For thousands of years filtration has been used to reduce the level of dirt, rust, suspended matter and other impurities from water. This is achieved by passing the dirty input water (influent) through a filter media. As the water passes through the media, the impurities are held in the filter media material. Depending on the impurity and the media, several different physical and chemical mechanisms are active in removing impurities from the water. Some of the equipment used to employ these mechanisms has changed dramatically over time. Other systems, such as depth filters, have undergone very little change. The fundamental physical and chemical mechanisms that occur during filtration have become better understood over the years. These advances have allowed water treatment specialists to optimize the removal of impurities from the water. Filtration systems remove particulate matter and, because of the large surface area of filter media, they also can be used to drive chemical reactions that result in the removal of several contaminants. The following mechanisms for the removal of impurities by a given media are as follows: • Occlusion - removal due to the impurity's particle size • Adsorption - removal due to the impurity's adherence to the media • Reduction - removal of free residual chlorine through conversion to chloride ions in the presence of activated carbon media • Oxidation - removal of iron and manganese using oxidation, precipitation and filtration in the presence of greensand media
The following processes are classic pretreatment examples:
Overview of Feedwater Pretreatment
The objective of the feedwater pretreatment process is to improve the quality of the feedwater to the level at which the RO membranes can be reliably operated. Proper pretreatment is critical to the stable operation of an RO system. Feedwater quality is defined in terms of the concentration of suspended particles and the saturation levels of the sparingly soluble salts. Turbidity and the Silt Density Index (SDI) are common measures of the concentration of suspended solids. Small particles that remain suspended in water are called turbidity. Turbidity is measured by the effect (in terms of scattering and absorption) of the suspended particles on a beam of light passing through the water. Silt Density Index (SDI) measures the ability of a feedwater to foul a membrane filter. Continuous operation of an RO system in which the feedwater has turbidity or SDI values above the design limits may result in significant membrane fouling. The Langelier Saturation Index (LSI) and the saturation ratio measure the saturation levels of sparingly soluble salts in the concentrate stream. The LSI provides an indication of calcium carbonate saturation. Negative values of LSI indicate that the water is aggressive and that it will have a tendency to dissolve calcium carbonate. Positive values of LSI indicate the possibility of calcium carbonate precipitation. The LSI was originally developed to measure the concentration of calcium carbonate in low-salinity potable water. A saturation ratio is the ratio of the product of the actual concentrations of the ions in the concentrate stream to the theoretical solubility of the salts, at a given temperature and ionic strength. These ratios are applicable mainly to silica and to sparingly soluble sulfates of calcium, barium and strontium. Other potentially scaleforming salts, such as calcium fluoride or calcium phosphate, seldom represent a problem at concentrations found in natural waters.
Depending upon the raw water quality, reverse osmosis pretreatment processes may contain some or a good number of the following treatment steps:
When a system is designed, the water to be used as feedwater should be tested and analyzed to provide an adequate feedwater pretreatment for the design. This assures proper and longer operation of the RO system.
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