Laboratory study of biological retention for urban stormwater management

Understanding a wood-derived biochar's impact on stormwater quality, plant growth, and survivability in bioretention soil mixtures

Bioretention systems are planted media filters used in stormwater infrastructure. Maintaining plant growth and survival is challenging because most designs require significant sand. Conventional bioretention soil media (BSM) might be augmented with biochar to make the BSM more favorable to plants, to improve nutrient removal efficiency, and enhance plant survivability during drought while replacing compost/mulch components that have been linked to excess nutrient export. Pots with BSMs representing high and moderate sand content were amended with wood biochar, planted with switchgrass, and subjected to weekly storms for 20 weeks, followed by a 10-week drought. After 20 weeks, 4% biochar amendment significantly increased stormwater infiltration (67%) and plant available water (52%) in the high sand content BSM (NC mix, which meets requirements for the state of North Carolina (US) and contains no compost/mulch), and these favorable hydraulic properties were not statistically different from a moderate sand content, biochar-free BSM with compost/mulch (DE mix, which meets requirements for state of Delaware (US)). While biochar amendment improved plant height (25%), the number of shoots (89%), and total biomass (70%) in the NC mix, these parameters were still less than those in the biochar-free DE mix containing compost/mulch. TN and NO3-1 removal were also improved (28-35%) by biochar amendment to NC mix, and the resulting TN and TP loadings to groundwater were 10 and 7 times less, respectively than biochar-free DE mix with compost/mulch. During the drought period, biochar amendment increased the time to switchgrass wilting by ∼8 days in the NC mix but remained 40% less than the biochar-free DE mix. A recalcitrant carbon-like biochar mitigates some of the deleterious effects of high sand content BSM on plants, and where nutrient pollution is a concern, replacement of compost/mulch with wood biochar in BSM may be desired.

Green engineered mulch for phosphorus and metal removal from stormwater runoff in bioretention systems

Phosphorus and metals in stormwater runoff are major causes of water quality degradation. Bioretention systems are increasingly implemented to improve stormwater quality and to better manage stormwater quantity. Many studies have focused on modifying the composition of the soil bed to improve pollutant removal. However, the pollutant removal performance of bioretention systems can diminish over time, such as when clogging of the media occurs. Sediment accumulation on the soil surface may inhibit infiltration into the soil bed, thus limiting pollutant removal. Soil replacement may be eventually required as pollutants accumulate in the soil. In this study, a green retrofit material, called green engineered mulch (GEM), was generated by coating regular wood mulch with aluminum-based water treatment residuals (WTR) via a simple and low-energy process (patent pending). The GEM was developed to serve as a green retrofit for bioretention systems to enhance the removal of phosphorus and metals from stormwater runoff. The GEM was placed in a rain garden in Secaucus, NJ, USA for 15 months, during which 12 storm events (ranging from 6.0 mm to 89.6 mm) were monitored. Runoff and infiltrate samples were analyzed for dissolved and total concentrations of phosphorus and metals, along with other key water quality parameters. The GEM significantly reduced (p < 0.05) the total concentrations of phosphorus and metals in stormwater infiltrate compared to the inlet, unlike the regular mulch. Minimal or no contact with the GEM resulted in no significant pollutant removal from surface runoff. No significant pollutant export from the GEM was observed. The spent GEM can be disposed of as non-hazardous waste in municipal landfills. This study demonstrates that the GEM is a safe and effective retrofit. Moreover, the GEM is a simple and economical retrofit solution that can be used in place of regular mulch in bioretention systems.

Stormwater Best Management Practices: Experimental Evaluation of Chemical Cocktails Mobilized by Freshwater Salinization Syndrome

Freshwater Salinization Syndrome (FSS) refers to the suite of physical, biological, and chemical impacts of salt ions on the degradation of natural, engineered, and social systems. Impacts of FSS on mobilization of chemical cocktails has been documented in streams and groundwater, but little research has focused on the effects of FSS on stormwater best management practices (BMPs) such as: constructed wetlands, bioswales, ponds, and bioretention. However emerging research suggests that stormwater BMPs may be both sources and sinks of contaminants, shifting seasonally with road salt applications. We conducted lab experiments to investigate this premise; replicate water and soil samples were collected from four distinct stormwater feature types (bioretention, bioswale, constructed wetlands and retention ponds) and were used in salt incubation experiments conducted under six different salinities with three different salts (NaCl, CaCl2, and MgCl2). Increased salt concentrations had profound effects on major and trace element mobilization, with all three salts showing significant positive relationships across nearly all elements analyzed. Across all sites, mean salt retention was 34%, 28%, and 26% for Na+, Mg2+ and Ca2+ respectively, and there were significant differences among stormwater BMPs. Salt type showed preferential mobilization of certain elements. NaCl mobilized Cu, a potent toxicant to aquatic biota, at rates over an order of magnitude greater than both CaCl2 and MgCl2. Stormwater BMP type also had a significant effect on elemental mobilization, with ponds mobilizing significantly more Mn than other sites. However, salt concentration and salt type consistently had significant effects on mean concentrations of elements mobilized across all stormwater BMPs (p<0.05), suggesting that processes such as ion exchange mobilize metals mobilize metals and salt ions regardless of BMP type. Our results suggest that decisions regarding the amounts and types of salts used as deicers can have significant effects on reducing contaminant mobilization to freshwater ecosystems.

Optimizing the maintenance strategies for a network of green infrastructure: An agent-based model for stormwater detention basins

Various stormwater best management practices and green infrastructures (GIs) are recommended to address flooding, stormwater runoff, water quality, and sustainability. While detention basins are considered one of the main GI strategies, their benefits cannot be fully realized without properly maintaining them and making sure that they stay operational. Therefore, this paper used agent-based modeling (ABM) to devise an optimal maintenance program for detention basins to ensure that they function properly and continue to perform their water quality and flood control functions. More specifically, the following 2 agent types were incorporated in the model: 1) the detention basins were considered as static agents, and 2) the service teams responsible for the operation (maintenance, repair, and replacement) of the detention basins were considered as active agents. The developed ABM was applied for the entire network of stormwater detention basins in Newark, NJ. Sensitivity analysis was conducted to identify the most critical variables affecting the total cost of operating the network of detention basins as well as the functioning percentage of detention basins. In addition, optimization was implemented to determine the best maintenance program or policy that minimizes the total cost of operations, while also making sure that a desired functionality level or threshold is achieved for the entire network of detention basins. Finally, the ABM was statistically validated using a total of 10,000 Monte Carlo runs and 99% confidence intervals. The optimization results showed that, in order to minimize the total cost of maintaining the entire network of detention basins and ensure that at least 80% of the basins are in a functioning state at the end of the planning horizon, the decision-maker should implement the following maintenance program or strategy: have 2 service teams for the operations of the detention basins, follow a replacement policy, and replace detention basins after 3 maintenance periods. Also, the identified optimal maintenance program or strategy would result with an average total annual cost of around $4,085,000, where the average annual repair cost is around $2,572,200, the average annual maintenance cost is around $19,700, the average annual replacement cost is around $763,100, and the average annual service team cost is around $730,000. The proposed ABM for detention basins can be extended to other GIs as well as to different geographical areas. The usage of ABM has the advantage to reduce the subjectivity in developing plans for managing GIs.

