The effect of various intermittent dry-wet cycles on nitrogen removal capacity in biofilters systems

Abstract

Stormwater biofilter systems have the potential to remove nutrients from urban runoff. These systems operate in unique intermittent dry-wet cycles that may affect their performance. Current consensus suggests that sediment drying promotes the release of potentially significant amounts of bio-available nitrogen and phosphorus upon re-wetting. We sought to investigate the impact of drying/wetting cycles on biofilter performance. Eighteen columns were planted with Carex appressa which reached maturity after eight months. The recovery of biofilter systems was tested in a range of drying periods from one to eight weeks with and without a Submerged Anoxic Zone (SAZ) and carbon supplement in the filter media. In all experiments, moisture content, adjacent soil and ambient temperature were logged in parallel to record the drawdown profile behaviour. A freely drained biofilter configuration was used as a control. The results indicated that having a submerged anoxic zone is critical for nitrogen removal at dry periods greater than two weeks. In addition, the anoxic zone was able to enhance biofilter nitrogen removal recovery and make it less dependent on drying/wetting.