softening; it is a polymer equipped with appropriate functional groups. Raw water is passed through an ion exchange resin adsorption column, where the Mg2+ and Ca2+ ions in the water exchange places with the cations on the resin, thereby removing the hardness from the water. Currently, scholars are developing various types of resins. Among them, the American company Orica Watercare has developed a magnetic weak-acid cation exchange resin that is highly effective at removing hardness. 2. Membrane technology: In the 1980s, membrane processes such as reverse osmosis, ion exchange, microfiltration, ultrafiltration, and nanofiltration gradually entered the stage of widespread application. The emergence and application of membrane technology have comprehensively improved water treatment technologies. To date, with the comprehensive development of membrane technology, many new technologies have emerged. Among them, the new type of polyvinylidene fluoride (PVDF) hollow fiber hydrophobic membrane can achieve a desalination efficiency of 99.9%, and the COD level of the effluent can be maintained within the range of 30–40 mg/L. Similarly, a new membrane separation technique—vacuum membrane distillation—is applied to the re-concentration of high-concentration solutions and the removal of Mg2+ and Ca2+. The advanced treatment technologies for low-hardness water mainly include RO/electrodeionization (EDI), reverse electrodialysis (EDR), electrodialysis (ED), and reverse electrodeionization (EDIR), among others. It is worth noting that the RO/EDI (also known as packed-bed electrodialysis) softening technology refers to a water treatment process that removes calcium and magnesium ions from water under the action of an external direct current field. This technology features high efficiency in removing hardness, continuous water production, and the absence of the need for regeneration chemicals. Nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF): Since the operating range of nanofiltration lies between that of ultrafiltration membranes and reverse osmosis membranes, and it is capable of retaining substances at the nanoscale (0.001 microns), it is referred to as “nanofiltration”. Its molecular weight for retaining organic substances is around 200–800 MW, and its ability to retain dissolved salts ranges from 20% to 98%. The removal efficiency for soluble monovalent ions is lower than that for polyvalent ions. Nanofiltration is generally used to remove organic substances and pigments from surface water, as well as hardness and radium from groundwater; it also helps to remove some dissolved salts. It is utilized in the food and pharmaceutical industries for the extraction and concentration of useful substances. The advantage is low operating pressure and a large throughput. Nanofiltration technology offers significant advantages and unique energy-saving effects in areas such as the desalination and purification of organic substances as well as water softening. Ultrafiltration can retain substances larger than 0.01 microns, allowing small molecules and soluble solids (inorganic salts) to pass through, while removing large molecular organic compounds, colloids, proteins, and microorganisms. It utilizes the microporous separation mechanism of ultrafiltration membranes and is mainly applied in drinking water treatment, industrial wastewater treatment, and the production of high-purity water. Microfiltration also utilizes the sieving mechanism of microfiltration membranes; under pressure, it retains viruses, particles, and other substances with sizes between 0.1 and 1 μm. Microfiltration can retain particles larger than 0.1–1 microns, allowing large molecules and soluble solids (inorganic salts) to pass through, but it retains suspended matter, bacteria, and colloids with high molecular weights. The operating pressure of microfiltration membranes is generally 0.3–7 bar. The separation mechanism of microfiltration membranes is primarily based on sieving and retention; it offers the advantages of low operating pressure and high membrane flux, but microfiltration membranes are generally prone to contamination and have a short service life. Ultrafiltration is applied in fields such as pharmaceuticals, chemicals, and water treatment. Microfiltration is widely used in water pretreatment, and it is also applied in fields such as pharmaceuticals, chemicals, and electronics. Ultrafiltration and microfiltration are also used in the treatment of high-salinity wastewater, but they are generally employed as pretreatment methods. Reverse Osmosis (RO): Reverse osmosis, also known as hyperfiltration, is a membrane separation process that uses a pressure difference as the driving force to separate the solvent from a solution. Currently, reverse osmosis technology has achieved good results when applied to pre-desalination treatment. After reverse osmosis treatment, 99.5% of the magnesium and calcium ions in the water, as well as 99% of the salts in the water, can be removed. It can reduce the load on ion exchange resins by over 90%, and also cut the amount of regenerant used for the resins by 90%. Therefore, it not only saves costs but also contributes to environmental protection. Reverse osmosis technology can also be used to remove particles, organic substances, and colloids from water, which helps to reduce the contamination of ion exchange resins and extend their service life. As membrane production technology becomes more mature and costs decrease, reverse osmosis also plays a significant role in the treatment of high-salinity wastewater. However, once the conductivity of high-salt wastewater exceeds 25,000 us/cm, the membrane flux declines rapidly, and scaling on the membrane becomes quite severe. It is worth noting that by combining the reverse osmosis process with efficient crystallization technology, it is possible to increase the volume of water that can be treated through reverse osmosis, extend the service life of the membranes, and handle more high-salinity wastewater. Osmotic forward (FO) has special advantages because its operating principle differs from that of traditional membranes. For example, membrane