Stormwater biofilter response to high nitrogen loading under transient flow conditions: Ammonium and nitrate fates, and nitrous oxide emissions

Nitrogen (N) in urban runoff is often treated with green infrastructure including biofilters. However, N fates across biofilters are insufficiently understood because prior studies emphasize low N loading under laboratory conditions, or use "steady-state" flow regimes over short time scales. Here, we tested field scale biofilter N fates during simulated storms delivering realistic transient flows with high N loading. Biofilter outflow ammonium (NH₄⁺-N) was 60.7 to 92.3% lower than that of the inflow. Yet the characteristic times for nitrification (days to weeks) and denitrification (days) relative to N residence times (7 to 30 h) suggested low N transformation across the biofilters. Still, across 7 successive storms, total outflow nitrate (NO₃^--N) greatly exceeded (3100 to 3900%) inflow nitrate, a result only explainable by biofilter soil N nitrification occurring between storms. Archaeal, and bacterial amoA gene copies (2.1 × 10⁵ to 1.2 × 10⁶ gc g soil^-1), nitrifier presence by16S rRNA gene sequencing, and outflow δ¹⁸O-NO₃^- values (-3.0 to 17.1 ‰) reinforced that nitrification was occurring. A ratio of δ¹⁸O-NO^3- to δ¹⁵N-NO₃^- of 1.83 for soil eluates indicated additional processes: N assimilation, and N mineralization. Denitrification potential was suggested by enzyme activities and soil denitrifying gene copies (nirK + nirS: 3.0 × 10⁶ to 1.8 × 10⁷; nosZ: 5.0 × 10⁵ to 2.2 × 10⁶ gc g soil^-1). However, nitrous oxide (N₂O-N) emissions (13.5 to 84.3 μg N m ^- 2 h ^- 1) and N₂O export (0.014 g N) were low, and soil nitrification enzyme activities (0.45 to 1.63 mg N kg soil^-1day^-1) exceeded those for denitrification (0.17 to 0.49 mg N kg soil^-1 day^-1). Taken together, chemical, bacterial, and isotopic metrics evidenced that storm inflow NH₄⁺sorbs and, along with mineralized soil N, nitrifies during biofilter dry-down; little denitrification and associated N₂O emissions ensue, and thus subsequent storms export copious NO₃^--N. As such, pulsed pass-through biofilters require redesign to promote plant assimilation and/or denitrification of mineralized and nitrified N, to minimize NO₃^--N generation and export.

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.

Considerations for evaluating innovative stormwater treatment media for removal of dissolved contaminants of concern with focus on biochar

Stormwater from complex land uses is an important contributor of contaminants of concern (COCs) such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), Copper, and Zinc to receiving water bodies. A large portion of these COCs bind to particulate matter in stormwater, which can be removed through filtration by traditional media. However, the remaining dissolved COCs can be significant and require special attention such as engineered treatment measures and media. Biochar is a porous sorbent produced from a variety of organic materials. In the last decade biochar has been gaining attention as a stormwater treatment medium due to low cost compared to activated carbon. However, biochar is not a uniform product and selection of an appropriate biochar for the removal of specific contaminants can be a complex process. Biochars are synthesized from various feedstocks and using different manufacturing approaches, including pyrolysis temperature, impact the biochar properties thus affecting ability to remove stormwater contaminants. The local availability of specific biochar products is another important consideration. An evaluation of proposed stormwater control measure (SCM) media needs to consider the dynamic conditions associated with stormwater and its management, but the passive requirements of the SCM. The media should be able to mitigate flood risks, remove targeted COCs under high flow SCM conditions, and address practical considerations like cost, sourcing, and construction and maintenance. This paper outlines a process for selecting promising candidates for SCM media and evaluating their performance through laboratory tests and field deployment with special attention to unique stormwater considerations.

Optimization and assessment of an electrochemical advanced oxidation system for synthetic stormwater treatment

Electrochemical advanced oxidation processes (eAOPs) such as the current advanced oxidation system (AOS) are a type of electrochemical wastewater treatment that creates oxidative species, such as iodide species, chloride species, and hydroxyl radicals, that can treat even the most recalcitrant contaminants. It is important to determine the concentrations and locations of oxidative species in eAOPs for optimization of the wastewater treatment process. In this study, a spectrophotometric methodology was used to determine concentrations of iodide and chloride oxidative species (starting at 10, 25, and 50 ppm) within an AOS under various input voltages (6, 12, and 24 V). Overall, it was found that iodate and chlorite were the dominant species created in their respective treatments. Additionally, the concentration of iodide oxidative species increased with increasing voltage, whereas the chloride species decreased with increasing voltage. The optimal conditions for the efficient creation of AOS oxidative species were 12 V and 10 ppm potassium iodide and 6 V and 10 ppm sodium chloride, respectively. In addition, the use of iodide is recommended for wastewater treatment using the AOS to effectively create oxidative species. Following optimization, the AOS performance was tested for synthetic stormwater. Results indicated that the AOS performed well for reduction of Escherichia coli; however, reduction of other contaminants was inconsistent as would be expected given the AOS was optimized for disinfection, not decontamination. Further AOS optimization for decontamination would be expected to result in improved decontamination performance.

Highly variable removal of pathogens, antibiotic resistance genes, conventional fecal indicators and human-associated fecal source markers in a pilot-scale stormwater biofilter operated under realistic stormflow conditions

Green stormwater infrastructure systems, such as biofilters, provide many water quality and other environmental benefits, but their ability to remove human pathogens and antibiotic resistance genes (ARGs) from stormwater runoff is not well documented. In this study, a field scale biofilter in Southern California (USA) was simultaneously evaluated for the breakthrough of a conservative tracer (bromide), conventional fecal indicators, bacterial and viral human-associated fecal source markers (HF183, crAssphage, and PMMoV), ARGs, and bacterial and viral pathogens. When challenged with a 50:50 mixture of untreated sewage and stormwater (to mimic highly contaminated storm flow) the biofilter significantly removed (p < 0.05) 14 of 17 microbial markers and ARGsin descending order of concentration reduction: ermB (2.5 log(base 10) reduction) > Salmonella (2.3) > adenovirus (1.9) > coliphage (1.5) > crAssphage (1.2) > E. coli (1.0) ∼ 16S rRNA genes (1.0) ∼ fecal coliform (1.0) ∼ intl1 (1.0) > Enterococcus (0.9) ∼ MRSA (0.9) ∼ sul1 (0.9) > PMMoV (0.7) > Entero1A (0.5). No significant removal was observed for GenBac3, Campylobacter, and HF183. From the bromide data, we infer that 0.5 log-units of attenuation can be attributed to the dilution of incoming stormwater with water stored in the biofilter; removal above this threshold is presumably associated with non-conservative processes, such as physicochemical filtration, die-off, and predation. Our study documents high variability (>100-fold) in the removal of different microbial contaminants and ARGs by a field-scale stormwater biofilter operated under transient flow and raises further questions about the utility of human-associated fecal source markers as surrogates for pathogen removal.

Bioretention Systems Optimization and Design Characterization Model Using Fuzzy Rough Set Theory

Urban stormwater has become a persistent concern on a global scale due to its adverse environmental implications. It is the prime vector of aquatic contaminants worldwide that causes pollutants when water bodies drain. Bioretention systems are increasingly used to alleviate setbacks associated with stormwater run-off in urban locales. It has played a substantial role in the implementation of low impact development (LID), a concept that addresses urban stormwater problems caused by land changes and development. The use of LID technologies is an innovative approach. However, it is beset with challenges, such as the insufficiency of data on rainfall distribution and difficulty in interpreting data. To address these research gaps, the present study developed a fuzzy rough set data algorithm for bioretention systems. Event mean concentration calculations and fuzzification of rainfall were performed to produce a rough set-based decision rule. Using the Weibull probability distribution, fuzzification of rainfall and parameter data, rule induction, and Preece testing, bioretention design considerations were determined. The bioretention characterizations generated evident pollutants present in the catch basin before and after filtration. In addition, the bioretention characterization conducted in this study was able to reduce the number of tests needed for rainfall identification based on the different attributes.

