Membrane fouling in reverse osmosis
- 2. Colloidal and Particulate Fouling The source of silt or colloids in reverse osmosis feed waters often includes : bacteria clay colloidal silica iron corrosion products
- 3. predict a colloidal fouling potential of feed waters, including : 1) Turbidity is an expression of the optical property of water that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample. Turbidity is caused by suspended and colloidal particulate matter such as clay, silt, finely divided organic and inorganic matter, plankton and other microscopic organisms
- 4. predict a colloidal fouling potential of feed waters, including : 2) Silt Density Index (SDI) can serve as a useful indication of the quantity of particulate matter in water and correlates with the fouling tendency of RO/NF systems. The SDI is calculated from the rate of plugging of a 0.45 μm membrane filter when water is passed through at a constant applied gauge pressure , SDI value of about < 3 SDI is sometimes referred to as the Fouling Index (FI)
- 5. predict a colloidal fouling potential of feed waters, including : 3) Modified Fouling Index (MFI) : is proportional to the concentration of suspended matter and is a more accurate index than the SDI for predicting the tendency of a water to foul RO/NF membranes The method is the same as for the SDI except that the volume is recorded every 30 seconds over a 15 minute filtration period . The MFI is obtained graphically as the slope of the straight part of the curve when t/V is plotted against V(t is the time in secondsTo collect a volume of V in liters) MFI value of < 1
- 6. Methods to prevent colloidal fouling 1) Media Filtration 2) Oxidation–Filtration 3) In-Line Filtration 4) Coagulation-Flocculation 5) Microfiltration/Ultrafiltration 6) Cartridge Microfiltration 7) Strong acid cation exchange resin softening 8) Antifoulants
- 7. 1) Media Filtration The removal of suspended and colloidal particles by media filtration is based on their deposition on the surface of filter grains while the water flows through a bed of these grains (filter media). The quality of the filtrate depends on the size, surface charge, and geometry of both suspended solids and filter media, as well as on the water analysis and operational parameters The most common filter media in water treatment are sand and anthracite.
- 8. 1) Media Filtration The effective grain size for fine sand filter is in the range of 0.35 – 0.5 mm, and 0.7 – 0.8 mm for anthracite filter. In comparison to single sand filter media, dual filter media with anthracite over sand permit more penetration of the suspended matter into the filter bed, thus resulting in more efficient filtration and longer runs between cleaning . The design depth of the filter media is a minimum of 31 inches (0.8 m).
- 9. 1) Media Filtration In the dual filter media, the filters are usually filled with 20 inches (0.5 m) of sand covered with 12 inches (0.3 m) of anthracite There are two types of filters employed, gravity and pressure filters As the filter vessel for pressure filtration is designed for pressurization, a higher pressure drop can be applied for higher filter beds and/or smaller filter grains and/or higher filtration velocities The available pressure is usually about 16 ft (5 m) of head for gravity filters, and 30 psi (2 bar) to more than 60 psi (4 bar) for pressure filters
- 10. 1) Media Filtration For feed waters with a high fouling potential, flowrates of less than 4 gpm/ft2 (10 m/h) and/or second pass media filtration are preferred If the flowrate has to be increased to compensate for one filter that goes out of service, the flowrate increase must be gradual and slow to prevent the release of previously deposited when the differential pressure increase between the inlet and outlet of the pressure filter is 4 – 9 psi (0.3 – 0.6 bar), and about 4.6 ft (1.4 m) for the gravity filter, the filter is backwashed and rinsed to carry away the deposited matter. Backwash time is normally about 10 minutes
- 11. 2) Oxidation–Filtration Some well waters, usually brackish waters, are in a reduced state . Typically, such waters contain divalent iron and manganese, sometimes hydrogen sulfide and ammonium, but no oxygen; therefore, they are also called anoxic. the oxygen has been used up (e.g., by microbiological processes) because the water is contaminated with biodegradable organic substances, or the water is from a very old aquifer.
