Bioretention Columns Research Articles

A Long-Term Assessment of Nitrogen Removal Performance and Microecosystem Evolution in Bioretention Columns Modified with Sponge Iron.

The nitrogen removal performance of bioretention urgently needs to be improved, and sponge iron has great potential to address this challenge. This study reported the results of a long-term investigation on bioretention columns improved by sponge iron, examining the durability of sponge iron from nitrogen removal performance, sponge iron properties, and the evolution of biological elements. The results showed that after 9 months of continuous operation, the removal rates of ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3--N), and total nitrogen (TN) in the bioretention columns with an appropriate proportion of sponge iron could reach 80% (some even over 90%). However, the long-term stress of sponge iron exposure, combined with the cumulative effect of pollutants, might lead to the excessive accumulation of reactive oxygen species (ROS) in plants, thereby posing risks of diminished chlorophyll content and enzyme activity. Simultaneously, the extended exposure could also have detrimental effects on microbial diversity and the abundance of dominant bacteria such as Proteobacteria and Sphingorhabdus. Therefore, it is necessary to select plant species and functional genes that demonstrate high adaptability to iron-induced stress.

Advancing microplastics remediation in bioretention systems using biochar/kaolin: Optimizing organics removal, plant health, and microbial community dynamics

Bioretention systems can efficiently eliminate microplastics (MPs) from stormwater and prevent their potential pollution in surface water. However, MPs dynamics in bioretention systems and their effects on microbes, plants, and organics removal are unknown. In this study, five lab-scale bioretention columns (i.e., control and four treatments) were established and filled with soil and fillers (zeolite and ceramsite). Various sorbents were utilized in columns, including biochar, kaolin and kaolin-biochar (KBC) composites for MPs adsorption. This study examines how biochar/kaolin amendment affects MPs and organics (COD and TOC) removal, plant health, and microbial community structure in bioretention systems. In the 60-day time-series column experiment, all amended columns removed over 90 % of MPs compared to the control. The biochar, kaolin and their combined composite eliminated MPs by 90 %, 94 %, and 97 %, respectively. Adding vegetation to the columns improved MPs removal. Moreover, bioretention systems were more effective in removing MPs ranging from 0.6 to 1 mm with a 71 % removal rate than MPs ranging from 0.3 to 0.6 mm, resulting in a 54 % removal. Organics were removed contrarily in the soil and filler layer of the bioretention system, with the soil layer removal higher due to increased microbial activity. The removal rate of total organic carbon was higher (90 %) than that of chemical oxygen demand (80 %). The most dominant phylum of the bacteria in the soil of the MPs-amended columns were Proteobacteria and Acidobacteriota, which constituted 16–27 % and 41–58 %, respectively. While the dominant phylum in that of fillers were Bacteroidota and Firmicutes, which constituted 18–42 % and 42–65 %, respectively. The maximum microbial enrichment was observed in the biochar and KBC vegetated columns. This work advances our understanding of the complex dynamics between microplastics and organic matter in stormwater and how, individually and in combination, vegetation, biochar, and kaolin vegetation, biochar, and kaolin, individually and in combination, enhance bioretention systems' effectiveness in managing multiple pollutants.

Migration of naphthalene in a biochar-amended bioretention facility based on HYDRUS-1D analysis

