Influence of intermittent wetting and drying conditions on heavy metal removal by stormwater biofilters

Intra-event variations of organic micropollutants in highway runoff and a presedimentation-biofilter treatment facility

The study assessed the quality of highway runoff and a stormwater treatment system, focusing on intra-event variations (IEVs: variations within a runoff/effluent event) of the concentration of organic micropollutants (OMPs) including bisphenol-A, alkylphenols, polycyclic aromatic hydrocarbons (PAHs), and petroleum hydrocarbons (PHCs). IEVs of OMPs varied considerably with no particular recurring pattern in highway runoff and presedimentation effluent, displaying sporadic strong first flushes. IEVs are significantly associated with rainfall intensity variations, especially for particle-bound substances such as PAHs and PHCs. However, phenolic substances showed distinct IEV patterns compared to total suspended solids, PAHs, and PHCs, likely due to their higher solubility and mobility. Downstream sand filter (SF) and vegetated biofilter (BFC) mitigated IEVs, leading to more uniform discharge during outflow events. Although BFC's IEVs were indiscernible due to low effluent concentrations, SF's IEVs often peaked at the beginning of events (within the first 100 of ⁓600 m3), exceeding the lowest predicted non-effect concentrations for five PAHs, bisphenol-A, and octylphenol. This study highlights the advantage of IEV analysis over conventional event mean concentration analysis for identifying critical effluent stages, crucial for developing control strategies to protect sensitive water recipients or for reuse applications.

Impacts of Deicer Salt on Water Quality Performance of Stormwater Bioretention Systems with Varied Vegetation and Hydrology

Sodium chloride (NaCl) deicers contaminate bioretention and influence effluent water quality, the effects of which are not yet fully understood. We tested this by constructing 48 mesocosms in a greenhouse, each having Panicum virgatum, Eutrochium purpureum, or no vegetation; having an internal water storage (IWS) zone or not; and being exposed to high or low NaCl doses in the late winters of 2022 and 2023. Synthetic stormwater was applied and effluent was monitored through May 2023 with an end-of-experiment analysis of soil and plant biomass for nitrogen, phosphorus, copper, zinc, and total suspended solids (TSS). Average effluent loads increased in spring, after NaCl application, for total phosphorus (+61%), copper (+61%), zinc (+88%), and TSS (+66%). These four analytes recovered by summer, with average annual percent removals >85%. Vegetation and IWS reduced annual phosphorus (by -33 and -70%, respectively) and copper (by -24 and -40%) loads, while higher NaCl concentrations increased annual phosphorus (+107%), copper (+22%), and TSS (+51%) loads. Nitrogen removal was not linked with NaCl but was dependent upon the presence of IWS or vegetation. Post-NaCl effluent spikes pose seasonal risks to aquatic ecosystems, emphasizing the need for active maintenance, redundant removal mechanisms, and minimized exposure to NaCl.

Soil, climate, and landscape drivers of base cation concentrations in green stormwater infrastructure soils

Deicing practices and infrastructure weathering can impact plants, soil, and water quality through the input and transport of base cations. Base cation accumulation in green stormwater infrastructure (GSI) soils has the potential to decrease soil infiltration rates and plant water uptake or to promote leaching of metals and nutrients. To understand base cation retention in GSI soils and its drivers, we sampled 14 GSI soils of different age, contributing areas, and infiltration areas, across 3 years. We hypothesized that soil, climate, and landscape drivers explain the spatial and temporal variability of GSI soil base cation concentrations. Sodium (Na), Calcium (Ca), and Magnesium (Mg) concentrations in GSI soils were higher than in reference soils, while Ca and Mg were similar to an urban floodplain soil. Neither the contributing area, contributing impervious area, nor their ratios to infiltration area predicted base cation concentrations. Age predicted the spatial variability of Potassium (K) concentrations. Ca and Mg were moderately predicted by sand and silt, while clay predicted Mg, and sand predicted K. However, no soil characteristics predicted Na concentrations. A subset of sites had elevated Na in Fall 2019, which followed a winter with many freezing events and higher-than-average deicer salt application. K in sites with elevated Na was lower than in non-elevated sites, suggesting that transient spikes of Na driven by deicer salt decreased the ability of GSI soils to accumulate K. These findings demonstrate the large variability of GSI soil base cation concentrations and the relative importance of soil, climate, and landscape drivers of base cation dynamics. High variability in GSI soil data is commonly observed and further research is needed to reduce uncertainties for modeling studies and design. Improved understanding of how GSI soil properties evolve over time, and their relation to GSI performance, will benefit GSI design and maintenance practices.