Enrichment Evaluation of Heavy Metals from Stormwater Runoff to Soil and Shrubs in Bioretention Facilities

Bioretention facilities with different inflow concentrations, growing media and plants were examined to determine whether the soil in these facilities was polluted with heavy metals and whether runoff had obvious toxic effects on plants. Using Beijing soil background value as the standard, the soils were evaluated by bioaccumulation index and single factor index. The results show that stormwater runoff containing Cu caused slight pollution in soils, and stormwater runoff containing Zn and Pb was not polluted. Nemerow comprehensive index evaluation revealed that the heavy metals content in the facilities containing vermiculite (a yellow or brown mineral found as an alteration product of mica and other minerals, used for insulation or as a moisture-retentive medium for growing plants) and perlite (a form of obsidian characterized by spherulites formed by cracking of the volcanic glass during cooling, used as insulation or in plant growth media) were higher than the standard. High influent concentration caused significantly higher heavy metals content in plants. While Pb accumulation in the two studied plants was the highest, Cu and Zn accumulation, which are essential for plant growth, was relatively low. The contents of the three heavy metals in the studied plants also exceeded their corresponding critical values.

Variability of Potential Soil Nitrogen Cycling Rates in Stormwater Bioretention Facilities

Low-impact development (LID) is a common management practice used to infiltrate and filter stormwater through vegetated soil systems. The pollutant reduction potential of these systems is often characterized by a single pollutant removal rate; however, the biophysical properties of soils that regulate the removal of pollutants can be highly variable depending on environmental conditions. The goal of this study was to characterize the variability of soil properties and nitrogen (N) cycling rates in bioretention facilities (BRFs). Soil properties and potential N cycling processes were measured in nine curbside bioretention facilities (BRFs) in Portland, OR during summer and winter seasons, and a subset of six sites was sampled seasonally for two consecutive years to further assess temporal variability in soil N cycling. Potential N cycling rates varied markedly across sites, seasons, and years, and higher variability in N cycling rates was observed among sites with high infiltration rates. The observed seasonal and annual changes in soil parameters suggest that nutrient removal processes in BRFs may be highly variable across sites in an urban landscape. This variability has important implications for predicting the impacts of LID on water quality through time, particularly when estimated removal rates are used as a metric to assess compliance with water quality standards that are implemented to protect downstream ecosystems.

Decontamination performance of a bioretention system using a simple sand-based filler proportioning method

This study investigated the decontamination performance of a bioretention system using a sand-based filler constructed using sand and peat soil. The filler was constructed according to a simple proportioning method that considers water turnover time and organic content. Different inorganic constituents were added to the filler including zeolite, volcanic rock, coal slag, vermiculite and perlite to further improve the decontamination effect. Total suspended solids (TSS), total phosphorus (TP), ammonium nitrogen (NH4+-N), total nitrogen (TN) and chemical oxygen demand (COD) were measured in the influent and effluent. The results showed that: (1) the overall removal effect of the sand-based filler was satisfactory, indicating a certain feasibility and practicality of the method; (2) bioretention based on the sand-based filler had a better performance in removing TSS and TP with the removal rate both over 95%, and the addition of inorganic constituents to the filler was beneficial to TSS removal due to the effect of cumulative filtration capabilities of multiple materials, while phosphate was easily adsorbed by the positively charged particles of the peat soil; (3) the high removal rate of NH4+-N was due to adsorption and it could reach more than 80% by adding inorganic constituents with good adsorption such as zeolite vermiculite and perlite. Similarly, the addition of vermiculite and coal slag could increase the removal rate of COD by 15-25%. This research offers a novel alternative for guiding the selection and proportion of fillers in bioretention systems.

Pollutant Removal Efficiency of a Bioretention Cell with Enhanced Dephosphorization

Low impact development can contribute to Sustainable Development Goals (SDGs) 2, 6, 7, 11, and 13, and bioretention cells are commonly used to reduce nonpoint source pollution. However, although bioretention is effective in reducing ammonia nitrogen and chemical oxygen demand (COD) pollution, it performs poorly in phosphorus removal. In this study, a new type of enhanced dephosphorization bioretention cell (EBC) was developed; it removes nitrogen and COD efficiently but also provides excellent phosphorus removal performance. An EBC (length: 45 m; width: 15 m) and a traditional bioretention cell (TBC) of the same size were constructed in Anhui, China, to treat rural nonpoint source pollution with high phosphorus concentration levels. After almost 2 years of on-site operation, the ammonium nitrogen removal performance of the TBC was 81%, whereas that of the EBC was 78%. The COD removal rates of the TBC and EBC were 51% and 65%, and they removed 51% and 92% of the total phosphorus, respectively. These results indicate that the TBC and EBC have similar performance in the removal of ammonium nitrogen and COD, but the EBC significantly outperforms the TBC in terms of total phosphorus removed.

Culex Mosquitoes at Stormwater Control Measures and Combined Sewer Overflow Outfalls after Heavy Rainfall

Mosquito borne diseases are increasingly problematic as climate change continues to alter patterns of precipitation, flooding, and temperatures that may favor mosquito habitats. Stormwater control measures (SCMs), ecologically sustainable methods of stormwater management, may have varying impacts on Culex mosquitoes, such as in areas with combined sewer overflows (CSOs). We studied spatial and temporal associations of SCMs and Culex mosquito counts surrounding the SCMs, stratifying our examination amongst those that do/do not use pooling and/or vegetation, as well as surrounding CSO outfalls after heavy rainfall (≥95th percentile) during summer 2018. Results indicate Culex mosquito counts after heavy rainfall were not significantly different at SCMs that use vegetation and/or ponding from at those that do not. We also found a 35.5% reduction in the increase of Culex mosquitoes the day of, and 77.0% reduction 7–8 days after, heavy rainfall at CSO outfalls treated with medium SCM density compared to those without SCMs. Our results suggest that SCMs may be associated with a reduction in the increase of Culex mosquitoes at the CSO outfalls after heavy rainfall. More research is needed to study how the impacts of SCMs on mosquito populations may affect human health.

Escherichia coli levels and microbial source tracking in stormwater retention ponds and detention basins

Little is known about the spatial and temporal changes that occur with Escherichia coli in urban stormwater systems. The goal of this project was to assess E. coli in urban stormwater detention basins and retention ponds, not connected to the sewer system, to determine temporal and spatial differences and evaluate the sources of E. coli utilizing microbial source tracking (MST). Surface water quality was sampled at three detention basins and five retention ponds during major storm events in the summers of 2018 and 2019. One week after each storm, groundwater and surface water were sampled. The MST samples were taken from storm events and normal flows, for both surface and groundwater. E. coli levels were higher during rain events in both detention basins and retention ponds and infrequently met a recreational standard. E. coli in groundwater was pervasive and infrequently met a recreational standard. The MST analysis found sewage, dog, human, and bird markers during storm events and sewage and bird markers during regular flows. PRACTITIONER POINTS: Rain events had significantly more E. coli than during normal flows in both retention ponds and detention basins. E. coli in groundwater was ubiquitous and fluctuates over time. Microbial source tracking (MST) found bird, dog, human, and sewage markers present during all storm events analyzed.

Removal of Cd (II) Ions from Bioretention System by Clay and Soil Wettability

In this work, a silane modifier with benzyl substitutes (OFS-B) and linear substitutes (OFS-L) was used to modify bentonite clay and soil, and the results were characterized by Fourier transform-infrared absorption spectroscopy (FT-IR) and powder-X-ray diffraction (XRD) analysis. A contact angle analysis was performed to determine the wettability of modified clay and soil. The findings revealed that silane-modified OFS-L clay and soil produced wettable surfaces, while OFS-B exhibited hydrophobic properties. These clays and soils were used in a bioretention system for Cd (II) removal. In the study, seven different types of bioretention systems, including natural, OFS-L, and OFS-B modified clay and soil, as well as natural, OFS-L, and OFS-B modified soil, were applied to Cyperus alternifolius plants without an additional layer. The removal capacity of Cd (II) was measured in the following order: modified clay > modified soil > original clay/soil > no layer, i.e., 99.48%, 92.22%, 88.10/78.5%, and 30.0%, respectively. OFS-L removed more Cd (II) than OFS-B during the modification. OFS-L now improves the bioavailability and accumulation of Cd (II) in the plant (18.5 µg/g) and has a higher chlorophyll-b concentration (1.92 mg/g fresh weight) than other systems. The wettable clay exhibited clay leaching into the various levels of the bioretention system. In the bioretention system, benzyl substituted clay prevented the penetration of water and formed a Cd (II) agglomeration. When compared to non-wettable modifiers, these results indicated that wettable clay material could be a capable material for removing Cd (II).

