Effective nitrogen removal during different periods of a field-scale bioretention system

Nitrogen removal performance of bioretention cells under polyethylene (PE) microplastic stress

The use of bioretention cells as a stormwater control measure allows stormwater runoff to be collected and filtered, effectively removing microplastics and other pollutants from stormwater. This study investigated the effect of polyethylene microplastics (PE-MPs) retention on the bioretention cell, in terms of denitrification performance and microbial community structure. Four PE-MP exposures were compared at different concentrations of 0, 250, 500 and 1000 mg/L under alternating dry and wet period conditions. Results showed that the removal efficiency reduced by 14.99%, 28.37% and 18.59% with PE-MP concentrations of 250, 500 and 1000 mg/L. The NO3--N removal efficiency increased by 36.19%, 20.19% and 35.39%. After 8 days of dry conditions, the NO3--N removal efficiencies of the bioretention cells were reduced by 36.66%, 46.86% and 31.11% compared to those after 2 days of dry conditions. Microbial sequencing results indicated that the accumulation of PE-MPs changed the microbial community structure within the bioretention cell filler material, promoting the growth of bacteria such as Actinobacteria, Bacteroidetes and Firmicutes. Furthermore, PE-MPs reduced the relative abundance of nitrifying bacteria (e.g. Nitrospira) within the bioretention cell and promoted denitrifying bacteria (e.g. Dechloromonas and Hydrogenophaga), along with numerous other genera such as Azotobacter and Nocardia.

Effects of freeze-thaw cycles on nutrient removal from bioretention cells

There have been numerous summaries of the runoff purification characteristics of bioretention cells in warm climates. However, little has been done on the effects of freeze-thaw cycles (FTCs) that frequently occur in cold regions on bioretention cell performance. Three experimental columns were constructed to simulate the operation of the bioretention cell under the FTCs. The effects of FTCs on the nutrient removal efficiency of different filling bioretention cells were analyzed. The results showed that the ammonia nitrogen (NH₄⁺-N) concentration in the effluent of the wood chip bioretention cell under the T3 conditions (WBCF) (2.35 mg/L) was significantly higher than that of the wood chip bioretention cell operating at room temperature (WBCR) (0.62 mg/L). The effluent NH₄⁺-N concentration of aluminum sludge bioretention cell (ABCF) (0.096 mg/L) under the FTCs was lower than that of WBCF (0.91 mg/L). Under the T3 condition, the effluent nitrate nitrogen (NO₃^--N) and total nitrogen (TN) concentrations of WBCF (5.33 mg/L and 8.86 mg/L) were higher than those of WBCR (5 mg/L and 6.11 mg/L) at room temperature. Under FTCs conditions, both WBCF and ABCF had high NO₃^--N removal efficiency (up to 85.87% and 24.75%) at the initial stage of thawing of the filler, and the efficiency gradually decreased with the thawing of the filler. With the increase of FTCs, the NO₃^--N removal efficiency of WBCF gradually decreased (always higher than 13.6%), while the removal efficiency of ABCF fluctuated wildly (the removal efficiency was primarily negative). The total phosphorus (TP) concentration in the effluent of WBCF (0.11 mg/L) under the T3 conditions was lower than that of WBCR (0.02 mg/L) at room temperature, and the TP concentration of ABCF (0.021 mg/L) in the effluent under the FTCs was slightly lower than that of WBCF (0.031 mg/L). The FTCs have a more significant impact on removing nitrogen pollutants in runoff, but have little effect on phosphorus. Compared with aluminum sludge, wood chips are more suitable for efficient removal of nitrogen pollutants in runoff under the FTCs. The experimental conclusions can provide a reference for the construction of bioretention cells in cold regions.

