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 conditions at different total ammonia concentrations (10-300 mg N l(-1)) and controlled 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 growth of the duckweed Lemna gibba. Relative growth rates (RGR) varied between 0 and 0.3 per day. The toxicity of total ammonia to duckweed was a result of the effect of both, ionised and un-ionised, forms at low NH3 concentrations (< 1 mg N l(-1)). At higher NH3 concentrations, the toxic effect of the ionised form could be disregarded. Relative growth rates of L. gibba decreased 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 concentrations as long as certain pH levels are not exceeded. Extrapolated relative growth 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. gibba becomes impossible at pH levels above approximately 9.8, depending on the temperature. (C) 2001 Elsevier Science B.V. All rights reserved.