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

Nitrate-nitrogen (NO-N) and nitrite-nitrogen (NO-N) are constituents of the nitrogen cycle. NO-N is toxic to humans, primarily due to its reduction to NO-N. In Japan, NO-N and NO-N levels in tap water must not exceed 10 mg/L and only NO-N alone not 0.04 mg/L, respectively. In this study, we verified the effect of microorganisms and ultraviolet (UV) to increase of NO-N in water. First, all tested drinking-waters including tap water and commercial mineral water in PET bottles had < 2 mg/L NO-N and undetectable levels (< 0.01 mg/L) of NO-N. However, we found that NO-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 NO-N. We also demonstrated that direct UV and sunlight irradiation of NO-N-containing drinking water generated NO-N in 1-2 h, with NO-N reaching > 0.04 mg/mL by 4-6 h. On the other hand, NO-N and NO-N were undetectable in commercially purified water.