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Mass retention in a floating treatment wetland with varying root lengths

IWRA 2021 Online Conference One Water, One Health
Theme 4: What are the synergies or trade-offs between ecosystem health and human health?
Author(s): TaĆ­s N. Yamasaki, Johannes G. Janzen

Taís N. Yamasaki, Johannes G. Janzen
Federal University of Mato Grosso do Sul, Faculty of Engineering, Architecture and Urbanism and Geography

Keyword(s): Floating treatment wetland, numerical modelling, nature-based solution


Urban surface waters, such as channels and retention ponds, are important drainage structures for buffering rainstorm events, but they are not designed to improve water quality. Consequently, the collected water that is later drained to other rivers and tributaries can carry pollutants that harm the ecosystem health. With urban growth and more frequent flooding events, this problem tends to aggravate. To tackle this problem, floating treatment wetlands are becoming a reliable nature-based solution that promotes several water treatment functions, such as filtration, sedimentation and uptake of nutrients directly from the water column. The advantage of floating treatment wetlands is that they can be placed directly on the water body surface, retrofitting the channel or pond. In addition, floating wetlands can have aesthetic features, which improve urban landscape. When it comes to treatment efficiency, it depends, among other factors, on how much untreated mass can actually be retained by the wetlands’ root zone. Thus, the objective of this study was to measure the temporary retention of mass inside a floating treatment wetland, focusing on how the variation in root length affected mass retention. The root length varied between 10% to 90% of the total water depth. The study was performed numerically, using Computational Fluid Dynamics. A pulse of conservative tracer was injected at the channel entrance, from which it travelled the domain. The mass of tracer temporarily retained by the wetland was monitored for all root lengths tested. The wetland was represented by a porous media. The results show that increasing the root length caused more tracer to pass through the wetland. A nine-fold increase in retained mass was measured between the shortest and the longest root length tested, which is good because more mass would be subjected to treatment. It is concluded that in terms of mass retention, it is preferrable to use plant species that develop long roots when designing
floating treatment wetlands.

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