Water is probably the most important natural resource in our environment. We drink it, we wash with it, cool with it, 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 deals with the control of microorganisms by disinfection with chlorine and DPD chlorine testing.
There are numerous reasons for disinfection, but all of them deal with protection: protect people, processes and product. The US EPA 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 20th century that are summarized in the Chick-Watson Relationship. At a given temperature the disinfection rate by chlorine is predictable for each type microorganism. This facilitates control of the disinfection process. As an example, a 1.8 minute contact time is required for 99.99% inactivation of Polio virus using a 1-mg/L dose of hypochlorous acid (HOCl).
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 the hypochlorous acid or free chlorine residual concentration.
The primary disinfecting agent used in the U.S. is chlorine. 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 version of chlorine has its pros and cons. Because 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 a hydroxide ion (OH-), which is readily available in an aqueous solution with a pH level above 7, to form a hypochlorite ion (OCl-).
A hypochlorite ion is less than one third as effective a disinfectant as hypochlorous acid. The sum of hypochlorous acid and hypochlorite ion is called free chlorine.
HOCl + OH- > > OCl- + H 2 O
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 less effective as a disinfectant than hypochlorite ions, 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 > > NH2Cl + H2O
HOCl + NH2Cl > > NHCl2 + H2O
HOCl + NHCl2 > > NCl3 + H2O
The process, which ultimately destroys both the combined 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.
Historically, five methods have been used to determine free chlorine concentrations:
Syringaldazine (FACTS) Method; and
SenSafe Aperture Strip Method.
DPD has become the preferred method for chlorine measurement. The most commonly used DPD method is colorimetric and typically involves a colorimetric instrument. DPD colorimetric methods determine concentration from the intensity of the color formed when the chlorine reacts with a color-forming indicator. DPD-FAS Titration method, which is more involved than colorimetric methods, determines chlorine concentration from the volume of a standardized solution that is added to the sample until a color change in the sample occurs by back titration with a standardized FAS titrant. Because the DPD colorimetric method is sanctioned by the US EPA, and is reliable and quick, it enjoys wide acceptance as one of the most often used tests.
Different versions of the DPD test are available from suppliers like Hach, Palintest, LaMotte, H & F Scientific, Taylor Technology, and Industrial Test Systems, Inc.
One version uses DPD tablets which must be removed from their foil, crushed completely and thoroughly mixed/dissolved in the sample before a colorimetric reading can be taken.
A second method utilizes DPD powder pillows and requires cautious opening of the foils, careful pouring of the reagent powder into the sample cell, and finally a thorough mixing of that powder into the sample before a colorimetric reading can be taken.
Following on the heels of powder pillows are mechanical DPD dispensers which release the reagent into samples on demand, which is more convenient in high volume testing facilities than opening and pouring powder pillows, but moisture in the apparatus has been known to cause incorrect amounts to be metered out.
Another method of chlorine measuring uses DPD liquids and is a bit more involved because two reagents are used; but DPD liquids often find usefulness in field testing.
A new patent pending delivery method when testing with DPD reagent is the DPD ReagentStrip. This DPD version, as with all the versions above, is EPA compliant for free chlorine testing.
Because the DPD ReagentStrip test procedure is less involved, the test is more beneficial. All DPD test methods up to now have clearly stressed the importance of matching cells for colorimetric reading. With the DPD ReagentStrip test method the same sample and cell is used for blanking the colorimeter and for testing the free chlorine result. This change in method also opens the door for testing with plastic cells, which up until now, have not been used because plastic cells scratch so easily. This makes this test a viable option for food processors, who are regulated by USDA rules that forbids nonfood grade chemicals or glass articles in the food processing area.
The fact that the ReagentStrip falls under OSHA 29CFR1910.1200(d) classification, which gives test strips exemption from being classified as a chemical article, allows this DPD test to be used in USDA facilities.
For proper disinfection the concentration of chlorine must be accurately known. Fortunately, today’s professionals have several DPD options from which to choose. Chlorine disinfection, aided by advances in testing will continue to find wide use and new applications.