Innovative Stormwater Management: How Using Biochar-Amended Iron-Enhanced Sand Filters Can Reduce The Flow of Bacteria (E. coli) and Phosphorus Into Our Water Bodies

INTRODUCTION
Elevated bacteria levels in the nation's surface waters are a public health issue – leading to sick pets and beach closures and posing immediate health risks to anyone who recreates in or consumes the water. The culprit? Pet, wildlife, and human waste carried by storm sewers, untreated, to nearby lakes, streams, and rivers. Until now, there have been limited ways to reduce bacteria levels in stormwater – most of which are prohibitively expensive. There are numerous Best Management Practices designed to remove conventional pollutants from stormwater, but very few that target bacteria. This project involved partnerships between Stantec, Coon Creek Watershed District, City of Coon Rapids (MN), City of Blaine (MN), the University of California at Los Angeles (UCLA), and the MN Board of Water and Soil Resources, with support from the MN Clean Water Fund. The project design featured creative, unique two-cell filters that effectively achieved pollutant reduction goals while also allowing the Client to perform head-to-head testing of different media mixtures to simultaneously achieve applied stormwater research objectives.
BACKROUND
As rainfall events become more frequent and intense, watershed management to limit flooding and prevent pollutant transport to lakes, streams, and wetlands becomes paramount within our communities. Stantec has been working closely with two Minnesota urban watersheds to design and construct regional stormwater management practices and retrofits to remove both conventional pollutants (total suspended solids and phosphorus) and bacteria for the past 5 years. Biochar, a charcoal-like substance made via pyrolysis of organic material, has recently been gaining attention as a potential filter media amendment for removing bacteria from stormwater runoff. Biochar has historically been used as a soil and media amendment in agriculture and for environmental remediation of sites contaminated with heavy metals or organic pollutants but has never been used in a full-scale application to target bacteria. Sustainably produced and carbon neutral, biochar has been used to help fight climate change by sequestering carbon while simultaneously providing energy to plants and increasing crop yields. Our methodology for this project was followed a series of successful small-scale field trials in the nearby Shingle Creek watershed that evaluated the impact of adding biochar to iron-enhanced sand filters (IESF) and achieved E. coli concentration reductions in urban stormwater between 49-97%. Based on those promising results, the Coon Creek Watershed District, Stantec, and the Cities of Blaine and Coon Rapids, teamed up to construct two full-scale biochar- and iron-enhanced sand filters (BIESF). Each filter is split into two identical cells, one with 30% biochar by volume added and one without. This design allows for head-to-head performance monitoring, testing the ability of biochar to remove E. coli while reducing nutrient and bacteria loading to two impaired creeks: Pleasure Creek and Woodcrest Creek. The combined filters are sized to treat runoff from 1.5 square miles of previously untreated drainage area before discharging to the two creeks and the Mississippi River.
DEMONSTRATED RESULTS
The Woodcrest BIESF is an inline, gravity filtration system located in the City of Coon Rapids which treats stormwater runoff form a 0.9 square mile drainage area. The large filter is 300 ft x 67 ft, approximately 1/3 the size of a football field. The Woodcrest BIESF filter went online in July 2020 and will remove an estimated 69 lbs of total phosphorus per year and over 80% of incoming E. coli loads. The Pleasure Creek BIESF is a pumped filtration system located in the City of Blaine which treats stormwater runoff from a 0.6 square mile drainage area. The system operates with a small lift station that pumps between 120-200 gpm. The lift station pulls stormwater from a large pond system upstream of Pleasure Creek and distributes it throughout one of the two cells for treatment. The 2020 sampling data from the two test locations showed: - All four filter cells reduced E. coli and total phosphorus (TP) concentrations and loads. - At the Woodcrest filter, the biochar cell removed 17% more E. coli than IESF cell (89% v 72% cumulative load reduction). - At Pleasure Creek, both filter cells performed similarly at removing E. coli (87% vs 84% cumulative load reduction). - TP load removals were comparable between media types; IESF outperformed BIESF at Woodcrest by 5%, but BIESF > IESF at Pleasure Creek by 13%. - For orthophosphorus, IESF outperformed BIESF by 15% at Woodcrest. At Pleasure Creek, insignificant amounts of leaching were observed from both media types, but slightly more export from IESF cell. - Removal efficiencies were variable across individual events; all cells generally performed better when incoming loads were higher. Overall, the BIESF and IESF cells at both filter locations were proven to be effective at removing phosphorus and bacteria. The BIESF cell showed an increased effectiveness of 5-12% at removing bacteria from stormwater while maintaining the effectiveness of phosphorus removal. It should be noted that at very low influent concentrations of orthophosphate (OP), the Pleasure Creek filter experienced some leeching of OP but still provided a net removal of total phosphorus. These biochar-enhanced iron-sand filters are the largest known stormwater treatment systems designed to remove bacteria from urban runoff in place. This innovative application has with far-reaching implications for the stormwater management industry. For example, biochar could be incorporated into several existing stormwater infiltration or filtration applications such as rain gardens, catch basin inserts, bio-swales, bench filter, and permeable check dams. This work was also the recipient of an ACEC of MN Grand Award and has led to the design of a third BIESF along Pleasure Creek set to be constructed in the winter of 2021-22 and begin operation during the summer of 2022.
SUMMARY
The BIESF Large-scale Demonstration Projects establish a new, effective, and innovative way of removing both dissolved phosphorus and E. coli from urban runoff. Until now, the scalability of this technology was uncertain. With many streams and lakes impaired for bacteria nationwide, including biochar in stormwater filter media could provide safer, cleaner recreational waters for humans and pets.