Oxidizing Biocide
Oxidizing biocides are powerful chemical oxidants, which kill virtually all micro-organisms, including bacteria, algae, fungi and yeasts. Common oxidizers are chlorine, chlorine dioxide, and bromine, ozone, and organo-chlorine slow release compounds. Chlorine is one of the most widely used, cost effective biocides and is available in liquid, gaseous or solid form. Its effectiveness is increased when used with non-oxidizing biocides and biological dispersants. Bromine chloride or chlorine dioxide should be considered for use in circulating water treatment for systems with high ammonia concentrations. Ozone is now days widely used to curb microbial growth.
Chlorine
Chlorine is the most widely adopted biocide for large circulating water systems. It provides a residual biocide in the treated water and can be readily checked. It is cheap and readily available as a pure gas, as well as in the form of various liquid and solid compounds. Its effectiveness increases when it is used with other non-oxidizing biocides and biological dispersants.
Chlorination systems are normally quite effective, but must be carefully controlled and monitored due to effluent quality control guidelines. Chlorination has several limitations in application, including:
a. Loss of effectiveness in alkaline waters (i.e. pH of 8 and greater)
b. Loss of effectiveness in the presence of contaminants, such as ammonia, methanol and ethylene glycol, etc.;
c. Corrosive towards common materials utilized in cooling tower installations;
d. Potential formation of less environmentally acceptable products;
e. Rapid degradation under heat and light
Storage and handing of chlorine must comply with the Dangerous Goods Ordinance. In 1974, the Best Practical Control Technology Currently Available (BPT), Best Available Technology Economically Achievable (BAT), and New Source Performance Standards (NSPS) of the EPA limited free available chlorine (FAC) to mass limitations of 0.2 mg/l daily average concentration and 0.5 mg/l daily maximum concentration for plants with either once-through cooling systems or closed cycle cooling systems. In addition, neither FAC nor total residual chlorine (TRC) could be discharged from any single unit for more than 2 hours per day, and multi-unit chlorination was prohibited. In the 1980s, further restrictions were placed on chlorine discharges by the EPA and state agencies. With the movement to reduce chlorine discharges to the environment, two options are offered to plants that use chlorine: chlorine minimization or dechlorination.
- Chlorine minimization is a program designed to ensure the most efficient use of chlorine to reduce the amount of TRC discharged. Plant personnel conduct tests to determine the minimum amount of chlorine necessary to control biofouling. Chlorination practices are then adjusted in accordance with test results. The condensers are periodically monitored and inspected to ensure minimum chlorine use and proper operation. Many plants in the United States have found that their current chlorine usage can be reduced significantly to comply with effluent limitations without other means or technologies for chlorine removal.
- Dechlorination involves using chemical means to remove a significant amount of TRC. The most commonly used dechlorination processes use sulfur dioxide, sodium bi-sulfite, or sodium thiosulfate.
Sodium Hypochlorite
Chlorine can be dosed in the form of sodium hypochlorite. A mixture of hypochlorous acid (HOCl), hypochlorite ion (OCl), and chloride ion (Cl) is formed when hypochlorite is added to water. The pKa (dissociation constant) for hypochlorous acid is approximately 7.53 at 25 °C. At pH levels below 6.0, most of the free chlorine is present as hypochlorous acid.
• At pH levels above pH 9.0, most of the free chlorine is present as hypochlorite ion.
• In the pH range from 6.0–9.0, the ratio of hypochlorous acid to free chlorine residual decreases with increasing pH. Hypochlorous acid is a much more effective biocide than hypochlorite ion.
• The effectiveness of sodium hypochlorite as a biocide decreases rapidly as the pH rises above 8.0.
Caution - Chlorine and hypochlorites must be applied carefully, because excessive chlorine will increase corrosion and may contribute to deterioration of cooling tower wood and reduction of heat transfer efficiency.
Chlorine dioxide
Chlorine dioxide is another strong disinfecting agent that is effective in controlling microbiological growth at high pH values. It is similar to free chlorine but having certain advantages:
1. Chlorine dioxide does not react with ammonia this reduces the disinfectant dose relative to chlorine.
2. Chlorine dioxide does not react with organics to the extent that chlorine or bromine does; this reduces the cooling water demand for chlorine dioxide relative to chlorine or bromine.
3. It is more effective than free chlorine at high pH values. Also, chlorine dioxide is very effective against Legionella and its relatively long half life allows chlorine residual remains in cooling tower water circuit for a relatively long period.
Chlorine dioxide is an unstable chemical that must be generated on site. Chlorine dioxide is produced by mixing the chlorinated water from a normal chlorinator and sodium chlorite solution. The reaction takes place very quickly; however, the process is more costly than simple chlorination. Chlorine dioxide is also generated electrolytically from a variety of chemicals. For cooling tower applications, the acid/sodium chlorite and acid/sodium hypochlorite/sodium chlorite generation method are typically used.
Chlorine dioxide and some of the chemicals used to generate it are hazardous.
Bromine
Bromine is produced either by the reaction of sodium hypochlorite with sodium bromide on site, or from pellets. Bromine has certain advantages over chlorine, including:
1. Bromine is a more effective disinfectant than chlorine in applications where the pH range is 8.0–9.0.
2. Effective disinfectant at low dose rates
3. Effective in the presence of nitrogenous compounds and organics such as methanol and ethylene glycol
4. It kills micro-organisms more quickly
5. Reduced potential for systems corrosion
6. Lower environmental impact
Iodine
Similar to chlorine and bromine, iodine is a good oxidizing biocide. However, it is relatively expensive.
Ozone
Ozone is a powerful disinfectant agent and virus deactivant that is capable to oxidize many organic and inorganic compounds. Ozone kills bacteria by rupturing their cell walls, a process to which micro-organisms cannot develop immunity. Residual ozone concentrations greater than or equal to 0.4 mg/L have been shown to result in a 100% kill in 2 to 3 minutes for any type of micro-organisms. The effectiveness of ozone is about 100 to 300 times more than chorine and can be successfully employed at a low concentration.
Since ozone has a short half life (usually less than 10 minutes), it readily decomposes into oxygen after oxidization. However, ozone is corrosive to some materials, and the cooling system construction materials need to be resistant to ozone attack. Also, injection equipment for ozone shall be designed to provide adequate contact of the ozone with the circulating water and in larger system; multiple injection point may be required.
Application of ozone is not suitable under the following situations where excessive organic material in the water or high operating temperature has a high depletion of applied ozone:
1. High organic loading from air, water or industrial processes that would require a high chemical oxygen demand (COD) since ozone oxidizes the organics and insufficient residual may remain for the water treatment.
2. Water temperatures that exceed 43.3°C since high temperatures decrease ozone residence time and reduce overall effectiveness of the ozone treatment.
3. Make up water is hard (>500 mg/L as CaCO3) or dirty make up water. Softening and / or prefiltering of make up water is recommended.
4. Long piping systems which may require long residence time to get complete ozone coverage.
5. Installation in dusty and smoky environment, and hot places such as boilers, kitchen and their chimney and exhaust.
Hydrogen Peroxide
Hydrogen peroxide (H2O2) is a powerful oxidizer, with its power stronger than chlorine and chlorine dioxide, but weaker than ozone. However, it can be catalyzed into hydroxyl radicals (OH-), which is more powerful than ozone, for micro-organisms control. Catalysts, such as iron, copper or other transition metals compounds can be added to hydrogen peroxide to generate hydroxyl radicals for more rigorous oxidation. This is the most powerful method to destroy micro-organisms and trace organics in water.
