Urban surface water quality enhancement; a case study on a floating treatment system

Abstract

Ongoing urbanisation and the subsequent extensive use of the urban water system can lead to degradation of its surface water quality. In the Netherlands, urban water bodies often suffer from the manifestations of eutrophication due to (historically) high nutrient loadings. This research focused on the enhancement of surface water quality of semi-confined urban water bodies with a case study on a floating treatment system. Urban water bodies often function as amenities of the urban area. Their ornamental and ecological value depends on the state of their aquatic ecosystem. Excessive nutrient loading, leading to the collapse of the system’s biodiversity, turns a water body into a turbid state without submerged macrophytes. Restoration of the clear water state through reduction of nutrient loadings alone is hindered by hysteresis caused by the ecosystem relations and could be supplemented with an approach focused on increasing the system’s nutrient carrying capacity or an internal approach directly targeting the manifestations of eutrophication. The Bright Water Company floating helophyte filter actively drains a filter bed with bog plants growing in it. The influent of this biofilter is provided by free inflow of surrounding surface water. Its water treatment ability depends predominantly on filtration and adsorption by specific nutrient absorbents. Additionally, its inner reservoir serves as a habitat for small aquatic organisms. Insights on the functioning and applicability of the biofilter were gained through in situ measurements. Two biofilter were applied in the Floresvijver in Groningen and measurements were conducted on influent, effluent and surface water. Visual observations and laboratory analyses of the water samples showed effective filtration and daphnia flourishing in the inner reservoir of the biofilter. Accumulation of the residual solids as a sludge layer on top of the filter bed and formation of biogas inside the filter material proved to reduce the hydraulic capacity significantly. Nutrient removal efficiency could not be determined with the monthly measurements of the water board but for optimal functioning of the phosphorous absorbent the current filter bed design should be adjusted while effluent samples indicated leaching of absorbent components. Application of the biofilter can contribute to the enhancement of urban surface water quality by increasing the nutrient carrying capacity of a water body. Especially in urban areas with various diffusive nutrient sources and physical constrains, the application of the biofilter can be efficient. Additionally, the biofilter functions as a habitat for zooplankton which are an important ecosystem element for the prevention of algae blooms. Furthermore, the effluent of the biofilter can provide a local increase in transparency and improve conditions for macrophyte development. The number of biofilters applied in a water body determine the significance of these contributions relative to the existing conditions.