Reverse Osmosis

Introduction to Reverse Osmosis

        A technique used in processes requiring high-quality, purified water, such in semiconductor processing or biochemical applications, is reverse osmosis. It can be used to treat boiler feedwater, industrial wastewater, or process water. Reverse Osmosis is a water purification technique that reduces the quantity of dissolved solids in solution (Kucera, 54). It was first developed in the 1950's by the US government to provide fresh drinking water for the Navy, and since then, advances have made it much more feasible for obtaining purified water from wastewaters produced in many industrial applications. RO uses waterline pressure to push raw wastewater against a special semipermeable membrane. It is essentially a molecular squeezing process which causes H20 molecules to separate from the contaminants. The separated water molecules then pass thru to the inside of the membrane on to a holding reservoir. The contaminants are washed from the membrane and disposed of. Recently, RO has been used in treating boiler feedwater, in addition to industrial and process wastewaters. Boilers are found throughout the chemical processing industry and the primary method to treat boiler wastewater is an ion-exchange based demineralization. However, RO has been demonstrated to be more cost effective than this demineralization process (Kucera, 54).

Example of RO system

Problems With Reverse Osmosis:

        It is necessary to establish feedwater quality guidelines to optimize system performance and prevent the three main problems associated with RO: scaling, fouling, and degradation of ROmembranes (Kucera 55) These problems tend to decrease system productivity because they reduce wastewater purity. Scaling occurs on RO membranes when the concentration of scale-forming species exceeds saturation, producing additional solids within the RO feedwater. Scalants include such chemical species as calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, and reactive silica (Kucera 55). Since these species have very low solubilities, they are difficult to remove from RO membranes. Scaling decreases the effectiveness of the membranes in reducing the solids and causes more frequent cleanings. A scale on a membrane provides nucleation sites that increase the rate of formation of additional scale (Kucera 55).

Methods to minimize scaling

        In order to minimize scaling, pretreatment methods involving chemical or ion exchange techniques are used. Ion exchange methods remove scale-forming species from the RO feedwater, while chemical techniques change the characteristics of the RO feedwater so that crystal formation is not favored. An example of a chemical technique to prevent fouling is lime softening, which involves chemical processes that reduce the hardness of the wastewater, essentially preventing material from precipitating out. (Kucera 56) Lime, soda, ash, and NaOH are used to convert soluble calcium and magnesium to insoluble calcium carbonate and magnesium hydroxide. Magnesium hydroxide tends to absorb silica, another scalant. These solids are then collected as sludge from the bottom of the "softener". Another softening procedure involves zeolite in an ion exchange process. A strong acid cation resin in the sodium is used to remove scale-forming cations, suchas calcium, magnesium, barium, and iron. (Kucera, 56) These cations are exchanged with the sodium to yield "soft water", that is, water of low hardness.
        Another pretreatment technique to prevent scaling is acidification, which specifically reduces the crystallization of calcium carbonate. Sulfuric acid is most commonly used in this process, but can often increase the formation of sulfate scales. Therefore, where sulfuric acid cannot be used, hydrochloric acid is substituted. (Kucera, 57). Often used with acidification, or by itself, are antiscalants. Antiscalants are chemicals added to wastewater to minimize scale carbonate or sulfate based scale (57). They consist of acrylates and phosphonates which inhibit the precipitation of carbonate or sulfanates.

Methods to prevent fouling

        The second problem with reverse osmosis is with the fouling of membranes. Fouling occurs when suspended solids, microbes and organic material deposit on the surface of the membrane. Soluble heavy metals, such as iron, can be oxidized within membrane modules and foul the membranes. Another problem is from colloidal sulfur, which when oxidized from H2S can foul RO membranes. (Kucera 55). Colloidal sulcar tends to be very sticky and therefore can attach easily to the surface of RO membranes Hydrogen sulfide would be found most commonly in well-water. The primary methods used to combat fouling are mechanical processes that physically remove the suspended solids or chemical treatments the deactivate the foulant.
        Coagulation is one technique that neutralizes the negative surface of the suspended solids, allowing the particles to cometogether. (Kucera 57) These large particles are then easy to remove from the water using filtration. The most common coagulants used are cationic polymers, inorganic salts, and aluminum and iron salts. Inorganic solvents tend to form large particles, while catonionic polymers require much less product for coagulation. Similar to coagulation is the clarification method, which destablizes suspended particles through charge neutralization (58). These particles conglomerate and are removed using sedimentation or filtration techniques. One particular type of filtration uses manganese greensand as a filter to remove soluble iron and manganese from the water source. This is generally done by oxidizing iron and manganese and physically removing the precipitates in the manganese greensand bed. Chlorination is the primary technique to minimize microbiological foulants, as it is very effective against a wide variety of microbes and can be easily deactivated using sodium metabisulfite. (59). After chlorination, activated carbon filters can be used to remove chlorine and reduce organics. However, activated carbon tends to foster microbial growth by providing nutrients for microbes, so it is not a very effective filtration technique. Finally, to treat H2S containing feedwater, which can form colloidal sulfur, a combination the above techniques is used. First, the water is oxidized to precipitate the sulfur, which is then coagulated and filtered. Any colloidal sulfur that may have formed is converted to thiosulfates with the addition of sulfite. Finally, chlorination is done to convert the thiosulfates to sulfates.(60)

Methods to Minimize Membrane Degradation

        The final problem with reverse osmosis is membrane degradation. It occurs when the membranes are exposed to conditions that destroy the polymers used to create the membranes. Some membranes are susceptible to hydrolysis at high and low pH, while others are degraded by exposure to oxidizers such as chlorine.(Kocera 56) To prevent membrane degradation by acidic or alkaline waters, a corrective amount of acid of base should be added to the feedwater to make the pH approximately neutral. To prevent oxidation reactions, dechlorination is used. (57)
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References

  • "Kucera, Jane". Properly Apply Reverse Osmosis Chemical Engineering Progress. February 1997. Pgs 54-61.