| Cooling Tower Treatment |
| Almost all well-managed cooling towers use a water treatment program. The goal of a water treatment program is to maintain a clean heat transfer surface and preserve capital while minimizing water consumption and meeting discharge limits. Critical water chemistry parameters that require review and control include pH, alkalinity, conductivity, hardness, microbial growth, biocide and corrosion inhibitor levels.
Depending on the quality of the make-up water, treatment programs may include corrosion and scaling inhibitors, such as organophosphate types, along with biological fouling inhibitors. Historically, chemicals have been fed into the system by automatic feeders on timers or actuated by conductivity meters. Automatic chemical feeding tends to decrease chemical dosing requirements. Current technology allows chemicals to be monitored and controlled online 24-7 in proportion to demand. This ensures results and can allow cycles to be increased. Where overfeed is prevalent, it can reduce chemical feed, too. |
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| Sulfuric “Acid” Treatment
Sulfuric acid can be used in cooling tower water to help control scale buildup. When properly applied, sulfuric acid will lower the water’s pH and help convert the calcium bicarbonate scale to a more soluble calcium sulfate form. In central North Carolina, most plants will be able to operate six to 10 cycles of concentration without acid feed. Along our coasts, acid can be used to increase cycles as water tends to be harder and higher in alkalinity. The same can be said if hard alkaline well water is used as tower make-up. Important precautions need to be taken when using sulfuric acid treatment. Because sulfuric acid is an aggressive acid that will corrode metal, it must be carefully dosed into the system and must be used in conjunction with an appropriate corrosion inhibitor. Workers handling sulfuric acid must exercise caution to prevent contact with eyes or skin. All personnel should receive training on proper handling, management and accident response for sulfuric acid used at the facility.
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| Side Stream Filtration
In cooling towers that use make-up water with high suspended solids, or in cases where airborne contaminants such as dust can enter cooling tower water, side stream filtration can be used to reduce solids buildup in the system. Typically, five to 20 percent of the circulating flow can be filtered using a rapid sand filter or a cartridge filter system. Rapid sand filters can remove solids as small as 15 microns in diameter while cartridges are effective to remove solids to 10 microns or less. High efficiency filters can remove particles down to 0.5 microns. Neither of these filters are effective at removing dissolved solids, but can remove mobile mineral scale precipitants and other solid contaminants in the water. The advantages of side stream filtration systems are reduced particle loading on the tower. This ensures heat transfer efficiency and may reduce biocide or dispersant demands. |
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| Ozone
Ozone can be a very effective agent to treat nuisance organics in the cooling water. Ozone treatment also is reported to control the scale by forming mineral oxides that will precipitate out to the water in the form of sludge. This sludge collects on the cooling tower basin, in a separation tank or other low-flow areas. Ozone treatment consists of an air compressor, an ozone generator, a diffuser or contactor and a control system. The initial capital costs of such systems are high but have been reported to provide payback in 18 months.
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| Side Stream Softening Hardness can cause scaling of heat exchanger surfaces and reduce energy efficiency. Chemicals and acid can be used to control hardness, but where very hard water is used as the makeup water, the hardness often must be removed to ensure efficient operations and maximum water savings. Side steam softening uses tried and true water softening technology to remove excess hardness. This allows the tower to operate at increased cycles of concentration thus saving water. |
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| Alternative Sources of Make-Up Water Some facilities may have an opportunity to reuse water from another process for cooling make-up water. Clean internal wastewater streams such as reverse osmosis (hyperfiltration) reject water is suitable for inprocess reuse. In some cases treated in-process effluent can be used as cooling tower make-up if the concentration ratio is maintained conservatively low. Similarly, blowdown streams may be suitable for use as in-process water in some applications. North Carolina’s Environmental Management Commission rules allow the use of reclaimed water, or tertiary treated municipal wastewater, for cooling tower make-up water. In reuse and reclaimed water applications for cooling towers, water quality and system dynamics must be fully understood. Factors such as mechanical design, metallurgy, water chemistry and fluid flow dynamics must be considered.
| | | | | | Information Courtesy of: “Water Efficiency Manual for Commercial, Industrial and Institutional Facilities” researched by the N.C. Department of Environment and Natural Resources, Division of Pollution Prevention and Enviornmental Assistance, Division of Water Resources, Land-of-Sky Regional Council, Waste Reduction Partners |
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