Black Carbon-Amended Engineered Media Filters for Improved Treatment of Stormwater Runoff

Abstract

Urban stormwater runoff is a significant driver of surface water quality impairment. Recently, attention has been drawn to potential beneficial use of urban stormwater runoff, including augmenting drinking water supply in water-stressed areas. However, beneficial use relies on improved treatment of stormwater runoff to remove mobile dissolved metals and trace organic contaminants (TrOCs). This study assesses six engineered media mixtures consisting of sand, zeolite, high-temperature gasification biochar, and regenerated activated carbon (RAC) for removing a suite of co-contaminants comprising five metals, three herbicides, four pesticides, a corrosion inhibitor, six per-and polyfluoroalkyl substances (PFASs), five polychlorinated biphenyls (PCBs), and six polycyclic aromatic hydrocarbons (PAHs). This long-term laboratory-scale column study uses a novel approach to generate reproducible synthetic stormwater that incorporates catch basin material and straw-derived dissolved organic carbon. Higher flow conditions (20 cm hr −1 ), larger sized media (0.42−1.68 mm), and downflow configuration with outlet control increase the relevance of this study to better enable implementation in the field. Biochar-and RAC-amended engineered media filters removed nearly all of the TrOCs in the effluent over the course of three months of continuous flow (480 empty bed volumes), while sample ports spaced at 25% and 50% along the column depth provide windows to observe contaminant transport. Biochar provided greater benefit to TrOC removal than RAC on a mass basis. This study used relatively high concentrations of contaminants and low biochar and RAC content to observe contaminant transport. Performance in the field is likely to be significantly better with higher biochar-and RACcontent filters and lower ambient stormwater contaminant concentrations. This study provides proof-of-concept for biochar-and RAC-amended engineered media filters operated at a flow rate of 20 cm hr −1 for removing dissolved TrOCs and metals and offers insights on the performance of biochar and RAC for improved stormwater treatment and field trials.

Predicting PFAS and Hydrophilic Trace Organic Contaminant Transport in Black Carbon-Amended Engineered Media Filters for Improved Stormwater Runoff Treatment

“…The synthetic stormwater consisted of a mixture of salts, catch basin material from Naval Weapons Station Seal Beach (NWSSB), CA, and straw. Detailed synthetic stormwater recipe and water quality and flow rate measurements are recorded in Pritchard et al 12 Batch isotherms were conducted to determine Freundlich sorption parameters of the HiTrOCs and PFASs for biochar and RAC. The procedures for the HiTrOCs isotherms are described previously in Pritchard et al 12 Experiments were conducted by adding increasing amounts of biochar (10 to 250 mg) and RAC (10 to 380 mg) to 1 L amber glass jars (for HiTrOCs) and 250 mL polypropylene (PP) tubes (for PFASs).…”

“…Detailed synthetic stormwater recipe and water quality and flow rate measurements are recorded in Pritchard et al 12 Batch isotherms were conducted to determine Freundlich sorption parameters of the HiTrOCs and PFASs for biochar and RAC. The procedures for the HiTrOCs isotherms are described previously in Pritchard et al 12 Experiments were conducted by adding increasing amounts of biochar (10 to 250 mg) and RAC (10 to 380 mg) to 1 L amber glass jars (for HiTrOCs) and 250 mL polypropylene (PP) tubes (for PFASs). Synthetic stormwater (1 L) with 9.5 ± 0.7 mg C L −1 DOC was added to 1 L amber jars, 200 mL of synthetic stormwater with 10.5 ± 1.8 mg C L −1 was added to 250 mL PP tubes, and each was spiked with all the contaminants together.…”

“…The larger effective tortuosity factor values observed in the RAC relative to the biochar may be due to the smaller pores of the RAC, which could increase pore diffusion hindrance. 12 The contaminant transport model was fit to observed contaminant transport at various locations in the column by finding the optimal Freundlich sorption parameters (K f , 1/n) and effective tortuosity factor (τ e ) for each replicate column for each BC mixture. The sorption and effective tortuosity parameters fit at the 15 cm sample port and 30 cm sample port may be found in Table S.6 and Table S.7 in the Supporting Information and the parameters fit using all available data listed in Table 2.…”

“…However, beneficial use of urban runoff relies on improved stormwater treatment, as current treatment techniques are largely insufficient for removing trace dissolved contaminants including hydrophilic TrOCs (HiTrOCs) and poly- and perfluoroalkyl substances (PFASs) . Recent laboratory studies have investigated the use of black carbon (BC) amendments such as biochar and regenerated activated carbon (RAC) to improve dissolved TrOC removal from stormwater through sorption. − However, long-term predictive modeling is necessary to better interpret laboratory study results and understand the suitability and expected lifetime for these filters to operate in the field.…”

“…While variable influent concentration has recently been incorporated into the model, further implementation of variable flow rate is necessary for situations when contaminant removal is kinetically limited and thus flow rates through the filter may impact contaminant removal. A previous study has shown that HiTrOCs and PFAS removal from stormwater using BC-amended filters is kinetically limited . As flow rates for urban runoff are variable, if this variability is not accounted for, it can significantly limit the usefulness of such models for predicting real-world conditions.…”