Woodchips bioretention column for stormwater treatment: Nitrogen removal performance, carbon source and microbial community analysis

This study chose Oak woodchips and gravel as media filter to enhance the denitrification in the bioretention system (saturated zone 7.7 L) treating synthetic stormwater runoff. It revealed that the denitrification process mainly occurred during the drying phase and enlarging volume of saturated zones to retain more stormwater during storm event was the direct method to promote nitrogen removal of the bioretention system. Nevertheless, it was noted that the nitrogen and dissolved organic carbon would be released into the effluent during the wetting period. The denitrification rate with different nitrate nitrogen (NO₃-N) concentrations did not show the obvious change with zero order kinetics constant of 2.91 mg/L∙d on average. Furthermore, it confirmed that woodchips were degraded and converted to volatile fatty acids (VFAs), especially acetic acid as carbon source, further utilized by the denitrifying bacteria, such as Dechloromonas, Acidoborax, Pseudomonas, Denitratisoma and Acinetobacter. In addition, genera of Lachnospiraceae and Lactobacillus, which had the ability to degrade the macromolecular organic components into low molecular VFAs, were observed in the woodchips bioretention system.

Nutrients and solids removal in bioretention columns using recycled materials under intermittent and frequent flow operations

This research investigated the fate and removal of nitrite (NO₂-N), nitrate (NO₃-N), orthophosphate (PO₄-P), and total suspended solids (TSS) in two bioretention columns, which were designed with three recycled materials. The first column was packed with Recycled Concrete Aggregate (RCA). The second column was a Layered Media (LM), which has layers of RCA with crushed glass and rice husks. The columns were tested under intermittent and frequent operations of synthetic runoff with low and high feed concentrations. The effect of inflow concentration, antecedent dry days (ADD), column age, and the anticipated number of events (EN) was also statistically analyzed on the performance of columns. Depending on column types, nutrient removal was significantly (p < 0.05) increased under frequent flow operations by 26-53% over intermittent. However, TSS removal was notably (p < 0.05) increased by 23-35% under intermittent operations over frequent. Overall, LM showed an increased NO₂-N (92 ± 2%) and NO₃-N (88% ± 2%) removal under low feed frequent operations and TSS removal (97% ± 2%) under initial intermittent operations. On the contrary, RCA showed a maximum of 99% PO₄-P removal under high feed frequent operations. Results showed that the nutrient outflow concentration was found to have a negative correlation with EN and column age and a positive correlation with ADDs throughout the experiments.

Biological nitrogen removal from stormwater in bioretention cells: a critical review

Excess nitrogen in stormwater degrades surface water quality via eutrophication and related processes. Bioretention has been recognized as a highly effective low-impact development (LID) technology for the management of high runoff volumes and reduction of nitrogen (N) pollutants through various mechanisms. This paper provides a comprehensive and critical review of recent developments on the biological N removal processes occurring in bioretention systems. The key plant- and microbe-mediated N transformation processes include assimilation (N uptake by plants and microbes), nitrification, denitrification, and anammox (anaerobic ammonia oxidation), but denitrification is the major pathway of permanent N removal. Overall, both laboratory- and field-scale bioretention systems have demonstrated promising N removal performance (TN: >70%). The phyla Bacteroidetes and Proteobacteria are the most abundant microbial communities found to be enriched in biofilter media. Furthermore, the denitrifying communities contain several functional genes (e.g., nirK/nirS, and nosZ), and their concentrations increase near the surface of media depth. The N removal effectiveness of bioretention systems is largely impacted by the hydraulics and environmental factors. When a bioretention system operates at: low hydraulic/N loading rate, containing a saturation zone, vegetated with native plants, having deeper and multilayer biofilter media with warm climate temperature and wet storm events periods, the N removal efficiency can be high. This review highlights shortcomings and current knowledge gaps in the area of total nitrogen removal using bioretention systems, as well as identifies future research directions on this topic.

Bioretention systems for stormwater management: Recent advances and future prospects

Bioretention is a popular stormwater management strategy that is often utilized in urban environments to combat water quality and hydrological impacts of stormwater. This goal is achieved by selective designing of a system, which consists of suitable vegetation at the top planted on an engineered media with drainage system and possible underdrain at the bottom. Bibliometric analysis on bioretention studies indicates that most of the original research contributions are derived from a few countries and selected research groups. Hence, most of the bioretention systems installed in diverse geographical locations are based on guidelines from climatically different countries, which often lead to operational failures. The current review critically analyzes recent research findings from the bioretention literature, provides the authors' perspectives on the current state of knowledge, highlights the key knowledge gaps in bioretention research, and points out future research directions to make further advances in the field. Specifically, the role and desired features of bioretention components, the importance of fundamental investigations in laboratory, field-based studies and modeling efforts, the real-time process control of bioretention cells, bioretention system design considerations, and life cycle assessment of full-scale bioretention systems are discussed. The importance of local conditions in guiding bioretention designs in difference climates is emphasized. At the end of the review, current technical challenges are identified and recommendations to overcome them are provided. This comprehensive review not only offers fundamental insights into bioretention technology, but also provides novel ideas to combat issues related to urban runoff and achieve sustainable stormwater management.

