Bioretention Cells Research Articles

Influence of double-layer filling structure on nitrogen removal and internal microbial distribution in bioretention cells

The nitrogen removal effect of traditional bioretention cells on runoff rainwater is not stable. The nitrogen removal effect of bioretention cells can be improved by setting up a layered filling structure, but the effect of changes in filling structure on the nitrogen removal process and microbial community characteristics is still unclear. Two types of porosity fillers were set up in the experiment, and a homogeneous bioretention cell and three bioretention cells with layered fillers were constructed by changing the depth range of the upper and lower layers to analyze the influence of the pore variation of different depth fillers on the nitrogen removal process and microbial community characteristics. The experimental results showed that, compared with the homogeneous filing structure, the layered filling structure can strengthen the adsorption of NH4+-N and the conversion of NO3−-N, so as to increase the removal rates of NH4+-N and NO3−-N by 20.71–81.56% and 9.25%–78.19%, respectively. Although the low porosity filler structure will reduce the nitrification activity and urease activity by 48.63%–66.68% and 8.00%–20.64% respectively, it can increase the denitrification activity by 19.14%–31.92%, thus significantly reducing the nitrate content in the filler. The low porosity filler structure can affect the growth and reproduction of various phylum bacteria such as Proteobacteria, Chloroflexi, Acidobacteria, and genus bacteria such as Nitrospira, Ellin6067, Rhizobacter, Pseudomonas, which can improve the diversity and richness of microorganisms.

Modeling multi-year phosphorus dynamics in a bioretention cell: Phosphorus partitioning, accumulation, and export

Phosphorus (P) export from urban areas via stormwater runoff contributes to eutrophication of downstream aquatic ecosystems. Bioretention cells are a Low Impact Development (LID) technology promoted as a green solution to attenuate urban peak flow discharge, as well as the export of excess nutrients and other contaminants. Despite their rapidly growing implementation worldwide, a predictive understanding of the efficiency of bioretention cells in reducing urban P loadings remains limited. Here, we present a reaction-transport model to simulate the fate and transport of P in a bioretention cell facility in the greater Toronto metropolitan area. The model incorporates a representation of the biogeochemical reaction network that controls P cycling within the cell. We used the model as a diagnostic tool to determine the relative importance of processes immobilizing P in the bioretention cell. The model predictions were compared to multi-year observational data on 1) the outflow loads of total P (TP) and soluble reactive P (SRP) during the 2012–2017 period, 2) TP depth profiles collected at 4 time points during the 2012–2019 period, and 3) sequential chemical P extractions performed on core samples from the filter media layer obtained in 2019. Results indicate that exfiltration to underlying native soil was principally responsible for decreasing the surface water discharge from the bioretention cell (63 % runoff reduction). From 2012 to 2017, the cumulative outflow export loads of TP and SRP only accounted for 1 % and 2 % of the corresponding inflow loads, respectively, hence demonstrating the extremely high P reduction efficiency of this bioretention cell. Accumulation in the filter media layer was the predominant mechanism responsible for the reduction in P outflow loading (57 % retention of TP inflow load) followed by plant uptake (21 % TP retention). Of the P retained within the filter media layer, 48 % occurred in stable, 41 % in potentially mobilizable, and 11 % in easily mobilizable forms. There were no signs that the P retention capacity of the bioretention cell was approaching saturation after 7 years of operation. The reactive transport modeling approach developed here can in principle be transferred and adapted to fit other bioretention cell designs and hydrological regimes to estimate P surface loading reductions at a range of temporal scales, from a single precipitation event to long-term (i.e., multi-year) operation.

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.

