Reverse Osmosis (RO) has emerged a reliable method for producing high-purity water. This membrane technology is often used in combination with other technologies such as ion exchange to meet the purity requirements for producing deionized water in the utility, pharmaceutical, semiconductor, and electronics industries. If you work with RO, however, you understand that the feed water must be preconditioned to protect the membranes from fouling and premature failure. What factors should be considered when designing an RO pretreatment system? Let's go back to the basics to find out.
Need For Pretreatment
An RO membrane functions much like a cross flow filter. The membrane is constructed of a porous material that allows water to pass through the membrane, but rejects up to 99% of the dissolved solids at the membrane surface. The dissolved salts are concentrated in the Reverse Osmosis reject water, or brine stream, where they are discharge to waste.
As the RO System continues to operate, the dissolved and suspended solids in the feed water tend to accumulate along the membrane surface. If these solids are allowed to build up, they eventually restrict the passage of water through the membranes, resulting in a loss of throughput. (The throughput capacity of the membranes is commonly referred to as the flux rate, and is measured in gallons per square foot of membranes surface area per day.)
Early in the development of membranes systems, little was known about which impurities in the Reverse Osmosis feed water are likely to cause fouling and a corresponding reduction in flux. Today, many of these troublesome impurity treatments have been identified, and preventive treatments have been devised that greatly reduce membranes fouling, thus prolonging the life of the RO System.
Autopsies of failed membranes modules have revealed a build-up of foulants caused by mineral scales such as calcium carbonate; colloidal materials like clays and silica; dead and living microorganism; carbon particles; and chemical attach by oxidizing agents like chlorine, ozone, or permanganate. Likewise, dissolved metals like iron and aluminum, whether naturally occurring or added as a coagulant, can cause premature fouling and failure of the membrane.
Start With a Water Analysis
A detailed chemical analysis of the RO feed water is an absolute necessity for identifying potential foulants. This should include a measurement of the hardness (calcium and magnesium), barium, strontium, alkalinity, pH, and chlorine. The data from the chemical analysis can be used by the RO equipment designers to determine the optimum membrane array that will both minimize the tendency of scale and deposit formation and maximize the recovery and flux rate.
For example, the Langelier Stability Index (LSI), a measure of the calcium carbonate scaling tendency of the water, is computed from the water analysis to determine the maximum permissible concentration of dissolved minerals in the reject stream before scale deposition becomes a problem. Because of the number of variables that must be considered, these calculations are difficult to do with pencil and paper. Fortunately, the membranes manufacturers have developed computer programs that make these computations fast and easy to perform where the user can project the performance of membranes at actual feed conditions.
Although a water analysis is helpful in predicting the tendency of dissolved minerals to cause problems in the RO System, it does not always forecast the fouling tendency of colloids and other finely dispersed suspended solids. The Silt Density Index (SDI) is a useful tool for quantifying the fouling tendency of the feed water. This test is conducted by filtering a sample through a 0.45 micron (µm) filter and measuring the time required to collect a unit volume of filtrate. An index number is calculated from this data. Traditionally, a SDI value of less than 3.0 is desirable for RO feed waters. The SDI measurement has certain limitations in that it does not model the cross flow design of an RO membrane.
The water analysis, LSI, SDI, or CFI values are used to determine the precise pretreatment requirements for a particular RO System. Since water supplies vary considerably from one location to another, each pretreatment requirement will be different. On average, however, the following pretreatment methods are commonly used in RO pretreatment systems.
Softening The RO Feed Water
Ion exchange is a popular method for reducing the potential for mineral scale formation on the membrane surface. Ion exchange softening uses sodium to replace scale-forming ions such as calcium, magnesium, barium, strontium, iron, and aluminum. The sodium forms very soluble salts that do not readily form mineral scales on the membrane surface.A sodium-cycle softener is regenerated with sodium chloride brine. The spent regenerant, along with the softener rinse water, must be discharge to waste. This, in combination with the event is generally recommended for this type of application.
A chemical reducing agent is often required to remove the last traces of oxidant prior to the RO System. Sodium bisulfite chemical injections used for this purpose. Sodium bisulfite reacts with chlorine to produce sodium bisulfite. The sodium bisulfite is readily rejected by the RO into the concentrated brine stream.
May be incorporated into the RO pretreatment system to control pH and minimize the scale-forming tendency of the feed water. Acid injection is indicated if the scale-forming tendency of the brine stream is above +0.3 as measured by the LSI. Either sulfuric or hydrochloric acid can be used for this purpose. However, sulfuric acid is less costly, and is more commonly used.
Have been shown to be effective in extending the intervals between chemical cleanings of the RO membranes. These products are generally formulated to include inorganic phosphates, organophosphonates, and dispersants. Use Antiscalant products that have been approved by the membrane manufacturer, and follow all direction in applying and controlling the product dosage. Some Antiscalant contain negatively charged polymers and dispersants that can react with cationic polymers that might be dosed up stream prior to the media filters. The Antiscalant must be compatible with these polymers; otherwise, the reaction product will foul the membranes.
Despite all efforts to protect the RO System from fouling and loss of flux, eventually the membranes will require chemical cleaning. A well-designed RO System will include provisions for a cleaning skid to facilitate the cleaning process. The skid should include a chemical tank, solution heater, recirculating pump, drains, hoses, and all other connection and fittings required accomplishing a complete chemical cleaning of the RO modules.
Various chemical cleaning agents are available for maintaining RO membranes. The type and amount of foulant will dictate the most effective cleaning agent. Acid cleaners’ best remove mineral scale deposits. Hydrogen peroxide is commonly used to clean and sanitize membranes to correct or prevent biofouling problems. In some cases, a mild solvent such as methanol is used. Because of the number of variables involved in the selection and application of these cleaning agents, contact the membrane manufacturer, equipment supplier, or a qualified chemical consultant for specific advice and recommendations on how to accomplish an effective cleaning.
The operation of the RO System should be carefully monitored to predict when the membranes would require cleaning. As a rule of thumb, cleaning is indicated when the normalized flux rate decreases by 10%. Under ideal condition, assuming that the RO pretreatment system is properly designed and operated, the frequency between membrane cleanings should be 3 months or more. Cleaning every 1 to 3 months is considered a fair performance, and suggests that some improvements in the pretreatment system should be considered. Cleaning frequencies every month or more indicate a change in raw water quality, a problem with the pretreatment system, or a problem with the operation of the RO unit.
Reverse osmosis is a reliable method for producing high-purity water. However, most water supplies require some form of RO pretreatment such as softening, media filtration, activated carbon, or chemical injection to protect the membranes from premature fouling or failure. The pretreatment requirements will vary from location to location, but the overall objective remains the same: to maintain the design flux rates, minimize the membranes cleaning frequency, and prolong the useful life of the RO equipment.