By Z. Amjad, Ph.D., J.F. Zibrida and R.W. Zuhl
The success of a reverse osmosis (RO) system depends upon membrane life and performance, the repeatability and the reproducibility of the process that the membranes are designed to perform and periodic cleaning of the membranes to restore capacity.
Membranes lose performance and are replaced due to the deposition of unwanted materials on the surface. In addition, a decrease in membrane performance may be due to other factors, i.e. degradation by chemical (oxidation, hydrolysis, etc.) and/or mechanical (compaction, telescoping) processes. For an RO process to be successful, the life of the membrane must be extended as much as possible to minimize replacement costs.
The types of foulants most commonly encountered in RO systems include inorganic fouling (scaling), colloidal fouling, biological fouling and organic fouling.
Scaling of RO membrane surfaces is caused by the precipitation of sparingly soluble salts from the concentrated brine. The presence of suspended solids in the water, such as mud and silt, tends to cause gross plugging of the device rather than fouling of the membrane surface.
Biofouling can be a serious problem; it is a special case of particulate fouling that involves living organisms. The biological material growing on membrane surfaces not only causes loss of flux but may physically degrade certain types of membranes. Hydrocarbon oils (naturally occurring or as a result of pollution) have also been known to cause performance deterioration. Synthetic cationic polymers have been known to carry over to the membrane system due to clarifier upset or media filters channeling. Cationic polymers are known to be incompatible with many of the acrylic acid-based antiscalants in use today and may influence membrane performance. In addition, it has been reported that, in high hardness water, polyacrylate based antiscalant can form an insoluble salt with calcium, thus leading to membrane fouling.
Scaling-foulant control alternatives
Several methods (discussed below) exist for reducing or preventing membrane fouling caused by the deposition of mineral scales and include softening, system recovery, acid and antiscalant/dispersant.
Hot and cold process lime softening and sodium cycle cation exchange are commonly applied methods to remove hardness ions from feed water. Sodium (which replaces the hardness ions) salts are rarely scale forming and, therefore, can be tolerated.
Adjusting System Recovery
In RO systems, membrane fouling by mineral scale can be controlled by operating the system under conditions where solubility of scale forming salts is not exceeded, i.e., operating the RO system at lower recovery. This technique is not always effective due to concentration gradients within the membrane not controlled by the bulk flow.
Acids are among the oldest treatments used to control calcium carbonate scale formation. Acid is injected into feed water to reduce alkalinity to prevent calcium carbonate precipitation. Normally, sulfuric acid is used and is relatively inexpensive. The use of sulfuric acid for alkalinity reduction increases the potential for sulfate scale (e.g., calcium sulfate, barium sulfate) formation. Though calcium sulfate is relatively soluble, strontium sulfate is becoming a problem in certain areas of the world and barium sulfate is extremely difficult to remove once it is formed. When acid is used to control pH, the product water is often degassed to remove the resultant carbon dioxide. Gasses are not rejected by RO membranes and will pass directly into the permeate stream which decreases permeate quality.
Nearly every RO water treatment program usefd today can benefit from the use of suitable pretreatment chemicals (e.g. antiscalants, dispersants, etc.). Depending on the system and the treatment program, the pretreatment chemicals can be hexametaphosphate, homopolymer based, or copolymer based (consisting of several monomers of varying functional groups, i.e., multifunctional). In some cases, blends of polymers and other scale control agents may be used to provide well-balanced treatment technology. Chemical suppliers have researched these proprietary blends which typically have both membrane manufacturer compatibility and NSF potable water approvals.
Silica (SiO2) commonly found in ground water deserves a special comment. Silica usually exists in the weakly ionized soluble form. As soluble silica is concentrated in the RO process, it polymerizes to form an insoluble colloidal silica or silica gel that will foul membranes. The easiest method for preventing silica fouling is to reduce the conversion rate. The solubility of silica increases with increasing temperature and at high pH values. Operating at warmer temperatures may lessen the chance of silica fouling. Silica can be removed from the feed by lime softening, but it is very expensive and usually not practical unless other pretreatment requirements dictate lime softening.
Certain polymers have been shown to be capable of dispersing fine particles of amorphous silica once they have formed. These polymeric dispersants are often used when the potential for particulate silica fouling exists. Although these dispersants may minimize the impact of the fouling, they do not address the root problem of silica polymerization. A new antifoulant was recently introduced which can effectively inhibit the silica polymerization and also disperse particulate matter. Development of this new antifoulant is a major technological breakthrough as it can facilitate the operation of RO systems with concentrate (reject) streams containing greater than 500 mg/L soluble silica.
