Removal and release of microplastics and other environmental pollutants during the start-up of bioretention filters treating stormwater

Untreated stormwater is a major source of microplastics, organic pollutants, metals, and nutrients in urban water courses. The aim of this study was to improve the knowledge about the start-up periods of bioretention filters. A rain garden pilot facility with 13 bioretention filters was constructed and stormwater from a highway and adjacent impervious surfaces was used for irrigation for ∼12 weeks. Selected plants (Armeria maritima, Hippophae rhamnoides, Juncus effusus, and Festuca rubra) was planted in ten filters. Stormwater percolated through the filters containing waste-to-energy bottom ash, biochar, or Sphagnum peat, mixed with sandy loam. Influent and effluent samples were taken to evaluate removal of the above-mentioned pollutants. All filters efficiently removed microplastics >10 µm, organic pollutants, and most metals. Copper leached from all filters initially but was significantly reduced in the biochar filters at the end of the period, while the other filters showed a declining trend. All filters leached nutrients initially, but concentrations decreased over time, and the biochar filters had efficiently reduced nitrogen after a few weeks. To conclude, all the filters effectively removed pollutants during the start-up period. Before being recommended for full-scale applications, the functionality of the filters after a longer period of operation should be evaluated.

Multi-media interaction improves the efficiency and stability of the bioretention system for stormwater runoff treatment

Bioretention systems are one of the most widely used stormwater control measures for urban runoff treatment. However, stable and effective dissolved nutrient treatment by bioretention systems is often challenged by complicated stormwater conditions. In this study, pyrite-only (PO), pyrite-biochar (PB), pyrite-woodchip (PW), and pyrite-woodchip-biochar mixed (M) bioretention systems were established to study the feasibility of improving both stability and efficiency in bioretention system via multi-media interaction. PB, PW, and M all showed enhanced dissolved nitrogen and/or phosphorus removal compared to PO, with M demonstrating the highest efficiency and stability under different antecedent drying durations (ADD), pollutant levels, and prolonged precipitation depth. The total dissolved nitrogen and dissolved phosphorus removal in M ranged between 64%-86% and 80%-95%, respectively, with limited organic matter and iron leaching. Pore water, microbial community, and material analysis collectively indicate that pyrite, woodchip, and biochar synergistically facilitated multiple nutrient treatment processes and protected each other against by-product leaching. Pyrite-woodchip interaction greatly increased nitrate removal by facilitating mixotrophic denitrification, while biochar further enhanced ammonium adsorption and expanded the denitrification area. The Fe3+ generated by pyrite aerobic oxidation was adsorbed on the biochar surface and potentially formed a Fe-biochar composite layer, which not only reduced Fe3+-induced pyrite excessive oxidation but also potentially increased organic matter adsorption. Fe (oxyhydr)oxides intermediate product formed by pyrite oxidation, in return, controlled the phosphorus and organic matter leaching from biochar and woodchip. Overall, this study demonstrates that multi-media interaction may enable bioretention systems to achieve stable and effective urban runoff treatment.

Rain garden hydrological performance - Responses to real rainfall events

Rain gardens, as bioretention facilities belonging to blue-green infrastructure solutions, are becoming increasingly implemented in cities. The main reason for this is to support traditional drainage systems in receiving runoff from impermeable surfaces and managing it through temporary retention and infiltration into the ground. However, as practice shows, investors focusing on the construction of the systems and their commissioning skip their monitoring during the operating period, thus missing the opportunity to obtain reliable data on their hydrological performance under actual field conditions. The purpose of the study was to evaluate the effectiveness of a rain garden, located in an urban area, to capture runoff from the roof of a building. The assessment was based on the results of measurements carried out in 2021 on the variability of the levels of water retained in the rain garden and on measurements of growing medium moisture content at several selected points in the rain garden depression against thermal and rainfall conditions. The results showed that the rain garden demonstrated good hydrological performance. This was proven by the observed direct infiltration of rainwater into the structural layer or the short retention time for rainfall events with a higher rainfall total. The highest growing medium moisture was observed in the area of rainwater inflow to the rain garden. The results of the research may be useful in the planning and realization of future investments with rain gardens, which are to be situated in areas of similar meteorological conditions.

