Most of the current power plants in the US were built in the 40s, 50s and 60s. Water is extremely important in making electricity; mainly as a cooling medium. For that reason, most of the power plants are built along the major rivers like the Allegany, Mississippi, Ohio, Missouri, etc. When the plants were built, water quality was not a major consideration, but that has changed. Now power plants are building water purification plants on site to clean up the water being used during power plant operations. Water that is high in Total Dissolved Solids (TDS), basic pH, organics and other pollutants can cause scale buildup, fouling, and corrosion of plant materials.
Water purification generally consists of several steps: flocculation/settling, filtration, reverse osmosis, and chemical addition. Flocculation is the process of adding a flocculant chemical to the water that causes sediment to “stick” together, making larger particles. The larger particles tend to settle in a slow or non-moving water column. Clarifiers are used to slow the water movement so the “Floc” can have the time to settle in the bottom of the Clarifier so it can be removed.
The next step towards water quality is filtration. Filters are used throughout the water purification process from traveling screens at the inlet to remove sticks and floating debris in the river to Ultrafiltration that can remove particles as small as viruses. A critical process of any filtration system is a way to clean the filter media. A traveling screen filter has a screen on a roller that continually moves to remove debris from the filter. Ultrafiltration units usually have both chemical addition and backwash systems. Chemicals are added (such as acids) to remove salt and/or scale buildup on the filters. Backwashing uses clean water to force water through the system in the opposite direction of the normal flow. The clean water is also used in conjunction with air scour systems to mechanically clean the filters by “boiling” the water with air in addition to flowing backwards through the filter to knock any items off of the filter so it can be carried away by the backwash water.
The final step in water purification at power plants is usually Reverse Osmosis (RO). The reverse osmosis process has a semi-permeable membrane that only allows extremely small particles through. Reverse osmosis semi-permeable membranes allow water to pass through while retaining 90 – 99 percent of all the inorganic substances in solution, 95 – 99 percent of the organic constituents, and 100 percent of the most finely-divided colloidal matter (bacteria, viruses, colloidal silica, etc.). The efficiency of membranes in the removal of salts varies from 90 to 99 percent. To maximize water quality and purity, the water is passed through a series of semi-permeable membranes in a reverse osmosis unit.
The flow rate through a reverse osmosis membrane is directly proportional to the effective pressure (the difference between the applied pressure and osmotic head). A pressure in excess of the osmotic pressure is generated by High-Pressure booster pumps to obtain a satisfactory flow of purified water through the RO units.
Multiple chemicals are added throughout the water purification process to aid in improving water quality or cleaning of the filters. Sodium Hypochlorite (chlorine) is added as a disinfecting agent that kills most biological foulants such as algae. Because chlorine can cause damage to filters and RO units, Sodium Bisulfite is added to remove chlorine. Acids are added to help clean filtration units of salt or scale buildup from high pH water.
Pure water improves power plant operations by minimizing maintenance issues and by helping power plants run more efficiently. By removing most of the impurities from the water, scale and organic fouling is minimized on heat transfer mediums and less corrosion of susceptible materials adds to the longevity of the plant components.
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