Phosphorus retention in the soil matrix of constructed wetlands

Abstract

Constructed wetlands may be described as soil/plant systems for wastewater treatment in which pollutant removal is based on general principles of nutrient transformation in soils. Currently perceived as “black boxes”; by engineers, the design and operation of these systems may be greatly improved based on the knowledge gained from several decades of studying nutrient cycling in soil‐plant systems. This paper reports on an attempt to operate this linkage. Three pilot scale systems planted with reed, cattail, and water hyacinth were used to study the role of the soil matrix in phosphorus (P) removal over a period of five months. Phosphorus removal was superior in the soil‐based systems with a mean P reduction from the influent concentration (24 mg‐mL‐1) of 80% compared with 54% in the soilless bed. Recycling the effluent into the system in order to increase the detention time did not contribute to improving removal, except in the soilless bed. This indicates that P removal in the soil‐based systems is rapid, and that an equilibrium value may be reached beyond which no further removal is possible. The effect of a lime amendment on the improvement of P removal was studied in batch tests in a decarbonated sand amended with 1.4%, 12.2%, 21%, 38%, and 49% calcium carbonate (CaCO3). Phosphorus removal from solution can be significantly improved by the addition of small amounts of lime (2–4%). Fixation is also faster and sustainable in lime‐amended sands. These results suggest that P removal from wastewater can be greatly enhanced by the addition of small amounts of lime to the soil substrate.