[Effect of inorganic nitrogen compounds and pH on the growth of duckweed]

The use of duckweed in domestic wastewater treatment was paied increasing attention recently. But inorganic nitrogen compounds and pH of wastewater possibly affect the growth of duckweed. The effect of ammonia, nitrate and pH on the growth of Lemna minor L., a common spiece in China, was assessed with laboratory scale experiments. The experiments used artificial culture with different levels of pH, ammonia and nitrate concentrations. The experimental results indicated that the lowest value of pH Lemna minor can tolerate was between 5-6, and it can grow well in pH from 6 to 9. The growth rate of Lemna minor was inhibited gradually with increasing concentration of ammonia. The toxicity of ammonia was a result of both forms, NH3 and NH4+. Compared to NH4+, the effect of NH3 was much stronger. Nitrate had few inhibitory on the growth of Lemna minor. The increase in ammonia and nitrate concentrations can increase the chloropyll content of Lemna minor. Activity of peroxidase of Lemna minor in ammonia culture was higher than that in nitrate culture because of the toxicity of ammonia.