Biochar has been proved as a promising and efficient filler in bioretention facilities for enhancing the stormwater pollutants removal. However, the migration behaviors of stormwater pollutants in biochar filled bioretention facilities is unclear. In this study, as one of the most typical stormwater pollutants, naphthalene was selected as an example and a HYDRUS-1D model was first used to understand the migration behavior of naphthalene in a bioretention facility. In comparison with the conventional bioretention soil media (sandy loam), the amended biochar filled bioretention cell showed that the naphthalene removal rate was enhanced by up to 10.1%. Meanwhile, the experimental data was well-fitted by the “two-site sorption model” in HYDRUS-1D model. Another, the effect of rainfall intensity on the naphthalene migration in both bioretention columns was further investigated. The HYDRUS-1D model fitting indicated that the increase in rainfall intensity promoted naphthalene migration by increasing hydraulic conductivity and water flux. In addition, static batch experiments revealed that the biochar filled fillers achieved about 50% higher adsorption capacity than sandy loam. The sensitivity analysis from the HYDRUS-1D model data verified adsorption coefficient Kd and longitudinal dispersivity λ are the main factors affecting naphthalene migration. Finally, the model simulation displays that the proportion of naphthalene retained by the fillers was highest during high rainfall intensities, indicating that the fillers remain the most important fate for naphthalene. This study presents research on the behavior and mechanisms of stormwater pollutant transport through improved bioretention facilities.

Hydrochemistry and Stable Isotopes for the Investigation of Water Movement in Bioretention Column Experiments

Hydrochemistry and Stable Isotopes for the Investigation of Water Movement in Bioretention Column Experiments

Effect of tire wear particle accumulation on nitrogen removal and greenhouse gases abatement in bioretention systems: Soil characteristics, microbial community, and functional genes

Tire wear particles (TWPs), as predominant microplastics (MPs) in road runoff, can be captured and retained by bioretention systems (BRS). This study aimed to investigate the effect of TWPs accumulation on nitrogen processes, focusing on soil characteristics, microbial community, and functional genes. Two groups of lab-scale bioretention columns containing TWPs (0 and 100 mg g−1) were established. The removal efficiencies of NH4+-N and TN in BRS significantly decreased by 7.60%–24.79% and 1.98%–11.09%, respectively, during the 101 days of TWPs exposure. Interestingly, the emission fluxes of N2O and CO2 were significantly decreased, while the emission flux of CH4 was substantially increased. Furthermore, prolonged TWPs exposure significantly influenced the contents of soil organic matter (increased by 27.07%) and NH4+-N (decreased by 42.15%) in the planting layer. TWPs exposure also significantly increased dehydrogenase activity and substrate-induced respiration rate, thereby promoting microbial metabolism. Microbial sequencing results revealed that TWPs decreased the relative abundance of nitrifying bacteria (Nitrospira and Nitrosomonas) and denitrifying bacteria (Dechloromonas and Thauera), reducing the nitrification rate by 42.24%. PICRUSt2 analysis further indicated that TWPs changed the relative abundance of functional genes related to nitrogen and enzyme-coding genes.

Rice husk hydrochar prepared by hydrochloric acid assisted hydrothermal carbonization for levofloxacin removal in bioretention columns

Hydrochars are promising adsorbents in pollutant removal for water treatment. Herein, hydrochloric acid (HCl) co-hydrothermally treated hydrochars were prepared from rice husk biomass at 180 °C via a one-step hydrothermal method. Adsorption behaviors of levofloxacin (LVX) on hydrochars were evaluated. The specific surface area and pore volume of the hydrochar synthesized in 5 mol/L HCl (5H-HC) were almost 17 and 8 times of untreated hydrochar, respectively. The 5H-HC sample exhibited the highest LVX adsorption capability at room temperature (107 mg/g). Thermodynamic experimental results revealed that adsorption was a spontaneous endothermic process. Yan model provided the best description of the breakthrough behavior of LVX in bioretention column, indicating that the adsorption on the samples involved several rate-limiting factors including diffusion and mass transfer. The results show that facile HCl co-hydrothermal carbonization of waste biomass can produce novel hydrochars with high LVX adsorption ability.