Abundance, distribution, and composition of microplastics in the filter media of nine aged stormwater bioretention systems

Bioretention systems are designed for quality treatment of stormwater. Particulate contaminants are commonly treated efficiently and accumulate mainly in the surface layer of the bioretention filter material. However, concerns exist that microplastic particles may not show equal accumulation behavior as other sediment particles. So far only two field and two laboratory studies are available on the fate of microplastics in few relatively newly built bioretention systems. Therefore, this study investigated the abundance and distribution of microplastics in nine 7-12 years old stormwater bioretention systems. It was found that microplastics generally accumulate on the surface of bioretention systems. Microplastic median particle concentrations decreased significantly from the surface layer (0-5 cm) of the filter material to the 10-15 cm depth layer from 448 to 136 particles/100 g, respectively. The distance to the inlet did not significantly affect the surface accumulation of microplastic particles, suggesting modest spatial variability in microplastics accumulation in older bioretention systems. Further, this study investigated the polymer composition in bioretention systems. It was shown that PP, EVA, PS and EPDM rubber are the most abundant polymer types in bioretention systems. Also, it was found that large percentages of microplastic particles are black particles (median percentage of black particles: 39%) which were found in 28 of the 33 investigated samples. This underlines the importance of including black particles in microplastic studies on stormwater, which has been overlooked in most previous studies.

Evaluating bioretention scale effect on stormwater retention and pollutant removal

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

A long term study elucidates the relationship between media amendment and pollutant treatment in the stormwater bioretention system: Stability or efficiency?

Media amendment has been more and more frequently tested in stormwater bioretention systems for enhanced runoff pollutant treatment. However, few studies systematically evaluated the amended system over a long time span, which hindered the further optimization of the proposed amended media. In this study, biochar-pyrite system (PB), conventional sand system (SB), and biochar-woodchip system (WB) were established and operated for 26 months. Media amendment greatly enhanced the dissolved nutrient removal, the highest total dissolved nitrogen removal in PB and WB were 65.6±3.6% and 68.2±2.5%, respectively. Compared with PB, WB could maintain excellent nitrogen removal under long-term operation. In contrast, PB demonstrated stable and more effective total dissolved phosphorus removal during all stages (73.1±3.1%-80.3±4.1%). A high content of phosphorus and organic matter was leached in WB especially at initial operation, while the initial pollutant leaching in PB and SB is much lower, about one-third of WB. Microbial and metabolic function analysis indicated that the microbial community in the bioretention system is complicated and stable. Media amendment enhanced microbial diversity and the relative abundance of functional genera related to nitrogen (Nitrospira, Thauera, Denitratisoma, etc.), sulfur (Thiobacillus, Geobacter, Desulfovibrio, etc.), and carbon cycles (cellulomonas, saccharimonadales, and SBR1031, etc.), which well explained the enhanced pollutant removal and by-product leaching in different systems. Overall, the current study indicates that although media amendment is conducive to enhanced dissolved nutrient removal in bioretention systems, it can hardly maintain both stability and efficiency from initial set-up to long-term operation. In practical application, catchment characteristics, prioritized pollutants, meteorological factors, etc. should all be considered before choosing suitable amended media and its design factors, thereby maximising the stability and efficiency of the bioretention system.

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.

Conventional and amended bioretention soil media for targeted pollutant treatment: A critical review to guide the state of the practice

Bioretention systems are widely used green infrastructure elements that utilize engineered bioretention soil media (BSM) for stormwater capture and treatment. Conventional bioretention soil media, which typically consists of sand, sandy loam, loamy sand or topsoil amended with compost, has limited capacity to remove and may leach some stormwater pollutants. Alternative engineered amendments, both organic and inorganic, have been tested to supplement BSM. Yet, municipalities and regulatory agencies have been slow to adopt these alternative amendments into their design specifications, partly because of a lack of clear guidance on how to select the right amendment to treat a target stormwater contaminant under highly variable climatic conditions. This article aims to provide that guidance by: (1) summarizing the current design BSM specifications adopted by jurisdictions worldwide, (2) comparing the performance of conventional and amended BSM, (3) highlighting advantages and limitations of BSM amendments, and (4) identifying challenges for implementing amendments in field conditions. The analysis not only informs the research community of the barriers faced by stormwater managers in implementing BSM amendments but also provides guidelines for their adoption by interested agencies to comply with existing regulations and meet design needs. This feedback loop could catalyze further innovation in the development of sustainable stormwater treatment technologies.

