Water Well Journal
Issue link: http://read.dmtmag.com/i/767379
The liquid form of chlorine, commonly known as sodium hypochlorite, is available in a variety of products and strengths. The consumer form available at the retail level is about 5% concentration and can include scented compounds and additives not safe for downhole use. Chemical suppliers to the water industry provide a more concentrated form of sodium hypochlorite in the range of 10% to 15% and generally carry industry certifications such as NSF 60, assuring the quality of the products. Sodium hypochlorite solutions degrade over time with ex- posure to sunlight and changes in temperature. It is estimated every month sodium hypochlorite can degrade in strength by as much as 1%. Therefore, using an old container of sodium hypochlorite after months of storage in the warehouse consid- erably reduces the effectiveness of the disinfection program. To that point, it is advisable to always assure the use of fresh, sealed containers of sodium hypochlorite for each well disin- fection project. When either the solid or the liquid form of chlorine are mixed in water, they disassociate into hypochlorite ion and hypochlorous acid, which is a function of the solution's pH value. Hypochlorous acid, being the most biocidal form, is pres- ent at pH values of 5.5 to 6.5. Therefore, control of the pH is important for effective well disinfection. Many failures investigated by our laboratory have been attributed to paying no attention to these control factors. For effective production of hypochlorous acid, you will need to identify the pH and alkalinity of the source water to evaluate the neutralizing potential prior to chlorine addition. Adjusting the pH of the mixed treatment solution to a range of 6.5 to 7.0 will improve the biocidal efforts by allow- ing for the maximum production of hypochlorous acid, the biocidal form of chlorine. There are a variety of buffering chlorine additive products available to the groundwater industry, designed to adjust the pH of the disinfection chemicals, based on the aquifer water characteristics. For use of those products, their suppliers can assist in the proper calculations. The Concentration Concentration of the disinfection chemicals is another important factor to consider for an effective treatment of the well. There are age-old theories that say shock chlorination has been found to be limited in effect since no consideration was given to pH and that alkalinity and large volumes of chlorine are added to achieve only minor biocidal activity. Research has shown, though, that at above 500 ppm chlorine, the effec- tive removal of coliform bacteria is significantly reduced. That same research found the most effective levels for well treatment is in the range of 200 to 300 ppm. In conjunction with the selection of the proper chlorine compound, providing the most effective concentration quan- tity and assuring proper pH buffering to provide the most biocidal activities, the application procedures are another important consideration for an effective disinfection process. Simply dumping a chlorine product into the top of the well and expecting it to evenly disperse through the standing water column has proven difficult, if not impossible, to accomplish. Although liquid chlorine products are heavier than water and will fall through the water column within the well, the dilution factor comes into play and the solution continually slows its descent, hence reducing its strength to the point of ineffectiveness. Our laboratory has performed experiments in the past to determine these factors and found a 300 ppm chlorine solu- tion applied to the top of the water table in a well took more than six hours to fall to the bottom of a 100-foot well and an additional six hours to disperse throughout the gravel pack. In consideration of these factors, an effective disinfection process should include placing the chlorine solution into the screened and gravel pack area and evenly throughout the water column by use of a tremie pipe or similar means. Significant biological accumulations within a relatively young well. Heavy biofouling such as this requires more invasive treatment than provided by chlorination. Iron oxide entrained biofilm accumulation on a column pipe within a shallow well completion. The iron will serve to readily neutralize chlorine, resulting in little penetration of biofilm. DISINFECTION from page 25 waterwelljournal.com 26 January 2017 WWJ