Do all reverse osmosis systems require chemical cleaning?
Sunday, 18 June 2017 05:50Despite 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.
What does an antiscalant do?
Sunday, 18 June 2017 05:50Antiscalants 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.
Is acid injection necessary?
Sunday, 18 June 2017 05:48
Just as acidic solutiosn aren't good for membranes, caustic solutions are equally damaging to membrane elements. Acid injection 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.
What is dechlorination used for?
Sunday, 18 June 2017 05:46Generally speaking, chlorination is a double-edged sword when it comes to Reverse Osmosis Systems. As a method of disinfection, chlorination is not only efficient and practical, but it is cost-effective as well. The only problem is that chlorine is too caustic for membrane elements, and can cause serious damage. Dechlorination is a kind of chemical injection that adds a chemical that forms salts with chlorine, making it readily rejected by the membrane elements. In this circumstance, dechlorination is a must as far as water treatment is concerned. Without dechlorination, reverse osmosis membranes are not only ineffective when it comes to chlorinated water, but the chlorine will simply destroy the protein membrane.
Does the feed water need to be softened by the RO?
Sunday, 18 June 2017 05:44Ion 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 to prevent damage to the membrane elements.. The sodium forms very soluble salts, which are readily rejected by the Reverse Osmosis System and 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 discharged to waste. It is because of this that ion exchange is recommended for applications that have high metal contents in the treated water.
What determines the precise pretreatments for a particular RO?
Sunday, 18 June 2017 05:43
n one word: analysis. Every source of water is different, and you never know what's in your water until you have it analyzed. 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, most Reverse Osmosis Systems need an antiscalant injector or water softener to prevent damage to the membrane.
Is it necessary to get a water analysis?
Sunday, 18 June 2017 05:41
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.
Is pretreatment necessary?
Sunday, 18 June 2017 05:40
If you work with an RO, you understand that the feed water must be preconditioned to protect the membranes from fouling and premature failure. 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 plant.
What types of water sources does Reverse Osmosis treat?
Sunday, 18 June 2017 05:40Reverse Osmosis is an ideal water treatment solution in most types of water. Generally speaking, all major water sources from a treatment standpoint can be broken down into three major categories: tap water, also known as municipal sources, groundwater, which includes brackish water, and saltwater. The biggest distinction between these three types is the Total Dissolved Solids (TDS) content of each type. As a rule of thumb, the American Health Association requires that drinking water is under 1,000 PPM TDS.
Tap water typically comes through a pre-existing infrastructure like city pipes or a damming system. Reverse osmosis is often used in a tap water environment to reduce hardness, or the debris deposited in water from traveling in metal pipes. Total dissolved solids is often a target of water purification in tap water systems. This type of Reverse Osmosis Systems are ideal in places like power plants, pharmaceuticals, laboratories, and hospitals, where an extreme purity of water is crucial to the industry. Tap water typically has a TDS of under 1,000 PPM.
Underground reservoirs of water are often brackish or highly brackish, meaning they contain large volumes of salt, but not enough to be considered salt water. Groundwater reverse osmosis is very common, and one of the best uses of a Reverse Osmosis System to date. Groundwater is most often purified for the agriculture industry, the mining industry, and for residential use. Groundwater is also a prized target of the bottling industry, because the unique mineral combinations often have an appealing taste. Brackish water usually has a TDS of 5,000 PPM or less, but can come in concentrations of up to 12,000 PPM.
Salt water reverse osmosis (sometimes referred to simply as desalination) is the turning of saltwater into drinking water. Ocean water has up to 45,000 PPM TDS. Typically, for environmental reasons, a bore hole is dug in the ocean for this kind of reverse osmosis, but an open intake is more cost effective. The biggest uses of desalination come in providing water in areas that lack a regular supply of fresh water.
What types of water sources does Reverse Osmosis treat?
Sunday, 18 June 2017 05:40Reverse Osmosis is an ideal water treatment solution in most types of water. Generally speaking, all major water sources from a treatment standpoint can be broken down into three major categories: tap water, also known as municipal sources, groundwater, which includes brackish water, and saltwater. The biggest distinction between these three types is the Total Dissolved Solids (TDS) content of each type. As a rule of thumb, the American Health Association requires that drinking water is under 1,000 PPM TDS.
Tap water typically comes through a pre-existing infrastructure like city pipes or a damming system. Reverse osmosis is often used in a tap water environment to reduce hardness, or the debris deposited in water from traveling in metal pipes. Total dissolved solids is often a target of water purification in tap water systems. This type of Reverse Osmosis Systems are ideal in places like power plants, pharmaceuticals, laboratories, and hospitals, where an extreme purity of water is crucial to the industry. Tap water typically has a TDS of under 1,000 PPM.
Underground reservoirs of water are often brackish or highly brackish, meaning they contain large volumes of salt, but not enough to be considered salt water. Groundwater reverse osmosis is very common, and one of the best uses of a Reverse Osmosis System to date. Groundwater is most often purified for the agriculture industry, the mining industry, and for residential use. Groundwater is also a prized target of the bottling industry, because the unique mineral combinations often have an appealing taste. Brackish water usually has a TDS of 5,000 PPM or less, but can come in concentrations of up to 12,000 PPM.
Salt water reverse osmosis (sometimes referred to simply as desalination) is the turning of saltwater into drinking water. Ocean water has up to 45,000 PPM TDS. Typically, for environmental reasons, a bore hole is dug in the ocean for this kind of reverse osmosis, but an open intake is more cost effective. The biggest uses of desalination come in providing water in areas that lack a regular supply of fresh water.