Water Well Journal

August 2015

Water Well Journal

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Field test to identify the pH and alkalinity to allow for proper buffering of the chlorine solution for effective disinfection. Heavy biomass growth overwhelming the producing zones will require cleaning prior to effective disinfection. waterwelljournal.com 22 August 2015 WWJ include perfumes or other compounds not safe for downhole use. From a chemical supplier, you should be able to get a stronger solution—ranging from 10% to 15% strength. Sodium hypochlorite degrades over time, with exposure to sunlight and changes in temperature. It is estimated with every month, liquid sodium hypochlorite degrades in strength by 1%. With this in mind, it is advised you always secure a fresh, sealed container of sodium hypochlorite for each well disinfection project. In its solid form, calcium hypochlo- rite is the common granular or tablet form of chlorine. As a mixture, it con- tains calcium carbonate and calcium chloride. It is marketed as chlorine pow- der or bleach powder for water treat- ment and as a bleaching agent. Calcium hypochlorite can be used as a 25%, 33%, 68%, and 73% active product— which means it contains that amount of available chlorine by weight. As with most powdered chemicals, it should be dissolved in water prior to introduction into the well. Calcium hypochlorite is commonly used in a variety of water systems—wells, pools, cooling towers, and storage tanks. Due to the amounts of calcium present, it is important not to use calcium hypochlo- rite in groundwater environments that exhibit high hardness as it can introduce calcium into an already saturated envi- ronment, causing scale formation. Most commercial forms of chlorine, including liquid and solid hypochlorite solutions, are buffered for safety. This is done to limit the potential for chlorine gas to form, which is dangerous, as well as to reduce the potential for corrosion or damage from the oxidizing effects of chlorine. Buffering to a higher, more al- kaline pH limits the biocidal effective- ness of hypochlorite solutions, resulting in a need for greater concentrations of the chlorine product. When sodium or calcium hypochlo- rite is mixed together with water, both hypochlorous and hypochlorite ions are formed. The hypochlorous ion is con- sidered to be several hundred times more biocidal than the chlorine residual formed by the hypochlorite ion. As such, the hypochlorous ion is the greater means of disinfection during treatment. The hypochlorous ion is present in greater concentration (strength) at lower pH values. For example, at a pH of 6.5, the hypochlorous ion content is near 95% strength. At a pH of 7.5, the hypochlorous and hypochlorite ions will be present in near-equal concentrations with only minimal biocidal effective- ness. As the pH passes 8.0, the hypochlorous ion content falls to less than 4%, hindering biocidal activity significantly. Above a pH of 8.3., the hypochlorite ion predominates. This phenomenon is why many dis- infection efforts fail. As such, you will need to identify the pH of your source water (for mixing) and the alkalinity to identify the neutralizing potential of the water prior to chlorine addition. Adjust- ing the pH of the treatment solution to a range of 6.5 to 7.0 will improve the bio- cidal efforts by allowing for the maxi- mum production of hypochlorous acid, the biocidal form of chlorine. The Disinfection Process Research has shown concentrations of chlorine above 500 ppm generally fail in effective removal of the offend- ing organisms (coliforms). The age-old theory of "shock chlorination" came about due to the limitations of pH and the need to apply more and more chlo- rine to achieve only minor biocidal activity. In new wells, with less biofilm and free iron present, chlorine concentra- tions of 50 to 150 ppm are typically needed. In older wells that have exhib- ited higher fouling, generally concentra- tions of 200 to 300 ppm are used. In addition to buffering the pH and selecting the chlorine compound, how you apply it is an important choice. Dumping a solution from the top often fails to reach the target area. While it is true chlorine has a higher specific gravity than water, it is gener- ally not sufficient to overcome the volumetric difference within the well column or remain effective as it dilutes in the well. Our laboratory conducted a series of experiments several years ago designed to determine the distribution and disper- sal patterns of hypochlorite solutions when added to the well. We found a SCHNIEDERS from page 21

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