Stormwater Bioretention Systems: Testing the Phosphorus Saturation Index and Compost Feedstocks as Predictive Tools for System Performance

Abstract

A replicated column trial was conducted to evaluate the potential for the phosphorus saturation index (PSI) to predict P movement in bioretention soil mixtures (BSMs). The impact of compost feedstock on BSM performance was also evaluated. Three composts (biosolids/yard, yard/food waste, and manure/sawdust) were each brought to PSI values of 0.1, 0.5, and 1.0 through the addition of Fe-based water treatment residuals (WTRs) to lower the PSI and P salts to increase the PSI. A synthetic stormwater solution was used for 12 leaching events. The PSI predicted total and dissolved P concentrations in column leachate. All composts removed P at PSI 0.1. All composts were a source of P for the higher PSI values tested, with P concentrations in the leachate decreasing over time. Ammonia and nitrate from all treatments decreased over time, with all treatments showing effective N removal. Copper removal (total and dissolved) was >90% for all treatments, with the highest removal observed at PSI 0.1 for all composts. Zinc removal (total) was also greatest in the 0.1 PSI for all composts. At PSI 0.5 and 1.0, the biosolids/yard compost was less effective than the other materials at removing Zn, with a removal efficiency of approximately 50%. Infiltration rates were similar across all treatments and ranged from 0.44 ± 0.1 cm min in the manure/sawdust at PSI 0.1 to 3.8 ± 2.8 cm min in the food/yard at PSI 1.0. Plant growth in the manure/sawdust compost was reduced in comparison to the other composts tested across all PSI levels. The results of this study indicate that the PSI may be an effective tool for predicting P movement in bioretention systems. Compost feedstock does not indicate the ability of composts to filter contaminants filtration, with all composts tested showing high contaminant removal.

Soil in the City: Sustainably Improving Urban Soils

“…In their study, Brown et al (2016b) propose that the P saturation index (PSI) could be used as a tool to evaluate whether the soil mixes used in GI systems could become a sink or source of nutrients, like N and P, and also of metals, like Zn and Cu. The PSI was developed as a predictive tool to determine the potential of P leaching from soils amended with organic residuals like manures and biosolids (Elliott et al, 2002; Agyin‐Birikorang and O'Connor, 2007) and is based on a strong correlation between the ratio of total P to Fe and Al oxides as determined by oxalate extraction (PSI = P ox /(Fe ox + Al ox ) and the P found in leachate.…”

“…The PSI was developed as a predictive tool to determine the potential of P leaching from soils amended with organic residuals like manures and biosolids (Elliott et al, 2002; Agyin‐Birikorang and O'Connor, 2007) and is based on a strong correlation between the ratio of total P to Fe and Al oxides as determined by oxalate extraction (PSI = P ox /(Fe ox + Al ox ) and the P found in leachate. Brown et al (2016b) evaluated three different composts adjusted with Fe‐based drinking water treatment residuals and P salts to PSI values of 0.1, 0.5, and 1.0 on nutrient and metal leaching using a synthetic stormwater solution and also evaluated plant performance. Results indicate that compost from manure/sawdust performed poorly in terms of plant performance.…”

“…Urban stormwater may contain a wide range of contaminants, including particulates, nutrients, metals, and organic matter like fats, oils, and grease. In general, the focus has been on heavy metals and nutrients as contaminants in urban runoff, and the reported concentrations have been typically <1 mg L −1 and <2 mg L −1 , respectively (Brown et al, 2016b). Removal efficiencies or retention of these pollutants will depend on the characteristics of soils used in the GI retention systems.…”

“…Removal efficiencies or retention of these pollutants will depend on the characteristics of soils used in the GI retention systems. Brown et al (2016b), referring to a review of soil specification from 16 states by Carpenter and Hallam (2010), indicate that composition of soils used in GI systems varies from 30 to 60% sand, compost ranging from 20 to 40%, and top soil ranging from 20 to 30%.…”

Urban Stormwater Characterization, Control, and Treatment

“…For Cu, the compost-amended media was a source during the first three to five storms, but then the media became a sink. Additionally,Brown et al (2016) indicated that the phosphorus saturation index (PSI) could be used as a predictive tool for estimating treatment capacity and that compost source was not a good indicator of the ability of the compost to filter contaminants Logsdon and Sauer (2016b). demonstrated leaching from compost-amended media and recommended that only small amounts of low-nutrient compost should be incorporated into bioretention media.…”