By Norah Kates, University of Washington

What happens to the water that falls on our cities every time it rains? A lot of the answer depends on what comes in contact with that water as it runs down rooftops, over streets, through gardens, and eventually – sometimes through creeks, rivers, or lakes – into the Puget Sound. What that water picks up along the way has big implications for environmental health.

So how do we keep the Sound clean, or help make it cleaner than it is now? And what do soil and biosolids have to do with that?

Green Stormwater Infrastructure is a way to help clean the water that falls over our urban landscapes by building special gardens, sometimes called rain gardens, bioswales, or bioretention basins, that use the natural ability of soil to filter water. Soil is a great filter, and by adding bioretention systems across the city, we get increased capacity to capture and treat stormwater, meaning cleaner water and less flooding, along with other benefits like more beautiful streets.

Not just any soil can be used for bioretention. Because it needs to work within a small footprint, and because we’re dealing with sensitive water quality issues, only specially-selected soil and plants are up to the challenge. The perfect bioretention soil balances three priorities: fast drainage (to prevent flooding), effective filtration, and the ability to support plant life. Too much of any one of these things can potentially jeopardize the others. 


We are still learning what kind of soil works best for bioretention. Most bioretention soil is a mixture of sand and an organic component like compost, but the sources of those materials, and the relative proportions of each, vary widely (Davis et al. 2009). Past research has shown that mixtures using many different kinds of organic components, including biosolids, can be effective at removing heavy metals and organic pollutants like PAHs (Brown et al. 2016, Jay et al. 2017, Paus et al. 2014). However, organic material can also export phosphorus into water. That’s a big problem, because when high-phosphorus effluent gets into surface waters, it can cause eutrophication.

So how do we balance bioretention soil, so that we get the performance we want (stormwater retention and water filtration), without too much phosphorus being flushed out? This was the subject of recent research at the University of Washington’s School of Environmental and Forest Sciences, with support from Northwest Biosolids and King County Resource Recovery.

We looked at the relationship between phosphorus, aluminum, and iron within a wide range of bioretention soil mixtures. Aluminum and iron are natural phosphorus-binding elements in soil and can also be added as amendments. For this study, we used water treatment residuals, byproducts of treating drinking water and excellent sources of aluminum and iron. These residuals have been used in agriculture to help control soils with too much phosphorus, and now researchers are looking into using them in a bioretention context, with promising results (Elliott et al. 2002, Agyin-Birikorang et al. 2007, Jay et al. 2016). Residuals from different drinking water treatment plants will have different amounts of aluminum and iron. For the organic component of the soil mixtures, we used compost from yard waste alone, food and yard waste combined, and biosolids and sawdust. We also used three different biosolids from the three King County wastewater treatment plants, and finally a potting soil blend that includes biosolids. 

In the study, we combined soil media and either DI water or a synthetic phosphorus solution in small cups, shook them up for 24 hours, ran them through a filter, and tested the liquid for dissolved phosphorus. We examined the organics for their ability to release phosphorus, water treatment residuals for their ability to adsorb phosphorus, and the two together. This gives us a good picture of what is going into and coming out of various soil components, and what the overall mixture is exporting to water. But the methods we used are very different from how real bioretention systems work, with water filtering down through the soil profile. So our results won’t tell us exactly what to expect from field conditions, especially in terms of the magnitude of phosphorus movement; for that, we need more research at larger scales.

All organic materials tested released a lot of phosphorus on their own (from 326 mg P/kg on the low end, to almost 10,000 mg P/kg). The King County biosolids released very high amounts of phosphorus, compared with the other organics tested, while high-iron biosolids from San Francisco (iron is added in the wastewater treatment process) released very low amounts on par with yard and food/yard waste composts. All water treatment residuals adsorbed a lot of phosphorus (rates varied by the concentration of phosphorus in solution but ranged from at least 1,000 to over 4,000 mg P/kg).
p release

The proportion of phosphorus to aluminum and iron in soil is called the soil’s phosphorus saturation index, or PSI (it can also be called a phosphorus saturation ratio, or PSR). In our research, we combined the different organic soil components with different water treatment residuals in varying proportions to get a wide range of mixtures. Each mixture had its own PSI, and when we looked at the relationship between PSI and the amount of phosphorus that was released by the mixture, we found a strong relationship. This means that PSI could be a good tool for predicting a bioretention soil’s expected phosphorus release, and for figuring out how much aluminum and iron amendment, like drinking water treatment residual material, may be needed to create a balance.

p release organicsWe found big differences in the amount of phosphorus release between all of the materials tested, and between mixtures with different amounts of each soil component (organics and water treatment residuals). In part, this was due to the differences in their PSI values. But there were additional differences, evidenced by the fact that the various materials did not all perform exactly the same, in terms of phosphorus release, even when mixtures were brought to the same PSI values. This is the interaction effect between PSI and type of organic, and between PSI and type of water treatment residual (see graph below with two of the 8 organics used in the study, as examples). However, all organics and all water treatment residuals released more phosphorus from mixtures with higher PSI values, and less phosphorus from mixtures with lower PSI values.

psi values

The results of this study present a strong endorsement for using PSI as a tool to control phosphorus release from bioretention soils, and for using water treatment residuals to manipulate PSI. By paying closer attention to soil chemistry and making use of amendments that can lower the PSI of soil mixtures, better-performing bioretention systems can be built. This means more opportunities to expand the areas where bioretention can be used, and cleaner water in our Sound.

Many thanks to Emma Leonard, Sally Landefeld, and Toby Una for help with lab work, and to Sally Brown, David Butman, Rebecca Singer, Fritz Grothkopp, Derrick Sanders, Matt Boyles, and Dongsen Xue for critical guidance on this research.

storm drain

Literature Cited

Agyin-Birikorang, S., & O’Connor, G. A. (2007). Lability of Drinking Water Treatment Residuals (WTR) Immobilized Phosphorus: Aging and pH Effects. Journal of Environment Quality, 36(4), 1076–1085.

Brown, S., Corfman, A., Mendrey, K., Kurtz, K., & Grothkopp, F. (2016). Stormwater Bioretention Systems: Testing the Phosphorus Saturation Index and Compost Feedstocks as Predictive Tools for System Performance. Journal of Environment Quality, 45(1), 98.

Davis, Allen P., Hunt, W. F., Traver, R. G., & Clar, M. (2009). Bioretention Technology: Overview of Current Practice and Future Needs. Journal of Environmental Engineering, 135(3), 109–117.

Elliott, H. A., O’Connor, G. A., Lu, P., & Brinton, S. (2002). Influence of Water Treatment Residuals on Phosphorus Solubility and Leaching. Journal of Environment Quality, 31, 1362–1369.
Jay, J. G., Brown, S. L., Kurtz, K., & Grothkopp, F. (2017). Predictors of Phosphorus Leaching from Bioretention Soil Media. Journal of Environment Quality, 46(5), 1098.

Jay, J. G., Brown, S. L., Tyler-Plog, M., Brown, S. L., & Grothkopp, F. (2018). Nutrient, Metal, and Organics Removal from Stormwater Using a Range of Bioretention Soil Mixtures. Journal of Environment Quality.

Paus, K. H., Morgan, J., Gulliver, J. S., & Hozalski, R. M. (2014). Effects of Bioretention Media Compost Volume Fraction on Toxic Metals Removal, Hydraulic Conductivity, and Phosphorous Release. Journal of Environmental Engineering, 140(10), 04014033.