Michael Ament
University of Vermont
Co-Authors: E. Roy, S. Hurley, E. Perkins, Y. Yuan, M. Voorhees
Green stormwater infrastructure, such as bioretention systems, are increasingly used to manage stormwater volumes and pollutant loads in urban landscapes. However, phosphorus (P) removal in bioretention field studies has been highly variable, prompting research into P-sorbing amendments for bioretention media. In this study, we investigated potential tradeoffs between hydraulic conductivity and P sorption capacity in drinking water treatment residuals (DWTRs). We quantified the maximum P sorption capacity (Smax) for three DWTR sources using batch isotherm and flow-through column experiments. A large column experiment was also used to determine how solid and mixed layers of DWTRs affect system hydraulics and P removal performance. Study results showed that Smax values vary greatly among DWTR sources and experimental methodologies, which has implications for regulatory standards. When applied to bioretention media, the impact of DWTRs on hydraulic conductivity and P removal depended on layering strategy. Although both the solid and mixed layer designs improved P removal relative to the control, the mixed layer exhibited higher flow rates and better P removal than the solid layer. It is therefore recommended that DWTRs be mixed with sand in bioretention media to simultaneously achieve hydraulic control and long-term P removal in bioretention systems.
Virtual Poster Symposium 
Thank you for taking the time to make this presentation! I also work on analyzing P sorbing materials, though not so much their hydraulic conductivity. I’m sure you said this in your presentation, but what were the layers for your three column-types? I assume sand is the lighter color and the DWTR is represented in black, but what are the gray colors and the orange-ish one?
Also, by what mechanism does DWTR sorb P? What is it composed of?
I’ve come across primarily steel slag or purposefully-designed iron-based sorption materials, so I’m unfamiliar with this one. Thank you!
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Hi Samantha,
Thanks for your questions. The top layer is a blend of 90% washed sand and 10% “low phosphorus” compost (derived from leaf litter). The next layer is either a layer of 100% washed sand, or 90% sand and 10% DWTR. The layers below that are pea stone and gravel for drainage purposes. The DWTRs are comprised of aluminum oxides, which have a net positive charge and they sorb the negatively charged phosphate ions via ligand exchange reactions. Ligand exchange reactions are much stronger than the cation exchange reactions that we think of in agricultural contexts involving organic matter, which is why P is often inaccessible to plants grown in soils dominated by metal oxides (tropical soils). DWTRs are a waste product generated by drinking water treatment plants and when processed (e.g. put through a freeze-thaw cycle) they can exhibit very high surface areas which gives them a high P sorption capacity. Hope this helps!
Mike
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Mike, thank you for such a comprehensive response! I really appreciate your explanation, that helps a lot!
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Michael – very nice work! I’m interested whether you looked at the DWT residuals themselves. I’ve seen reports that there is a high level of variability in WTR characteristics. These characteristics could have substantial impacts on the P sorption capacity that is left in the residuals. Did you do an analysis of the makeup (chemically) of the WTR? If so, how did that impact the removal rates you saw?
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Hi Ryan,
Yes, we did investigate the DWTRs themselves and found that the 3 sources varied 4-7 fold in their P sorption capacities. Interestingly, this variation did not correspond with metal oxide content, amorphous metal oxide content, or P saturation ratio. It did, however, correspond with surface areas, which aligns with our understanding of sorption as a surface process.
Mike
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