Date of Award

2000

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

Faculty

Faculty of Communications, Health and Science

First Advisor

Dr Paul Lavery

Second Advisor

Dr Mark Lund

Abstract

The ability of natural wetlands to act as effective nutrient sinks and to absorb new nutrient loadings is well documented. Constructed wetland systems (CWSs) aimed at optimising these nutrient removal mechanisms have been used for the removal of nutrients and pollutants from a variety of waters and wastewaters over the past thirty years. Over the past decade, the use of CWSs has extended to the removal of nutrients from urban stormwater, as a more ecologically sensible management option to the traditional method of discharging stormwater into natural wetlands. Stormwater CWSs on the Swan Coastal Plain are designed to remove phosphorus. Phosphorus is a commonly limiting nutrient affecting plant growth and the soils of the Coastal Plain have traditionally been heavily supplemented with phosphorus for urban and agricultural purposes. Despite the aims of these systems, stormwater CWSs on the Swan Coastal Plain have indicated poor phosphorus removal, typically 60-70% lower than their designed target. In contrast, natural wetlands on the Swan Coastal Plain have indicated significantly higher phosphorus removal. Conceptual models of phosphorus removal for CWSs suggest that phosphorus is predominantly removed by the biofilm component, suggested to account for more than half the cumulative phosphorus removal in the long-term. One hypothesis proposed to account for poor phosphorus removal in CWSs on the Swan Coastal Plain has been a lack of an active biofilm component. Biofilms cover every surface of aquatic systems in a thin film, and consist of an organic matrix of algae, fungi and bacteria embedded in polysaccharides. This study compared the biofilms of two CWSs with four physico-chemically distinct natural wetlands on the Swan Coastal Plain in order to justify or reject the proposed hypothesis. The study consisted of two distinctly separate experimental components. The first of these components aimed at quantifying the composition and biomass of biofilms, by investigating biofilm biomass in terms of organic, inorganic and percentage organic biomass, as well as biofilm composition in terms of the algal, fungal and bacterial component percentage cover. The second component aimed at determining the rate at which biofilm can remove phosphorus from the water column by a series of controlled nutrient depletion 'batch-culture' experiments. The results indicated that biofilms in natural wetlands on the Swan Coastal Plain were highly variable in terms of both biomass and composition. The two CWSs sampled indicated comparable biofilm biomass and composition, with the measured parameters generally falling within the ranges observed between the natural wetlands. The composition of biofilms appeared to be a reflection of the Photosynthetically active radiation (PAR) intensity at the sediment, with the biofilms in wetlands observed having high colour (low PAR intensity) being fungal/bacterial dominated, and biofilms in wetlands observed having low colour (high PAR intensity) being algal dominated. The biofilm composition of both CWSs was fungal/bacterial dominated because of high colour. The phosphorus removal rate by biofilm appeared to be concentration dependant, with negligible phosphorus removal at low concentrations. However, at high concentrations, the phosphorus removal rates established were significantly higher than those previously published, confirming that biofilms have the potential for significant phosphorus removal from CWSs. This research demonstrated that biofilms have the ability to remove significant quantities of phosphorus at reasonably high rates. Poor phosphorus removal of stormwater CWSs on the Swan Coastal Plain likely result from biofilm compositions poor at phosphorus removal, resulting from CWS design that fails to optimise both biofilm biomass and biofilm composition. The research results indicated that the engineering of algal-dominated biofilm composition by manipulating CWS design, as well as increasing the surface area for biofilm growth, may significantly increase phosphorus removal.

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