Bioretention for removal of nitrogen: processes, operational conditions, and strategies for improvement

As one of the low-impact development measures, bioretention plays an important role in reducing the runoff peak flow and minimizing runoff pollutants, such as heavy metals, suspended solids, and nutrients. However, the efficiency of nitrogen removal in the bioretention system is unstable, owing to the different chemical properties of various forms of nitrogen and the limitations of current bioretention system for nitrogen transformation. This review article summarizes the recent advances in bioretention system in treatment of urban stormwater and agricultural runoff for nitrogen removal. The microbial characteristics and main processes of nitrogen transformation in bioretention are reviewed. The operational conditions affecting nitrogen removal, including climatic conditions, pH, wet-dry alternation, influent loads and nitrogen concentration, and hydraulic residence time are discussed. Finally, measures or strategies for increasing nitrogen removal efficiency are proposed from the perspectives of structural improvement of the bioretention system, optimization of medium composition, and enhancement of the nitrogen removal reaction processes.

Cost-benefit analysis of low-impact development at hectare scale for urban stormwater source control in response to anticipated climatic change

Investigation of the cost-effectiveness of low-impact development (LID) practices at the hectare scale in response to impacts of possible climate change was conducted using representative concentration pathways (RCPs). An LID project in Guangzhou has been selected to illustrate changes in the hydrologic performance for alternative source control strategies for a variety of future climate models and scenarios. Frequent storms of shorter duration in RCP 8.5 cause more dramatic fluctuation of hydrologic performance. Hydrologic performance of LID practices on reducing runoff volume and peak flow in test catchment are different in climate scenarios. Based on the constraints of life cycle costs and environmental impacts of LID alternatives, comprehensive strategies were found effective in managing surface runoff at the source to adapt to the influence of climate change. The methodology described herein could be useful in considering LID practices for critical source management with limited budgets and considering environmental impacts under long-term climate change.

The impact of media, plants and their interactions on bioretention performance: A review

Bioretention systems have gained considerable popularity as a more natural approach to stormwater management in urban environments. The choice of bioretention media is frequently cited as one of the critical design parameters with the ultimate impact on the performance of the system. The goal of this review is to highlight data that challenge the importance of media as being the dominant design parameter and argue that the long-term performance is shaped by the interactions between media and the living components of a bioretention system, especially vegetation. Some of the key interactions are related to the impact of plant roots on media pore structure, which has implications on infiltration, storage capacity, and treatment. Another relevant interaction pertains to evapotranspiration and the associated impacts on the water balance and the water quality performance of bioretention systems. The impacts of vegetation on the media are highlighted and actual, as well as potential, impacts of plant-media interactions on bioretention performance are presented.

A bilayer media bioretention system for enhanced nitrogen removal from road runoff

Bioretention has been widely used in urban non-point source (NPS) pollution management for effectively reducing downstream pollution loads and peak flows. However, nitrogen (N) removal in conventional bioretention systems has been uniformly unstable and highly variable due to a lack of anaerobic denitrification. To improve the stability and effectiveness of N removal, two bioretention columns with bilayer media (C1 and C2) were designed. High permeability quartz sand (~2 mm diameter) was used as the upper media, and low permeability modified media (~0.6 mm diameter, adding 5% organic substance) as the lower media. The bilayer media structure formed an anaerobic zone for promoting denitrification processes. The results showed that the retrofitted columns performed well and that the removal efficiencies of various forms of N were considerably enhanced to 76.8%-95.3%, 85.1%-98.3%, and 87.5%-97.4% for TN, NH₄⁺-N, and NO₃^--N, respectively. Additionally, copying numbers of the denitrification functional genes detected via FQ-PCR in the lower media of C1 and C2 were accounted for 46.06% and 44.16% of the 16S rDNA gene, respectively. These results confirmed the presence of anaerobic denitrification processes. Consequently, bilayer media bioretention systems are worth promoting in cities where nitrogen in urban runoff poses a threat to the receiving surface water, due to the systems' remarkable performance in nitrogen removal, simple structure, and easy implementation.