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.

Urban stormwater nutrient and metal removal in small-scale green infrastructure: exploring engineered plant biofilter media optimisation

The present study evaluated engineered media for plant biofilter optimisation in an unvegetated column experiment to assess the performance of loamy sand, perlite, vermiculite, zeolite and attapulgite media under stormwater conditions enriched with varying nutrients and metals reflecting urban pollutant loads. Sixty columns, 30 unvegetated and 30 Juncus effusus vegetated, were used to test: pollutant removal, infiltration rate, particulate discharge, effluent clarity and plant functional response, over six sampling rounds. All engineered media outperformed conventional loamy sand across criteria, with engineered attapulgite consistently among the best performers. No reportable difference existed in vegetation exposed to different material combinations. For all media, the results show a net removal of NH₃-N, PO₄^3--P, Cd, Cu, Pb and Zn and an increase of NO₃^--N, emphasizing the importance of vegetation in biofilters. Growth media supporting increased rate of infiltration whilst maintaining effective remediation performance offers the potential for reducing the area required by biofilters, currently recommended at 2% of its catchment area, encouraging the use of small-scale green infrastructure in the urban area. Further research is required to assess the carrying capacity of engineered media in laboratory and field settings, particularly during seasonal change, gauging the substrate's potential moisture availability for root uptake.

A review on plant-microbial interactions, functions, mechanisms and emerging trends in bioretention system to improve multi-contaminated stormwater treatment

Management and treatment of multi-polluted stormwater in bioretention system have gained significant attraction recently. Besides nutrients, recent source appointment studies found elevated levels of Potentially toxic metal(loid)s (PTMs) and contaminants of emerging concern (CECs) in stormwater that highlighted many limitations in conventional media adsorption-based pollutant removal bioretention strategies. The substantial new studies include biological treatment approaches to strengthen pollutants degradation and adsorption capacity of bioretention. The knowledge on characteristics of plants and their corresponding mechanisms in various functions, e.g., rainwater interception, retention, infiltration, media clogging prevention, evapotranspiration and phytoremediation, is scattered. The microorganisms' role in facilitating vegetation and media, plant-microorganism interactions and relative performance over different functions in bioretention is still unreviewed. To uncover the underneath, it was summarised plant and microbial studies and their functionality in hydrogeochemical cycles in the bioretention system in this review, contributing to finding their interconnections and developing a more efficient bioretention system. Additionally, source characteristics of stormwater and fate of associated pollutants in the environment, the potential of genetical engineered plants, algae and fungi in bioretention system as well as performance assessment of plants and microorganisms in non-bioretention studies to propose the possible solution of un-addressed problems in bioretention system have been put forward in this review. The present review can be used as an imperative reference to enlighten the advantages of adopting multidisciplinary approaches for the environment sustainability and pollution control.

A case in subtropical climate city: Assessing the bioretention hydraulic performance on storm in response to poor permeability soil

Bioretention has been widely used in China for the purpose of sponge city construction. In subtropical climate areas, the performance of bioretention cell under condition of low infiltration underlying soil and heavy storms is still poorly understood. This study aimed to assess the effects of low infiltration underlying soil and precipitation characteristics on the hydraulic performance of a bioretention cell using the Storm Water Management Model (SWMM). The hydraulic performance of a bioretention cell were investigated under a Typical year rainfall event (P_(total) (total precipitation) = 1299.2 mm) and seven heavy storms (i.e., P_total range from 53.1 mm to 287.3 mm), at different S_F(i) (seepage rates of the underlying soil) (i.e., range from 2.5 mm/h to 15 mm/h). Then, sensitivity of the optimal design to the different design parameters, including the hydraulic conductivity of soil medium layer and the berm height of surface layer, was examined. The results show that the increase in S_F(i) was effective in increasing the AR_VR(i) (annual runoff volume reduction) and R_VR(i) (runoff volume reduction), while little effective in increasing P_FR(i) (peak flow reduction). Moreover, the AR_VR(i) could meet the designed goal of 70% when the S_F(i) was more than 7.5 mm/h. For R_VR(i), the key variable of precipitation characteristic changes from P_total to P_4h(max) (maximum precipitation in 4 h) as S_F(i) increases, while P_4h(max) remains as the key variable for P_FR(i) all the time. The sensitivity studies demonstrate that the hydraulic conductivity is more effective in increasing P_FR(i) than the berm height. For the bioretention cell under condition of low infiltration underlying soil and heavy storms, in order to simultaneously achieve expected reduction goal of both peak flow and runoff volume, and make the best comprehensive performance of bioretention cell, it requires not only a maintenance action to increase the hydraulic conductivity of soil medium layer, but also a drain pipe to be added in the storage layer, and meanwhile other LID practices should be combined.

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.

Removal characteristics of heavy metal ions in rainwater runoff by bioretention cell modified with biochar

As a form of pollution source control and a low-impact development measure, bioretention is a convenient, economical, and effective method for the removal of heavy metals from stormwater runoff, which can adapt to the randomness and uncontrollability of non-point source pollution. However, few studies have assessed the performance of bioretention in the simultaneous removal of multiple heavy metals and the impact of heavy metal migration on the bioretention life cycle. In this study, the removal rates of various heavy metals: copper (Cu), zinc (Zn), lead (Pb), and cadmium (Cd), were enhanced using a biochar modified bioretention cell, as compared to the traditional sandy soil bioretention process. Following treatment with the biochar modified bioretention cell, the average concentrations of Cu, Zn, Pb, and Cd were 55%, 61%, 19.66%, and 36.43% lower than the traditional sandy soil bioretention effluent, respectively. These results show that biochar significantly improves the removal of heavy metals by the bioretention process, especially Cu and Zn. This study also evaluated the effect of biochar on the inhibition of heavy metal migration in the filler material, by sampling and analysing the filler and retained water at different filler depths, then repeating the filler leaching experiment after simulated rainfall. The content of heavy metals at a filler depth of 45 cm in the traditional sandy soil bioretention system, was significantly higher than in the biochar modified bioretention system, showing that biochar plays an important role in the inhibition of heavy metal migration.

Treatment efficiency of synthetic urban runoff by low-cost mineral materials under various flow conditions and in the presence of salt: Possibilities and limitations

Urban runoff belongs to important carriers of pollutants that during infiltration can accumulate in the soil/water environment. One of the protection solutions may be the enhancement of infiltration systems by horizontal permeable treatment zones. The article presents the results of column tests carried out in order to determine (1) the influence of the hydraulic loading rate on the dynamic capacities of selected reactive materials: low-cost mineral materials (zeolite, limestone sand, halloysite) and reference material (activated carbon), and control soils (topsoil and Vistula sand) against Zn, NH₄⁺ and PO₄^3-, and (2) remobilization of contaminants under the influence of salt (NaCl 5 g/L) present in synthetic runoff water. The research has revealed that the most useful for the removal of zinc ions was limestone sand (>4.36 mg/g), of orthophosphates - halloysite (2.29 mg/g on the average), and of ammonium ions - zeolite (2.75 mg/g on the average). The control soils were characterized by low ability to immobilize the contaminants tested. In addition, increase in the hydraulic loading rate of synthetic runoff water reduced the dynamic capacity of materials to a variable degree depending on the material applied and the contamination removed (by 24% for limestone sand-PO₄^3- system to 95% for activated carbon-NH₄⁺ system). The presence of NaCl caused significant leaching of ammonium ions from zeolite and halloysite filter beds (up to 99.3%), and phosphates from the activated carbon filter bed (up to 41.3%). All tracer contaminants tested leached intensively from the Vistula sand filter bed, while only ammonium ions leached from the topsoil filter bed. It seems justified to support the performance of infiltration systems by layers of: limestone sand, to enhance the processes of heavy metal precipitation and ammonium ion volatilization by increasing the pH, and halloysite for the sorption of phosphates.