Evaluating the Effectiveness of Bioretention Cells for Urban Stormwater Management: A Systematic Review

Bioretention cells (BRCs) are a promising low-impact development (LID) practice that are commonly used in urban settings to improve the water quality and mitigate the hydrological effects of stormwater runoff. BRCs have been the subject of extensive research in order to better comprehend their function and improve their effectiveness. However, BRC performance differs greatly among regions in terms of hydrologic performance and quality enhancement. Due to this variance in BRC effectiveness, the current study conducted a comprehensive systematic review to answer the question, “Are BRCs an effective LID method for urban catchment stormwater management?”. This review study analyzed the effectiveness of BRCs in mitigating hydrologic impacts and enhancing the quality of stormwater runoff in urban catchments. A review of 114 field, laboratory, and modeling studies on BRCs found that the promising BRCs may be one of the most successful approaches to restore urban hydrology cycle and improve stormwater water quality. With further development of BRCs, their performance in terms of quantity and quality will become more reliable, helping to develop long-term solutions to stormwater urban drainage issues. At the end of this review, the knowledge gaps and future prospects for BRC research are presented. In addition to providing a foundational grasp of BRC, this review study outlines the key design recommendations for BRC implementation in order to address the issues raised by certain BRC design errors.

Assessing Hydrological Performances of Bioretention Cells to Meet the LID Goals

BRCs (bioretention cells), one of many low-impact development (LID) practices, are increasingly utilized to lessen the amount of runoff while simultaneously improving the runoff quality. Because the goal of BRCs and LID designs, in general, is to mimic or replicate the pre-development hydrology, it is critical to evaluate the hydrologic and ecologic performances of the BRC facility from the perspective of replicating the pre-development hydrology. The metrics developed in this study were intended to represent the hydrologic regime including the runoff volume control metrics, peak flow frequency exceedance curve, and flow duration curve. We also used a hydrological indicator of T0.5, the fraction of a multi-year period in which the flow exceeds the 0.5-year return period storm to represent the performances regarding downstream ecology. The indicators were compared to their pre-development values to determine how closely they reflected and replicated the pre-development state. A long-term stormwater management model (SWMM) model was developed to examine conditions before and after development and water movement in BRCs. When the BRCs facilities areas are 5% of the entire impervious study area, key findings show that: (1) BRCs have significant runoff volume control performances. (2) The peak flow frequency exceedance curve with BRCs could fully match the pre-development scenario for minor rainfall events compared to the 0.1-year storm. Flow duration curves with BRCs showed that, the frequency, magnitude, and duration of small flows that occurred for more than 90% of the total time closely matched those of pre-development hydrology. (3) T0.5 with BRCs showed significant improvement compared with the value of the area with no BRCs and was close to the pre-development T0.5. The findings presented in this study indicated the significant performance of BRCs in improving downstream ecology.

Enhanced nutrient removal of agricultural waste-pyrite bioretention system for stormwater pollution treatment

Pyrite as filler applied to bioretention cells has the potential for simultaneous nitrogen and phosphorus removal, but its low electron supply rate cannot adapt to the demand for nitrogen and phosphorus removal under high rainfall intensity and continuous rainfall conditions. In this study, novel pyrite-based bioretention cells with agricultural waste amendment (corn cobs, rice husks, wood chips) were developed, and the pollution removal of each cell was evaluated under simulated complex rainfall conditions. The bioretention cell amended with woodchips achieved significantly higher efficiency and stability for dissolved nutrient removal compared with the pyrite bioretention cell. Its average removal efficiency of nitrate nitrogen, total nitrogen, and phosphate phosphorus was 69.39%, 45.60%, and 60.01% higher than that of the control group. The average chemical oxygen demand (COD) and total iron (TFe) concentrations in the effluent were slightly higher than those from the pyrite bioretention cell but were significantly lower than those from the corn cob and the rice husk-pyrite bioretention cell. Microbiological analysis revealed that the addition of woodchips significantly increased the relative abundance of Thiobacillus (the main functional genus of autotrophic denitrification) and Thauera (the main functional genus of heterotrophic denitrification) in the bioretention cell to 25.17% and 3.06%. The results indicated that woodchips promoted the proliferation of both heterotrophic and autotrophic microorganisms, therefore enhancing the nitrogen removal efficiency of mixotrophic denitrification.

