The Misunderstandings of Chlorine in Well Disinfection

Chlorine should be part of the solution and not a nuclear bomb.

By Edd Schofield

We frequently get calls from customers about their failed chlorination efforts or shock treatments on highly congested or fouled wells.

Diagram of chlorine interacting with biofilm on a well surface.

The contact typically indicates that he or she has dumped three times the recommended amount of chlorine down a given well and is perplexed when the test results still come up positive for coliform bacteria and the well still continues to behave poorly.

These operators are all working under the age-old mistaken belief that with well disinfection, the higher the concentration of chlorine, the better. It stems from the adage of a little is good, but a lot is better. As such, operators believe shock chlorinating problematic wells is the solution.

Regretfully, we have known for years that the opposite is true. The truth is the more chlorine we use in treating a well, the more complicated problems downhole become.

As has been discussed in previous columns, if a well is severely fimpacted, we typically look to address the issues with a complete well rehab initially with an acid and biocide (enhancer) treatment. We then follow with a disinfection treatment.

With any well that needs cleaning or rehabilitation, we typically need to acidize the well with a true acid enhancer or biocide in the initial effort to clean up both the biofilm as well as the mineral scale within the screen and casing section of any impacted well. Chlorine is an excellent disinfectant but a relatively poor remover of mineral scale, iron oxide deposits, and deep-seated biofilms.

Eliminating the Shock

Now, I want to discuss successful well disinfection treatment practices that may seem counter-intuitive. We need to somehow eliminate the term “well shocking” from our way of thinking.

We’ve known for decades that shocking a well does not promise guaranteed results. In fact, the exact opposite is more likely true.

We need to abandon the premise of shocking a well with exceedingly high amounts for chlorine concentrations. We need to instead rely upon 100 ppm to 300 ppm to get the necessary results of full-depth penetration in the killing of bacteria within a well system.

You read that correctly; we need to go in with lower concentrations of chlorine to be assured of the desired depth of penetration and assured destruction of the multiple layers of biofilm that we typically encounter when addressing slime.

The belief that lower concentrations will actually be more effective is counter-intuitive, I know. Yet the reality is we are dealing with a living organism. As such, when we go too hot and heavy with chlorine, we burn and oxidize the exposed outer membrane. In this effort, the outer layer of the slime is sacrificed. In turn, the balance of the lower-level critters live another day and eventually sluff off the burnt sacrificed layer, expand, and defend themselves, further affecting well operation and future bacteria test results.

The shocking truth is we need to go in at lower concentrations of chlorine with longer contact time and then monitor the solution downhole to assure sufficient PPM of chlorine is better penetrating the biofilm versus shocking the external organisms and sealing the pathway shut.

So, if you want to save time and greatly enhance your success rates in disinfecting a well, stop dropping a nuclear bomb and using 1000 ppm of chlorine. Go in at 300 ppm and be a ninja.

By way of dropping the concentrations of chlorine concentrations, providing agitation, and periodically monitoring the downhole solution of the chlorine mix, you will have far better success in treating any
well as you will successfully penetrate the entire biofilm colonies you are attempting to treat.

One Step in a Program

In groundwater wells, it is common to see chlorine residuals disappear within a few minutes or hours when
significant biofouling or existing mineral encrustation is present. This is often an indication the chlorine is being consumed by biofilm, and as such, little chlorine is left for complete disinfection. This is one reason chlorine alone is frequently insufficient with well rehabilitation work.

So, it is self-evident that if we are going in with lower concentrations of chlorine to get the desired depth of
penetration into the biomasses, then we need to check our chlorine downhole residuals on an hourly basis. In doing so, we assure ourselves that the chlorine has not been prematurely chewed up and neutralized and that we have adequate residual.

We need to abandon the premise of shocking a well with exceedingly high amounts for chlorine concentrations.

If the lower concentration of chlorine has been exhausted, then the treatment’s effectiveness will be greatly reduced. This is why many well rehabilitation professionals view chlorine usage without the introduction of true biocide as an incomplete rehabilitation solution. So, if the objective is restoring specific capacity
and removing plugging deposits, chlorine is typically only one step in a broader rehabilitation program.

Several factors cause chlorine to dissipate more quickly. Organic matter (bacteria, biofilm, and organic material like leaves) rapidly consumes chlorine. Iron, manganese, ammonia, and sulfides react with chlorine and reduce its residual. Higher temperatures accelerate chlorine decay. pH levels affect chlorine effectiveness with lower pH generally increasing disinfecting power. Also know that sunlight (UV radiation) and simple time will break down free chlorine quickly, even if it is in sealed containers.

In a water well, a 1000 ppm chlorine treatment does not effectively penetrate a mature biofilm colony. Instead, the chlorine reacts almost immediately with the outer layer of the biofilm, oxidizing the extracellular polymeric substance (EPS) matrix and consuming available chlorine before it can diffuse deeply into the colony. When chlorine residual drops rapidly, additional treatment (mechanical cleaning, acids, biodispersant, or other rehabilitation chemistries) is often required.

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Chlorine is excellent at killing independent organisms, but it does not effectively dissolve iron oxide scale,
calcium carbonate deposits, silts, clays, or other mineral-plugging materials. Rarely do we see just biological fouling in a given well but rather mineralization in conjunction with biofilm deposits and formation fines. As such, we typically use acids with biodispersants or acid enhancers as part of a complete rehabilitation chemistry that targets and removes the complex nature of downhole deposits

Consequently, many successful rehabilitation programs use chlorine (disinfection) last as a true final attack
of any biological fouling. This approach typically yields much greater restoration of well efficiency than just chlorination alone. For rehabilitation work, chlorine should generally be viewed as a disinfectant and biofilm oxidizer.

Edd Schofield is the technical sales manager–chemicals for Johnson Screens and has been with the company since 1988. He served as the Groundwater Foundation’s McEllhiney Distinguished Lecturer in Water Well Technology in 2007. He can be reached at edd.schofield@johnsonscreens.com.