Ammonium Release and Adsorption Characters of Polyurethane-Biochar Crosslinked Material as an Additive Filler in Stormwater Treatment

The additive fillers in bioretention facilities play a leading role in stormwater treatment to purify polluted runoff. At present, many traditional materials could not meet the requirements at the same time, including low ammonium leaching quantities, high water storage volume and strong ammonium adsorption. This study investigated a polymer material, polyurethane-biochar crosslinked material (PCB), to evaluate the feasibility of using it as an additive filler in stormwater treatment compared with its raw material hardwood biochar (HB), and two traditional fillers. Successive leaching and ammonium isothermal adsorption experiments were conducted in deionized water and artificial stormwater. PCB leached 4.98-5.31 μmol/g NH₄-N, less than the leaching quantities of compost, the traditional filler. After polyurethane modification, ammonium adsorption of PCB was improved: at a typical ammonium concentration of 2 mg/L in stormwater, PCB could adsorb 43.6 mg/kg ammonium versus 34.6 mg/kg for HB. With the addition of PCB in sand column, the ammonium adsorption improved from 31.34 to 84.72%. To improve the performance of bioretention facilities, PCB is recommended to be added into filter layers in stormwater treatment, taking advantage of its high cation exchange capacity and spongy internal structure to minimize overland flooding and enhance removal of ammonium from stormwater.

Investigate the influence of microplastics weathering on their heavy metals uptake in stormwater

Plastic debris as the main portion of urban litters could be transported via storm runoff to the water resources. In this study the influence of microplastics (MPs) weathering on their Pb²⁺ and Zn²⁺ uptake in stormwater was examined. Low-density polyethylene (LDPE) and polyethylene terephthalate (PET) MPs were subjected to weathering through mechanical interaction with a mixture of silt/sand, and in synthetic stormwater. The surface analysis revealed significant physio-chemistry alterations of LDPE MPs due to the silt/sand weathering. However, this weathering mostly resulted in the surface morphology alterations of PET MPs. The kinetics of heavy metals adsorptions onto the new and stormwater weathered LDPE MPs were best described by pseudo 1st and 2nd models, respectively. Despite increasing Pb²⁺ uptake by weathered PET MPs, Zn²⁺ uptake by both new and weathered PET MPs was below the detection limit. Both Pb²⁺ and Zn²⁺ were released from new and silt/sand weathered LDPE MPs during five days exposure to the synthetic stormwater. This study underscores the critical role of plastic type and weathering conditions on heavy metal transport by MPs from the urban environment to the water resources.

The effect of intermittent drying and wetting stormwater cycles on the nutrient removal performances of two vegetated biofiltration designs

Vegetated biofiltration systems (biofilters) are now a well-established technology for treatment of urban stormwater, typically showing high nutrient uptake. However, the impact of high temporal variability of rainfall events (further exacerbated by climate change) on nitrogen and phosphorus removal processes, within different biofiltration designs, is still unknown. Hence, a laboratory-based study was conducted to uncover mechanisms behind nutrient removal in biofilters across different drying and wetting regimes. Two sets of experimental columns were based on (1) the standard biofiltration design (unsaturated zone only), and (2) combination of unsaturated and saturated (submerged) zone (SZ) with additional carbon source. Columns were watered with synthetic stormwater according to three drying and wetting schemes, exploring 1, 2, 3, 4 and 7-week drying. Hydraulic performance, soil moisture and pollutant removal were monitored. The results show that hydraulic conductivity of SZ design experiences less change over time compared to standard design, due to slower media drying, crack formation and lower plant die-off. Varied drying lengths challenged both designs differently, with 2-week drying resulting in significant drop of performance across most pollutants in standard design (except ammonia), while SZ design was able to retain high performance for up to four weeks of drying, sustaining microbial and plant uptake. Increased oxygenation of SZ columns during short-term drying was beneficial for ammonia and phosphorus removal. While SZ design showed better performance and quicker recovery for nitrogen removal, in regions with inter-rain event shorter than two weeks, the standard design (no saturated zone, no carbon source) can achieve similar if not better results.

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.

The Common Approaches of Nitrogen Removal in Bioretention System

Bioretention is considered one of the best management practices (BMPS) for managing stormwater quality and quantity. The bioretention system has proven good performance in removing total suspended solids, oil, and heavy metals. The nitrogen (N) removal efficiency of the bioretention system is insufficient, however, due to the complex forms of nitrogen. Therefore, this paper aims to review recent enhancement approaches to nitrogen (N) removal and to discuss the factors influencing bioretention efficiency. To improve bioretention efficiency, several factors should be considered when designing bioretention systems, including nitrogen concentration, climate factors, and hydrological factors. Further, soil and plant selection should be appropriate for environmental conditions. Three design improvement approaches have been reviewed. The first is the inclusion of a saturated zone (SZ), which has been used widely. The SZ is shown to have the best performance in nitrogen removal. The second approach (which is less popular) is the usage of additives in the form of a mixture with soil media or as a separated layer. This concept is intended to be applied in tropical regions with wet soil conditions and a short dry period. The third approach combines the previous two approaches (enhanced filter media and applying a SZ). This approach is more efficient and has recently attracted more attention. This study suggests that further studies on the third approach should be carried out. Applying amendment material through filter media and integrating it with SZ provides appropriate conditions to complete the nitrogen cycle. This approach is considered a promising method to enhance nitrogen removal. In general, the bioretention system offers a promising tool for improving stormwater quality.

Nitrate removal uncertainty in stormwater control measures: Is the design or climate a culprit?

Eutrophication is caused by excess nitrate and other nutrient exported via stormwater runoff to surface waters, which is projected to increase as a result of climate change. Despite recent increases in the implementation of stormwater control measures (SCM), nutrient export has not abated, indicating poor or inconsistent removal capacities of SCM for nitrate. However, the cause of the variability is unclear. We show that both design and local climate can explain nitrate removal variability by critically analyzing data reported on the international BMP database for nitrate removal by four common types of SCM: bioretention cells, grass swales, media filters, and retention ponds. The relative importance of climate or design on nitrate removal depends on the SCM type. Nitrate removal in grass swales and bioretention systems is more sensitive to local climate than design specifications, whereas nitrate removal in the retention ponds is less sensitive to climate and more sensitive to design features such as vegetation and pond volume. Media filters without amendment have the least capacity compared to other SCM types surveyed, and their removal capacity was independent of the local climate. Adding amendments made up of carbon biomass, iron-based media, or a mixture of these amendments can significantly improve nitrate removal. The type of carbon biomass is also a factor since biochar does not appear to affect nitrate removal. This analysis can help inform the selection of SCM and modification of their design based on local and projected climate to maximize nitrate removal and minimize eutrophication.

Phosphorus Release and Adsorption Properties of Polyurethane-Biochar Crosslinked Material as a Filter Additive in Bioretention Systems

Bioretention systems are frequently employed in stormwater treatment to reduce phosphorus pollution and prevent eutrophication. To enhance their efficiency, filter additives are required but the currently used traditional materials cannot meet the primary requirements of excellent hydraulic properties as well as outstanding release and adsorption capacities at the same time. In this research, a polyurethane-biochar crosslinked material was produced by mixing the hardwood biochar (HB) with polyurethane to improve the performance of traditional filter additives. Through basic parameter tests, the saturated water content of polyurethane-biochar crosslinked material (PCB) was doubled and the permeability coefficient of PCB increased by two orders of magnitude. Due to the polyurethane, the leaching speed of phosphorus slowed down in the batching experiments and fewer metal cations leached. Moreover, PCB could adsorb 93–206 mg/kg PO₄³⁻ at a typical PO₄³⁻ concentration in stormwater runoff, 1.32–1.58 times more than HB, during isothermal adsorption experiments. In the simulating column experiments, weaker hydropower reduced the PO₄³⁻ leaching quantities of PCB and had a stable removal rate of 93.84% in phosphate treatment. This study demonstrates the potential use of PCB as a filter additive in a bioretention system to achieve hydraulic goals and improve phosphate adsorption capacities.