Decontamination and Ecological Restoration Performance of a Bioretention Cell-Microbial Fuel Cell under Multiple-Antibiotics Stress

Antibiotics are refractory pollutants that have been widely found in various environmental media such as soil and surface water. Existing sewage treatments perform poorly at preventing antibiotics in urban sewage from polluting natural environments. In this study, we designed a bioelectrically enhanced bioretention cell system (bioretention cell-microbial fuel cell, BRC-MFC) that utilizes the unique structure of the BRC system to improve the removal of sewage antibiotics. This new system can efficiently remove antibiotics by using a synergy of plant absorption, filler adsorption, filler filtration and microbial degradation. To study the influences of multiple-antibiotics stress on the decontamination performance of BRC-MFC, ofloxacin (OFLX) and tetracycline (TC) were selected as target antibiotics, and five BRC-MFCs were built to treat sewage containing antibiotics of different concentrations. The concentrations of pollutant in the influent and effluent were measured and the pollutant removal performance of BRC-MFC was studied. The diversity of rhizosphere microorganisms and the abundance of denitrifying functional genes were analyzed. Experimental results showed that over 90% of OFLX and TC in each BRC-MFC were removed, with the removal rates positively correlating with the concentration of antibiotics. In addition, the removal rates of chemical oxygen demand (COD) in BRC-MFC were both over 90%, while the removal rate of total nitrogen (TN) was around 70%. Meanwhile, antibiotics could significantly improve the removal of ammonia nitrogen (NH4+-N, p < 0.01). The microbial richness decreased, and we found that combined antibiotic stress on microorganisms was stronger than single antibiotic stress. The abundance of denitrifying functional genes was reduced by antibiotic stress. The results of this study provide reference values for other projects focusing on removing various antibiotics from domestic sewage using BRC-MFC.

Nitrogen transfer and transformation in bioretention cells under low temperature conditions

The nitrogen removal effect of traditional bioretention cells is generally poor under low temperature conditions, with significant levels of fluctuation and leaching often reported. Therefore, the migration characteristics of nitrogen were explored in bioretention cells under low temperature conditions, with the aim of improving the nitrogen removal effect. Four groups of modified collapsible loess bioretention cells were constructed and operated at 1, 5, 10 and 25 °C. The nitrogen removal effect of the cells was determined at different temperatures and the nitrogen migration and transformation characteristics under low temperature conditions were discussed. Experimental results showed that during the rainfall period, the ammonia nitrogen removal efficiency remained similar at different temperatures (above 97 %), while the nitrate nitrogen removal efficiency varied significantly at 1, 5, 10 and 25 °C, from 28.15 %–65.22 %, 96.68 %–98.8 %, 96.75 %–98.88 % and 80.14 %–96.72 %, respectively. In addition, nitrate nitrogen accumulation occurred in the filler during rainfall events, with lower temperature conditions increasing the final concentration of nitrate nitrogen accumulated. Following a rainfall event, the content of nitrate nitrogen in the filler decreased significantly over a 60 h dry period. However, the nitrate nitrogen reduction rate was significantly lower under low temperature conditions, than at 25 °C. Overall, low temperature conditions had a negative effect on the accumulation of nitrate nitrogen in the filler during rainfall events, as well as the transformation and migration of nitrate nitrogen within the filler during drought periods, with the adverse effects most significant at temperatures lower than 5 °C.

Numerical simulation study of urban hydrological effects under low impact development with a physical experimental basis

Global climate change and rapid urbanization led to frequent extreme rainstorms and waterlogging. Low impact development (LID) as an essential measure of urban stormwater control, it is necessary to explore the urban hydrological response under low impact development combined with physical experiment and numerical simulation. This paper takes the Beiyangyuan campus of Tianjin University, where numerous LID practices have been implemented, as the study area. The prototype permeable pavement in the campus was scaled down to a laboratory-scale for infiltration experiments. The results show that the Horton infiltration model could describe the infiltration process of permeable concrete and grass-planting brick. The initial infiltration rate of the permeable concrete is large but decays faster. In contrast, the infiltration process of the grass-planting brick decays slower, and the stable infiltration rate and infiltration capacity are higher than that of the permeable concrete. The mean infiltration rates measured for the permeable concrete and the grass-planting brick were 95 mm/hr and 190 mm/hr, respectively. A SWMM-MIKE21 coupled model was constructed based on the LID experimental results to explore the effects of the removal or density reduction of the LID practices on surface hydrological processes. The results show that permeable pavement performs best in the reduction of runoff volume, inundation area, and inundation depth, while the bio-retention cell takes the second place, and the sunken greenbelt has limited effect. Frequent light rainfall is sensitive to the density of LID practices, and the implementation of even a tiny percentage (4% of the catchment area) of LID practices can be effective in controlling runoff. The removal or density reduction of LID practices has little effect on the inundation region but leads to an increased possibility of flood risk due to increasing inundation depth.

