The effect of pH variation at the ammonium/ammonia equilibrium in wastewater and its toxicity to Lemna gibba

Laboratory scale batch experiments were performed under controlled conditio ns at different total ammonia concentrations (10-300 mg N l(-1)) and contro lled pH values of 6.8-8.7 using settled domestic wastewater to measure the effect of the ionised (NH4+ or ammonium) and un-ionised form (NH3) on the g rowth of the duckweed Lemna gibba. Relative growth rates (RGR) varied betwe en 0 and 0.3 per day. The toxicity of total ammonia to duckweed was a resul t of the effect of both, ionised and un-ionised, forms at low NH3 concentra tions (< 1 mg N l(-1)). At higher NH3 concentrations, the toxic effect of t he ionised form could be disregarded. Relative growth rates of L. gibba dec reased linearly with increasing NH3 concentrations up to a maximum level (8 mg N l(-1)), above which duckweed died. These data indicate that L. gibba can be used to treat wastewater containing high total ammonia concentration s as long as certain pH levels are not exceeded. Extrapolated relative grow th rates resulting from different combinations of pH and total ammonia are given for the examined ranges. Up to a pH of 7.8, a substantial production of 55 kg DW ha(-1) per day was achieved. Wastewater treatment using L. gibb a becomes impossible at pH levels above approximately 9.8, depending on the temperature. (C) 2001 Elsevier Science B.V. All rights reserved.