Let's start by pulling, what is reverse osmosis concentrate? Simply put, it refers to the water that has not passed through the membrane and has been "intercepted" when filtered by reverse osmosis equipment. This water is not clean, with salt, organic matter, heavy metals, and other substances several times more concentrated than the original water. If it is directly discharged, whether into the river or underground, it will cause pollution, and environmental protection will definitely not agree. So, this reverse osmosis concentrated water treatment is a must solve problem. Today, let's have a good talk with everyone about what reliable methods are available for treating concentrated water.
First of all, when dealing with reverse osmosis concentrate, it's not just about finding a solution. You need to first look at the condition of the concentrate itself. For example, how high is the salt content inside? Are there any particularly difficult pollutants to handle? Also, after processing, how do you want to use the water? Do you want to continue using it for recycling, or just discharge it directly to meet the emission standards? There are significant differences in the processing methods chosen for different needs.
Let's first talk about the commonly used "reduction" treatment, which means reducing the amount of concentrated water to save effort in subsequent processing. The most common method is "reverse osmosis concentrated water re concentration", which simply means sending the concentrated water from the first reverse osmosis to a new set of reverse osmosis equipment and filtering it again. In this way, there will be an additional amount of clean water, and the remaining concentrated water will have a higher concentration, but the amount will be much less. However, there is a problem here. When the concentration of concentrated water is high, the salt inside tends to crystallize and stick to the reverse osmosis membrane. Over time, the membrane will become useless. So generally, pre-treatment needs to be added beforehand, such as adding scale inhibitors to prevent salt scaling, and the membrane needs to be cleaned regularly to ensure that the equipment can work normally.
There is also a method of reduction called "evaporation concentration", as the name suggests, which relies on heating to evaporate the water in concentrated water, leaving behind even stronger saltwater or solid residue. This method is suitable for handling concentrated water with particularly high salt content. No matter how much salt is in it, it can be concentrated to a very small amount. But it also has its drawbacks, it's too energy consuming! Heating requires a large amount of electricity or steam, and the cost is not low. And during evaporation, some organic matter in the concentrated water may decompose, producing harmful gases that need to be treated, otherwise it will cause air pollution. There are also improved evaporation technologies now, such as multi effect evaporation and mechanical vapor recompression (MVR), which can save a lot of energy compared to traditional evaporation methods. However, the equipment investment is relatively large and needs to be weighed according to the actual situation.
After reduction, the amount of concentrated water is reduced, but the pollutants inside are still present. The next step is to "harmless" treatment, either removing the pollutants or turning them into harmless substances. The most commonly used method is "advanced oxidation technology", which is mainly used to treat organic matter in concentrated water. Its principle is to generate something called "hydroxyl radicals", which are particularly active and can decompose organic matter into carbon dioxide and water, or into smaller molecules that are easier to handle. Common advanced oxidation technologies include ozone oxidation, Fenton oxidation, photocatalytic oxidation, and so on. For example, Fenton oxidation involves adding ferrous sulfate and hydrogen peroxide to concentrated water to produce hydroxyl radicals under acidic conditions, which are particularly effective in dealing with difficult to degrade organic compounds. However, when using this method, it is necessary to control the dosage of chemicals and reaction conditions, otherwise either the treatment will not be thorough, or the chemicals will be wasted, and sludge will also be generated. The subsequent treatment of sludge is another matter.
If the concentration of heavy metals in concentrated water is high, then the "chemical precipitation method" comes in handy. It is to add chemical agents such as lime, sodium hydroxide, and sodium sulfide to concentrated water, allowing the agents to react with heavy metal ions to form insoluble precipitates. Then, the precipitates can be separated by precipitation and filtration, reducing the amount of heavy metals in the water. For example, when dealing with concentrated water containing chromium, adding sodium sulfide can generate chromium sulfide precipitate, and when dealing with concentrated water containing lead, adding lime can generate lead hydroxide precipitate. However, this method will generate a lot of sludge, which contains heavy metals and is classified as hazardous waste. It must be handed over to qualified units for treatment and cannot be thrown away casually, otherwise it will still cause secondary pollution.
There is another method for treating heavy metals called "adsorption method", which uses materials with adsorption capacity, such as activated carbon, zeolite, ion exchange resin, nanomaterials, etc., to "absorb" heavy metal ions from concentrated water onto the surface of the material, thereby achieving the goal of removal. For example, activated carbon has a particularly large surface area and strong adsorption capacity. In addition to adsorbing heavy metals, it can also adsorb some organic matter. Ion exchange resin is more precise, capable of selectively adsorbing certain heavy metal ions, and can be regenerated and reused after saturation, making it more economical. However, adsorbent materials have a certain adsorption capacity. Once they are fully adsorbed, they are useless and need to be replaced or regenerated regularly. If the concentration of heavy metals in concentrated water is too high, the adsorbent material will quickly become saturated, and the processing cost will increase.
In addition to reduction and harmlessness, we now advocate for "resource utilization", which means extracting useful substances from concentrated water and turning waste into treasure. For example, isn't there a lot of salt in concentrated water? Salt can be extracted using membrane separation or evaporative crystallization methods to produce industrial salt. For example, using the evaporation crystallization method, concentrated water is heated and evaporated to crystallize salt, and then purified to obtain industrial standard sodium chloride and potassium chloride salts, which can be used in industries such as chemical and building materials. However, when extracting salt, it is necessary to ensure that there are no toxic or harmful substances in the concentrated water, otherwise the extracted salt cannot be used. Moreover, the purification process also requires technology to remove impurities from the salt, which is not low in cost. However, in the long run, it can solve pollution problems and recover resources, making it very cost-effective.
Also, the treated water can be recycled if it meets the corresponding standards. For example, the treated water can be used to water plants, plant grass, spray road surfaces, or as supplementary water for industrial circulating water, such as cooling water for steel plants and power plants. In this way, not only does it reduce the amount of fresh water used, but it also reduces the amount of wastewater discharged, killing two birds with one stone. However, before reuse, the water quality needs to be tested according to its intended use to ensure compliance with requirements. For example, when used as cooling water, the hardness and chloride ion concentration of the water must be controlled to prevent equipment corrosion or scaling.
Finally, to summarize with everyone, there is no one size fits all solution for reverse osmosis concentrated water treatment. It is necessary to combine several treatment technologies based on the water quality, quantity, treatment goals, and cost budget of the concentrated water, known as the "combination process". For example, first pretreat to remove some impurities, then use reverse osmosis to concentrate and reduce, then use advanced oxidation technology to remove organic matter, use chemical precipitation to remove heavy metals, and finally recycle the treated water to extract salt for resource utilization. And in the process of handling, attention should also be paid to energy conservation and consumption reduction, reducing secondary pollution, so as to be both environmentally friendly and economical.
Nowadays, environmental protection requirements are becoming increasingly strict, and reverse osmosis concentrated water treatment is also receiving more and more attention. We believe that in the future, more efficient and economical treatment technologies will emerge, allowing concentrated water to truly become a "resource" from "wastewater".
