Innovative Bio retention Strategies: Enhancing Dissolved Nitrogen Removal with Biochar-Amended High-Permeability Media for Urban Storm water Treatment

Abstract

Nutrient pollution from stormwater leads inland and coastal water bodies to become eutrophic, which results in sea grass receding and harmful algal blooms (HAB) flourishing. These human-induced effects destabilize the ecosystems, and some HABs threaten human health directly. A possible solution to this problem is bioretention, defined as the storage and controlled discharge of storm water runoff in an ecologically constructed setting. But since it is largely reliant on particle settling as a nutrient removal process, it struggles with pollutants such as dissolved nitrogen. This is particularly inconvenient in India, where nitrogen is the limiting nutrient for HAB growth owing to the geological significance of phosphorus. Several conventional bio retention systems are limited by their low hydraulic loading rates (HLRs) and extensive footprint requirements, rendering them impractical for densely urbanized areas. To address these constraints, we performed a series of bench-scale, column, and pilot-scale experiments using a novel mixed media of high-permeability gravel amended with biochar. These tests were designed to characterize the hydraulic and physical properties of the amended media, assess its runoff treatment performance under a high HLR representative of extreme precipitation events, and quantify its mass removal efficiency for dissolved nitrogen species at a low HLR corresponding to median storm conditions. Collectively, these investigations aim to identify a more space-efficient solution capable of handling both extreme and typical storm water loads. To determine the air-to-water ratio in the soil and assess whether aerobic nitrification or anaerobic denitrification is promoted, HPG was amended with biochar at different volumetric ratios and tested for porosity and moisture retention at the bench scale. Porosity and moisture?retention capacity increased in direct proportion to the percentage of biochar in the media. Saturated hydraulic conductivity was determined through column tests, and found to be slightly reduced. This is an indication of decoupling of porosity and saturated hydraulic conductivity, which can be attributed to the internal nanoscale porosity of biochar but still warrants further study.

Introduction

The spread of harmful algal blooms (HABs), driven largely by nitrogen and phosphorus contamination, negatively impacts ecosystems and human health. Nutrient pollution mainly comes from sediment leaching and stormwater runoff, especially in urban areas where impervious surfaces increase runoff and nutrient transport. Traditional stormwater management systems like bioretention are effective at removing sediment but struggle to control dissolved nutrients such as nitrogen.

Bioretention systems (BRS) require large land areas and long hydraulic retention times (HRT) to treat stormwater effectively, making them challenging to implement in dense urban settings. Modifications like biochar amendments and raised drainage systems aim to improve nutrient removal. Biochar, produced by pyrolysis of organic waste, enhances water retention, adsorption of pollutants, and microbial activity, facilitating nitrogen removal through processes such as nitrification and denitrification.

The study investigates biochar-amended, high-permeability gravel in bioretention cells, examining nitrogen removal efficiency under varying hydraulic loading rates and drainage configurations (free-draining vs. raised-drainage). The goal is to optimize nitrogen transformation and removal from urban stormwater while addressing challenges posed by urban watershed hydrology, media characteristics, and land constraints.

Conclusion

The selection of drainage type should be based on site-specific conditions such as peak flow rates and influent concentration ratios of ammonia to nitrate, but more research is needed to completely comprehend which to implement by monitoring the specific impacts of IWSZ depth and size. These systems can be the first step in a treatment train for water quality treatment and ought to be an effective passive storm water quantity treatment in urban catchments. The study’s primary hypothesis is confirmed: both systems effectively processed high?rate HLR runoff, but achieved even greater dissolved?nitrogen removal—approximately 30%—under the lower, more typical HLR representative of real storm events, rather than the <20% removal seen at extreme loading. These mass?removal results indicate that biochar amendments to high?permeability media offer a promising urban storm water treatment strategy. Moreover, the free?draining configuration consistently removed more NH??—indicative of enhanced nitrification—while the elevated?drain system preferentially removed NO?—reflecting denitrification—thereby supporting our secondary hypothesis that free?drain BRSs favour nitrification and elevated?drain BRSs favour denitrification. Total inorganic nitrogen (TIN) removal did not differ significantly between free? and elevated?drain configurations for either design storm, indicating that denitrification—the sole permanent nitrogen?removal pathway—occurred at similar rates in both systems. Drain style thus had negligible influence on overall dissolved?N removal and no statistically significant effect on species?specific targeting. In contrast, storm intensity exerted a clear influence: average TIN reduction was 34 % under the median?event HLR versus 23 % under the 10?year storm HLR (11.8 cm/min).

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Copyright © 2025 Debashish Panda, Prof. Sanam Sarita Tripathy. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.