A guide to smart disinfection.
By Roger Miller
In the normal operational life span of a water well there are repair and maintenance procedures not only to keep the well operating and producing water but also to assure the water is safe for potable use.
The process of well disinfection falls into both these areas as the buildup of bacteria can foul the well and impede capacity. However, in this case disinfection is only part of the resolution and not a standalone process. Secondly—and perhaps more commonly—disinfection is the resolution to unsafe bacterial conditions in the well and a common practice that should be administered with full knowledge of the source problem.
Disinfection is mandated after initial construction and development of the well, following service and repairs to the well, and of course, if unsafe conditions are found through testing of the well. However, well inspection procedures for identifying the source are important in the case of unsafe bacterial conditions. These include noting surface contamination or a septic system influence in the subsurface. Once these sources have been corrected, the well can be disinfected as the final step in resolving the problem.
Use of disinfection chemicals in the water industry was initiated in the late 1800s after the development of the “Germ Theory” in the 1850s identified specific waterborne bacteria as the cause of epidemics such as cholera and typhoid. The initial chemistry of choice was, and still remains today, chlorine.
Chlorine is available in gas, liquid, and solid forms. Gas chlorine is a dangerous chemistry to transport, store, and apply. Consequently, it is limited to a controlled environment such as a water treatment plant and not to field application procedures.
The solid form, calcium hypochlorite, is the common tablet or granular products seen throughout the water industry. It is actually a mixture of calcium carbonate and calcium chloride marketed as chlorine powder or bleach powder.
Calcium hypochlorite comes in a variety of product forms and concentrations representing the available chlorine by weight. It is used in the disinfection of water wells, but the chemical is not active until dissolved in water. As such, dissolving it at the surface, then applying it to the well is recommended to assure the chemical is active and biocidal.
In wells and aquifers exhibiting high hardness content, the use of calcium hypochlorite should be avoided as the further addition of calcium can help enhance the formation of scale deposits downhole.
The dynamics of the well and downhole conditions must be fully understood before using potentially dangerous oxidizers like hydrogen peroxide or household bleach for disinfection
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 exposure 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 considerably 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 disinfection 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 present 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 allowing 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.
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 effective 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 quantity 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 solution 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.
The calculated treatment volume should be sufficient to flood the entire well, borehole, and near well aquifer. The standard recommendation for domestic wells is three to four times the standing well volume.
The application of surging or agitation is recommended to effectively disperse the chemical solution throughout the treatment zone and continually bring new chemical in contact with the bacteria which are the target of disinfection.
Surge blocks are effective in providing this activity and are proven to move fluids in and out of the well to accomplish the agitation required.
In conjunction with agitation, sufficient contact time to allow the chlorine solution to effectively do its job is important. The rule of thumb used by our lab for water wells is a multiplier of the chlorine concentration and the hours of contact time to equal a minimum of 1000 contact units. Therefore, if you use 500 ppm of chlorine, you should allow a minimum contact time of at least two hours. Or if you use 100 ppm of chlorine, you should allow a minimum contact time of 10 hours.
When the chlorination process is complete, the well should be pumped off from the bottom to assure effective removal of the chlorine solution and associated debris. It is recommended at a minimum the chlorine level be monitored to make certain the solution is removed. Measuring conductivity can be helpful as well to determine if and when evacuation of the solution is complete.
Disposal of chlorine solutions is becoming a growing concern and many states have requirements for dechlorination of these discharge fluids. However, most of the regulations are based on fluid volumes, and domestic wells may not provide regulated volumes to consider.
Therefore, knowledge of your state’s requirements will guide the need to treat disinfection fluid discharges. When required, there are products available to the industry based on standard chlorine neutralizing chemistries such as sodium bisulfite, sodium thiosulfate, and ascorbic acid.
In summary, an effective well disinfection process should include:
- Assessing the source of any noted issues that could be related to surface contamination
- Assessing the need to potentially clean the well prior to disinfection
- Selecting the appropriate chlorine product and concentration required for your project
- Evaluating the aquifer water pH and alkalinity to determine the potential for a pH buffering additive
- Dispersing the treatment volume of chlorine solution throughout the well, agitating the solution, and allowing an adequate contact time
- Fully evacuating the well when completed, neutralizing the chlorine discharge fluids, and disposing of them in accordance with all laws and regulations
- Placing the well back into active service and monitoring its performance.
Effective water well disinfection is not a simple process and certainly not recommended for a well owner. Well disinfection requires a trained water well contractor who understands disinfection chemicals, their proper application procedures, and the rules and regulations under which to operate. That is what provides safe and successful results.
Roger Miller is a senior consultant at Water Systems Engineering, specializing in water chemistry. He has worked over the past 40 years in research and development, analytical procedures, site assessment, and project oversight in the groundwater and water treatment industries. He can be reached at firstname.lastname@example.org.