Statistical evaluation on hydrologic performance of bioretention design parameters under different rainfall conditions

Effectiveness of a bioretention cell (BC) in mitigating stormwater peak flow load to an urban drainage system is highly dependent on the design configuration and retrofitting scale. The selection of appropriate BC design parameters has always been critical to achieving its maximum benefits. The BC modeling in the stormwater management model (SWMM) relies on numerous design parameters (in total of 18) with wide selection ranges, making parameter selection challenging. This study investigated the effect of hydrologic dynamics of BC design parameters under different rainfall conditions (i.e., intensity, duration, time-to-peak) on surface infiltration, surface outflow, and storage. Firstly, the seven influential BC design parameters (i.e., conductivity, berm height, vegetation volume, suction head, porosity, wilting point, and soil thickness) were selected based on one-factor-at-a-time (OAT) method, and 1000 random samples of each parameter were generated across the factor space by following respective uniform distributions. The simulations were performed with a python wrapper of SWMM (PySWMM) that used random samples for one parameter while leaving the others constant each time. In general, the statistical results of each parameter represent a significant variation in the volume distribution under various design rainfall conditions. Comparatively, the parameters of conductivity, berm height, and vegetation volume are categorized as the most influential compared to other BC design parameters for surface infiltration, storage, and surface outflow, respectively.

Hydrogen peroxide levels in biochar-based bioretention cells with three environmental conditions and its potential risks

Hydrogen peroxide levels in biochar-based bioretention cells with three environmental conditions and its potential risks

Optimization of integrating life cycle cost and systematic resilience for grey-green stormwater infrastructure

The trade-offs for alternative grey-green infrastructure (HGGI) solutions between life cycle cost (LCC) and systematic resilience may impose many limitations in planning and implementation of urban stormwater strategies in high-dense urban catchment. This study presents a generic approach in the context of a multi-objective optimization, with the aim to (1) optimize and design HGGI under various uncertainties, and conflicting objectives (e.g., LCC, technological and operational resilience); and (2) determine the trade-offs between the multi-objectives and degrees of system decentralization in areas with various development intensities. Taking two areas in Guangzhou, China, as case studies, this study shows that decentralized HGGI exhibits economic advantages. However, to maintain a high systematic resilience in high-density catchments, it is often necessary to rely on solutions with high degree of centralization, which may incur higher cost. HGGI is found to perform better in terms of technological resilience under extreme storms, than operational resilience to the uncertainty associated with structure performance. In addition, porous pavement is found to exhibit high potential for wider application compared to bioretention cells as green infrastructure in HGGI. The framework developed in this study could support decision-making for stormwater management via enhancing both hydraulic reliability and hydrologic resilience.

Nitrogen and phosphorus removal in a bioretention cell experiment receiving agricultural runoff from a dairy farm production area during third and fourth years of operation.

