Bioretention Column Study: Fecal Coliform and Total Suspended Solids Reductions

Surface water impairments for pathogens are a major problem in waterways throughout the United States, especially in New Jersey. Fecal coliform (FC) counts are commonly used as an indicator of pathogens. In this study, bioretention systems, which are stormwater best management practices, were modeled with columns. Unlike traditional detention basins, bioretention systems are meant to manage water quality in addition to quantity and peak flow rates. Typical rainfall conditions for New Jersey were mimicked in the laboratory with regard to rainfall intensity and stormwater composition (bacterial colony counts) using a diluted manure slurry. The applied influent flow rate was 77.0 mL/min, while the average leachate flow rate observed in the bottom spout was 20.8 mL/min. The hydraulic performance of the columns was adequate in that the influent completely infiltrated well before the New Jersey Department of Environmental Protection (NJDEP) performance standard of 72 h. However, ponding of more than 30.5 cm (the NJDEP performance standard for ponding) was observed in some of the simulations. The mean, median, and range of reduction coefficients for fecal coliform (FC) were 91.6%, 98.6%, and 54.5% to 99.8%, respectively, for 13 simulations. The mean, median, and range of reduction coefficients for total suspended solids (TSS) were 91.5%, 91.9%, and 81.0% to 99.4%, respectively, for 15 simulations. The average pH of the influent water was 6.87, while the average leachate pH was 4.61. It is likely that both adsorption and filtration were responsible for the FC and TSS reductions. Die-off factors that may have influenced FC reduction in addition to adsorption and filtration are discussed. Reduction coefficients were observed to not necessarily be the ultimate indicator of system performance or effectiveness, as they are conditional upon the leachate concentration.