The pretreatment for most RO systems can be divided into four types of pretreatment:
1) Sediment control
The prefilter itself in a factory assembled RO system should not be considered as the only sediment control for the system. An anthracite flint multimedia backwash sediment filter is preferred as the first piece of equipment in the pretreatment line and also serves as protection for other pretreatment technologies where more sediments are present. Ideally, the sediment control method should be selected to ensure a silt density index (SDI) value <3. SDI is discussed later in the topic of flow rates. If not addressed, SDI values> 3 can cause premature membrane plug. The RO system should be blocked when the sediment filter is washed countercurrently so that there are no sediments or high SDI values in the RO feed water.
2) Dechlorination
It is usually done with activated carbon sized at 3.7 gpm (gallons per minute) per cubic foot of carbon service flow. The carbon filter must be sized to meet the demand of the industrial reverse osmosis system feed rate, not the permeate rate. The feed flow rate will always be greater than the permeate rate. Activated carbon also reduces organic compounds; however, much lower flow rates are required to achieve an organic reduction. For chlorine removal an EBCT (Empty Bed Contact Time) of 4 minutes may be acceptable, while for organic removal we should not work below 10. RO should be blocked when the carbon filter is washed against the current so that the chlorinated water Do not damage RO membranes.
The chemical injection of sodium metabisulfite can also be used for dechlorination, injected in practice at 3 ppm (parts per million) per ppm of free chlorine. Sediment and carbon filters must be programmed to backwash at different times. This will ensure that a large volume of water and pressure is available for backwashing of each filter. The blocking of the RO, so that it cannot work while the sediment and carbon filters are in backwash, guarantees that the RO pump will not cavitate if these filters do not have an automatic bypass function. If the filters have an automatic bypass function, then the RO system should be turned off independently to prevent damage to the membrane by chlorine and sediment.
3) Scaling control
This is most commonly done by traditional water softeners. The water softener must be an alternate double system, unless the demand for RO feed water is low, the water hardness is low, there is adequate permeate storage and the water softener can block the RO during regeneration. The injection of antiscale chemicals instead of decalcifying or softening the water is also a popular means of scale control. A mineral analysis is especially important when chemical antifouling agents are used because all the minerals and metals are still in the feedwater. Therefore, the recovery of the system will depend on the levels of these constituents and the performance of the antiscalante chemical. Without some type of embedding control, an RO membrane could be embedded in a matter of hours, depending on the chemistry of the feed water and the recovery of the system.
4) Biological control
An ultraviolet (UV) sterilizer, which produces germicidal UV radiation of 254 nanometers and doses at 30,000 microwatt seconds per cm2, is a good defense against general microbes. Microbes can form biofilms on the surface of the membrane, which reduces permeate production. The UV system must be prefiltered with a 5 micron cartridge filter to maximize UV transmission in the water. It is true that these sterilizing equipment have evolved positively and today we have equipment with multiband optical effect lamps from 200 to 400 nanometers, much more effective in order to protect the aseptic membrane, preventing certain microorganisms from proliferating and excreting exopolymers (EPS) ). If a traditional water softener is used as an inlay control method for the RO system, then the UV sterilizer will receive the additional benefit of reduced scale formation in the UV quartz lamp where we have higher temperature, lower salt solubility dissolved. This will improve the UV transmission to the water and reduce the cleaning intervals of the quartz jacket, so the UV system must be installed after the water softener. If a chemical antiscalante is being used, instead of the traditional water softener for the control of membrane embedding, check with the antifouling manufacturer to confirm that the UV system will not have an adverse effect on the performance of the chemical used.