This study assessed the performance of three bioretention cells during the third and fourth years post establishment with respect to their ability to capture nitrogen (N) and phosphorus (P) in runoff from a dairy farm production area. The effects of two treatments across the three cells were evaluated: a vegetation treatment using switchgrass (Panicum virgatum L.) and a soil amendment treatment using low-P compost (derived from leaf litter). Cell 1 has neither vegetation nor compost; Cell 2 includes vegetation without compost; Cell 3 includes both vegetation and compost. The system was installed in 2016; performance was monitored in 2018 and 2019, after vegetation was well established. In 2019, bioretention cell hydrology was modified to create an internal storage zone (ISZ) and increase hydraulic retention time (HRT), targeting improved nitrate removal. In 2018, all three cells reduced effluent concentrations of total N by >50% and of both total P and soluble reactive P (SRP) by >90%. Similar trends were found in 2019 with the ISZ, except SRP effluent concentrations were significantly higher compared with 2018, indicating a tradeoff of P leaching associated with increased HRT. Averaging eight monitored storms, median mass removals of all nutrients for Cell 2 (with vegetation and without compost) was >94%. System performance improved during the third and fourth years of operation compared with results of the initial monitoring, highlighting the importance of monitoring once plant and soil media have become established.

Global sensitivity analysis of bioretention cell design for stormwater system: A comparison of VARS framework and Sobol method

Bioretention cell (BC) design variation significantly changes its hydrological dynamics at unit-scale leading to major changes in its target design goals when designed unit replicated at the catchment-scale. Improved understanding of the behaviors of BC design parameters under different rainfall conditions is critical for effective implementation of BCs to achieve design goals. This paper illustrates and compares two global sensitivity analysis methods (i.e., VARS and Sobol) for the identification of influential parameters and their variations in hydrological dynamics (model responses) under different rainfall conditions and perturbation scales, so as to quantify their uncertainty and reliability for influential factors ranking. From the application results of both sensitivity analysis methods, six parameters out of the total seventeen including conductivity, berm height, vegetation volume, suction head, porosity and wilting point were indicated for their significant sensitivities to surface infiltration, surface outflow and peak flow. In addition, soil thickness, conductivity slope and field capacity (in a total of nine) were categorized as influential to storage outputs. Conductivity and vegetation volume were ranked as the most influential parameters, followed by berm height and suction head, porosity and wilting point. The results analysis also demonstrated that the behavior of design parameters towards model response significantly changes with different rainfall conditions and perturbation scales, as well as the used sensitivity analysis methods. In particular, it is indicated from this study that the VARS is preferred over other sensitivity analysis approaches including the Sobol method (i.e., variance-based method) because of its higher accuracy, reliability, and computational efficiency.

Surrogate-based multiobjective optimization to rapidly size low impact development practices for outflow capture

The optimal implementation of low impact development (LID) practices remains an open research issue. Multiobjective optimization (MOO) based on physically-based models facilitates LID sizing but requires large computational budgets. We developed and validated a surrogate MOO framework based on machine learning (ML) models to obtain satisfactory optimal solutions with an affordable computational cost. First, a calibrated and validated storm water management model (SWMM) was driven by a 2-year, 120 min Chicago pattern storm with random LID areas for generating 10,000 samples. Second, ML models, including multiple linear regression (MLR), generalized regression neural network (GRNN), and backpropagation neural network (BPNN), were trained and validated to predict catchment total outflow (CTO) and catchment peak outflow (CPO). Third, using the outperformed ML models as surrogates, the LID areas were optimized by non-dominated sorting genetic algorithm-II to minimize construction costs, CTO, and CPO. The results obtained for an urban catchment in Fengxi, China, demonstrated that: (1) BPNN outperformed MLR and GRNN in terms of accuracy, while the training time (∼40 min) was ∼6857 times and ∼706 times that of MLR (∼0.35 s) and GRNN (3.4 s) models, respectively. (2) The optimization process using BPNN models as surrogates converged well, indicating the efficiency of the proposed framework. (3) Under the marginal effect, the permeable pavement was identified as the leading practice for most Pareto solutions, followed by bioretention cell and green roof. Moreover, a subcatchment with a higher runoff coefficient was deemed preferable for LID implementation. (4) Outflow reduction by the LIDs was significant (CTO and CPO were reduced by 28.6–56.9 % and 27.5–48.3 %, respectively) but negligible for retarding the time of CPO. (5) The optimization time was reduced by ∼95.82 % using the surrogate-based MOO instead of the SWMM-based model when the training time of the BPNN models was ignored. This study provides insights into the efficient optimization of LIDs.

