The application of alkali addition in reverse osmosis systems

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Application of Alkali Addition in Reverse Osmosis Systems 1 Introduction Reverse osmosis (RO) is a highly effective membrane separation process, which relies on reverse osmosis membranes to separate the solvent from the solutes in a solution under pressure. Reverse osmosis technology can reduce the load on ion exchange resins by over 90%, and it can also cut the amount of regenerant needed for these resins by 90%. In addition to desalination, it is capable of removing particles, organic substances, and colloidal materials from water, which helps to reduce contamination of the ion exchange resins and extend their service life. However, for a long time, reverse osmosis technology has seemed ineffective in removing free CO2 present in water, as traditional reverse osmosis systems require controlling the LSI value (i.e., the Langley saturation index) of the feed water in order to prevent scaling on the reverse osmosis membranes. An important parameter for controlling the LSI value is the pH level of the incoming water; if the pH is acidic, scaling is less likely to occur, whereas if it is alkaline, the tendency to scale is quite significant. Therefore, in traditional processes, adding HCl solution before the reverse osmosis system effectively prevents the formation of scale caused by precipitates such as CaCO3 and MgCO3 ； And NaOH solution is rarely added before the reverse osmosis system. 2 Common methods for removing CO2 and their reaction mechanisms 2.1 CO2 removal towers The most widely used device for this purpose is the carbon dioxide degassing tower. Since water contains a large amount of bicarbonate alkalinity, after treatment through an H-type ion exchanger (i.e., a cation exchange bed), the H+ ions on the resin are replaced by water, resulting in the formation of carbonic acid. Therefore, the CO2 removal tower is usually placed after the cation exchange bed and before the anion exchange bed. When the pH of water is less than 4.3, carbonic acid in the water exists almost entirely in the form of carbon dioxide, as shown in the following reaction: H+ + HCO3- → H2CO3 → CO2 + H2O. As the concentration of H+ increases, that is, as the pH becomes lower, the above reaction proceeds to the right. In this case, water is sprayed from above while air is blown in from below, ensuring thorough contact between the air stream and the water droplets. Since the amount of carbon dioxide in the air is very small, its partial pressure is low – only 0.03% of atmospheric pressure. According to Henry’s law, water treated with an H-type ion exchanger has a high carbon dioxide partial pressure, so this carbon dioxide is carried away by the air stream with its lower partial pressure, thereby removing carbon dioxide from the water and also eliminating a large portion of the anions HCO3- present in it. This helps to **reduce the load on the anion exchange resin, increase the amount of water that can be processed per cycle, and decrease the consumption of regenerants». However, since the carbon dioxide degassing tower draws in air from the production environment, it is inevitable that impurities present in that air will be introduced, contaminating the water source upon contact with it and resulting in a decrease in the water production rate of the anion exchange bed. 2.2 Vacuum degassing tower: The vacuum degassing tower is also a method for removing CO2; in addition, it can remove substances such as O2 from water, and its removal efficiency is much higher than that of the CO2 removal tower. Its working principle involves using a device to spray water from above; after being dispersed by the filter media, the CO2 and O2 in the water escape. A vacuum pump creates a vacuum in the middle of the tower, drawing away the CO2 and O2. The degassed pure water flows to the bottom of the tower, where it is pressurized by a mixed-bed supply pump before entering the post-treatment mixed bed. If the water fed into the vacuum degassing tower is slightly acidic, the carbonates in the water can also be removed in large quantities, as when the pH value of the water is below 4.3, carbonic acid in the water exists almost entirely in the form of carbon dioxide. Since the reverse osmosis output water that has not been degassed is generally slightly acidic, the vacuum degassing tower is usually placed after the reverse osmosis unit. The structure of vacuum degassing towers is relatively complex; major components include the degassing tower itself and vacuum pumps. To ensure uniform dispersion of the water inlet, the height of the filter media layer must be kept high, which results in the degassing tower reaching heights of over ten meters and requiring a large amount of space. Additionally, since liquid level control in the degassing tower is very precise, too high a liquid level can reduce the degassing efficiency, while too low a level can cause the water pump to draw in air, leading to production accidents. Therefore, the level gauges used must be highly sensitive. Although vacuum degassing towers do not introduce new impurities from the outside, their complex structure, large space requirements, and high operating costs impose certain limitations on their use in actual production. In addition to the two commonly used degassing methods mentioned above, there are also methods such as membrane degassing; however, due to their high investment costs, they are generally used only in ultra-pure water systems where very high standards for water quality are required. 