Key Takeaways & Executive Findings
- •• • Achieved removal rates of 81.1% for COD, 98.1% for TN, 79.1% for TP, and 90.3% for TSS, meeting GB/T 18921-2019 standards for landscape water reuse, demonstrating practical viability for urban runoff treatment. • • The alkali-modified straw zone was the primary removal unit, supporting the highest microbial richness (Proteobacteria 54.3%, Firmicutes 21.9%), which enhanced denitrification and hydrolysis-acidification, critical for TN and COD removal. • • The system reduced PAH toxicity equivalent by 86.5%, with complete removal of BaP, DahA, BghiP, and IcdP in the straw zone, and removal rates of 49.1% for NaP and 59.2% for Acy via plants and biochar, indicating effective mitigation of carcinogenic risks. • • Despite high hydraulic and pollutant loads and low temperatures causing slight performance declines, effluent remained compliant, underscoring system resilience for field applications.
Abstract
Surface runoff pollution has become a significant source of water contamination. This study constructed an integrated composite bioretention system comprising straw, aquatic plant, and biochar zones for purifying urban surface runoff, aiming to meet the standards for reuse as landscaping water. The system's performance in removing conventional pollutants and polycyclic aromatic hydrocarbons (PAHs) was investigated, along with microbial community structure analysis. Results showed removal efficiencies of 81.1% for COD, 98.1% for TN, 79.1% for TP, and 90.3% for TSS, with effluent meeting the 'Water Quality for Scenic and Recreational Use' (GB/T 18921-2019) standard. The system exhibited robust resistance to pollutant and hydraulic loading. The alkali-modified straw zone was the primary pollutant removal region, facilitating physical adsorption and capture of suspended solids, while released carbon sources enhanced total nitrogen removal. This zone exhibited the highest microbial richness, with relative abundances of Proteobacteria and Firmicutes at 54.3% and 21.9%, respectively. The system effectively removed all 16 priority PAHs, reducing effluent toxicity equivalent by 86.5%. The straw zone completely removed four high-molecular-weight PAHs (BaP, DahA, BghiP, IcdP), while aquatic plants and biochar effectively removed medium- and low-molecular-weight PAHs.
1. Introduction
Urban surface runoff, laden with nutrients, suspended solids, and emerging contaminants such as polycyclic aromatic hydrocarbons (PAHs), poses a significant threat to aquatic ecosystems and drinking water sources. Conventional bioretention systems, while effective for flow control, often suffer from substrate clogging, poor nitrogen removal due to undefined aerobic/anoxic zones, and limited capacity for emerging pollutants. These bottlenecks necessitate innovative designs that enhance treatment efficiency and operational stability.
This study introduces a composite bioretention system integrating alkali-modified straw, aquatic plants, and biochar zones to address these limitations. The straw zone provides a carbon source and habitat for denitrifying bacteria, while the biochar zone offers high adsorption capacity for final polishing. This multi-zone configuration optimizes hydraulic retention and creates distinct redox environments, thereby improving removal of conventional pollutants and PAHs. The system's performance under varying loads and temperatures was rigorously evaluated, offering a scalable solution for urban runoff management.
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HE Lin, ZHU Zheng, MA Shun, HE Hua, LIANG Wenyan, JU ... (2026). Removal of Pollutants from Urban Surface Runoff by a Straw-Based Composite Bioretention System. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202410085
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Frequently Asked Questions
What is the long-term operational stability of the system regarding clogging and maintenance frequency?
The study did not specify long-term clogging behavior, but the use of alkali-modified straw as a top layer likely reduces clogging by capturing TSS (90.3% removal) and preventing penetration to lower layers. Regular maintenance would involve replacing the straw zone periodically, though frequency was not determined.
How does the system perform under extreme hydraulic or pollutant loads, and what are the failure thresholds?
The system maintained effluent compliance even under high hydraulic and pollutant loads, with only slight decreases in removal efficiencies. Specific thresholds were not provided, but the design appears robust for typical urban runoff variability.
What is the cost-effectiveness compared to conventional bioretention systems?
The use of agricultural waste (straw) as a low-cost material and its modification with NaOH is economically attractive. However, a full cost-benefit analysis including construction, operation, and maintenance was not presented, but the potential for carbon source reduction and enhanced performance may offset initial costs.
Are there any potential secondary pollution risks from the release of nutrients or organic matter from the straw zone?
The study indicates that the straw zone releases carbon sources to enhance denitrification, but no leaching of pollutants was reported. The system's effluent met standards, suggesting minimal secondary pollution risk under tested conditions.
How scalable is this system for real-world applications, and what are the key design parameters?
The system dimensions (65 cm × 25 cm × 55 cm) are laboratory-scale. Scaling up would require consideration of hydraulic loading rates (50 mL/min), media composition, and plant selection. The modular design could be adapted, but pilot-scale studies are needed to validate performance.
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