Metal speciation in stormwater bioretention: Removal of particulate, colloidal and truly dissolved metals

For comprehensive estimation of the metal treatment efficiency of bioretention systems, information on metal speciation in the stormwater and the effluent is needed. However, so far, most bioretention studies only considered total metal concentrations. Despite their environmental importance, dissolved metals (defined as fractions < 0.45 μm) have only been evaluated in few studies. This study represents the first bioretention study to subdivide the <0.45 μm fraction further by filtration through a 3 kDa ultrafilter (corresponding to appr. 2-3 nm), thus enabling distinction between particulate, colloidal and truly dissolved metals. Higher bioavailability of the truly dissolved fraction has been indicated by previous research, underlining the importance of this study. Since vegetation and salt in stormwater both may be explanatory variables for metal fractionation, these have been added as factors in the utilized full factorial pilot-scale column experiment. While total metal removal was often >95%, detailed fractionation revealed that Cu and (when no salt was added) Zn removal in the <0.45 μm and <3 kDa fractions was significantly lower. Further, mean concentrations of Cu and (in one treatment) Cd in the <0.45 μm effluent fraction did not meet Swedish receiving water quality guidelines. By calculating the particulate, colloidal and truly dissolved fractions, it was shown that bioretention systems affect metal speciation of Cu and Zn. Colloidal and truly dissolved fractions were mostly prevalent in the effluent rather than the influent. Salt affected metal removal mostly negatively. Fractionation was affected by salt mainly in the influent where it increased the concentrations of Cd and Zn in the truly dissolved fraction (no effects on Cu and Pb fractions). In the effluent, Cu and Zn were only slightly affected by salt. Vegetation had mostly no significant effects on metal removal and fractionation. Further integration of detailed metal fractionation into sampling routines in bioretention research is recommended.

Evapotranspiration in green stormwater infrastructure systems

Evapotranspiration (ET) is a viable runoff reduction mechanism and an important player in the hydrologic cycle of vegetated green stormwater infrastructure (GSI). As a dynamic process, ET is dependent on both meteorological factors (e.g., rainfall characteristics, relative humidity, and air temperature) and GSI properties (e.g., soil media type). This paper investigates the role of ET in runoff volume reduction of green roofs and rain gardens through a comprehensive literature review. Evapotranspiration is mostly unaccounted in the design and crediting of GSI systems because of the complex interaction of soil, plants, and climate that makes its quantification difficult. To improve vegetated GSI design for runoff volume reduction, design methods should consider ET and infiltration processes concurrently. Two methods, complex and simple, are reviewed and discussed herein. The simple method requires minimal input information compared to the more complex continuous simulation method; however continuous simulation yields volume reduction values more similar to field observations. It is demonstrated that modifying the drainage structure and using fine-grained in-situ soils can potentially increase ET in vegetated GSI systems. None of the available ET predictive equations, mostly derived from agricultural sciences, are found to precisely match observed GSI ET data. Until further research is conducted on GSI ET estimation methods, the 1985 Hargreaves method is recommended when performing continuous simulations. The 1985 Hargreaves method is simple, requires limited input data that are readily available, and generates reasonable results. Technical recommendations and directions for future research are provided.

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.

Analysis of rainfall infiltration and its influence on groundwater in rain gardens

The dynamic observation data on groundwater level and water quality were obtained from rain gardens #2 and #3 from May to October 2016. The water balance method and 2D numerical simulation of variable saturation zone were used to calculate rainfall infiltration recharge coefficient, water supply, and evaporative discharge of rain garden. These parameters were used to simulate and explore the impact of rainfall infiltration in rain gardens on groundwater level and water quality. The groundwater depth of rain gardens was mainly affected by the concentrated infiltration of rainfall. The variation range of groundwater depth was approximately 4.298 ± 0.031 mm for J1, 3.9364 ± 0.097 mm for J2, and 4.0958 ± 0.064 mm for J3, and the specific yield was 0.208. Groundwater quality was naturally attenuated and would not threaten the safety of groundwater at a certain scale. Visual MODFLOW was used to simulate groundwater flow and conduct parameter sensitivity analysis to determine the main influencing factors of garden groundwater level change. Results showed that rainfall recharge was crucial to module sensitivity.