A Need for Standardized Reporting: A Scoping Review of Bioretention Research 2000–2019

Bioretention cells are a type of low-impact development technology that, over the past two decades, have become a critical component of urban stormwater management. Research into bioretention has since proliferated, with disparate aims, intents and metrics used to assess the “performance” of bioretention cells. We conducted a comprehensive, systematic scoping review to answer the question of “How is the field performance of bioretention assessed in the literature?”, with the aim of understanding (1) how is the performance of bioretention defined in the literature? (2) what metrics are used to assess actual and theoretical performance? A review of 320 studies (mostly peer reviewed articles) found that performance was defined in terms of hydrologic controls, while investigations into water quality pathways and mechanisms of contaminant transport and fate and the role of vegetation were lacking; additionally, long term field and continuous modelling studies were limited. Bioretention field research was primarily conducted by a small number of institutions (26 institutions were responsible for 50% of the research) located mainly in high income countries, particularly Australia and the United States. We recommend that the research community (I) provide all original data when reporting results, (II) prioritize investigating the processes that determine bioretention performance and (III) standardize the collection, analysis and reporting of results. This dissemination of information will ensure that gaps in bioretention knowledge can be found and allow for improvements to the performance of bioretention cells around the world.

Effect of struvite and organic acids on immobilization of copper and zinc in contaminated bio-retention filter media

Few studies have been carried out to connect nutrient recovery as struvite from wastewater and sustainable utilization of the recovered struvite for copper and zinc immobilization in contaminated soil. This study revealed the effect of struvite on Cu and Zn immobilization in contaminated bio-retention soil in the presence of commonly exuded plant organic acids. The research hypothesis was that the presence of both struvite and organic acids may influence the immobilization of Cu and Zn in soil. The outcome of this research confirmed that more than 99% of Cu and Zn was immobilized in bio-retention filter media by struvite application. Water-soluble Cu and Zn concentrations of struvite treated soil were less than 1.83 and 0.86 mg/kg respectively, and these concentrations were significantly lower compared to the total Cu and Zn content of 747.05 mg/kg in the contaminated soil. Application of struvite to Cu- and Zn-contaminated soil resulted in formation of compounds similar to zinc phosphate tetrahydrate (Zn₃(PO₄)₂•4H₂O) and amorphous Cu and Zn phases. Struvite was effective in heavy metal remediation in acidic soil regardless of the presence of Ca impurities in struvite and the presence of plant organic acids in soil. Overall, this study revealed that struvite recovered from wastewater treatment plants has potential for use as an amendment for heavy metal remediation in contaminated bio-retention soil.

Dynamic testing in columns for soil heavy metal removal for a car park SUDS

The increase in urban runoff brought about by a rise in impermeable surfaces has triggered the alteration and pollution of many aquatic systems. The overall goal of this research was to design a 'Sustainable Urban Drainage System' (SUDS) for the retention of heavy metals from a car park consisting of mixing autochthonous soil (70%) with sand (30%) to improve the hydrological conductivity and adsorption capacity. To quantify the retention of metals we characterize the adsorption kinetics and isotherms of the soil mixture and perform dynamic experiments. The proposed methodology allowed us to work out the amount of heavy metal retention by the adsorbent and the retention mechanisms. The retention capacity of the adsorbent mixture was as follows: Cr³⁺ ≈ Cu²⁺ ≫ Zn²⁺ > Ni²⁺ > Cd²⁺. Chromium and copper ions were mainly retained by precipitation, whereas zinc, nickel and cadmium were retained by ionic exchange with calcium ions that saturate the soil colloids. The soil mixture buffered pH was found to change when fed with an acid solution of metallic ions.

Long-term simulation of potentially toxic elements (PTEs) accumulation and breakthrough in infiltration-based stormwater management practices (SMPs)

Stormwater management practices (SMPs) rely on infiltration and adsorption capabilities of soil and vegetative cover to mitigate the harmful impacts of contaminants in stormwater runoff, including potentially toxic elements (PTEs). Under chemical equilibrium conditions, the soil-water distribution coefficient (K_d) quantifies the relationship between the solid and aqueous phase PTE concentrations, and thus the PTE removal efficiency and mobility through the SMP soil layers during the infiltration process. The SMP loading ratio (LR), the ratio of the drainage area to the SMP infiltration area, combined with runoff concentration determines SMP mass loading and is also expected to impact PTE transport. In this study, a simulation model was developed to investigate PTE breakthrough and build-up in SMP media, considering the impacts of K_d and LR. Eight PTEs were simulated (Cl^-, Cr, Fe, Zn, Cu, As, Cd, and Pb), and Cl^- was the only PTE that showed high mobility and reached the groundwater table (e.g., ~ 1 year for breakthrough). Conversely, other PTEs were effectively immobilized in the top ~60 cm of soil for a simulated lifespan of 20 years. Soil and porewater contaminant indices, as indicators of SMP lifespan, were estimated based on the ratio of PTE porewater and soil concentrations after 20 years to published standards, suggesting the following order of environmental significance (most concern to least): Cl^- > Cr > As > Pb > Fe > Cu > Cd > Zn. After 20 years of simulated use, only Cl^- pore water concentrations at the groundwater table exceeded regulatory values, with porewater contamination index values of 4 to 7.5. Chloride also exceeded the surficial media soil contamination index, as did As and Cr, though these exceedences were largely associated with media background concentrations. Generally, higher LR and K_d contributed to higher accumulation of PTEs in top layers; however, simulations showed that the combination of low LR and high K_d may result in lower PTE accumulation in the media, such that the PTE concentration in soil may decrease in deeper layers. In these scenarios, a notable fraction of PTE load was adsorbed on top layers and considerably lower PTE concentrations reached the lower layers. Sensitivity analysis revealed that dispersion, infiltration rate, and kinetically-limited sorption did not impact the PTE accumulation and mobility to a practical extent. The results from this simulation may be adapted to various environmental conditions to enhance the design and maintenance of SMPs.

Potential use of floating treatment wetlands established with Canna flaccida for removing organic contaminants from surface water

Surface water bodies worldwide may be contaminated with various organic contaminants. In many cases, the actual toxicity thresholds to nontarget organisms are unknown, thus presenting unknown risks. This study evaluated the potential use of floating treatment wetlands (FTWs) planted with Canna flaccida (common name: Canna) for removing two pharmaceuticals (acetaminophen and carbamazepine) and one herbicide (atrazine) from contaminated water. Triplicate FTWs with varying plant densities were established in 378 L mesocosms. After dosing the mesocosms with the contaminants, water samples were collected over a 12-week period for analysis. The planted FTWs showed differing abilities for removing acetaminophen, atrazine, and carbamazepine. Plant densities on the FTWs did not affect dissipation of acetaminophen or atrazine, but did carbamazepine. All acetaminophen residues were removed from the water within 2 weeks, while all atrazine residues were removed within 12 weeks. Approximately, 79-92% of these residues removed were associated with the FTWs. In contrast, all of the carbamazepine was not removed after 12 weeks, at which time only 29-36.7% of the total removed was associated with the FTWs. Overall results suggest that FTWs established with C. flaccida are promising for removing trace concentrations of acetaminophen, atrazine, and carbamazepine from surface water.

