The vertical flow constructed wetland in Ellenbrook was constructed in 2014 to reduce nutrient concentration within the Ellen Brook and, therefore, eutrophication within the Swan River. The current filter media was selected for its ability to sorb and remove phosphorus from the Ellen Brook. Over the period of operation, the filter media has declined in phosphorus removal efficiency and the rate of infiltration has decreased. The spent filter media will require replacement within the next two years.
The study investigates the performance of several filter medium employed in pilot scale vertical flow constructed wetlands with specific focus on phosphorus removal. Five pilot trial wetlands, containing four different filter medium, were assembled inside intermediate bulk containers. The filter media underwent soil characterisation surveys for analysis of particle size distribution and physical and chemical parameters. Each wetland was fed influent water from the Ellenbrook and the effluent was sampled for analysis of the water chemistry. Measurements were taken to determine the infiltration rate of each pilot trial wetland.
Infiltration rates ranged between 217.0mm/h and 2418.9mm/h. Phosphorus removal efficiency was, on average, between 13.7% and 68.3%. A significant correlation between infiltration rate and phosphorus removal efficiency was observed - pilot trial wetlands with a lower infiltration rate appeared to remove significantly more phosphorus. Taking both factors into account, phosphorus removal on an hourly basis was calculated and ranged between 18mg/h and 404mg/h. Nitrogen removal ranged between 43mg/h and 450mg/h. The release of other contaminants was observed from all filter media. Comparison with several literature and guideline values indicated that no filter media conformed to the specified limits. This may result in operational issues overtime and restrict the ability to repurpose filter media once it is saturated with phosphorus.
The results from this study will assist the Department of Biodiversity, Conservation and Attractions in the decisions and planning to replace the filter media within the Ellenbrook wetland