The fate of three typical persistent organic pollutants in bioretention columns as revealed by stable carbon isotopes

There is a lack of simple and effective methods to quantify the fate processes of persistent organic pollutants (POPs) in bioretention systems. In this study, the fate and elimination processes of three typical 13C-labeled POPs in regularly added bioretention columns were quantified using stable carbon isotope analysis techniques. The results showed that the modified media bioretention column removed more than 90% of Pyrene, PCB169 and p,p'-DDT. Media adsorption was the dominant removal mechanism for the reduction of the three exogenous organic compounds (59.1–71.8% of the input) although plant uptake (5.9–18.0%) was also important. Mineralization was effective in degrading pyrene (13.1%) but had a very limited effect on p,p'-DDT and PCB169 removal (<2.0%), the reason for which may be related to the aerobic conditions of the filter column. Volatilization was relatively weak and negligible (<1.5%). The presence of heavy metals inhibited the removal of POPs to some extent: media adsorption, mineralization and plant uptake were reduced by 4.3–6.4%, 1.8–8.3% and 1.5–3.6% respectively. This study suggests that bioretention systems are an effective measure for the sustainable removal of POPs from stormwater and that heavy metals can inhibit the overall performance of the system. Stable carbon isotope analysis techniques can help to investigate the migration and transformation of POPs in bioretention systems.

Hydrologic and water quality modelling of bioretention columns in cold regions

AbstractBioretention is widely used in urban sustainable stormwater management; however, limited numerical research has been conducted on its performance in cold regions, particularly for winter snowmelt, spring runoff and summer large storms (&gt;50 mm) for urban flood mitigation. In this study, HYDRUS 1D was used to explore these knowledge gaps. The model was comprehensively calibrated and validated against 2‐year hydrologic and water quality data of four bioretention columns with different designs under lab‐simulated cold region conditions. The Morris method was used to measure the sensitivity and interaction of the calibrated hydraulic parameters. The model revealed that the effective hydraulic conductivity (KS) values of the soil media were similar for winter snowmelt and spring runoff when the soil temperature was around −0.5°C. Preferential flow is likely to occur in soil media during winter or spring in cold regions. The summer modelling showed that bioretention could substantially reduce peak flow, ponding depth and duration for large storm events (even for a 1:100 local storm with 83.4 mm in 4 h). The water quality modelling confirmed experimental results that the bioretention effectively removed phosphate and ammonium but had leaching issues for chloride and nitrate. Finally, optimization and recommendations for bioretention columns were provided.

Clogging mechanism of bioretention cell with fine-grained soil medium

In order to explore the clogging mechanism of bioretention cells containing fine-grained soil medium, four groups of bioretention cell were constructed. The clogging process was studied in bioretention cells, under simulated runoff conditions with varying suspended solid (SS) concentrations, determining the microbial clogging process at different degrees of SS clogging. Results showed that SS caused clogging in the upper and shallow layers of the filler material. When the concentration of SS was low, clogging of the bioretention column occurred mainly in the shallow layer of the filler, while at SS concentrations higher than 300 mg/L, clogging mainly occurred in the upper layer of the filler. Microbial clogging caused by continuous rainfall occurred in all filler layers. Furthermore, bioretention cells suffering from severe clogging by SS were more likely to also suffer from microbial clogging. Microbial clogging can be effectively mitigated by ensuring that the bioretention cell has adequate drying periods, although this approach has a limited effect on the restoration of bioretention cells with a submerged zone. In bioretention cells exhibiting serious physical clogging, the proportion of microbial clogging can reach 44 %. In the practical application of bioretention cells, replacement of the upper filler layer is the most direct method to effectively relieve clogging, while setting a drying period is effective for alleviating microbial clogging. The experimental results revealed the clogging mechanism in the bioretention cell from two aspects, establishing effective approaches for the reduction of filler clogging and providing a theoretical basis for the practical operation of bioretention cells.

