Executive Summary
Various contaminants exist in stormwater runoff, including nutrients, metals, pathogens, and organic compounds. Filtration systems built with engineered media are used to manage stormwater
runoff by the North Carolina Department of Transportation (NCDOT), but the department’s current standard filtration media mix specification does not take the optimization of contaminant removal into consideration. To better meet regulatory compliance, it is beneficial for NCDOT to
explore different material options to build filtration systems with a better capacity for contaminant removal, while maintaining desirable support for plant growth and water infiltration. This project aims to evaluate various affordable media materials that can remove multiple contaminants
simultaneously and maintain high performance in runoff filtration measures under various field conditions.
Biochar, zeolite, vermiculite, woodchips, iron products, and granular activated carbon were identified as potential amendments for stormwater filters during the literature review. These materials were tested in this project for contaminant removal, together with Sand (type II fine
aggregate) and Stalite (MS16 gradation) as the filter media bases. The following five groups of contaminants were selected to assess the removal performance of stormwater filtration media due to their ubiquitous presence in stormwater and their health/environmental impacts:
1) Suspended particles, measured as TSS and turbidity;
2) Nutrients, measured as nitrite, nitrate, total nitrogen (TN), Total Kjeldahl Nitrogen (TKN), orthophosphate, and total phosphorous (TP);
3) Heavy metals, measured as dissolved aluminum (Al), chromium (Cr), copper (Cu), magnesium (Mn), lead (Pb), and zinc (Zn);
4) Organic compounds, measured as dissolved organic carbon (DOC), the absorbance of ultraviolet light at 254 nm (UV254), and specific UV absorbance (SUVA);
5) Pathogen indicators, measured as total coliform, fecal coliform, Enterococci, and E. coli.
Real stormwater was collected off NC Highway 29 near the UNC Charlotte campus and used for all tests. In most cases, the measured contaminant concentrations in the stormwater collected for this project were on the lower end of typical ranges reported in stormwater runoffs. To better evaluate the contaminant removal performance of different media, stormwater used for this project was further spiked with nutrients, heavy metals, and organics unless otherwise specified.
A total of 17 different filter materials (four types of biochar, two types of zeolite, two types of vermiculite, three types of woodchips, three types of iron products, one type of granular activated carbon, sand, and Stalite) were tested in the first round of batch tests to screen for contaminant
removal efficiencies. The activated carbon, two types of biochar, two types of iron products, and one type of vermiculite showed better performance than the remaining materials tested, as they exhibited high contaminant removal percentages and minimal contaminant release.
The six best-performing media from the first round of batch tests, together with the base materials (sand and Stalite), were examined in the second round of batch tests for their contaminant removal capacity in the reoccurring loading mode. Both activated carbon F400 and Biochar Basic
effectively removed a broad range of contaminants, particularly nutrients. Therefore, Biochar Basic was selected as the first choice for stormwater filter amendments in the following tests due to cost considerations. The two iron products demonstrated similar performance: both had good
removal of phosphorus and some metals but leached out nitrogen and other metals. Therefore, one iron product (Iron-8/50) was selected to be tested as a filter amendment in the following tests to complement the performance of Biochar Basic. Stalite was proven to be a better base material for the stormwater filters than sand, as Stalite did not leach any contaminants and even removed some phosphorus and Al in the batch tests.
The Biochar Basic and Iron-8/50, which demonstrated high removal percentages and did not lose their removal capacities with recurrent contaminant input during the second round of batch tests, were examined in bench-scale column tests. These columns were packed with sand or Stalite as the base material, while filter amendments (biochar and/or iron) were added at 5% by weight. The stormwater was filtered downward the columns by gravity flow, with the average water infiltration rates ranging from 0.48 to 1.92 inch/day in fresh columns. The column tests included five major phases simulating various field conditions (baseline tests, alternating concentration tests, antecedent drought tests, winter tests, and aged media tests). These conditions did not decrease the contaminant removal by the tested column; however, the column performance decreased after
long-term use. Overall, the combination of Stalite with 5% Biochar Basic turned out to be the best performer in removing all tested contaminants, and is suggested for constructing stormwater filtration systems for contaminant removal from highway runoff.
The information gained through this project will enable NCDOT to improve the performance of its stormwater filtration systems to effectively remove contaminants, including nutrients, metals, pathogens, and organic contaminants. Such improvement will help NCDOT comply with the
NPDES permit, state nutrient load reduction rules, future dissolved metals TMDLs, and potential regulations on organic contaminants in the future. The testing protocols developed in this project can be applied to test future media materials of interest or additional contaminants. The performance data of a wide variety of media materials generated in this project can be used as benchmarks to assess other materials in future studies.
