Performance and Longevity of Stormwater Biofilter Media Under Hydrologic and Physical Stressors
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Performance and Longevity of Stormwater Biofilter Media Under Hydrologic and Physical Stressors


Stormwater treatment systems such as biofilters have been increasingly incorporated in urban areas because they can alleviate water scarcity issues in urban areas, decrease pollutions from contaminated runoff, and provide an alternative source of water. Despite all benefits, large-scale application of biofilters is limited, partly due to limited knowledge of how the anthropogenic stressors (e.g., compaction) and filter media design could decrease their longevity. The overall objective of this dissertation is to investigate the processes that affect the longevity of stormwater biofilter media. Specific objectives include: (1) to examine the effect of one of the physical stressors—compaction—on the longevity of biofilter media and performance of biofilters; (2) to evaluate the effect of chemical stressors—stormwater constituents such as dissolved organic carbon—on longevity and performance of iron amendment; (3) to examine the persistence of antibiotic resistance genes, an emerging contaminant, in stormwater biofilters with different design configurations. To examine the effect of compaction on the capacity of biochar-augmented roadside biofilters to infiltrate stormwater and remove pollutants, columns were packed with a mixture of sand and amendments (biochar or compost) using energy equlivalent to the standard Proctor test. The effects of compaction on hydraulic conductivity and removal of Escherichia coli (E. coli) were evaluated. The results revealed that the negative impact of compaction on hydraulic conductivity can be alleviated by using moist biochar. The addition of moisture during compaction can increase contaminant removal, initial particle release, and infiltration capacity of biochar-augmented sand filters for road runoff treatment. To evaluate the designs that could increase the long-term removal of bacterial pathogen in biofilters and minimize their leaching in the first flush, conventional biofilter media, a mixture of compost and sand, was augmented with iron fillings. The results show that the addition of iron amendments could increase removal and decrease first-flush leaching of E. coli. However, the presence of compost decreased the adsorption capacity: exposure of 1 g of iron filings to 1 mg of dissolved organic carbon (DOC) from compost reduces E. coli removal by 8%, potentially due to the alteration of the surface charge of iron and blocking of adsorption sites shared by E. coli and DOC. To examine the effect of two design features—addition of iron amendment and submerged layer—on biofilter ability to mitigate the dissemination of antibiotic-resistant genes (ARGs), conventional biofilter media was augmented with iron filings with and without a submerged layer. Natural stormwater containing ARGs was applied intermittently, and the effluent was analyzed for ARGs. The results show that conventional biofilter design including organic compost, without any submerged layer, removed nearly all antibiotic resistance genes, whereas the presence of submerged layer in the same column significantly (p<0.05) reduced the removal or increased the relative gene abundance in the effluent. While the addition of iron improved compost’s ability to remove ARGs in the partially submerged biofilter, it decreased removal in the unsubmerged biofilter. Collectively, all these results help engineers improve the design of stormwater biofilters by modifying filter media and increase their longevity.

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