Industrial Byproducts as a Treatment for Bioretention Underdrain Effluent: A Benchtop Proof of Concept
Abstract
Around the world, stormwater is a frequent and growing source of contamination for waterbodies. Common sources of pollution include heavy metals such as, but not limited to aluminum (Al), arsenic (As), cadmium (Cd), cobalt (Co), copper (Cu), lead (Pb), manganese (Mn), nickel (Ni), selenium (Se), and zinc (Zn). Nutrients such as phosphorus (P) also pose a large risk to waterbodies. Neonicotinoid pesticides are the most commonly used pesticide in the United States and are of growing concern to waterbodies, aquatic life, and non-aquatic life. Low impact development (LID) structures such as bioretention have been effective in the management of stormwater pollutants for decades. More recently, areas for improvement in bioretention have been identified and a need for additional water quality improvements has been recognized. Industrial byproducts have been suggested as an augmentation to bioretention design as a way to provide low-cost water quality improvement while giving an otherwise disposed byproduct a second and sustainable use. This study focused on using fly ash (a coal burning byproduct), steel slag (a steel smelting byproduct), and biochar (wood waste heated with little to no oxygen) in treating a complex synthetic stormwater. This proof of concept benchtop study looked at influent and effluent concentrations in flowthrough column studies for a suite of metals, phosphorus, and three selected neonicotinoids to identify which media yielded the best removal potential for the various contaminants. Columns were set up in two orientations, one vertical with constant flow and full saturation and another horizontal which included a bioretention simulation with head-driven flow which flowed into a treatment column filled with the experimental media. Results showed that aggregated fly ash and steel slag yielded a noticeably higher pH, likely from the dissolution of different -oxide species into solution. This dissolution also contributed to a noticeable increase in specific conductivity. These trends were observed for biochar as well but at noticeably lower magnitudes. In vertical columns, the small-sized fraction of steel slag proved to be the best to remove most contaminants from solution. Some exceptions existed where either the large-sized fraction of aggregated fly ash was more effective, or results were mixed. Removal is generally understood to be via sorption, however further studies would be required to confirm the mode of removal. Major cations such as Ca+2, Na+, Mg+2, and K+ were generally leached from the media with aggregated fly ash and steel slag as the most common culprits due to their high composition of these elements. Because biochar posed a lesser leaching risk and showed comparable contaminant removal to aggregated fly ash, vertical column experiments suggest biochar as the next best treatment media behind small steel slag. Horizontal column experiments had the additional condition of results being compared to the removal rates of a bioretention simulation control column. Where the control failed to treat all three neonicotinoids, P, As+5, and Na+, the experimental columns containing the media of study proved to be effective treatments. Small steel slag again showed to be the best medium to remove As+5, and P where biochar proved to yet again prevent meaningful major cation leaching and was a sink for Na and all three neonicotinoids. Depending on the contaminant of concern being treated, either biochar or small steel slag would be the standout options for bioretention effluent treatment. Further studies are recommended to confirm the mechanism by which contaminants are removed, the stability of the removed contaminants to stay out of solution, and the environmental safety of deploying these media into the real world. Modeling is also recommended to help guide future studies by predicting the state and species of contaminants in solution and their likelihood to sorb to the experimental media or precipitate as a result of treatment.
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