Biochar and fungi as bioretention amendments for bacteria and PAH removal from stormwater

Bioretention has been widely used to mitigate hydrologic impacts of stormwater runoff and is increasingly being relied upon to treat chemical and biological pollutants transported by stormwater. Despite this reliance, we still lack an understanding of treatment performance for certain organic and biological contaminants which may interact with biotic and abiotic components of bioretention systems. We evaluated the treatment of fecal indicator bacteria (FIB) and polycyclic aromatic hydrocarbons (PAHs) in stormwater runoff by bioretention. We compared treatment performance by Washington's standard bioretention mix of 60% sand: 40% compost (by volume), and by three other mixtures amended with biochar, fungi (Stropharia rugosoannulata), or both. All bioretention columns were conditioned with clean water and then dosed with collected roadway runoff at a rate equivalent to a 6 month, 24 h storm in this region during 8 events over a 14-month period. Effluents for each column were analyzed for 23 PAHs, Escherichia coli, fecal coliform, dissolved organic carbon (DOC), and total suspended solids (TSS). The fate and transport of PAHs within the bioretention columns was tracked by measuring soil PAHs in media cores taken from the columns. ΣPAH were almost completely removed by all treatments across all storms, with removal rates ranging from 97 to 100% for 94 out of 96 samples. Compost appeared to be a source of PAHs in bioretention media, as biochar-amended media initially contained half the ΣPAHs as treatments with the standard 60:40 sand:compost mixture. We observed a net loss of ΣPAHs (19–73%) in bioretention media across the study, which could not be explained by PAHs in the effluent, suggesting that bioremediation by microbes and/or plants attenuated media PAHs. E. coli and fecal coliform were exported in the first dosing event, but all columns achieved some treatment in subsequent dosing events. Overall, these findings suggest that PAHs in stormwater can be remediated with bioretention, are unlikely to accumulate in bioretention media, and that biochar amendments can improve the treatment of E. coli.

Evaluating bioretention scale effect on stormwater retention and pollutant removal.

Bioretention column studies are commonly used in laboratory to assess the performance of such structures in removal of pollutants and to investigate different conceptions aiming to increase their efficiency. However, no studies were found recommending suitable diameters or sizes, or about the uncertainties related to the transfer of results among the different scales (i.e., among different experiments or from the laboratory to field scale). This study assessed the effect of the varying diameters in experimental bioretention columns on the retention and removal of pollutants from stormwater runoff. Three sets of columns with diameters of 400mm, 300mm, and 200mm were assessed. The results showed that runoff retention (R) was affected by the time interval between stormwater events, but not by the bioretention diameter, although the diameter influenced the variability of R results. The removal of TSS (95%), nitrite (98%), and phosphate (96%) did present variability among the different bioretention diameters. However, the nitrate removal was statistically different among the bioretention columns, with removal efficiency above 50% in the 300-mm and 200-mm columns, while the 400-mm columns acted as a source of nitrate by increasing its concentration in the outflow stormwater by up to 285%, suggesting that the removal of this pollutant can be influenced by the scale effect of the bioretention columns and the experiments with small bioretention diameters may not provide reliable results.

Nutrient removal performance and microbial community analysis of amended bioretention column for rainwater runoff treatment

Bioretention is a promising approach for rainwater runoff treatment, while effective performances can hardly be expected from traditional systems. Several amended bioretention columns were configured for enhanced pollutants removal with different media designs and submerged depths. Results revealed that amended media largely stimulated simultaneous nutrients removals under different influent concentrations, with the highest removal rates by 90% traditional bioretention soil media +5% water treatment residuals +5% coconut coir. Further experiments showed that increase of submerged depth significantly improved the removal of nitrate and total nitrogen, though led to slight decrease of ammonia nitrogen, total phosphorus and chemical oxygen demand removals. HiSeq sequencing elucidated that the submerged depth altered the diversity and structure of the microbial community in the bioretention columns, as well as some putative bacteria for biological pollutants removal. This study might stimulate the cost-effective treatment of rainwater runoff by bioretention approaches.

Influence of urban runoff pollutant first flush strength on bioretention pollutant removal performance.

