Behavior of Nitrate-Nitrogen and Nitrite-Nitrogen in Drinking Water.

Nitrate-nitrogen (NO3 -N) and nitrite-nitrogen (NO2 -N) are constituents of the nitrogen cycle. NO3 -N is toxic to humans, primarily due to its reduction to NO2 -N. In Japan, NO3 -N and NO2 -N levels in tap water must not exceed 10 mg/L and only NO2 -N alone not 0.04 mg/L, respectively. In this study, we verified the effect of microorganisms and ultraviolet (UV) to increase of NO2 -N in water. First, all tested drinking-waters including tap water and commercial mineral water in PET bottles had < 2 mg/L NO3 -N and undetectable levels (< 0.01 mg/L) of NO2 -N. However, we found that NO2 -N was generated in tap water left to stand at room temperature for several days, leading to increases in CF and TC counts and reduction of NO3 -N. We also demonstrated that direct UV and sunlight irradiation of NO3 -N-containing drinking water generated NO2 -N in 1-2 h, with NO2 -N reaching > 0.04 mg/mL by 4-6 h. On the other hand, NO3 -N and NO2 -N were undetectable in commercially purified water.