Evaluating the impact of climate change on future bioretention performance across the contiguous United States

In light of shifting precipitation patterns induced by climate change, communities are seeking to build resiliency in urban drainage systems through interventions such as green stormwater infrastructure (GSI). Bioretention cells are one of the most commonly implemented forms of GSI for their ability to reduce peak discharge, retain runoff, and filter pollutants. However, they may be at risk of reduced function in the future due to deviations from historic precipitation frequency and intensity patterns which are essential to their design. Further, changes in future function are likely to vary regionally as the magnitude of future climate changes will differ across the globe. To explore the range of impacts to future bioretention function, an ensemble of 10 regional climate models at 17 locations across the contiguous United States were evaluated to provide the widest range of potential future outcomes using a probabilistic approach to capture the uncertain nature of climate change. Bioretention cells were modeled using USEPA’s Storm Water Management Model (SWMM) to compare existing and future performance under a range of climate change projections. Median annual rainfall and 99th percentile rainfall event depths and intensities were projected to increase across all 17 locations while antecedent dry period (i.e., the time between consecutive rain events) was projected to increase for 11 locations. Correspondingly, bioretention cell hydrologic performance decreased across all 17 locations under future scenarios: relative to performance under current climate conditions, annual volumes of infiltration decreased between 4.0 and 24.0% across all 17 locations while overflow increased between 0.4 and 19.6% for 15 locations. Results suggest that bioretention cells in the southern United States are at significant risk of reduced function in the future while those in the Midwest and Northeast are at moderate risk. Bioretention cells in the Northwest/West performed the best under future climate scenarios; that is, they showed similar function in the future to that of the present. Findings demonstrate that most, if not all, bioretention cells across the contiguous United States will require some degree of modification to maintain existing function under future conditions.

Verification of PCSWMM's LID processes and their scalability over time and space

IntroductionThis paper explores the scalability of PCSWMM's Low Impact Development (LID) modeling tools within the urban stormwater computer model.MethodsThe scalability is assessed for a variety of spatial and temporal scales and for event (50-year return storm) and continuous inputs (daily rainfall for an 11 month period), and with a focus on bioretention cells. The model is calibrated for a moderate to large scale, semi-urban watershed on Vancouver Island, British Columbia, Canada. Sensitivity analysis and specialized metrics are used to verify internal model processes at a variety of scales.ResultsWith regard to spatial scaling, changes in flow path length and slope derived from Digital Elevation Models were the most impactful spatial information when modeling flood event and the model's surface layer was the dominant contributor to peak flowrate and volume mitigation by the bioretention cell. However, when modeling the continuous rainfall inputs, storage layer related parameters dominated model outputs. Aside from the soil layer's depth, soil layer parameters such as hydraulic conductivity, showed negligible influence on response to time series rainfall. Parameters that are kept static by the model such as vegetation cover, hydraulic conductivity and storage void ratio (but are naturally dynamic), were tested for their impact on response if allowed to change seasonally or with excessive loading. Runoff coefficients were greatly impacted by storage layer parameter dynamics with very little impact from vegetation. For event simulations, the berm height in the surface layer was the dominant player in reducing peak flow as well as total volume. An analysis to help illustrate sensitivity across spatial scales is proposed.DiscussionThe Spatial Dynamic Sensitivity Analysis shows that parameter sensitivity changes dynamically as LID implementation percentage changes. In particular, the clogging factor, which is a parameter associated with the storage layer, was highly influential for time series rainfall analysis. The LID model concepts in PCSWM seem appropriate for events because the surface layer dominates the response for very large storms. For smaller storms, continuous time series, and larger spatial scales, the model could be revised to better represent soil layer dynamics and vegetation cover, which were both currently inconsequential to the model's output.

