[Grete Gillebo]

I thought the EU legislation says 0.5 ppm AMMONIUM for drinking water, and that it actually means TAN (Total Ammonia Nitrogen) ? If that is the case, then your 0.5 NH3/NH4+ equals 0.39 ppm (NH3/NH+)-N. For the low pH, I have disregarded the tiny amount of NH3-N.
Anyhow, I do not think it is correct to assume that NH4+ is “non-toxic”. I have seen aquarium fish suffer long time damage from exposure to such low levels. But of course, the damage would be much worse if it was NH3.