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Constructed Wetlands for Wastewater Treatment: Optimizing Biogeochemical Cycling in an Engineered Ecosystem

2010Ecology and nutrient cycling in constructed wetlands with artificial aerationBORST Barbara (USA)
Organisation: Christian-Albrechts Universit´┐Żt, Kiel (DE) - UNESCO-IHE Institute, Delft (DE)Supervisor :Martin Zimmer (CAU), Diederik Rousseau (UNESCO)
Summary: The concept of ecosystem services has gained attention among ecologists in recent years, and wetlands in particular have been celebrated for their important contributions to society. Wetlands provide essential regulating ecosystem services, particularly in their contribution to biogeochemical cycling and nutrient retention. Constructed wetlands (CWs) are semi-open systems that are engineered to mimic and accelerate purification processes that occur in natural wetlands in a more controlled environment, explicitly for the purpose of treating contaminated wastewaters. They are currently used world-wide to treat various types of wastewaters. CWs are traditionally very effective at BOD, DOC, and suspended solids removal, while nutrient removal or immobilization can be a more elusive goal to achieve. Nutrient removal is thus often a key design goal for CWs, as anthropogenic release of nitrogen and phosphorus are recognized as the cause of ever-increasing problems of eutrophication in waters throughout the world. Therefore, much current research focuses on understanding the primary removal mechanisms for N and P in CWs and research to design CWs for optimization of these ecological processes. Research indicates that the primary mechanism for nitrogen removal is via bacterial nitrification and denitrification, the effectiveness of which can vary significantly depending on the design and conditions of the system. This nitrification and denitrification is attributed to the biological activity of bacterial biofilms that colonize the granular medium or sediment. A limiting factor of CW design is that nitrification requires aerobic conditions while dentrification requires anaerobic conditions. Horizontal sub-surface flow wetlands (HSSFWs) are popular because the can eliminate odor, mosquito, and evaporation problems. however, they also tend to foster anaerobic conditions, limiting nitrification, which is the first step in the nitrogen removal process. Artificial aeration has recently been introduced as a way in increase nitrification rates in CWs. The current research contributes to the body of knowledge to optimize the N and P removal from wastewater treated by CWs by investigating the effects of artificial aeration on the performance and ecology of the system. The research is being conducted at the lab of the UNESCO-IHE Institute in Delft, The Netherlands and involves three laboratory-scale HSSFWs consisting of lined beds filled with gravel, planted with phragmites reed and fed with diluted primary effluent from a nearby wastewater treatment plant. The three CW mesocosms were subjected to three different aeration treatments: one system is conventional (non-aerated), one system is aerated, and the final system is a hybrid system in series, where the first section aerated and the second system non-aerated. The research questions are:
What is the impact of aeration on nitrogen and phosphorus cycling in HSSFWs? What is the impact of different aeration treatments on micro, meio, and megafauna in HSSFWs?
What can be inferred about the relationship between the biodiversity of the system and nutrient removal performance? N and P forms were tracked by measuring ammonium, nitrate, total nitrogen, phosphate, and total phosphorus in the influents, midpoints, and effluents. Additionally, the ecology of the system was investigated to assess the impact of the different aeration treatments on the bacterial biofilm communities as well as the protozoa and invertebrate communities.