Bioretention is commonly used for runoff pollution control. The first flush strength of pollutants can affect bioretention performance. To examine the influence of the first flush strength on bioretention performance, bioretention columns filled with garden soil as the main media were established. Activated carbon and water treatment residuals (WTR) were added and compared for their ability to enhance phosphorus removal. Waste edible fungus culture medium (WEFCM) as a carbon source was also explored. When WEFCM was used as a carbon source instead of wood chips, total nitrogen (TN) removal increased from 60.83 ± 21.22 to 62.21 ± 16.43%, but chemical oxygen demand (COD) leaching was observed. WTR was better able than activated carbon to enhance phosphorus removal (87.97 ± 8.87 vs. 81.66 ± 9.27%) without impacting TN removal. NH4+-N removal increased with increasing first flush strength, but there was no trend for suspended solids (SS), COD, TN, or total phosphorus. First leaching phenomenon in bioretention outflow was proposed in this study. A low first leaching was observed in the outflow when the inflow had a uniform pollutant mass (i.e., no first flush) because of media leaching. A weak first leaching outflow was observed for SS and COD when they were present at strong first flush inflow.

Nitrogen process in stormwater bioretention: effect of the antecedent dry days on the relative abundance of nitrogen functional genes.

In this study, we evaluated the relative abundance of nitrogen functional genes (amoA, nirK and nirS) involved in ammonia oxidation and denitrification bacteria in laboratory-scale bioretention columns in response to environmental factors (e.g., moisture content, pH, soil organic matter, soil nitrogen) under different antecedent dry days (ADDs). We observed a decrease tendency of the relative abundance of ammonia-oxidizing bacteria at first and then increased when increasing ADDs from 1 to 22 day, while the relative abundance of denitrifying bacteria showed a downward trend. The abundance of bacteria gene amoA was positively associated with soil ammonia nitrogen concentration (r2 = 0.389, p < 0.05) and soil organic matter concentration (r2 = 0.334, p < 0.05), while the abundance of bacteria gene nirS was positively correlated with soil ammonia nitrogen (r2 = 0.730, p < 0.01), soil organic matter (r2 = 0.901, p < 0.01) and soil total nitrogen (r2 = 0.779, p < 0.01). Furthermore, gene counts for bacteria gene nirS were correlated negatively with plant root length (r2 = 0.364, p < 0.05) and plant biomass (r2 = 0.381, p < 0.05). Taken together, these results suggest that both nitrification and denitrification can occur in bioretention systems, which can be affected by environmental factors.

Corncob-pyrite bioretention system for enhanced dissolved nutrient treatment: Carbon source release and mixotrophic denitrification

Solid biomass waste amendment and substrates modification in bioretention systems have been increasingly used to achieve effective dissolved nutrients pollution control in stormwater runoff. However, the risk of excess chemical oxygen demand (COD) leaching from organic carbon sources is often overlooked on most occasions. Pyrite is an efficient electron donor for autotrophic denitrification, but little is known about the efficacy of autotrophic-heterotrophic synergistic effect between additional carbon source and pyrite in bioretention. Here, four bioretention columns (i.e., corncob column (C), pyrite column (P), the corncob-pyrite layered column (L-CP), and the corncob-pyrite mixed column (M-CP)) were designed and filled with soil, quartz sand, and modified media to reveal the synergistic effects. The results showed that the corncob-pyrite layered bioretention could maintain low COD effluent concentration with high stability and efficiency in treating dissolved nutrients. When the influent nitrogen and phosphorus concentrations were 8.46 mg/L and 0.94 mg/L, the average removal rates of ammonia nitrogen, total inorganic nitrogen, and phosphate were 83.6%, 70.52%, and 76.35%, respectively. The scouring experiment showed that placing the corncob in the mulch layer was beneficial to the sustained release of dissolved organic carbon (DOC). Erosion pits were found in the SEM images of used pyrite, indicating that autotrophic denitrifying bacteria in the bioretention could react with pyrite as an electron donor. The relative abundance of Thiobacillus in the submerged zone of the corncob-pyrite layered bioretention reached 38.39%, indicating that the carbon source in the mulch layer increased the relative abundance of Thiobacillus. Coexisting heterotrophic and autotrophic denitrification in this bioretention created a more abundant microbial community structure in the submerged zone. Overall, the corncob-pyrite layered bioretention is highly promising for stormwater runoff treatment, with effective pollution removal and minimal COD emission.

