Micropollutants in water recycling: a case study of N-Nitrosodimethylamine (NDMA) exposure from water versus food
One of the xenobiotics of most concern currently under discussion by regulators and treatment experts is N-nitrosodimethylamine (NDMA). NDMA is a carcinogen known to induce cancer in a variety of animals, causing DNA damage at low doses. Human exposure occurs through cigarettes, food, personal care products, and drinking water, in addition to endogenous formation in the stomach. NDMA is produced when water containing precursors is treated (in particular during disinfection or advanced oxidation) and recycled water contains a significant amount of precursors. The topic is becoming increasingly relevant as recycled water is considered as a source of potable water, directly or indirectly.
The daily tolerable limit for intake has been identified to be 4.0–9.3 ng/(kg day) [D.J. Fitzgerald, N.I. Robinson, Development of a tolerable daily intake for N-nitrosodimethylamine using a modified benchmark dose methodology, J. Toxicol. Environ. Health A 70 (2007) 1670–1678.]. Water at the WHO proposed guideline value of 100 ng/L would contribute about 2.9 ng/(kg day) of this intake, while intake from food varies from 5.7 to 44.2 ng/(kg day). Smoking and the workplace are additional exposure routes. This outlines that the exposure is often higher than tolerable limits. In the food and drinks industry, this has in recent decades resulted in improved manufacturing processes.
Awareness of the presence of NDMA in drinking water is a relatively recent issue. NDMA stems from precursors in raw water and can be generated during treatment. Generally removal of precursors is more achievable than the removal of NDMA itself. For example, the potent NDMA precursor dimethylamine is rapidly removed in biological pretreatment, while many other precursor amines are more persistent. These precursor amines include some ion exchange resins and coagulants, used in water treatment processes, which have been shown to generate NDMA during chlorination. Ozonation has also been shown to produce NDMA in treatment. UV oxidation is the preferred method for removal of NDMA in water treatment, although reverse osmosis (RO) membranes are possible alternatives if effective retention can be achieved.