Testing to Ensure Proper Disinfection

 By Ivars Jaunakais and Dick Wood

Why is Disinfecting Important?

Water is probably the most important natural resource there is. We drink it, we wash with it, we cool with it, we transfer energy with it, and we even use it to move things around. If water was a pure substance it could be used without concern, but water is far from pure. It contains microorganisms and dissolved minerals. Both of these constituents need to be controlled before water can be safely used. This article will deal with the control of microorganisms by disinfection with chlorine and chlorine confirmation.

There are numerous reasons for disinfection, but all of them deal with protection:

                   Protect people

                   Protect  process

                   Protect  product

The USEPA defines disinfection as a “5-log reduction (99.999% inactivation) in microorganisms in 10 minutes.” This definition is based upon the results of disinfection studies conducted in the early 20^th century that are summarized in the Chick-Watson Relationship:

- ln [N/N_o]  =   λCt

where:              N_o is the initial organism concentration

            N is the final organism concentration

λ is the specific lethality of disinfectant
toward the microorganism

C is the disinfectant concentration

t is the contact time

ln is the natural logarithm

At a given temperature λ or the Ct value is constant for each type of disinfectant and microorganism. This allows disinfectants to be compared on a level playing field. It also facilitates control of the disinfection process. Let me use the following example to illustrate how this works. How much contact time is required for 99.99% inactivation of Polio virus using a 1-mg/L dose of hypochlorous acid (HOCl)?

                   The specific lethality of hypochlorous acid toward Polio virus is 5 L/min-mg.

                   N/N_o = 1 – Removal = 1 – 0.9999 = 0.0001

t   =                  - ln(0.0001)                     =   1.8 min

                                                     (5 L/min-mg)(1 mg/L)

In actual practice the contact time is usually set by engineering constraints, such as tank size, or pipe length and water flow. In these cases the treated water is monitored for a specific disinfectant concentration or residual.

A Brief Review of Chlorine Chemistry

Since the primary disinfecting agent used in the United States is chlorine, the remainder of this article will focus on its testing and use. As shown in the table below, chlorine is available in different forms. Despite their chemical and physical differences they all form hypochlorous acid when added to water. Hypochlorous acid (HOCl) is the actual disinfecting agent.

Each chlorine chemical has its pros and cons; but since they all form the same disinfecting agent, the same testing methods can be used for all of them.

Two chemical reactions impact the performance of chlorine (hypochlorous acid) as a disinfectant. The first is its reaction with hydroxide ion (OH¯) which is more present  at pH above 7 to form hypochlorite ion (OCl¯). Hypochlorite ion is less then one third as effective a disinfectant as is hypochlorous acid. The sum of hypochlorous acid and hypochlorite ion is called free chlorine.

HOCl  +  OH¯   à   OCl¯  +  H₂O

The second reaction with ammonia (NH₃) is actually a series of reactions that form monochloramine (NH₂Cl), dichloramine (NHCl₂), and nitrogen trichloride (NCl₃). In addition to being even less effective as a disinfectant than hypochlorite ion, chloramines typically impart an objectionable taste and odor to the water. At the pH of most waters (6.5 to 8.5) monochloramine is mostly found.  The sum of the three chloramine species is called combined chlorine.

   HOCl  +  NH₃   à   NH₂Cl  +  H₂O

HOCl  +  NH₂Cl   à   NHCl₂  +  H₂O

                                          HOCl  +  NHCl₂   à   NCl₃  +  H₂O

The process which ultimately destroys both the combine chlorine and the ammonia responsible for it is called Breakpoint Chlorination.

Another chlorine term you may commonly run into is Total Chlorine. Simply stated, total chlorine is the sum of free chlorine and combined chlorine.

Chlorine Test Methods

Historically, four methods have been used to determine free chlorine concentrations:

                   Amperometric Method

                   DPD-FAS Method

                   DPD Method

                   Syringaldazine (FACTS) Method

Both the DPD and the Syringaldazine methods are colorimetric methods; and the remaining methods are all titration methods. The colorimetric methods determine concentration from the intensity of the color formed when the chlorine reacts with a color-forming indicator. Titration methods, which are more involved than colorimetric methods, determine chlorine concentration from the volume of a standardized solution that is added to the sample until an electrical or color change in the sample occurs.   Because the DPD colorimetric method is sanctioned by the US EPA, it enjoys a wide acceptance as one of the most often used tests.  Versions of the DPD test are available from suppliers like Hach and LaMotte.

Now a fifth and sixth free chlorine test methods can be added:

                 Syringaldazine Test Strip Method

                 TMB Aperture Test Strip Method

Back in 1978 Rupe et al. of Miles Laboratories (a medial diagnostic company) was issued a US Patent (#4092115) for a chlorine test strip using Syringaldazine as the indicator.  The test was specifically attractive for the pool and spa industry; and was quick and relatively accurate as long as the chlorine level was above 1 ppm (mg/L).  Another advance in chlorine test strip development occurred in 1996 when Ramana et al of Industrial Test Systems, Inc was issued a US Patent (#5491094) and later a second US Patent (#6541269) in 2003. This test strip uses a new indicator, 3,3’,5,5’Tetramethylbenzidine (TMB) and also features an aperture for exposing the indicator to the water sample so more sensitive detection of chlorine is possible.  The TMB indicator has one important benefit in that no chloramine interference is found; chloramine interference has been reported for the DPD Method.  The aperture test strip gives results in 40 second with detection of chlorine down to 0.05 ppm (mg/L).  Several benefits can be found with the use of test strips: 

1. According to the guidelines issued by OSHA 29CFR 1910.1200 (d) test strips can be considered non-hazardous.  Therefore, used test strips can be disposed of as regular trash, and the tested water sample should be discarded down a drain.
2. Test strips contain a very small amount of chemical.  One test strip typically contains 3% of the chemical as one DPD test. This makes  test strips very safe to use.
3. The USDA has allowed test strips to be used on site at process control stations for fruit and vegetable rinsing since no glass or chemicals are involved.
4. They are easy to transport  for field use.
5. Non-technical, where almost anyone can learn to use test strips accurately. 

Recently the US EPA has stated that the TMB Aperture Test Strip Method will be recommended as an official test for free chlorine testing in drinking water.  What made this possible is performance as demonstrated in Chart 1 (study performed by Galbraith Laboratories), where the DPD Method (EPA 8021) is compared to the TMB Aperture Method (referred to as “Proposed Method” on the graph).

For proper disinfection the concentration of chlorine must be accurately known.  Fortunately, today’s professionals have several options from which to choose. Chlorine disinfection, aided by advances in testing will continue to find wide use and new applications.

Ivars Jaunakais, owner of Industrial Test Systems, Inc., located in Rock Hill , SC , has been developing and manufacturing his own brand of test strips for water quality, SenSafe brand, since 1989. With 30-years of test strip development experience, Mr. Jaunakais has taken the principles upon which test strips were founded, simplicity, affordability, and accuracy, and built upon them to provide innovative products to many different industries. For more information, or to contact Mr. Jaunakais, contact ITS at    1-800-861-9712.