Effects of Vegetation and Saturated Zone in Cascaded Bioretention on Enhancing Nutrient Removal

Bioretention is a water management practice that is increasingly being applied for runoff quality control. Although previous Bioretention studies have used some techniques to improve nutrient removal, some nutrients still leach out. Therefore, this study used Cascaded Bioretention (CB) by connecting three Bioretention columns in series. The planted Bioretention Column was retrofitted by adding a subsurface drainage module (SDM) below the gravel layer to create a dual saturated zone. This study aimed to investigate the effect of the number of treatments, vegetation, and modified saturated zone on enhancing nutrient removal from agricultural runoff and to understand nutrient removal mechanisms. The removal efficiencies of NH3-N, NO3-N, NO2-N, and TN improved to 89.8%, 49.7%, 49.2%, and 53.4%, respectively. The only negative removal was ON, which significantly decreased by incorporating vegetation and a modified saturated zone. Increasing the number of treatments significantly enhanced TN and ON while maintaining stable removal for other nitrogen compounds. However, phosphorus was less sensitive to increasing the number of treatments. Nitrogen removal could be enhanced by different removal processes such as nitrification, denitrification, mineralization, assimilation by plant uptake, and Anammox. However, phosphorus removal was less complicated, as adsorption and infiltration are likely to be the main removal mechanisms.

Microbial community and nitrogen transformation pathway in bioretention system for stormwater treatment in response to formulated soil medium

Bioretention system has been commonly recognized as an emerging management method to control urban rainwater runoff. However, nitrogen conversion mechanisms in bioretention system in response to formulated soil medium remains unknown. In this work, five bioretention columns with different formulated soil medium were established, and results indicated that all bioretention columns achieved high NH4+-N removal rate, but exhibited significantly different NO3--N removal performance. High-throughput sequencing results suggested that nitrification might be achieved by heterotrophic nitrifying bacteria, while Pseudomonas and Rubellimicrobium may be the main denitrifying bacteria, which served as anaerobic and aerobic denitrifier, respectively. Heterotrophic nitrification-anaerobic/aerobic denitrification may the microbial pathways to realize effective nitrogen removal performance in these bioretention systems. Bioretention system with moderate silt content (NC, 10%) displayed higher abundant potential denitrifier, leading to better nitrogen removal performance. Besides, soil adsorption played an important role in nitrogen removal, particularly when activated carbon was added. This work could provide guidance of formulated soil medium optimization for the application of bioretention systems.

The impact of bioretention column internal water storage underdrain height on denitrification under continuous and transient flow

Internal water storage (IWS), a below-grade saturated layer, is a bioretention design component created by adjusting the underdrain outlet elevation. Anaerobic conditions and the presence of a carbon source in IWS facilitates denitrification. Yet it remains unclear how underdrain height within the IWS impacts nitrate (NO3−) removal. This study applied synthetic stormwater with NO3− to three laboratory columns with underdrains located at the bottom, middle, or top of a 32 cm thick gravel-woodchip IWS. Under steady state conditions, underdrain nitrogen removal demonstrated a positive linear relationship with increasing hydraulic residence time (HRT). For a 1 cm/h hydraulic loading rate (HLR), nitrogen removal efficiency increased from 52 to 99% as underdrain height moved from the top to the bottom. Despite identical IWS thickness across columns, immobilize zones below the middle and top underdrains limited the steady state nitrogen removal. Dual isotopes in NO3− also indicated denitrification occurred in mobile zones and showed little or no denitrification in immobile zones due to limited mass transport. Transient flow conditions were applied, to mimic storms, followed by dry conditions. Lower effluent nitrogen concentrations and mass fluxes were observed from the bottom underdrain across the range of HLRs tested (1 to 5 cm/h) but performance of all three underdrains converged after the application of one pore volume. The top underdrain enhanced mixing between new incoming low-DOC stormwater and old IWS water with high-DOC which minimized effluent DOC concentrations. NO3− isotope enrichment factors indicated denitrification during transient flow for all three underdrain heights and enrichment increased for the 5 cm/h HLR. For sites with narrow IWS geometries (width to depth ratio < 1), optimal underdrain height is likely located between the bottom and top of the IWS to promote mixing with old IWS water high in DOC and sustain denitrification during storms.

