Phosphorus Retention by Bioretention Mesocosms Using Media Formulated for Phosphorus Sorption: Response to Accelerated Loads

Recent research indicates that phosphorus (P) retention by bioretention systems comprising sandy media may not be effective for even a decade of urban runoff loads. To improve P retention for longer durations, this paper present findings from bioretention mesocosms using media amended with red mud (RM), a by-product of bauxite processing; water treatment residuals (WTRs), a by-product of water treatment; or Krasnozem soil (K), a highly aggregated clay soil. All treatments were vegetated except for one (K20nv). All treatments had outlets to restrict outflows except for one (WTR-Knr). To simulate the effect of long-term nutrient loads, the mesocosms were loaded weekly with secondary treated effluent with P concentrations averaging 3.3 mg-L-1. Over 80 weeks, this comprised hydraulic loads from 24.5 to 29.3 m-year(-1) at a flow-weighted average between 2.8 and 3. 2 mg-L-1 PO4-P, or mass loads from 1,115 to 1; 284 kg-ha(-1) PO4-P. These cumulative P loads represent the equivalent of over three decades of runoff loads. After 80 weeks, cumulative PO4-P retention in the K and RM soil treatments ranged from 79% to 95%, whereas PO4-P retention in the WTR treatments ranged from 95% to 99% of the input load. At 6-month intervals, the treatments were dosed with at least four sequential dosing runs of synthetic storm water with concentrations less than 0.8 mg-L-1 PO4-P. After 56 weeks of effluent loading, removal of PO4-P from storm water was negative 109% in the unvegetated K20nv treatment, compared with 33% retention in the corresponding vegetated K20 treatment. The K40 treatment with the most K retained 69% PO4-P, while the RM10 treatment with the most RM retained 78% PO4-P. After 80 weeks of effluent loading, removal of PO4-P from storm water was negative in both the K20nv and vegetated K20 treatments. The K40 treatment retained 76% PO4-P, and the RM10 treatment retained 55% PO4-P, while the total dissolved P (TDP) retention was 72% and 52%, respectively. After 110 weeks of effluent loading comprising 1; 598 kg-ha(-1) PO4-P, equivalent to 48 years of bioretention loads, PO4-P retention from storm water by the K40 treatment increased to 85%, and retention by the RM10 treatment increased to 91%. TDP retention also increased to 78% and 75%, respectively. These observations of P retention increasing after exposure to additional loads are uncharacteristic of typical sorption responses. After 80 weeks of effluent loading equivalent to 32 years of bioretention loads, the flow-restricted WTR-K treatment removed 99% of the storm-water PO4-P load while the corresponding free-discharge WTR-Knr treatment retained 94%. The WTR-K treatment was less effective than the WTR-Knr treatment in the earlier storm-water runs. The restricted outlet WTR30 treatment, which contained the most WTRs, retained 99% of the storm-water PO4-P load. These high rates of P retention from storm water after accelerated P loads indicate that these amendments can provide effective P retention for the expected lifetime of bioretention facilities. DOI: 10.1061/(ASCE)IR.1943-4774.0000243. (C) 2011 American Society of Civil Engineers.