Uptake of phosphorus and nitrogen by Myriophyllum aquaticum (Velloza) Verd. Growing in a wastewater treatment system

Abstract

The potential of the aquatic macrophyte M. aquaticum to remove nitrogen and phosphorus from secondarily treated wastewater effluent was investigated over 13 months. The flow-through wastewater treatment system consisted of three separate lagoons, the first lagoon with a floating mat of aquatic plants, the second with aquatic plants and aerated, and the third with aquatic plants under a polythene canopy and aerated. The aerated, canopy-covered lagoon was more efficient than the two other lagoons at removing influent ammonia- nitrogen (41 .I%), total nitrogen (16.7%), total phosphorus (4.2%), suspended solids (65%) and biochemical oxygen demand (18.1%). Influent phytoplankton numbers were also reduced by 98.6%. Following an initial planting density of 10 kg m^-2 on a wet weight basis, the standing crop of aquatic plants in the aerated, canopy-covered lagoon had a mean daily growth rate of 6.27 g m^-2 from May to October, resulting in daily accumulation rates of phosphorus and nitrogen of 0.12 g m^-2 and 0.22 g m^-2, respectively. Test pieces of M. aquaticum grown in separate enclosures within each lagoon had a mean daily growth rate of 7.0 g m^-2 on a dry weight basis through winter in the aerated, canopy-covered lagoon and 18.0 g m^-2 through summer in the exposed. aerated lagoon. Highest daily accumulation rates of the macronutrients were obtained in the exposed, aerated lagoon during spring and summer, with 0.43 g m^-2 phosphorus and 1.26 g rn^-2 nitrogen. Harvesting of plants resulted in an increase in ammonia-nitrogen in the lagoon effluent, caused by the removal of bacterial nitrifiers present on plant roots. Without harvesting, aufwuch accumulation on M. aquaticum and light restriction by the canopy resulted in a decrease in the phosphorus and nitrogen demand.