By Leo Zappa and William Zavora
Water disinfectants are an essential element of potable water treatment because of the barrier they provide against waterborne disease-carrying microorganisms. Diseases such as typhoid, hepatitis, and cholera have been greatly reduced in incidence since the introduction of chemical disinfection.
For over one hundred years, the predominant disinfectant chemical has been chlorine. While a highly effective disinfectant, chlorine has been found to react with naturally occurring matter (NOM) in the water to form disinfectant by-products (DBPs). DBPs have been linked to a number of human health concerns and have been regulated by the United States EPA. Public water system operators will soon face compliance with the U.S. EPA's Stage 2 Disinfectants and Disinfectant By-products Rule (US-EPA Stage 2 DBPR). Specifically, water utilities will be required to achieve locational running annual averages of 80 ug/l for total trihalomethanes (TTHM) and 60 ug/l for haloacetic acids (HAA5) starting in 2012.
One of the methods employed by some water utilities to come into compliance with US-EPA Stage 2 DBPR is to switch disinfectant chemicals, moving away from chlorine and converting to alternative means of disinfection such as chloramine. It is known that chloramines can provide satisfactory disinfection while producing lower levels of TTHMs and HAA5s. However, recent research has discovered that use of the alternate disinfectant, while reducing the levels of the currently regulated DBPs, can have unintended consequences. Specifically, the use of chloramines can lead to the formation of new classes of DBP's.
These emerging, and currently unregulated DBPs, can include nitrogen and iodine-based compounds (N-DBPs, Iodo-DBPs). Examples of these new DBPs include iodo acids such as iodoacetic acid, iodo-THMs such as dichloroiodomethane, haloaldehydes, halomides, and NDMA. The formation potential of these emerging DBPs is enhanced by the increased use of impaired waters as supplies of pristine waters decrease. Impaired waters can encompass such factors as the impact of wastewater (including the reuse of wastewater in states such as Florida and California) and algal growth. Impaired waters often have heightened levels of organic nitrogen, which provides precursors for nitrogenous DBPs.
The major concern regarding these new classes of DBPs is their toxicity to humans. Current research is focused on determining the cytoxicity and genotoxicity of these emerging DBPs. Cells exposed to a cytotoxic compound can suffer necrosis, where the cell membrane loses integrity and dies. In contrast, cells exposed to genotoxic compounds can suffer genetic mutations, which can in turn lead to the formation of cancerous tumors.
A number of the emerging N-DBPs and iodo-DBPs appear to be significantly more genotoxic and cytotoxic than the currently regulated TTHMs and HAA5s. Examples include halonitromethanes, which appear to be up to 10 times more cytotoxic than regulated THMs, and iodo-acids, which have been shown to be twice as genotoxic as the currently regulated DBPs.
Due to the competing demands to provide safe, disinfected drinking water to their customers while at the same time meeting current DBP regulations and limiting the formation of emerging DBPs, municipal water providers are investigating other means to prevent or limit DBP formation. One such alternative approach is the removal of naturally occurring matter (NOM) from the water prior to adding disinfectant chemicals. By removing the organic precursors, the formation potential for DBPs, both regulated and emerging, is greatly reduced.
There are a number of technologies which have been evaluated and are now being employed by municipal water providers for precursor removal. Membrane filtration, activated carbon, and the enhanced coagulation process have emerged as the three most commonly applied technologies for NOM reduction. All three of these technologies have been thoroughly researched for their effectiveness relative to NOM reduction, and there are numerous technical papers which describe how these technologies can be applied to help municipalities meet their Stage 2DBPR compliance requirements.
A key feature common to these technologies is that they are able to accomplish the goal of meeting current DBP regulations without any detrimental side-effects, such as the formation of emerging disinfection by-products. Another point worth noting is that each of these technologies and processes were originally developed to accomplish other water quality goals, but have been repurposed to provide a solution to the Stage 2 DBPR challenge.
Municipal water providers are presented with the dilemma of balancing the need for supplying disinfected water with the prevention of forming hazardous disinfection by-products. Short term solutions such as switching disinfectant chemicals may be relatively inexpensive and easy, but can create as many problems as they solve. The long term solution to this challenge should be to encourage the water industry to research and develop innovative approaches to applying new and existing technologies. Emphasis should be placed on those technologies and processes that reduce or remove contaminants and precursor compounds from water in lieu of adding more chemicals to our drinking water.
[Note: The views expressed in this article are not necessarily those held by WWEMA, but offer the perspective of one of its member companies.]
About the Authors: Leo Zappa is Municipal Industry Manager with Calgon Carbon Corp., a WWEMA member company. William Zavora, PE, is Senior Applications Engineer with Calgon.