3 Methods and Reaction Mechanisms for Removing CO2 by Adding Alkali Before RO 3.1 Reaction Mechanism Reverse osmosis is an efficient desalination process; therefore, it is possible to remove carbon dioxide from water by converting it into bicarbonate. Since the chemical reaction H + HCO3- = H2CO3 = CO2 + H2O is reversible, when the concentration of OH- increases, this reaction proceeds in the reverse direction. When the OH value of water exceeds 8.2, all of the CO2 in the water is converted into HCO3-, and thus all of the HCO3- can be removed through the reverse osmosis system, thereby indirectly achieving the removal of CO2. However, when alkali is added, the scaling tendency of the reverse osmosis membrane becomes very evident, as the LSI value is far above the allowable range. Practice has shown that when alkali is added before the reverse osmosis system without taking any other measures, crystallization in the RO concentrate stream occurs very rapidly; white crystals can typically be observed on the concentrate side within 1 to 2 hours. The reaction equations are: Ca2+ + 2OH- + CO2 = CaCO3 + H2O; Mg2+ + 2OH- + CO2 = MgCO3 + H2O. Therefore, it is necessary to add scale inhibitors before RO to prevent scaling. With the advancement of science and technology, there are now a wide variety of anti-scaling agents for reverse osmosis, and their scaling inhibition efficiency is continually improving. For example, the MDC220 product developed by Hangzhou Impu Water Treatment Technology Company can achieve an LSI value of 3.0 on the concentrated water side. Our workshop uses this scale inhibitor; that is, the inhibitor is added while adding alkali before the RO process. In practice, this allows the reverse osmosis system to require cleaning only once every three months or more, and the problem of scaling caused by alkali has been effectively resolved. 3.2 Problems encountered in practical application and their solutions 3.2.1 Raw water quality The quality of raw water has a significant impact on the reverse osmosis cleaning cycle. Our company uses industrial tap water as the raw water source. Since this tap water is treated water from the eutrophic Taihu Lake, it contains a high level of organic substances, and its properties are greatly affected by seasonal changes; moreover, its conductivity is quite high (above 600 μs/cm). It has been observed that after the RO system operates for a while, the inlet operating pressure increases significantly, while the pressure difference between different stages remains unchanged. The system can return to normal after appropriate cleaning, but this problem reoccurs after some time. Through analytical studies on the addition of alkalis and scale inhibitors to the RO system, we suspected that it was related to changes in the organic matter content in the raw water quality; subsequently, the problem was resolved by replacing the scale inhibitor with another one. If deep well water with high hardness is used as the raw water, and alkali is added before the RO system, although scale inhibitors are used to delay scaling in the system, the tendency for scaling on the RO membrane remains faster compared to when tap water is used as the raw water. Practice has shown that the two-stage membrane generally needs to be chemically cleaned every about 1 month. 3.2.2 Control method: Since NaOH solution is added before the reverse osmosis system, the LSI value becomes much higher than the allowable range, which easily leads to scaling on the second-stage reverse osmosis membranes. If the scale inhibitor is not added in a timely manner, for example due to a failure in the dosing pump or a power outage ； Or the amount added may not be sufficient; for example, if the stroke of the dosing pump is set too low, production accidents can also occur. This imposes stricter requirements on the control method. It is recommended that once a fault occurs in the scale inhibitor dosing pump, the alkali dosing pump should be turned off promptly to prevent rapid crystallization of the RO concentrate. Since CO2 in the water does not have a significant impact on the entire pure water system in a short period of time, and its effect on the capacity of the mixed-bed resin is only noticeable over a longer duration, it is possible to stop the pumps for repair when a fault occurs, without affecting production operations ； In terms of control methods, we can link the scale inhibitor pump with the alkali pump so that if one of them stops operating, the other will also stop immediately. Alternatively, an alarm signal can be generated in case there is a malfunction in the dosing pump, allowing the issue to be resolved as quickly as possible. In addition, it is essential to strengthen inspections at the production site; for example, if it is observed that the output of fresh water from the second stage of reverse osmosis has decreased significantly, it is necessary to check immediately the dosage of scale inhibitors and carry out chemical cleaning of the reverse osmosis system promptly. Regular inspections are very beneficial for reducing equipment failures and preventing production accidents. 3.2.3 RO Operation Mode The best way to prevent scaling in the second stage of RO is to use a two-stage RO operation scheme and add NaOH solution before the second-stage reverse osmosis system. Since the feed water for the second-stage reverse osmosis is the product water from the first-stage reverse osmosis, which has an extremely low salt content, the addition of alkali has minimal impact on the desalination capacity of the second-stage reverse osmosis; therefore, scaling does not occur in this stage, and there is no need to add scale inhibitors. However, the drawback of this operating mode is the higher initial investment; yet it is the most ideal way to operate an entire pure water system, as it not only increases the yield from the post-treatment mixed-bed (with the conductivity of the second-stage RO water being ≤1μs/cm) but also enhances the operational efficiency of the whole pure water system (with a low failure rate). 4 Conclusion The method of adding NaOH solution before the reverse osmosis system to remove CO2 from water has lower initial investment and operating costs compared to traditional degassing towers and degassing membranes; it requires no additional large-scale equipment, and it makes full use of the desalination capacity of the RO system. Furthermore, the addition of alkaline solution is also beneficial for removing organic substances from water. However, strict requirements must be imposed on the control method, which is especially important when reverse osmosis uses a single-stage approach rather than a two-stage one.