devices have a simple structure and are easy to operate ； Forward osmosis membranes require low or even no pressure, thus saving energy and reducing operating costs ； Osmotic forward process has a strong ability to separate pollutants, with a very high salt rejection rate ； The fouling of forward osmosis membranes is almost reversible, and the cleaning efficiency is relatively high. Under ideal conditions, a forward osmosis membrane requires an active layer with high retention efficiency, good hydrophilicity, and high water flux; the support layer, on the other hand, should have thin thickness, low tortuosity, high porosity, and high mechanical strength. It should also possess strong resistance to contamination and be applicable in various fields. The osmotic membranes used in early studies were mainly reverse osmosis membranes and modified nanofiltration membranes. As research progresses, it has been found that due to the thick porous support layer in reverse osmosis, the concentration polarization is very high, which leads to a rapid decrease in water flux. Membrane distillation is a membrane separation technique that combines distillation methods with membrane technology. The separation principle of vacuum membrane distillation is that one side is evacuated to create a vacuum, thereby using the pressure difference between the two sides to facilitate the transfer of vapor. The membrane prevents other substances in the solution from passing through, and after distillation, the liquid is condensed to achieve separation or concentration. In the process of vacuum membrane distillation, the operating temperature can be lower compared to other membrane distillation processes, and the permeation flux can be higher; this makes it possible to easily utilize inexpensive heat sources such as geothermal energy, solar energy, and waste heat. In recent years, there has been a growing number of studies on the use of vacuum membrane distillation technology for treating concentrated brine from seawater desalination. Some scholars have conducted studies on vacuum membrane distillation of concentrated brine from RO seawater desalination using polyethylene and polypropylene microporous membranes, respectively. Research shows that the maximum retention rate of the membrane can reach 99.999%, so this technology enables efficient concentration of concentrated brine in RO seawater desalination. This technology uses the pressure difference across the membrane to generate driving force, and it offers advantages such as low mass transfer resistance, high heat utilization efficiency, high separation efficiency, large membrane flux, and no evaporation of permeates. However, when this process is used to treat concentrated brine, it also suffers from problems such as scaling and membrane fouling. 3. Final evaporation technology: The discharge of saltwater at high concentrations can have adverse effects on the environment. There are two main reasons for this effect; one is the high concentration of the saltwater ； Second, saltwater contains more components. One of the goals of evaporation technology is to reduce the volume of highly concentrated saltwater, thereby causing the salts within it to crystallize ； On the other hand, it aims to create a circular industrial economy by supplying the extracted salts to manufacturers that use them as raw materials, thereby achieving a \"zero emissions\" goal. Natural evaporation: The principle of natural evaporation involves using sunlight to remove water from the saltwater with a high concentration in the pool, thereby bringing it to its saturation point for crystallization and allowing the salt to precipitate out. This type of setup is known as an “evaporation pond”. The device is powered by sunlight, which makes it suitable for use in areas that are relatively dry, have low annual rainfall, and abundant solar radiation. This facility has the following advantages: since its energy comes from sunlight, the heat source is not subject to limitations related to wear and tear, daily maintenance is relatively easy, the cost of treating high-concentration saline is low, and it can withstand fluctuations in load. The downside is that the “evaporation pond” system is an open-type device, and the volatile components in the concentrated brine can easily enter the atmosphere, causing air pollution. At the same time, the sides of the \"evaporation pond\" system as well as the underlying impermeable barriers are also crucial; if not handled properly, they can cause severe pollution to the rock and soil masses and underground water sources ； Generally, “evaporation ponds” require a large amount of land, and their use in areas with limited land resources can result in certain waste ； During the operation of “evaporation ponds”, the fresh water produced through evaporation is difficult to utilize, resulting in some waste. Thermal zero-discharge technology is developed based on thermal brine desalination systems. Due to its relatively low energy consumption, multi-effect distillation is one of the three most commonly used brine desalination technologies today. Building upon this technology, the theory of multi-effect distillation and evaporative crystallization has been developed and is being increasingly applied. Foreign scholars have explored “zero-emission” systems utilizing evaporative crystallization. The evaporated steam is used to heat the water fed into the evaporator; its efficiency is much higher than that of conventional evaporative crystallization facilities. Multi-effect evaporation (MEE) is generally carried out with 3 to 6 effect evaporations; too few effects result in insufficient energy savings, too many effects lead to inadequate temperature differences, and overly long systems are prone to problems. The first-stage evaporator is heated by steam, while the subsequent evaporators use the secondary steam generated by the previous evaporator as a heat source, thereby enabling multiple reuse of thermal energy – this is what is known as multi-effect evaporation. Compared to multi-stage flash evaporation, multi-effect evaporation suffers from more severe scaling.