The influence of Low Impact Development (LID) on basin runoff in a half-urbanized catchment: A case study in San Antonio, Texas

Low Impact Development (LID) was promoted as an alternative to conventional urban drainage methods. The effects of LID at the site or urban scales have been widely evaluated. This project aims to investigate the impact of LID implementation on basin runoff at a regional scale in a half-urbanized catchment, particularly the overlap of urban and rural sub-flows at peak times. A SUPERFLEX conceptual model framework is adapted as a semi-distributed model to simulate the rainfall-runoff relationship in the catchment for San Antonio, Texas, as a case study. Scenario analyses of both urban development and LID implementation are conducted. Results show that (1) the infill urban development strategy benefits more from runoff control than the sprawl urban development; (2) in non-flood season, permeable pavements, bioretention cells, and vegetated swales decrease peak runoff significantly, and permeable pavements, bioretention cells, and green roofs are good at runoff volume retention; (3) contrary to the general opinion about the peak reduction effect of LID, for a partly urbanized, partly rural basin, the LID implementation delays urban peaks and may cause larger stacking of rural and urban peak runoffs, leading to larger basin peaks under extremely wet conditions.

Application of electrochemical oxidation for the enhancement of antibiotic resistant bacteria removal in stormwater bioretention cells

Recently, increasing attention has been paid to the removal of antibiotic resistant bacteria (ARB) during electrochemical advanced oxidation processes. However, there is still no available literature about the application of electrochemical oxidation (EO) to enhance ARB removal in stormwater bioretention cells. Batch experiments were conducted to investigate target ARB (E. coli K-12 carrying blaTEM, tetR and aphA) removals in bioretention cells with different current densities and ratios of air to water (A/W). ARB removals for bioretention cells with 17.6 μA/m2 of current density and 24:1 of A/W ratio was the largest with 5.28 log reduction, which was obviously higher than the one (3.68 log reduction) in the control (without EO). H2O2 production could be responsible for ARB removals in the used bioretention cells, where H2O2 levels increased at first and then decreased with the increase of current densities and A/W ratios. The evaluation for the application of EO implied that the highest antibiotic resistance (AR) conjugation frequency (3.8 × 10−3) at 3.5 μA/m2 of current density and 48:1 of A/W ratios was 124.5 % of the one in the control, while the largest AR transformation frequencies at 17.6 μA/m2 of current density and 48:1 of A/W ratios was 366.9 % (tetR) and 216.2 % (aphA) of the corresponding in the control, and there were still stable for both dominant microflora and metabolic activities in bioretention cells with electricity and aeration, suggesting that EO could be promising for the enhancement of ARB removals in bioretention cells.

Optimal design of stepped bioretention cells for slopes

Stepped bioretention cells (SBCs) refer to multiple bioretention cells (BCs) built in series along slopes. They are green infrastructures that can possibly be implemented on slopes. However, their optimal design has not been thoroughly investigated. The use of multiple cells makes the design of SBCs distinct from that of traditional BCs. In general, the design of a single BC focuses on water distribution among different horizontal layers (i.e., surface, soil, and storage layers), while that of SBCs additionally focuses on water distribution among cells. To examine the optimal design of SBCs, this study constructed a physical model of SBCs and conducted eight representative experiments. A numerical model was built, calibrated, and validated based on the experimental results and then used to evaluate the effects of design measures, namely changes in the number of cells and installation of pipes (i.e., underdrains and intermediate pipes). Intermediate pipes connect the storage layers of two adjacent cells. Four metrics were used to compare the effectiveness of design measures under various environmental conditions: peak runoff reduction, runoff volume reduction, exfiltration, and water distribution uniformity (WDU). The first three metrics reflect the overall hydrologic performance of SBCs, while the last metric reflects the distribution of infiltrated water among the cells. The findings showed that reducing the number of cells increased peak runoff reduction under less-intense rainfall events and increased WDU. Underdrains significantly improved peak runoff reduction and runoff volume reduction but decreased exfiltration and WDU under all environmental conditions. Elevated underdrains encouraged more exfiltration than bottom underdrains. Intermediate pipes enhanced exfiltration but had a limited effect on peak runoff reduction. However, intermediate pipes decreased runoff volume reduction and even led to a poor WDU when underdrains were absent. Overall, the findings suggest that SBCs are an effective form of green infrastructure for slopes as they provide comparable hydrologic performance (22%–97% runoff volume reduction) to traditional BCs.