Predicting bioretention pollutant removal efficiency with design features: A data-driven approach

The objective of this study is to synthesize previous research findings from bioretention experiments and identify design features that lead to the best performance of bioretention pollutant removal with a data-driven approach. A bioretention database was built from 79 bioretention publications, composed of 182 records of bioretention cells with their design features and the corresponding pollutant removal efficiency data. Non-parametric correlation analysis, multiple linear regression (MLR), and decision tree classifiers were applied to investigate the relationships between bioretention design features and pollutant removal efficiencies. Non-parametric statistics and MLR results indicated that bioretention surface area, media depth, the presence of an internal water storage (IWS) layer, soil composition, and vegetation cover are all significantly correlated with pollutant removal efficiencies. The impacts of design features are significantly different under different climate and inflow conditions. Decision tree classifiers showed that non-vegetated bioretention cells with sand filter media generally have higher than 80% total suspended solid (TSS) mass removal efficiencies; bioretention cells with minimum organic matter and greater than 0.58 m soil media depth tend to remove more than 51% of total nitrogen (TN); and vegetated bioretention cells with minimum organic matter remove more than 67% of total phosphorus (TP). The overall accuracy of decision tree classifiers in the test set is around 70% to predict TSS, TN, and TP mass removal efficiency classes. This study suggests that the data-driven approach provides insights into understanding the complex relationship between bioretention design features and pollutant removal performance.

Hydrological modeling and field validation of a bioretention basin

An emerging green infrastructure, the bioretention basin, has been deployed world-wide to reduce peak flows, encourage infiltration, and treat pollutants. However, inadequate design of a basin impairs its treatment potential and necessitates the development and validation of a suitable hydrological model for design and analysis of bioretention basins. In this study, an existing numerical model, RECHARGE, has been adopted to simulate hydrological performance of a basin in the tropical climate of Singapore over a half year that included 80 storm events. Comparison of the model predictions with field observations shows that RECHARGE successfully simulates the basin hydrology of 80 events of varying rainfall characteristics with mass balance error of 5.1 ± 7.5% per event and 0.3% overall. Using the verified model, we develop new design curves that predict bioretention basin performance as a function of three basin design parameters: detention depth; ratio of drainage basin area to bioretention area; and saturated hydraulic conductivity of the basin soil media. We evaluate basin performance in terms of the percentage of water that infiltrates and is treated in the subsurface portion of the basin and define an infiltration index to measure the change in infiltrated percentage caused by unit change in the basin design parameters. The marginal improvement in basin performance drops significantly when the basin depth (h_d) is increased above 40 cm, when the ratio of drainage area to bioretention area (R) is decreased below 20, or when the saturated hydraulic conductivity (K_s) is increased above 10 cm/h.

Public perception towards river and water conservation practices: Opportunities for implementing urban stormwater management practices

The effectiveness of urban stormwater management practices (SMPs) on local water quality is dependent on adoption rates reaching a critical mass. While numerous studies have measured the effectiveness of practices on controlling water quantity and improving water quality, few have focused on the perspective of the public. The purpose of this study was to identify individuals' perceptions of urban SMPs implementation in the public and private realms, and how longitudinal perceptions about the local river could inform future water resource management. Through the lens of environmental behavior theories, we performed statistical analyses on four surveys - 2006, 2009, 2014 and 2016 - administered to urban residents in the Wabash River watershed in Tippecanoe County, Indiana. Our findings show that residents' water quality awareness and sense of personal responsibility increase over the ten years studied. In particular, rain garden adopters have higher appreciation of the Wabash River and care about how the river functions than other SMP adopters and non-adopters. In terms of urban SMP adoption, results indicate that residents are supportive of integrating rain barrels and rain gardens into public spaces. Perceptions of SMP benefits related to functional benefits, rather than environmental benefits, are prevalent when considering implementing SMPs on personal property. In addition, respondents support reducing stormwater charges for adopters of such practices on private property. Although cognitive barriers exist in those who have yet to adopt the practices, including concerns about SMP effectiveness, maintenance, aesthetics, and risk of bugs and insects, adopters are less likely to perceive such barriers. This research suggests that making resources (i.e., skills, knowledge, equipment, funding) more accessible to the public is essential, but not sufficient to encourage pro-environmental behaviors. Promoting public involvement in watershed activities, increasing their awareness about how urban SMPs function, and emphasizing the functional benefits of practices can be effective in motivating adoption.