Accumulation of Cu, Cd, Pb, Zn and total P from synthetic stormwater in 30 bioretention plants

The uptake and distribution of four heavy metals, including copper (Cu), cadmium (Cd), lead (Pb) and zinc (Zn), and those of total phosphorus (TP) in 30 plants in North China were investigated through pot trial experiments. Accumulation and distribution of heavy metals or TP were associated with plant species, tissues, metal elements and pollutant loading. The highest amount of heavy metal and TP accumulation was found in the whole plants of Hylotelephium erythrostictum (Miq.) H. Ohba (C1) and Chlorophytum laxum R. Br. (L4), respectively. Considering the biological concentration factor, translocation factor, retention factor and biomass indices, C1 is the suitable plant for Cd and Cu uptake, whereas Hosta plantaginea (Lam.) Aschers (L3) and Viola verecunda A. Gray (V1) are the suitable plants for Pb removal. Rehmannia glutinosa (Gaetn.) Libosch. ex Fisch. (S1) and L4 can be chosen for Zn and TP removal, respectively. Cluster analysis was applied to select suitable plants for heavy metal and TP removal. Results showed that C1, L4 and Pennisetum alopecuroides (L.) Spreng (G1) have a good capability of accumulating heavy metals and TP. Results demonstrated that the plant species rather than the families considerably influenced the accumulation of pollutants.

Bioretention cell age and construction style influence stormwater pollutant dynamics

In urbanized landscapes, green infrastructure is proposed as a method for using relatively small plots of land to manage stormwater and protect receiving ecosystems from pollutants. Bioretention cells can infiltrate stormwater from roads and parking lots, and as stormwater passed through the soils, metals can be removed. Metal removal and storage has been demonstrated in laboratory media columns and field-scale test cells, but we have an incomplete understanding of metal removal and accumulation in aging bioretention cells in the field. We surveyed 25 bioretention cells (0-7 years of service) for soil physicochemistry to determine which characteristics related to soil metal (Cu, Pb, and Zn) concentrations. We collected soil cores and treated them with simulated stormwater to measure potential rates of metal removal under different metal and salt concentrations. Older bioretention cells had higher Cu, Pb, and Zn concentrations in soil, which indicates accumulation and storage over time. The oldest cells had metal concentrations that were not a risk to human health but which approached screening thresholds for potential impairment of plants. Soil organic matter content (OM) was positively associated with metal concentrations which highlights the importance of OM in the functioning of cells. Retrofit bioretention cells were younger with less OM and more clay than cells built concurrently with new parking lot construction (i.e., de novo), but after 2.7 years of service, soil OM was similar between retrofit and de novo builds. Soil cores from de novo bioretention cells retained more stormwater metals than soil cores from retrofit cells, and this was likely due to differences in soil OM. Elevated road salt in stormwater was associated with less effective metal removal and leaching of Zn from soils. Overall, these data suggest that management (e.g., salting) and design (e.g., construction) decisions can greatly influence metal removal function of bioretention cells.

Impact of vegetation selection on nitrogen and phosphorus processing in bioretention containers

A year-long bioretention container study in Maryland, USA, measured the relationship between three plant species (Eutrochium dubium, Iris versicolor, and Juncus effusus) and N ( NO 3 - , NO 2 - , NH 4 + , total nitrogen [TN], total dissolved nitrogen [TDN], dissolved organic nitrogen, particulate organic nitrogen [PON]) and total phosphorus (TP) removal from synthetic stormwater. Statistically significant removal was only found for NO 3 - and TP. Plant-independent NO 3 - removal occurred 9 months after planting, and then changed to removal only by the least-densely planted Juncus treatment. Removal in higher-density Juncus plantings was suspected to be limited by preferential pathways created by high root density. Juncus' low-density NO 3 - removal success correlates with its high growth rate, root mass and length, and large biomass, matching previous literature. TP removal was plant-independent. Shoot harvesting of one plant of each species after 1 year would remove 0.61 g N. Of the plant species in this study, Juncus effusus is most highly recommended for bioretention for its nutrient removal dynamics and year-round green aesthetics. PRACTITIONER POINTS: Only the one-Juncus density treatment had significant NO 3 - removal. All Juncus treatments as well as non-Juncus treatments prevented the PON, TN, or TDN export seen in the No-plants control. TP removal was plant-independent. Juncus had the greatest biomass increase and biomass N. Shoots contain the majority of biomass N for each plant species. Juncus and Iris had high survivorship. Joe Pye had low survivorship. These, and all other study results, need field-scale verification.

Bioretention cell incorporating Fe-biochar and saturated zones for enhanced stormwater runoff treatment

Nitrogen (N) and phosphorus (P) removal in conventional bioretention systems is highly variable. Therefore, five novel experimental columns with different media configurations and constituents, and incorporating a saturated zone were developed and assessed to optimize the removal of N, P and other nutrients. Three types of media composed of the conventional mixed sand and soil media (T₁), biochar-amended media (T₂), and iron-coated biochar (ICB)-amended media (T₃) were evaluated. Two of the experimental columns were designed with double-layer configurations, while the other three were of a single-layer structure. Removal efficiencies of nutrients in the experimental columns were evaluated and compared using simulated runoff. Also, the effect of media depth on the retention of P and denitrifying enzyme activity (DEA) in the bioretention columns were evaluated. The experimental column only filled with T₃ showed the best performance for COD, ammonia (NH₄⁺-N) and total phosphorus (TP) removal (94.6%, 98.3% and 93.70%, respectively), whereas columns filled with T₂ performed poorly for TP removal (57.36%). For the removal of nitrate (NO₃^--N) and total nitrogen (TN), the columns using a single-layer and only filled with either T₃ or T₂ exhibited the best performance (93% and 97% TN removal, respectively). Overall, this study demonstrates that our proposed single-layered bioretention cell only filled with T₃ and incorporating a saturated zone effectively improves the runoff quality, and can provide a new bioretention cell configuration for efficient stormwater treatment.

Influence of fillers on the removal of rainwater runoff pollutants by a permeable brick system with a frame structure base

To fully investigate the effectiveness of fillers in the removal of pollutants from rainwater, gravel, zeolite, slag, volcanic rock and iron filings with a 3-5 cm particle size were applied to construct a brick paving system with a frame structure for the removal of pollutants. Total suspended solids (TSS), chemical oxygen demand (COD), ammonia nitrogen (NH₃-N), total nitrogen (TN), total phosphorus (TP) and heavy metals (Cu, Zn, and Pb) in the influent and effluent were measured, and the effectiveness and mechanism of pollutant removal were further investigated. The results showed that the permeable brick system effectively reduced TSS, TP, Zn, Cu and Pb and was relatively ineffective in reducing NH₃-N, TN and COD. The removal results obtained using different materials show that (1) physical interception is the main reason for TSS and TP removal, (2) the adsorption and ion exchange properties of zeolite enable it to highly absorb ammonia nitrogen, (3) iron filings can effectively reduce NO₃-N, and (4) adding fillers rich in iron oxide, such as volcanic rock or slag, can contribute to COD adsorption. The study provides a feasible technical path for improving the practicability of permeable pavement.

Laboratory analysis on the surface runoff pollution reduction performance of permeable pavements

Permeable pavements are used to address the water quality impacts of urbanization. However, few quantitative relations are available on their pollutant removal performance with respect to varying conditions, especially for different components of a permeable pavement. Individually, the water quality performance of the surface pavement layer and gravel layer of a permeable pavement under various conditions was determined in laboratory-scale pavement cells. Our aim was to reveal the manner in which different factors influence the ability of these two layers to remove total suspended solids (TSS), nutrients, including nitrate (NO_x-N), ammonia (NH₄-N) and phosphorous (TP), chemical oxygen demand (COD), and heavy metals (copper (Cu), lead (Pb), cadmium (Cd), and zinc (Zn)), and to provide quantitative understanding of these influences. The removal efficiencies of most stormwater runoff pollutants showed a significant variation with changing rainfall intensity. NH₄-N, NO_x-N, TP, and TSS removal exhibited statistically negative linear relationship with rainfall intensity. TSS removal was negatively correlated with TSS concentration for the gravel layer, whereas no obvious trend was observed for the surface pavement layer. The statistical results obtained demonstrate that TSS, NH₄-N, NO_x-N, TP, and COD were removed mainly by the surface pavement layer. A smaller gravel gradation was more effective for removing most pollutants, except for NO_x-N and COD. Positive linear relationships were observed between the gravel layer thickness and pollutant (TSS, TP, NH₄-N, Cu, and Cd) removal. More importantly, a simple mathematical model was developed to predict the overall performance of the permeable pavement system. By comparing with the overall measured performance, a good performance was achieved, illustrating its promising application in the design of permeable pavements.

