Chlorine may purify drinking water, but what does it leave behind?

The use of chlorine to disinfect drinking water is a staple of public health efforts around the world. This purification process has made water-borne diseases like cholera largely a memory in the developed world, but chlorinating drinking water is not without risk. Chlorine can react with organic matter in water creating substances known as disinfection byproducts, or DBPs, which—when present in high enough concentrations—can pose serious health risks. Thus, a better understanding of these factors is needed for future wastewater treatment efforts.

To that end, Virender Sharma, PhD, Thomas McDonald, PhD and Leslie Cizmas, PhD, faculty in the Environmental and Occupational Health Department of the Texas A&M School of Public Health, conducted a study focusing on DBP formation and the various factors involved. The research, published in the Chemical Engineering Journal, looked at a wide range of existing research into DBPs, focusing on the roles that chemicals like iodide and bromide, metal ions, metal oxides and nanoparticles play in DBP formation.

“Bromide and iodide are chemicals found in water that form brominated and iodinated DBPs when water is disinfected with chlorine,” said Sharma. “Many of these compounds have a higher toxicity than chlorinated DBPs, making them an important topic of study. In addition, the rate at which DBPs form can be affected by water conditions and the presence of other substances like metals and nanoparticles.”

These substances include calcium and magnesium ions, which are common in hard water, and copper and iron ions found in surface water. They all affect DBP formation in different ways, with some ions enhancing the reaction and others slowing it down. Studies have found that these effects are complex, depending on factors like water pH. For example, changes in pH will change the available types of copper ions, leading to increased DBP formation in more alkaline water.

Water pipes are also a source of metal oxides, with lead, copper, iron and zinc being the most prevalent. Another study found that iron and copper ions coming from water pipes and storage systems might even alter newly formed DBPs, turning them into more toxic forms.

“Copper resists corrosion and has antimicrobial properties, which is why it’s so commonly used for water pipes,” Sharma said. “Leftover chlorine can promote corrosion in copper, and zinc oxides from stainless steel pipes and iron oxides from cast iron ones can also speed up DBP formation, particularly in the case of brominated DBPs.”

A new and growing source of concern related to DBP formation is the increasing use of nanoparticles in industrial and medical applications. Nanoparticles are substances ranging from one to 100 nanometers, made of carbon, silicon and metals like silver and titanium. Because of their small size, nanoparticles have a large surface area in proportion to their mass, which makes them highly reactive. However, there has been little research into how nanoparticles might affect DBP formation, with a few studies on silver nanoparticle use in wastewater treatment giving contradictory results.

The results of this study provide a starting point for further research into DBP formation, particularly as influenced by metal ions, metal oxides and nanoparticles. Research findings highlight the need for further systematic studies, especially related to the increased use of nanoparticles, and how we need more information on how factors ranging from pH to light exposure play a role in DBP formation.

“With more data, researchers and policy makers will be more able to make good decisions on wastewater treatment and public health,” said Sharma.