Effect of total ammonia nitrogen concentration and pH on growth rates of duckweed (Spirodela polyrrhiza)

The use of duckweed in domestic wastewater treatment is receiving growing attention over the last few years. Duckweed-based ponds in combination with anaerobic pre-treatment may be a feasible option for organic matter and nutrient removal. The main form of nitrogen in anaerobic effluent is ammonium. This is the preferred nitrogen source of duckweed but at certain levels it may become inhibitory to the plant. Renewal fed batch experiments at laboratory scale were performed to assess the effect of total ammonia (NH3+NH+4) nitrogen and pH on the growth rate of the duckweed Spirodela polyrrhiza. The experiments were performed at different total ammonia nitrogen concentrations, different pH ranges and in three different growth media. The inhibition of duckweed growth by ammonium was found to be due to a combined effect of ammonium ions (NH+4) and ammonia (NH3), the importance of each one depending on the pH.