The Effects of Rainfall Runoff Pollutants on Plant Physiology in a Bioretention System Based on Pilot Experiments

Bioretention facilities have been widely used in the construction of Sponge City in China, but there have also been doubts about whether road runoff pollutants have adverse effects on plant growth. In response to this problem, this paper explored the effects of bioretention on the removal of pollutants and explored the effects of runoff on plant growth and physiology. The results showed that (1) the average concentration reduction rate and load removal rate of TN and NO3--N were above 70%, the average NH4+-N concentration reduction rate and load removal rate were greater than 90%, and the removal of elemental N was affected by the influent concentration. The removal effect of the four heavy metals was not very great. The average concentration reduction rate and load removal rate of heavy metals were 65.4–95.7% and 85.4–99.4%, respectively. The cumulative load removal rate of various pollutants was above 87.0%. (2) The runoff of high–concentration pollutants had a negative or no significant effects on the net photosynthesis rates (Pn), chlorophyll contents (CC), and electrolyte leakage (EL) of most plants (e.g., Iris tectorum Maxim, Rosa xanthina Lindl, and Ligustrum vicaryi). It had a significantly negative effect on the plant height of shrub plants (e.g., Rosa xanthina Lindl and Ligustrum vicaryi), but had a positive effect on Pn and CC of Iris lactea var. chinensis. (3) The runoff of low–concentration pollutants had a positive or no significant effects on the physiological indexes of herbaceous plants (e.g., Iris tectorum Maxim and Iris lactea var. chinensis), but there were no explicit conclusions regarding the physiological indicators of shrub plants (e.g., Rosa xanthina Lindl and Ligustrum vicaryi). It had no obvious effects on the plant height of these four species of plants.

Performance of a kerb side inlet to irrigate street trees and to improve road runoff water quality: a comparison of four media types

The TREENET inlet is an emerging water-sensitive urban design technology that consists of a novel kerb side inlet coupled with a leaky well infiltration system. The inlets have been retrofitted to existing roads since 2006; however, there is currently little information available on the effectiveness of these inlet and leaky well systems. This study investigated the performance of the kerb side inlets and leaky well system for water quality improvement prior to infiltration to native soil. The leaky wells included four filter media types, namely gravel, water treatment solids, sandy loam and clay. To compare the performance of the four filter media types, batch and column studies were performed in the laboratory. The best performance was observed using the sandy loam as a filter media, followed by clay, water treatment solids and then gravel. The selection of effective media for removal of heavy metals is important as each media type has different pollutant removal capacity, infiltration and clogging performance.

Influences of stormwater concentration infiltration on soil nitrogen, phosphorus, TOC and their relations with enzyme activity in rain garden

Rain garden is a typical facility with many applications in urban low impact development (LID). It plays an important role in regulating runoff water quantity and quality. Two rain gardens with the discharge ratios of 20:1 and 15:1 were used as studied facilities. Seven soil sampling events were conducted from April 2017 to February 2019 to study the influences of stormwater concentration infiltration in rain gardens on soil nitrogen (N), phosphorus (P) and TOC and their relations with enzymes. The results showed that the contents of soil TN and NO₂-N + TON in gardens gradually decreased with time, while those of NH₃-N and TP increased with time. The content of NO₃-N varied greatly with time, and there was no obvious rule. TOC increased first and then decreased. Vertical distributions of N, P and TOC showed that the contents of NH₃-N, NO₂-N + TON and TN at 0-50 cm were high, so the upper soil was the sensitive area to the influence of stormwater concentration infiltration in rain gardens. The content of NH₃-N decreased gradually with the increase of soil depth, but those of NO₃-N and TP increased with the soil depth. Therefore, NO₃-N and TP migrated down with water infiltration in soil, and preventing NO₃-N and P leaching was critical for effective N and P removal though rain gardens. Soil urease (SU), sucrose (SS), protease (SP) and acid phosphatase (SAP) had a good linear relationship with N, P and TOC, and R²were all greater than 0.5.

A Review of Nitrogen Removal for Urban Stormwater Runoff in Bioretention System

One of the best management practices (BMPs) for stormwater quality and quantity control is a bioretention system. The removal efficiency of different pollutants under this system is generally satisfactory, except for nitrogen which is deficient in certain bioretention systems. Nitrogen has a complex biogeochemical cycle, and thus the removal processes of nitrogen are typically slower than other pollutants. This study summarizes recent studies that have focused on nitrogen removal for urban stormwater runoff and discusses the latest advances in bioretention systems. The performance, influencing factors, and design enhancements are comprehensively reviewed in this paper. The review of current literature reveals that a bioretention system shows great promise due to its ability to remove nitrogen from stormwater runoff. Combining nitrification and denitrification zones with the addition of a carbon source and selecting different plant species promote nitrogen removal. Nevertheless, more studies on nitrogen transformations in a bioretention system and the relationships between different design factors need to be undertaken.

Development Trend and Frontier of Stormwater Management (1980–2019): A Bibliometric Overview Based on CiteSpace

The threat of urban floods due to climate change and urbanization has enabled sustained attention to the stormwater management field. Numerous scholars and countries have successively proposed innovative concepts for stormwater management. To grasp the current research focus and status quo and determine the development trend and dynamic direction, this work used CiteSpace, a scientific bibliometric analysis software, to analyze and identify 3080 articles based on the core database of Web of Science from 1980 to 2019. Results show a comprehensive overview of the stormwater management field, including the changes of annual articles with time; the most influential countries, institutions, authors, and articles; and the periodical keywords, highly cited papers, and burst time in the field. A knowledge table in the stormwater management field was obtained, the development context of the field and the research focus of each stage were understood, and the future development trend of the field is inferred. This study aims to provide reference for researchers and practitioners in the stormwater management field.

Statistical analyses of flow rates of stormwater treatment bioretention media

The design and performance of stormwater controls is affected by the treatment flow rates of bioretention media. This article presents the results of a large number of laboratory column tests conducted to examine the treatment flow rates for various mixtures of stormwater bioretention media. Statistical analyses were conducted to identify the treatment media having targeted treatment flow rates. It was found that the bioretention media treatment flow rates were most affected by the median particle size (D₅₀ ) and uniformity coefficient (C_u ) of the media, and the amount of organic matter. Statistical models were developed to evaluate and compare the treatment flow rates for various bioretention media mixtures. The findings of previous research (Sileshi, 2013) using two level, four factors (2⁴ , with varying texture [T], uniformity [U], organic content [OC], and compaction [C]) full-factorial experiment study indicated that T and U of the media mixture have the greatest effect on the measured final infiltration rates of the media, followed by interactions of T and U; C; interactions of T and OC of the material; and interactions of U and OC of the material. As expected, media containing primarily larger particles (higher sand percentage) and, that is, uniformly graded (small uniformity coefficients) had the largest treatment flow rates. Compaction had minor effects if the organic matter content was low, but had significant effects on the flow rates for high organic matter content. PRACTITIONER POINTS: Bioretention media treatment flow rates were most affected by the median particle size and uniformity coefficient of the media, and the amount of organic matter. Statistical models were developed to evaluate and compare the treatment flow rates for various bioretention media mixtures. The media containing primarily larger particles and, that is, uniformly graded (small uniformity coefficients) had the largest treatment flow rate.