Agricultural Byproducts as Amendments in Bioretention Soils for Metal and Nutrient Removal

Abstract

This study investigated the effectiveness of metal and nutrient removal from stormwater in bioretention systems amended with agricultural byproducts. Both batch and column studies were conducted to evaluate three amendments: hazelnut shells, pecan shells, and spent grain from the brewing process. Batch studies using buffered synthetic water containing copper and zinc evaluated adsorptive properties of the three amendments. Of the three amendments, hazelnut shells had the highest sorption coefficient based on Kd ranges of 19,200–106,000 L/kg and 8,610–18,900 L/kg for zinc and copper, respectively. Both pecan shells and spent grain had significantly lower Kd values for zinc (2,160–6,030 L/kg and 1,702–55,932 L/kg for pecan shells and spent grain, respectively) and copper (1,090–1,760 L/kg and 1,270–2,030 L/kg for pecan shells and spent grain, respectively). However, the spent grain contained zinc that potentially could add to zinc concentrations in the stormwater. Column studies using stormwater collected from an industrial site evaluated metal and nutrient removal from stormwater. Six columns were packed with 90% bioretention soil mix and 10% hazelnut shells, pecan shells, or spent grain, and two columns were packed with 100% bioretention soil mix as a control. Five tests were conducted with stormwater collected from a nearby industrial site. Influent and effluent samples were analyzed for copper, zinc, nitrate, ammonia, total nitrogen, phosphate, and total phosphorus. The columns with pecan shells had the highest removal, with 53% removal of copper and 87% removal of zinc. Removal in the columns with hazelnut shells and spent grain was 47% and 19% for copper and 83% and 65% for zinc, respectively. All columns exported nutrients. Although hazelnut shells had the highest sorption coefficient, the pecan shells removed more metals from the stormwater. This study indicates both hazelnut and pecan shells improve metals removal potential of bioretention systems.