Assessment of imidacloprid removal from agricultural runoff by the bioretention treatment train system

Imidacloprid pesticide is commonly used in the agricultural sector to protect from pest attacks and avoid crop losses. However, the leak of Imidacloprid to the environmental system could result in dire consequences to the aquatic life and humans as the toxic contaminant produced by this pesticide may disturb the ecosystem through the pesticide cycle, which could penetrate the soil and pollute the water body especially runoff. Therefore, this paper explains the new implementation in managing pesticide pollution that occurred in the agricultural runoff, specifically Imidacloprid pesticide. The assessment was conducted by introducing a treatment train system that focuses on a few removal mechanisms. The system included a series of bioretention columns that are expected to remove Imidacloprid through infiltration, plant uptake, and pesticide degradation. The system was also designed to have different configurations in order to analyze whether various additives and plants may affect the ability of treatment train in removing Imidacloprid. Hibiscus and Ti plants were used in the bioretention columns, while coconut husk and rice husk ash were used as additives. In addition, Imidacloprid that was present in all samples was analyzed using High-Performance Liquid Chromatography-Ultraviolet (HPLC-UV). The removal efficiency for every configuration was recorded based on Imidacloprid concentration in influent and effluent at every column in the treatment train system. The results indicated that the vegetation set achieved 90% of Imidacloprid removal efficiency while the control set (only soil media) removed Imidacloprid at a range between 64.9 to 65.9%. Furthermore, the vegetation set with additives also resulted in the increment of Dissolved Oxygen (DO) with about 16% and pH increment between 2 to 11% for all treatment train sets with a better water quality improvement.

Unconventional microbial mechanisms for the key factors influencing inorganic nitrogen removal in stormwater bioretention columns

Bioretention systems are environmentally friendly measures to control the amount of water and pollutants in urban stormwater runoff, and their treatment performance for inorganic N strongly depends on various microbial processes. However, microbial responses to variations of N mass reduction in bioretention systems are complex and poorly understood, which is not conducive to management designs. In the present study, a series of bioretention columns were established to monitor their fate performance for inorganic N (NH4+and NO3−) by using different configurations and by dosing with simulated stormwater events. The results showed that NH4+ was efficiently oxidized to NO3−, mainly by ammonia- and nitrite-oxidizing bacteria in the oxic media, regardless of the configurations of the bioretention systems or stormwater conditions. In contrast, NO3− removal pathways varied greatly in different columns. The presence of vegetation efficiently improved NO3−mass reduction through root assimilation and enhancement of microbial NO3− reduction in the rhizosphere. The construction of an organic-rich saturation zone can make the redox potential too low for heterotrophic denitrification to occur, so as to ensure high NO3− mass reduction mainly via stimulating chemolithotrophic NO3− reduction coupled with oxidation of reductive sulfur compounds derived from the bio-reduction of sulfate. In contrast, in the organic-poor saturation zone, multiple oligotrophic NO3− reduction pathways may be responsible for the high NO3− mass reduction. These findings highlight the necessity of considering the variation of N bio-transformation pathways for inorganic N removal in the configuration of bioretention systems.