Impacts of nonpoint source pollutants on microbial community in rain gardens

Low-impact development (LID) techniques are being applied to reduce non-point source (NPS) pollution which are generated from various land uses. Cost-effective LID design requires consideration of influent runoff properties as well as physical and ecological pollutant-removing mechanisms. However, current LID technology design has failed to reflect the different properties of influent water from various land uses, and the biological design factors in LID facilities causing low efficiency and difficulties in maintenance. This study was conducted to identify biological design factors by analyzing the impact of the pollutants included in influent runoff and physical environment on microbial growth in rain garden facilities applied to different land uses. The results showed that the non-point source pollutant loadings were about 1.5-3 times higher in the runoff from parking lots, which are frequently visited by automobiles than in roof runoff. Type of soil, chemical species, and chemical composition were assessed as internal environmental factors having significant impact on the phylum and the count of microorganisms in the facilities. The growth of Cyanobacteria, Streptophyta, Chlorophyta, Bacillariophyta, and Xanthophyceae was good when there was appropriate water content in the soil, light, and sandy soil. Based on these results, the future design of rain garden facilities should be performed by considering a microorganism appropriate to the properties of the influent pollutants, determining appropriate water content, nutrient content and soil type, and choosing plants that contribute to microbial growth.

Removal of organic contaminants in bioretention medium amended with activated carbon from sewage sludge

Bioretention, also known as rain garden, allows stormwater to soak into the ground through a soil-based medium, leading to removal of particulate and dissolved pollutants and reduced peak flows. Although soil organic matter (SOM) is efficient at sorbing many pollutants, amending the bioretention medium with highly effective adsorbents has been proposed to optimize pollutant removal and extend bioretention lifetime. The aim of this research was to investigate whether soil amended with activated carbon produced from sewage sludge increases the efficiency to remove hydrophobic organic compounds frequently detected in stormwater, compared to non-amended soil. Three lab-scale columns (520 cm³) were packed with soil (bulk density 1.22 g/cm³); activated carbon (0.5% w/w) was added to two of the columns. During 28 days, synthetic stormwater-ultrapure water spiked with seven hydrophobic organic pollutants and dissolved organic matter in the form of humic acids-was passed through the column beds using upward flow (45 mm/h). Pollutant concentrations in effluent water (collected every 12 h) and polluted soils, as well as desorbed amounts of pollutants from soils were determined using GC-MS. Compared to SOM, the activated carbon exhibited a significantly higher adsorption capacity for tested pollutants. The amended soil was most efficient for removing moderately hydrophobic compounds (log K _ow 4.0-4.4): as little as 0.5% (w/w), carbon addition may extend bioretention medium lifetime by approximately 10-20 years before saturation of these pollutants occurs. The column tests also indicated that released SOM sorb onto activated carbon, which may lead to early saturation of sorption sites on the carbon surface. The desorption test revealed that the pollutants are generally strongly sorbed to the soil particles, indicating low bioavailability and limited biodegradation.

