Submitted to: Water Quality Modeling International Symposium
Publication Type: Proceedings
Publication Acceptance Date: April 1, 1995
Publication Date: N/A
Interpretive Summary: Wetlands are nature's way of removing nutrient pollutants from water. Constructed wetlands are an attempt to use the water purifying capacity of a wetland to remove nutrients or other contaminants from a water source. In many cases, nutrient removal by constructed wetlands is limited because of an imbalance of nutrients in the wastewater. These nutrient imbalances can be overcome by the expensive addition of nutrients; however, a more efficient approach is to design a wetland which is less sensitive to the balance of nutrients in the water being treated. The water-flow pattern in most wetlands in use today is horizontal. In horizontal-flow wetlands, plants absorb nutrients in excess at the inlet, leaving little to sustain growth in the rest of the wetland. In this study, we tested if nutrient removal was increased by water flowing horizontally or vertically through a wetland. We showed that a vertical-flow pattern in wetlands distributes nutrients evenly throughout the wetland, thereby reducing the sensitivity of the wetland to nutrient imbalances and increasing nutrient removal. Constructed wetlands are used to clean water by small communities and industries, because they are more economical than standard mechanical treatment technology; these groups will benefit from this improvement in wetland design.
As wetland productivity increases, so does the P removal potential of that wetland. To maximize both productivity and P removal, P must be the most limiting nutrient. Phosphorus can be made the most limiting nutrient in the wastewater being treated by adding the other more limiting nutrients. However, an easier, more maintenance-free approach is to design a wetland which is less sensitive to the balance of nutrients in the water being treated. A model was developed using STELLA II which focuses on the effect of flow pattern on P removal by plants in wetlands. Model simulations showed that a vertical flow pattern in wetlands reduces the sensitivity of the wetland to nutrient balance by distributing nutrients evenly throughout the wetland. By making the distribution of nutrients more uniform throughout the wetland, luxury consumption of nutrients limiting productivity is minimized, thereby increasing limiting nutrient use efficiency. The result is that wetland productivity is maximized with a vertical flow pattern. Phosphorus removal is maximized because luxury consumption of P can still occur, and productivity is not diminished by other nutrient limitations.