The very name signifies what the process of reverse osmosis method is. While all other types of water filtration devices reduce impurities in the water, the process of reverse osmosis entails removal of water from the impurities. The units of reverse osmosis achieves this by forcing water through a specially constructed, semi-permeable, nonporous synthetic membrane (usually cellulose acetate). This membrane separates soluble and suspended particles from the water and provides filtered water with some quality of purification. The process of reverse osmosis removes a wide variety of substances from the water. Of the impurities that are removed from water, more than 75 percent of these are sodium, calcium and chloride. Reverse osmosis also may be effective in removing fluoride, nitrate and some forms of arsenic. While most of the times the membranes may last a year, its life truly depends upon the quality of water entering the unit as this directly impinges on the membrane.
The only reason why the water filtration systems based on reverse osmosis units are expensive is because of the number of accessories that are needed to operate them. They require in-line installation and a pre-filter to remove all dirt and sediment. The pre-filter may be in the form of a mechanical filter. To prevent buildup of contaminants on the membrane, if a back flush system is employed – this further shoots up the cost of the system.
Normally, household drinking water filtration systems, including a reverse osmosis step, are commonly used for improving purification of water for drinking and cooking. Such systems typically include four or five stages:
1. a sediment filter to trap particles including rust and calcium carbonate
2. optionally a second sediment filter with smaller pores
3. a carbon filter to trap organic chemicals and chlorination
4. a reverse osmosis filter with a thin film composite membrane (TFM or TFC)
5. optionally a second carbon filter
In some systems, the carbon pre-filter is omitted and cellulose triacetate membrane (CTA) is used. The CTA membrane is prone to rotting unless protected by the chlorinated water, while the TFC membrane is prone to breaking down under the influence of chlorine. In CTA systems, a carbon post-filter is needed to eliminate the chlorine.
Waste water are also given treatment by the process called Wet Oxidation. Incineration of waste water containing a large amount of sulfur or nitrogen can produce sulfur oxides (SOx) or nitrogen oxides (NOx), which are typical atmospheric pollutants and their quality in purification are very low. Therefore, if this waste water is to be incinerated, equipment to process these pollutants must be installed. Such equipment is not necessary, however, if waste water is given treatement by a process called “wet-oxidation” or “supercritical pressure method,” in which liquid is directly oxidized under high pressure and at high temperature.
a. Wet-oxidation has been used to process waste water from synthetic rubber manufacturing plants. However, since the process requires a high temperature and high pressure, it is practical only for large-scale plants.
b. Production of certain kinds of dyestuffs and organic pigments, such as sulfur dyes and azo dyes, uses raw materials which result in waste water containing a large amount of sulfur or nitrogen. Incineration of such waste water would cause atmospheric pollution their quality are low. Wet-oxidation of such waste water under high pressure and at high temperature would require a large capital investment. In addition, the wet-oxidation process is difficult to manage. For these reasons, wet-oxidation is impractical for small-scale plants.
When sulfuric acid or nitric acid is added to this waste water and heated, organic matter in the waste water are oxidized and decomposed at about 100ÂºC, so that wet-oxidation can be conducted without causing oxides of nitrogen or sulfur to vaporize.By adding sulfuric acid to waste water from sulfur dye production to adjust the pH to 1, and boiling the waste water for 30 minutes, the BOD of the waste water, originally about 50,000 mg/L, can be reduced to 1,000 mg/L, and hazardous materials in the waste water can be decomposed.