A flexible modeling framework for hydraulic and water quality performance assessment of stormwater green infrastructure

A flexible framework has been created for modeling multi-dimensional hydrological and water quality processes within stormwater green infrastructure (GI) practices. The framework conceptualizes GI practices using blocks (spatial features) and connectors (interfaces) representing functional components of a GI. The blocks represent spatial features with the ability to store water (e.g., pond, soil, benthic sediments, manhole, or a generic storage zone) and water quality constituents including chemical constituents and particles. The hydraulic module can solve a combination of Richards equation, kinematic/diffusive wave, Darcy, and other user-provided flow models. The particle transport module is based on performing mass-balance on particles in different phases, e.g., mobile and deposited in soil with constitutive theories controlling their transport, settling, deposition, and release. The reactive transport modules allow constituents to be in dissolved, sorbed, bound to particles, and undergo user-defined transformations. Four applications of the modeling framework are presented that demonstrate its flexibility for simulating urban GI performance.

Do salt and low temperature impair metal treatment in stormwater bioretention cells with or without a submerged zone?

Although seasonal temperature changes and (road) salt in winter and/or coastal stormwater runoff might interfere with the metal treatment performance of stormwater bioretention cells, no previous study has evaluated the effect of these factors and their interactions under controlled conditions. In this 18week long study 24 well established pilot-scale bioretention columns were employed to evaluate the individual and combined effect(s) of low/high temperature, salt and presence of a submerged zone with an embedded carbon source on metal removal using a three factor, two-level full factorial experimental design. In most instances, the three factors significantly influenced the metal outflow concentrations and thus the treatment performance; the effect of temperature depended on the metal in question, salt had an overall negative effect and the submerged zone with carbon source had an overall positive effect. Despite these statistically significant effects, the discharge water quality was generally markedly improved. However, leaching of dissolved Cu and Pb did occur, mainly from bioretention cells dosed with salt-containing stormwater. The highest concentrations of metals were captured in the top layer of the filter material and were not significantly affected by the three factors studied. Overall, the results confirmed that bioretention provides a functioning stormwater treatment option in areas experiencing winter conditions (road salt, low temperatures) or coastal regions (salt-laden stormwater). However, validation of these results in the field is recommended, especially focusing on dissolved metal removal, which may be critically affected under certain conditions.

Small scale green infrastructure design to meet different urban hydrological criteria

As small scale green infrastructures, rain gardens have been widely advocated for urban stormwater management in the contemporary low impact development (LID) era. This paper presents a simple method that consists of hydrological models and the matching plots of nomographs to provide an informative and practical tool for rain garden sizing and hydrological evaluation. The proposed method considers design storms, infiltration rates and the runoff contribution area ratio of the rain garden, allowing users to size a rain garden for a specific site with hydrological reference and predict overflow of the rain garden under different storms. The nomographs provide a visual presentation on the sensitivity of different design parameters. Subsequent application of the proposed method to a case study conducted in a sub-humid region in China showed that, the method accurately predicted the design storms for the existing rain garden, the predicted overflows under large storm events were within 13-50% of the measured volumes. The results suggest that the nomographs approach is a practical tool for quick selection or assessment of design options that incorporate key hydrological parameters of rain gardens or other infiltration type green infrastructure. The graphic approach as displayed by the nomographs allow urban planners to demonstrate the hydrological effect of small scale green infrastructure and gain more support for promoting low impact development.

Multi-purpose rainwater harvesting for water resource recovery and the cooling effect

The potential use of rainwater harvesting in conjunction with miscellaneous water supplies and a rooftop garden with rainwater harvesting facility for temperature reduction have been evaluated in this study for Hong Kong. Various water applications such as toilet flushing and areal climate controls have been systematically considered depending on the availability of seawater toilet flushing using the Geographic Information System (GIS). For water supplies, the district Area Precipitation per Demand Ratio (APDR) has been calculated to quantify the rainwater utilization potential of each administrative district in Hong Kong. Districts with freshwater toilet flushing prove to have higher potential for rainwater harvest and utilization compared to the areas with seawater toilet flushing. Furthermore, the effectiveness of using rainwater harvesting for miscellaneous water supplies in Hong Kong and Tokyo has been analyzed and compared; this revives serious consideration of diurnal and seasonal patterns of rainfall in applying such technology. In terms of the cooling effect, the implementation of a rooftop rainwater harvesting garden has been evaluated using the ENVI-met model. Our results show that a temperature drop of 1.3 °C has been observed due to the rainwater layer in the rain garden. This study provides valuable insight into the applicability of the rainwater harvesting for sustainable water management practice in a highly urbanized city.