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> Cleaning RO membranes
> Scaling
> Fouling
> SDI
Reverse osmosis is used as pretreatment and final treatment steps in many biopharm water systems. Impurities present in the feed water to the reverse osmosis system can build up on the membrane surface and adversely affect performance. RO membranes see two modes of performance degredation due to these impurities: scaling and fouling. Proper pretreament is essential to maximize the performance and longevity of the RO system.
As scalants and foulants build up on the RO membrane surface, higher and higher feed pressure is required to maintain a constant output. The pressure drop across an array of RO elements is usually measured. This differential pressure is a good indicator of the amount of fouling occurring in the membrane array. The differential pressure is calculated by subtracting the RO membrane reject pressure from the feed pressure.
Most commercially available RO membranes have a maximum allowable differential pressure. Operation beyond this maximum differential pressure can result in serious mechanical damage to the membranes due to the large forces that are generated in the flow direction in the membrane. Each membrane sees a larger and larger force as the differential pressure builds towards the end of the array. The permeate tubes see the majority of this force and the last element sees the largest of all the forces created by the pressure drops of the upstream elements.
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Cleaning RO membranes
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In order to return the membranes to their original operational levels of permeate flow and water quality, the membranes must be chemically cleaned. A low pH cleaner is used to remove mineral scale while a high pH cleaner is used to remove biofilm. Chemically cleaning a RO membrane is typically a lengthy manual procedure. At some point, the membranes may become permanently fouled and the production levels of the membrane cannot be restored to acceptable levels of production. The membranes must then be replaced to restore the performance of the RO system.
A major source of the fouling is the suspended solids contained in the RO feed water. The typical measurement of these suspended solids is turbidity expressed in nephelometric turbidity units, or NTU. This is a measure of the light scattered at a particular wavelength by a sample of the water. The higher the level of suspended solids present in the water, the higher the NTU reading.
In order to prolong the time between cleanings and the overall useful life of the RO membranes, it is necessary to pre-treat the RO feed water to remove the suspended solids. To be effective, the water produced from the RO pretreatment system must remove solids to such a degree that turbidity is no longer a viable method to measure the remaining solids present, i.e. the turbidity measurement is zero. At this point, the suspended solids are measured by observing the decrease in flow through a filter paper. The standard method for this measurement is the Silt Density Index or SDI.
Based on vendor literature, RO manufacturers specify an SDI of 3 or less as acceptable for feed to their membranes. Feed water with a higher SDI will result in a significantly lower lifetime for the membranes.
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Scaling
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Scaling occurs when soluble inorganic compounds in feed water concentrate and precipitate onto the membrane surface and form a scale. This scale can be very difficult to remove and treatment often depends on the type of scale that is on the membrane. Scale formation due to exceeding the solubility limits of these compounds is detrimental to RO system effectiveness and membrane life. Common inorganics that lead to scaling are Calcium Carbonate and Silica.
The feed water is often treated by methods such as acid injection, softening, or addition of a scale inhibitor to reduce or eliminate these scaling compounds. Another way to control scale formation is to reduce the system recovery. Higher recoveries may cause the precipitation of these inorganics due to increasing their concentration above the solubility limits in the concentrate stream. The system recovery can be reduced to bring this concentration below the solubility levels.
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Fouling
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Fouling may occur due to either colloids or biological growth … or both. Colloidal matter such as clay, colloidal silica, and dead microorganisms often lead to non-biological fouling. Measuring and calculating the Silt Density Index (SDI) can find the content of these colloids in source water. Pretreatment options that help reduce or eliminate colloidal matter in feed water are media filtration, coagulation-flocculation, cartridge filtration, microfiltration, or ultrafiltration.
Fouling due to biological growth on the membrane surface is known as biological fouling. With biological fouling, organisms such as bacteria, fungi, viruses, etc. collect on the membrane surface and form a living film under the correct conditions. Biofilms are very hard to clean due to the fact that the film on the membrane surface is often thick and thus provides a layer of protection for these microorganisms. These organisms feed on the deposited organic material located on the membrane surface and can quickly multiply, adversely effecting the membrane flux rates. This causes a rapid build up of differential pressure across the RO membrane array.
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SDI
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The SDI of a water is calculated by the following formula;
SDI 15 = [ 1 - T0/T15 ] * 100
where T0 is the time required to collect a specific volume of water (typically 500 mls) flowing through a .45 micron filter paper at 30 psig inlet pressure and T15 is the time required to collect the same specific volume after 15 minutes. The apparatus commonly used for collecting the SDI measurement is shown in figure 2.
Based on vendor literature, RO manufacturers specify an SDI of 3 or less as acceptable for feed to their membranes. Feed water with a higher SDI will result in a significantly lower lifetime for the membranes.
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