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 $3.3\mathrm{mg}\mathrm{\text{-}}{\mathrm{L}}^{-1}$. Over 80 weeks, this comprised hydraulic loads from 24.5 to $29.3\mathrm{m}\mathrm{\text{-}}{\mathrm{year}}^{-1}$ at a flow-weighted average between 2.8 and $3.2\mathrm{mg}\mathrm{\text{-}}{\mathrm{L}}^{-1}$ ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$, or mass loads from 1,115 to $1,284\mathrm{kg}\mathrm{\text{-}}{\mathrm{ha}}^{-1}$ ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$. These cumulative P loads represent the equivalent of over three decades of runoff loads. After 80 weeks, cumulative ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$ retention in the K and RM soil treatments ranged from 79% to 95%, whereas ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$ 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 $0.8\mathrm{mg}\mathrm{\text{-}}{\mathrm{L}}^{-1}$ ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$. After 56 weeks of effluent loading, removal of ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$ 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% ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$, while the RM10 treatment with the most RM retained 78% ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$. After 80 weeks of effluent loading, removal of ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$ from storm water was negative in both the K20nv and vegetated K20 treatments. The K40 treatment retained 76% ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$, and the RM10 treatment retained 55% ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$, while the total dissolved P (TDP) retention was 72% and 52%, respectively. After 110 weeks of effluent loading comprising $1,598\mathrm{kg}\mathrm{\text{-}}{\mathrm{ha}}^{-1}$ ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$, equivalent to 48 years of bioretention loads, ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$ 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 ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$ 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 ${\mathrm{PO}}_4\mathrm{\text{-}}\mathrm{P}$ 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.