- 12. 2) Oxidation–Filtration One method of handling anoxic waters is to oxidize iron and manganese by : • air • sodium hypochlorite • potassium permanganate (KMnO4). The hydroxides formed can then be removed by media filtration. Hydrogen sulfide will be oxidized to elemental sulfur that can be removed by media filtration as well
- 13. 2) Oxidation–Filtration Greensand is such a granular medium, which is a green (when dry) mineral glauconite. It can be regenerated with KMnO4 when its oxidizing capability is exhausted. After regeneration, the residual KMnO4 has to be thoroughly rinsed out to avoid oxidation damage of the membranes . This technique is used when < 2 mg/L Fe2+ is present in the raw water For higher Fe2+ concentrations, KMnO4 can be continuously dosed into the inlet stream of the filter , In this case, however, measures have to be taken to ensure that no permanganate can reach the membranes
- 14. 2) Oxidation–Filtration Birm filtration has also been used effectively for Fe2+ removal from RO feed water. With birm filtration a pH increase and consequently a shift in the LSI value might occur, so care should be taken to avoid CaCO3 precipitation in the filter and in the RO system Instead of media filtration, microfiltration or ultrafiltration , can be used to remove small iron and manganese hydroxide particles formed from an oxidation process. This is a rather new technology for iron and manganese removal.
- 15. 3) In-Line Filtration In-line filtration can be applied to raw waters with a SDI only slightly above 5. The optimization of the method, also named in-line coagulation or in-line coagulation-flocculation . A coagulant is injected into the raw water stream, effectively mixed, and the formed microflocs are immediately removed by media filtration Ferric sulfate and ferric chloride are used to destabilize the negative surface charge of the colloids and to entrap them into the freshly formed ferric hydroxide microflocs Aluminum coagulants are also effective, but not recommended because of possible fouling problems with residual aluminum.
- 16. 3) In-Line Filtration Aluminum coagulants are also effective, but not recommended because of possible fouling problems with residual aluminum. The optimum dosage is usually in the range of 10–30 mg/L, but should be determined case by case . To strengthen the hydroxide microflocs and thereby improving their filterability, and/or to bridge the colloidal particles together, flocculants can be used in combination with coagulants or alone Flocculants are soluble high molecular weight organic compounds (e.g., linear polyacrylamides). Through different active groups, they may be positively charged (cationic), negatively charged (anionic), or close to neutral (nonionic).
- 17. 3) In-Line Filtration Flocculants are soluble high molecular weight organic compounds (e.g., linear polyacrylamides). Through different active groups, they may be positively charged (cationic), negatively charged (anionic), or close to neutral (nonionic). Coagulants and flocculants may interfere with an RO membrane indirectly or directly • Indirect interference occurs when the compound forms a precipitate that is deposited on the membrane. For example, channeling of the media filter may enable flocs to pass through and deposit on the membrane. A precipitate can also be formed when concentrating the treated feed water, such as when aluminum or ferric coagulants are added without subsequently lowering pH to avoid supersaturation in the RO stage
- 18. 3) In-Line Filtration reaction with a compound added after the media filter can cause a precipitate to form. This is most noticeable with antiscalants. Nearly all antiscalants are negatively charged and will react with cationic coagulants or flocculants present in the water. The membranes in several RO plants have been heavily fouled by a gel formed by reaction between cationic polyelectrolytes and antiscalants . • Direct interference occurs when the compound itself affects the membrane resulting in a flux loss. The ionic strength of the water may have an effect on the interference of the coagulant or flocculant with the membrane
- 19. 3) In-Line Filtration To minimize the risk of direct or indirect interference with the RO membrane, anionic or nonionic flocculants are preferred rather than cationic flocculants. Overdosing must be avoided.
- 20. 4) Coagulation-Flocculation For raw waters containing high concentrations of suspended matter resulting in a high SDI, the classic coagulation-flocculation process is preferred The hydroxide flocs are allowed to grow and settle in specifically designed reaction chambers. The hydroxide sludge is removed, and the supernatant water is further treated by media filtration.
- 21. 5) Microfiltration/Ultrafiltration Microfiltration (MF) or ultrafiltration (UF) membrane substantially removes suspended matter Hence, an SDI <1 can be achieved with a well-designed and properly maintained MF or UF system . There is both dead-end and crossflow filtration : • Dead-end filtration has two streams, inlet and outlet. 100% of the feed passes through the UF or MF filter medium (i.e.,100% recovery) • In crossflow filtration, there are three streams: feed, concentrate, and permeate
- 22. 5) Microfiltration/Ultrafiltration In UF and MF hollow-fiber membranes, there are two different types of configurations: flow can be from outside-in or inside-out : • For outside-in configuration, there is more flexibility in the amount of feed to flow around the hollow fibers, whereas inside-out configuration has to consider the pressure drop through the inner volume of the hollow fibers . • Inside-out configuration, however, offers much more uniform flow distribution through the bore of hollow fiber compared to the outside-in configuration .
