Water Treatment Residuals in Bioretention Planters to Reduce Phosphorus Levels in Stormwater

Abstract

Abstract

Recent studies have shown that phosphorus leaches from bioretention soil mixes (BSMs), which can lead to algal blooms in receiving waters. Water treatment residuals (WTRs), by-products of the water treatment process (which commonly contains alum [Al₂(SO₄)₃·14H₂O]), may help retain phosphorus in soil. Aluminum complexes with phosphate to form a precipitate (AlPO₄), effectively removing it from stormwater. Many water treatment plants have to pay to dispose of WTRs at a landfill. Using WTRs in bioretention can be a beneficial reuse and cost-saving measure for municipalities. However, the vast majority of studies have been small-scale column and batch experiments conducted in a laboratory or other controlled setting. In this study, large-scale testing was conducted in the field by adding WTRs to BSMs to evaluate phosphorus retention. Five planters were constructed: a control with BSM only, two planters with bioretention soil mixed with WTRs, and two planters with layers of compost, WTRs, and sand. Compared with the control, total phosphorus (TP) and phosphate concentrations in mixed planters were 58% and 67% lower on average, respectively. TP and phosphate concentrations in layered planters were 89% and 95% lower than the control on average, respectively. Aluminum levels in the effluent from mixed and layered planters were very similar to levels in the effluent from the control planter. After the first test, aluminum levels in the effluent from all planters were below 0.55 mg/L. This study shows that the use of WTRs in bioretention beds, particularly when soil components are layered, is an effective method for reducing the amount of phosphorus leached from the soil mix at the field scale. Stormwater managers, particularly in watersheds where phosphorus is a concern in receiving waters, should consider using WTRs and layering for future bioretention installations.