When municipal water treatment plants disinfect drinking water or wastewater, the process kills harmful microbes but also generates potentially carcinogenic compounds. Now, in a large survey of wastewater treatment plants across the U.S., researchers report that these compounds, known as N-nitrosamines, often end up in treated sewage sludge, or biosolids. Because biosolids are widely used as fertilizer on farmland, researchers say the findings show the need to better understand the risks associated with this class of contaminants (Environ. Sci. Technol. 2014, DOI: 10.1021/es5001352).
N-nitrosamines, which are generated during chlorination and chloramination steps at water treatment plants, have come under increased scientific and regulatory scrutiny in recent years. The International Agency for Research on Cancer and the National Institute of Environmental Health Sciences designate a number of these compounds as likely carcinogenic. Given both the compounds’ toxicity and their known presence in drinking water, the Environmental Protection Agency is considering setting drinking-water standards for five of these compounds, including the most-studied one, N-nitrosodimethylamine (NDMA).
Biosolids’ use in agriculture could present another pathway for human exposure to N-nitrosamines. For example, these compounds could leach from soil into groundwater, and some research suggests that plants can take up NDMA. But only a few studies looking at a small number of treatment plants have been published on the abundance of N-nitrosamines in biosolids.
Arjun K. Venkatesan of Arizona State University and his colleagues, including Rolf U. Halden, sought to address this knowledge gap by analyzing biosolids from across the U.S. The team acquired 80 samples of biosolids from the EPA, which had previously collected these samples from 74 randomly selected U.S. wastewater treatment plants between August 2006 and March 2007. The researchers didn’t know where each sample originated from, or the details about the sludge-treatment steps taken by each plant, because the EPA didn’t provide that information.
After extracting the nitrosamines from the samples, the researchers separated the compounds via liquid chromatography and then identified and quantified each with mass spectrometry.
The researchers found seven N-nitrosamines, five of which had never been seen in U.S. biosolids before. Overall, 88% of samples contained some level of N-nitrosamines. Some of the nitrosamines were found in many samples but occurred in low amounts. N-nitrosodiphenylamine, for example, occurred in 79% of samples at 10 ng per g of sample on average. Others occurred in only a small number of samples but at high levels. For example, NDMA occurred in just 3% of samples but at 504 ng/g on average.
The researchers conclude that the N-nitrosamines’ regular presence in biosolids underscores the need for further study to understand where the compounds ultimately end up after biosolids are applied to fields and determine the resulting risks to human health.
Other follow-up studies should use freshly collected biosolid samples whose origins are fully known, says Susan D. Richardson, an environmental chemist at the University of South Carolina and a former EPA water researcher who wasn’t involved in the study. Knowing details about the treatment plants, such as the volume of water treated and the nature of the treatment and disinfection steps, could help researchers understand the conditions under which certain N-nitrosamines form and persist in treatment plants. Still, the study looks at a larger, more geographically diverse set of samples than previous studies have, Richardson says. The findings offer strong evidence that N-nitrosamine contamination is not just a drinking water issue, she adds.