Floating Treatment Island configuration for optimum nutrient removal

Abstract

Floating Treatment Islands (FTIs) have been studied as a method to mitigate the risks associated with high nutrient levels in contaminated water. The goal of this project was to compare fractional treatment rates by a series of FTIs located at the edge of a channel, allowing the center channel to remain clear. Experiments were performed using a scaled-down model floating treatment island (FTI) with a 19 x 24.5 cm x 10 cm root zone modeled using 3.6 mm diameter dowel rods (n = 75/135 cm 2, low flow blockage) attached to the inside wall of a 1.2 m wide x 16 m long flume. Three cases were considered, with four FTIs spaced at various distances based on the length scale L of the FTI: the closest spacing had each FTI located 2L downstream of the last, the mid-range spacing were placed 4L apart, and the farthest spacing had each FTI 8L past the last. Based on the cross-sectionally averaged flow rates measured at the leading and trailing edges of the root zone, treatment rates within the root zone were estimated using a first-order kinetic model, and an iterative method was used to solve for the fractional treatment by the series of FTIs. This paper explores the effects of various parameters on treatment, including flow rates and velocity recovery, biological uptake rate, and island size. Finally, the researcher evaluated which setup provided the most treatment for a given number of treatment islands. It was found that the 8L spacing provided the best treatment, significantly more than the 4L or 2L spacing for k < 10 day⁻¹ according to the results of a two-sample t-test. For a set of 8L spaced FTIs with an uptake rate of 1 day⁻¹ with a cross-sectional coverage of 13.6%, it would be possible to reduce the upstream nutrient concentration by 25% over a channel length of roughly 3.5 km, 50% over a channel length of 10 km, and 90% over a channel length of approximately 32 km. These treatment rates could have the potential to mitigate the risk of eutrophication in sufficiently long channels.