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.
Keywords: Disinfectant; Microbicidal activity; Nitrate; Nitrite; Stability.