Growth of floating aquatic macrophytes in alkaline: Industrial wastewaters

Ponds holding industrial wastewater can quickly develop algal blooms, which in turn results in high pH excursions. Such deterioration in water quality is experienced in Portland Aluminium's retardation pond. A covering of flo ating aquatic plants, such as Azolla filiculoides and Lemna minor, will red uce the Light entering the water body, in rum reducing alga! production and lowering pH levels. In the case of Portland Aluminium's retardation pond, the initial installation of such a system has to be established under alkal ine conditions and in high fluoride concentrations (up to 10 mg/L). The sur vival and growth of A. filiculoides and L. minor under alkaline conditions (pH levels of 7-9.5) and under different fluoride concentrations (0-10 mg/L ) were assessed in the laboratory and in field conditions. L. minor grew we ll under alkaline and high fluoride conditions in shortterm laboratory test s, but appeared unable to compete with itinerant algae in the field. A. fil iculoides survived only in media of pH 7 and 0-mg/L fluoride in the short-t erm laboratory tests, but grew very well in the longterm field experiments. A. filiculoides was also observed to inhibit algal growth in the field. Bo th species absorbed fluoride. Fluoride concentrations in the plants rose an d fell in relation to the fluoride concentrations in the pond, suggesting t hat the fluoride was readily desorbed and that the absorption itself was by a passive mechanism.