Water Disinfection By-Products (DBPs) and Their Toxicity

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Disinfection is essential to obtain safe drinking water. Photo by gugacurado

Haloacetic acids (HAAs) are toxic by-products formed during the disinfection phase of the water treatment process. Researchers are currently conducting tests to confirm the cause of the toxicity, as found in a 2011 study performed at the University of Illinois. The study showed that HAA toxicity is not due to a direct interaction with DNA, but to an interaction with the enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH).

Water Treatment & Disinfection

The disinfection of water is a process that works to eliminate all possible toxic bacterial species. This part of the water treatment process makes it safe for us to drink the treated water without catching any disease and/or infection. Indeed, drinking disinfected water reduced remarkably the diffusion of water-born diseases into the society; infections such as cholera and typhus, spread by bacteria present in the water, have almost disappeared in developed countries.

Disinfection can also be applied in other cases, for instance in the treatment of industrial wastewaters, or to water used in swimming pools.

Most Common Types of Water Disinfection

Many disinfectants used for water cleaning are chlorine-based molecules; examples are chlorine (Cl2), chlorine dioxide (ClO2) or chloroamine (NH2Cl). Ozone (O3) is also employed in water purification. All these treatments are quite effective for sterilization; some particular bacteria, however, can be more resistant, and may need treatment with more powerful molecules.

Possible By-Products Formed During the Disinfection Process

Although water disinfection is an essential process for our well-being, there are some risks associated with the treatments, which have to be considered.

In some cases, the disinfectant(s) react(s) with some of the compounds already present in the water, to form other compounds, which are normally referred to as water disinfection by-products (DBPs). Depending on the disinfectant used, DBPs may have a different nature; in many cases they are toxic.

The hydrogen atom(s) in the acetic acid can be replaced by halogen(s). Photo by Clara Piccirillo

The hydrogen atom(s) in the acetic acid can be replaced by halogen(s). Image by Clara Piccirillo

Haloacetic Acids

The haloacetic acids (HAAs) are a class of molecules often present in disinfected waters. They are formed when a halogen atom (chlorine (Cl), bromine (Br) or iodine (I)) replaces a hydrogen atom in acetic acid (see the formula to the left).

The most common HAAs detected as DBPs are bromoacetic acid (BAA), dibromoacetic acid (DBAA), chloroacetic acid (CAA), dichloroacetic acid (DCAA), and trichloroacetic acid (TCAA). Together, these are also referred to with the acronym HAA5.

Legislation from the Environmental Protection Agency established a maximum concentration of 60 mg/L for the sum of all HAA5 present in a water sample due to the HAA5 toxicity.

Toxicity of HAAs

The toxicity of HAAs is well known, and it has been previously reported in several scientific publications; they are associated, for instance, with some cancers, such as those of the bladder or colon. Not much information, however, is available about the actual mechanisms causing the toxicity.

A recent study on this topic was performed by Professor Michael Plewa and his coworkers at the Department of Crop Sciences of the University of Illinois.

Water treatment research works to improve the safety of disinfection methods. Photo by LittleMan

Inhibition of Glyceraldehyde-3-phosphate Dehydrogenase (GAPDH)

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is an enzyme that catalyzes the conversion of the glucose into other molecules (pyruvate and ATP); this process is essential for living organisms. If such conversion does not take place, reactive oxygen species (ROS) can be formed; these can then interact and damage the DNA.

In their research, Professor Plewa and his coworkers showed that HAAs’ toxicity is due to their interaction with GAPDH, and their capacity for inhibiting its enzymatic activity.

The inhibition effectiveness, and hence the toxicity, depends on the nature of the halogen atom present in the acid. According to the study’s results, the toxicity scale is: iodine > bromine >> chlorine. This is due to the different activity of each of these atoms in the reaction with the GAPDH, with iodine being the more reactive, and chlorine the least.

Important Implications of Water Treatment

Professor Plewa told Decoded Science in a November, 2011 interview:

“Although HAAs have been regulated for nearly 15 years, the mechanism for their toxicity was unknown. These results are important, as now things are getting clearer: the toxicity is not due to a direct interaction with DNA, but via the GAPDH. We are performing more studies to test our hypothesis. The inhibition of GAPDH could also be associated with the induction of neurological disease; this is also an issue deserving more investigation.

This kind of study can help us in reviewing and improving our disinfection practices: we should use those technologies that generate the highest quality of water, free from pathogenic agents, but also that generate the least amount of toxic DBPs.”

Professor Plewa and his coworkers published the original results in Environmental Science and Technology, in June, 2011.

Sources

H. Galal-Gorchev. “Chlorine in water disinfection.” Pure and Applied Chemistry, 68(9), 1731-1735 (1996). Accessed November 21, 2011.

Environmental Protection Agency. “Water: Basic Information about Regulated Drinking Water Contaminants.” Accessed November 21, 2011.

J.A. Pals et al. “Biological Mechanism for the Toxicity of Haloacetic Acid Drinking Water Disinfection Byproducts.Environmental Science and Technology. 45, 5791-5797 (2011). Accessed November 21, 2011.

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