- 23. 5) Microfiltration/Ultrafiltration Crossflow UF/MF systems operate at high recovery and flux rate and so backwashing and air-scouring techniques are frequently used to reduce fouling. If a chlorine-resistant membrane material is used (e.g., polysulfone or a ceramic membrane), chlorine can be added to the wash water in order to retard biological fouling
- 24. 6) Cartridge Microfiltration A cartridge filter with an absolute pore size of less than 10 μm is the suggested minimum pretreatment required for every RO system It is a safety device to protect the membranes and the high pressure pump from suspended particles • Usually it is the last step of a pretreatment sequence • A pore size of 5 μm absolute is recommended • The better the pre-filtration the less RO membrane cleaning required • If there is a risk of fouling with colloidal silica or with metal silicates, cartridge filtration with 1 – 3 μm absolute pore size is recommended
- 25. 6) Cartridge Microfiltration The filter should be sized on a flowrate according to the manufacturer’s recommendation and replaced before the pressure drop has increased to the permitted limit, but at least every 3 months . Back flushable filters as final safety filters are generally not recommended because of their risk of breakthrough in case of a malfunction of their backflush mechanism, their lower efficiency and the higher biofouling risk . Back flushable fine filters may be used upstream of the cartridge filters to protect them
- 26. 6) Cartridge Microfiltration The cartridge filter should be made of a synthetic non-degradable material (e.g., nylon or polypropylene) and equipped with a pressure gauge to indicate the differential pressure, thereby indicating the extent of its fouling . Regular inspections of used cartridges provide useful information regarding fouling risks and cleaning requirements: • If the differential pressure across the filter increases rapidly, it is an indication of possible problems in the raw water supply or in the pretreatment process • Replacing cartridge filters more often than every 1 – 3 months usually indicates a problem with the pretreatment
- 27. 7) Strong acid cation exchange resin softening not only removes hardness, but it also removes low concentrations of iron and aluminum that otherwise could foul the membrane . Softened water is also known to exhibit a lower fouling tendency than un softened (hard) water because multivalent cations promote the adhesion of naturally occurring colloids, which are usually negatively charged . The ability to minimize iron depends on the Fe species present. Fe2+ and Fe3+ are substantially removed by the SAC resin and, if in excess of 0.05 ppm, have a tendency to foul the membrane and catalyze its degradation .
- 28. 7) Strong acid cation exchange resin softening Colloidal or organo-Fe-complexes are usually not removed at all and will pass through into the product water. Insoluble iron-oxides are, depending on their size, filtered out depending on the flowrate and bed-depth used . When dealing with higher concentration of ferrous iron, one needs special care to avoid ferric iron fouling. It was reported that addition of SMBS was able to prevent membrane fouling
- 29. measuring the silt density index Equipment: including , • 47 mm diameter membrane filter holder • 47 mm diameter membrane filters (0.45 μm pore size) • 10 – 70 psi (1 – 5 bar) pressure gauge • needle valve for pressure adjustment
- 30. measuring the silt density index • Procedure 1. set the pressure regulator at 207 kPa (30 psi or 2.1 bar). 2. Place the membrane filter carefully on its support. 3. Make sure the O-ring is in good condition and properly placed. Replace the top half of the filter holder and close loosely. 4. Bleed out trapped air, close the valve and tighten the filter holder. 5. Open the valve. Simultaneously, using a stopwatch, begin measuring the time required for the flow of 500 mL. Record the time ti. Leave the valve open for continued flow.
- 31. measuring the silt density index 6. Measure and record the times to collect additional 500 mL volumes of sample, starting the collection at 5, 10, and 15 minutes of total elapsed flow time. Measure the water temperature and check the pressure as each sample is collected. 7. After completion of the test, the membrane filter may be retained for future reference. Alternatively, the filter may be left in operation after the test until clogged in order to collect suspended matter for analysis with analytical methods. 8. Calculation: Note: For this test method, 1-(ti/tf) should not exceed 0.75. If 1-(ti/tf) exceeds this value, use a shorter time for T; (i.e., 5 or 10 minute