A pilot bioretention system with commercial activated carbon and river sediment-derived biochar for enhanced nutrient removal from stormwater

Bioretention is an effective technology for urban stormwater management, but the nutrient removal in conventional bioretention systems is highly variable. Thus, a pilot bioretention column experiment was performed to evaluate the nutrient control of systems with commercial activated carbon and river sediment-derived biochar. Significant chemical oxygen demand (COD) and total phosphorus (TP) leaching were found with the addition of activated carbon and biochar, but total nitrogen (TN) leaching was significantly improved when activated carbon was used as the medium. During a semi-synthetic runoff experiment, the bioretention systems containing two types of fluvial biochar showed relatively better COD and TN control (average mass removal efficiencies and cumulative removal efficiencies) than commercial activated carbon. However, the average TP mass removal efficiency with commercial activated carbon (95% ± 3%) was significantly higher than biochar (48% ± 20% and 56 ± 14%). The addition of biochar in the media increased the nitrogen removal efficiency, and the addition of activated carbon significantly increased the phosphorous removal efficiency. Therefore, both biochar and activated carbon are effective materials for bioretention, and fluvial biochar provides an alternative approach to comprehensively utilize river sediment.

Nutrient removal from urban stormwater runoff by an up-flow and mixed-flow bioretention system

Bioretention is one of the most popular technical practices for urban runoff pollution control. However, the efficiency of nutrient removal from urban stormwater runoff by bioretention systems varies significantly. To improve the nutrient removal performance, innovative up-flow and mixed-flow bioretention systems were proposed in this study, and a laboratory study was conducted to investigate the runoff retention and nutrient removal performance. During the leaching experiment using tap water as the inflow, turbidity, chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) leaching phenomenon was obvious. COD and TN leaching controls were obviously improved when the up-flow and mixed-flow bioretention systems were adopted comparing with the conventional bioretention. During the semi-synthetic runoff experiments, after the leaching experiments' performance (accumulated 2.78 times of empty bed volume), there were no significant differences in COD mass removal efficiencies of conventional and up-flow bioretention processes (p > 0.05); however, the COD mass removal efficiencies of the mixed-flow bioretention processes increased by 10% when compared with conventional bioretention. The TN mass removal efficiencies of the up-flow and mixed-flow bioretention increased obviously from 17% ± 13% (conventional) to 41% ± 23% (up-flow) and 31% ± 16% (mixed-flow). However, there were no significant differences in TP mass removal or runoff reduction among the three bioretention columns (p > 0.05). Both up-flow and mixed-flow bioretention can effectively improve TN mass removal, and the mixed-flow bioretention did not show a better TN removal performance than the up-flow bioretention because these two bioretention had almost the same volume of the saturated zone. Overall, the results indicate the mixed-flow bioretention proposed in this study can effectively improve TN mass removal and slightly improve COD mass removal relative to conventional methods via increases in hydraulic retention time and in-flow paths, respectively.

Assessing Hydrological Effects of Bioretention Cells for Urban Stormwater Runoff in Response to Climatic Changes

An investigation into the effectiveness of bioretention cells (BCs) under potential climatic changes was conducted using representative concentration pathways. A case study of Guangzhou showed changes in peak runoff in climate change scenarios, with obvious growth in RCP8.5 and slight growth in RCP2.6. The performance of BCs on multiple parameters, including reduction of runoff volume, peak runoff, and first flush, were examined in different design storms using a hydrology model (SWMM). The effectiveness of BCs varied non-linearly with scale. Their performance fell by varying amounts in the various scenarios. BCs could provide sufficient effects in response to short-return-period and short-duration storms, but the performance of BCs decreased with heavy storms, especially considering climate change. Hence, BCs cannot replace grey infrastructure but should be integrated with them. The method developed in this study could be useful in the planning and design of low impact development in view of future climate changes.