Interactions between design, plant growth and the treatment performance of stormwater biofilters

Plants play a critical role in the nutrient removal performance of stormwater biofilters. However, the influence of biofilter design on plant growth and subsequent implications for treatment performance are not well understood. A 12 month, laboratory-scale biofilter column experiment was conducted to investigate the response of Carex appressa to variations in biofilter design and implications for nutrient removal performance. Plant growth in Skye sand, a natural iron-coated sand with a strong capacity to immobilize phosphorus, was evaluated against a typical loamy sand filter media in biofilters with and without a saturated zone. Plant biomass correlated strongly with nutrient removal and was significantly greater in biofilters with a saturated zone, suggesting that inclusion of a saturated zone facilitates nutrient uptake. In the presence of a saturated zone, plants grown in Skye sand had a significantly higher specific root length, surface area and volume than plants grown in loamy sand, illustrating C. appressa's ability to adapt root morphology to maintain growth under nutrient limited conditions. These root traits also correlated strongly with nutrient removal, suggesting that use of Skye sand in biofilters rather than loamy sand would be advantageous for nutrient removal. However, root adaptations, in particular increased etiolation, can make plants vulnerable to stressful environments (e.g. prolonged drying). Therefore, it is critical that a saturated zone be included in stormwater biofilters to increase growth and protect against drying.