Developing Biosolids Based Bioretention Soils for Green Stormwater Infrastructure

Abstract

Bioretention systems are increasingly used as alternatives to engineered systems for stormwater management in urban areas. These systems typically consist of sand and compost and are designed to allow for rapid infiltration and filtering of stormwater. In some cases, these systems drain directly into streams. In other cases, the water percolates through the subsoil into groundwater. The systems are also designed for a wide range of settings including curbside systems, systems in parking lots and systems alongside buildings. The contaminants of concern in the stormwater include nutrients (N and P), metals (primarily Cu, Pb and Zn), organics (primarily PAHs) sediment and pathogens. Recent work has shown that untreated stormwater is toxic to fish and that filtering stormwater through a sand, compost and bark mixture was sufficient to eliminate any negative impacts (McIntyre et al., 2016). The current regulations on bioretention systems for green stormwater infrastructure in WA State specify that the compost used not include municipal biosolids or animal manures. The rational for this guidance is that biosolids and manures are considered to have higher nutrient and metal concentrations than yard waste or food scraps. The current guidance does not take into account that composts can have widely different characteristics based on feedstock blends and different ratios of each component of the mixture. Previous work demonstrated that feedstocks were not a significant predictor of performance for bioretention systems . In contrast, the phosphorus saturation index (PSI), a test to estimate the potential for phosphorus leaching, was shown to be highly predictive of efficacy of the different mixtures for both nutrients and metals. MethodsWe conducted a replicated field trial to test a range of different bioretention soil mixtures (BSM) for their ability to remove metals and nutrients from stormwater. The study included 40% food yard waste compost and 60% sand by volume, the current recommended mixture in WA State and a 100% sand as control treatments. Other treatments included biosolids: yard waste compost produced by King County added at different rates with sand, and +/-Fe based water treatment residuals, and high Fe biosolids from DC Water, used both directly and following composting. A list of the treatments used in the study is shown below. The control treatments and the treatments that include biosolids from DC Water are shown in bold.