Stormwater runoff has become a primary non-point source of pollution that contains many pollutants that have deleterious effects on the environment. To manage stormwater, different techniques that target specific stormwater pollutants may be implemented in watersheds. One such treatment practice is called bioretention, or rain gardens. This thesis discusses the use of bioretention media as a treatment technique for the removal of toxic metals and investigates the release of phosphorus. A review of previous research and the laboratory experiments will be discussed in two chapters.
Chapter 1 reviews the literature on the concentrations, sources, and effects of toxic metals found in stormwater runoff. The review also discusses relevant management practices and parameters related to the removal of toxic metals using materials commonly found in bioretention practices, such as MNDOT Grade 2 compost and C-33 sand. Third Chapter 1 reviews the sorption mechanisms and important variables that aid or hinder sorption of toxic metals to organic materials. Lastly, previous research on sorption of cadmium, copper, lead, and zinc to organic and inorganic sorbent materials is discussed.
Batch and column experiment were performed to investigate the removal of cadmium, copper, and zinc form synthetic stormwater by compost-amended sand. The results of these experiments are discussed in Chapter 2. The batch sorption capacities for Cd and Zn are 2.13 mg/g and 3.82 mg/g, respectively, for Minnesota Compost 1 and 0.02 and 0.07 mg/g, respectively for sand. Copper precipitates as tenorite (CuO) at the pH of the stormwater (7.2) and was only 28% dissolved in the influent, so a sorption capacity was not computed. Column studies using four different ratios of compost (0, 10, 30, and 50%, by volume) were conducted to develop metal breakthrough curves. The breakthrough curves for Cd and Zn were fit to the Thomas Model. The resulting sorption capacities are 0.07, 0.23, 0.37, 0.78 mg Cd/g and 0.10, 0.23, 0.33, 0.61 mg Zn/g for 0, 10, 30, and 50% compost fractions, respectively. These sorption capacities, when adjusted for mass of sand and compost, are consistent with the sorption capacities determined from the batch experiments. Assuming representative values for dissolved metal concentrations, the estimated lifespan of bioretention cells for removal of Cd and Zn ranged from 163 to 205 years for bed depths of 30 cm for a bioretention practice constructed with 30% compost. Copper was removed in the columns due to filtration and removal increased with increasing compost content.
In the batch and column studies, concentrations of phosphorus exceeded the initial concentrations indicating that phosphorus is exported from the bioretention media to the infiltrating stormwater. The phosphorus concentrations exiting the columns were initially high (0.5 mg P/L), but then decreased to a steady state value of 0.20 - 29 mg P/L (that exceeded the influent value of 0.13 0.03 mg P/L) for the remainder of the experiment. The total yearly load exported from a bioretention practice containing 30% compost is 1.44 g dissolved phosphorus per year per square meter of bioretention area.
Overall, the results suggest that bioretention cells are not likely to fail because of loss of dissolved toxic metal removal capacity as the breakthrough times on the order of hundreds of years far exceed the typical design life of engineering systems of 30 years. Although only one compost was tested in the column experiments, the similarity in batch sorption capacities for several compost samples obtained from Minnesota and around the country suggest that the source of compost is not a strong factor in determining dissolved metal removal performance. For metals that are in particulate form or particle-associated, removals will be dictated by the filtration performance of the bioretention cell which is a function of particle size, bioretention media grain size and porosity, and other factors. Copper, 72% of which was in the particulate form in the column influent, was effectively removed by the bioretention columns and removal improved with increasing fraction of compost. Finally, one significant concern regarding bioretention media is that not only are nutrients not removed effectively, but the compost that is key to metals removal may actually release nutrients (i.e., phosphorus). Thus, it is important to consider the installation of alternative media beneath the compost-amended sand to remove phosphorus, such as iron-amended sand. Such hybrid approaches require more investigation.