Antiscalant selection based on water chemistry
The prediction of reject (brine) and permeate chemistry based on feed water is integral to the design and optimization of RO technology. The dissolved salts that concentrate in the brine develop a scaling potential dependent on make-up water chemistry, pH and recovery. Several predictive tools have been developed to predict the scaling potential of water. While scaling indices such as Langelier, Ryznar, or Stiff and Davis and are a good indication of calcium carbonate scaling, they do not include other potential scales such as calcium sulfate, barium, sulfate, etc. Recent developments in system simulation can facilitate prediction of scale potentials for a number of scale forming salts in RO systems. A scale inhibitor dosage model can be correlated to predict the required product (i.e., antiscalant, dispersant, etc.) level to reduce the potential of scaling using predictive modeling computer technology. The following example (presented in the text, Table 6, Figure 5 and Figure 6) demonstrates the use of a predictive model for selecting a product to achieve desired performance from an RO system.
Table 6 shows analyses of an RO system raw water, feed water (pH adjusted with sulfuric acid to depress alkalinity), product water and brine (@ 75 percent recovery).
Insert table 6 here
Figure 5 relates to the RO system described above the associated brine stream data shown in Table 6 above. Figure 5(a) shows that calcite saturation varies as a function of pH. Figure 5(b) shows that gypsum saturation increases only with the increased sulfate ion from the sulfuric acid injection for pH suppression. The use of an antiscalant is important in this case in order to control the precipitation of calcite and gypsum.
Insert Figure 5 (a) and (b) here
Figure 6 relates to the RO system described above and the associated brine stream data shown in Table 6 and Figure 5. Figure 6 portrays dosage projections for two proprietary RO antiscalants. Figure 6(a) is a dosage projections for two proprietary RO antiscalants. Figure 6 (a0 is a dosage projection for a particular antiscalant; Figure 6 (b) is a dosage projection for another antiscalant. If an RO system feed water contains trivalent cations such as iron or aluminum in concentrations above 0.05 ppm, it is likely that a high performance multifunctional antiscalant will be required, especially for higher LSI conditions.
If the product flux descreases to unacceptable values (typically >10 percent decrease), the membrane must be cleaned. The cleaning method and frequency depend on the type of foulant and the membrane’s chemical resistance. Generally, it is easier to clean a membrane that is slightly fouled (check manufacturer flux decrease guideline for cleaning).
The cleaning method typically includes (a) mechanical cleaning (i.e., direct osmosis, flushing with high-velocity water, ultrasonic, sponge ball or brush cleaning, air sparging, etc.), (b) chemical cleaning (use of chemical agents) and (c) a combination of mechanical and chemical cleanings. The most prevalent method is chemical cleaning that frequently incorporates specially formulated membrane cleaners.
A large number of chemical agents are available for removing deposits. Chemical c leaning essentially involves the use of chemicals to react with deposits, scales, corrosion products and other foulants that affect flux rate and product water quality. These chemical agents can be classified into four categories, as follows:
Acid cleaning has a limited effect on sand, clay and biological matter. Alkaline cleaners can do little to dissolve and disperse hardness scale. Chelating agents are effective in dissolving calcium and barium based scales and iron oxides, but exhibit poor performance in removing oily substances and biological foulants.
Limitations of commodity chemicals have led membrane manufacturers and others to publish non-proprietary formulations for use as membrane cleaning agents. Several companies are offering proprietary formulated cleaners specially developed for removing foulants from membrane surfaces.
Clean membranes are critical for maintaining the efficient operation of RO systems. Membrane cleaning is a complex subject due to the variety of potential foulants. Characterizing deposits on fouled membranes is essential to the selection of the most economical and effective cleaner. Analysis of feed waters and spent cartridge filters, as well as evaluation of chemical changes (if any) resulting from pretreatment, provide valuable insight into foulant characteristics.
Once the magnitude and types of deposits are identified, membrane cleaning is required to restore system performance. If a deposit analysis reveals a variety of foulants (e.g., calcium carbonate scale, silica/silicate and metal oxides/hydroxides), single-function commodity-type cleaners such as citric acid and laundry detergents may not suffice. In such instances, the use of proprietary formulated cleaners should be considered. Prepackaged proprietary membranes typically have membrane manufacturers’ compatibility approvals and proven field performance.
The following factors should be considered when selecting an RO system cleaning program:
- Cleaning equipment requirement
- Membrane type and cleaner compatibility
- Foulant identification
- Ease of application
- Environmental impacts (requirements for discharging spent solutions