Performance of a gross pollutant trap-biofilter and sand filter treatment train for the removal of organic micropollutants from highway stormwater (field study)

This field study assessed the occurrence, event mean concentrations (EMCs), and removal of selected organic micro-pollutants (OMPs), namely, polycyclic aromatic hydrocarbons (PAHs), petroleum hydrocarbons (PHCs), nonylphenol (NP), 4-t-octylphenol (OP), and bisphenol A (BPA), in a gross pollutant trap (GPT)-biofilter/sand filter stormwater treatment train in Sundsvall, Sweden. The effects of design features of each treatment unit, including pre-sedimentation (GPT), sand filter medium, vegetation, and chalk amendment, were investigated by comparing the units' removal performances. Overall, the treatment train removed most OMPs from highway runoff effectively. The results showed that although the sand filter provided moderate (<50 % for phenolic substances) to high (50-80 % for PAHs and PHCs) removal of OMPs, adding a vegetated soil layer on top of the sand filter considerably improved the removal performance (by at least 30 %), especially for BPA, OP, and suspended solids. Moreover, GTP did not contribute to the treatment significantly. Uncertainties in the removal efficiencies of PAHs and PHCs by the filter cells increased substantially when the ratio of the influent concentration to the limit of quantification decreased. Thus, accounting for such uncertainties due to the low OMP concentrations should be considered when evaluating the removal performance of biofilters.

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

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

Synthetic stormwater for laboratory testing of filter materials

Synthetic stormwater was tested to determine the ageing effects on dissolved metal concentrations and used in a column experiment to determine efficiency of four different filter materials (milkweed, bark, peat, polypropylene) in removing total and dissolved metals. Synthetic stormwater was created by adding metal salts, oil and collected stormwater sediment to tap water. Two ageing experiments were performed to determine the change of synthetic stormwater quality over time. One experiment lasted for 11 days and another focused on rapid concentration changes one day after preparation. The one-day ageing experiment showed rapid decrease in dissolved concentration of certain metals, specifically Cu. To consider this change, correction coefficients for each metal were developed and used to estimate the average dissolved metal concentration in the synthetic stormwater during the experiment to determine filter treatment efficiency. During the 11-day experiment on metal concentrations, no noticeable quality changes were observed for at least six days after the preparation of synthetic stormwater. Furthermore, a column experiment was run with duplicate filter columns. Inflow and outflow samples were analysed for total and dissolved metals, turbidity, particle size distribution, and pH. High removal of total metal concentrations was noticed in all tested filter media (58-94%). Dissolved metal concentration removal varied among different filter media. In general, columns with bark and peat media were able to treat dissolved metals better than polypropylene and milkweed. The level of treatment of dissolved metals between the different filter media columns were bark > peat > milkweed > polypropylene.

The control efficiency and mechanism of heavy metals by permeable pavement system in runoff based on enhanced infiltration materials

As one of the commonly used stormwater management measures, permeable pavement system (PPS) played a prominent role in controlling runoff pollution and alleviating urban waterlogging. In this study, new enhanced infiltration materials (construction waste brick, coal gangue, activated carbon, multi-walled carbon nanotube, multi-layer graphene) were applied in PPS and the control efficiency and mechanism of typical heavy metals (HMs, Mn²⁺, Pb²⁺, Zn²⁺, Cu²⁺, Cd²⁺, Ni²⁺) was investigated in runoff. Furthermore, the influences of different rainfall intensities and antecedent dry periods on HMs removal by PPS were evaluated. The results showed that all PPS with enhanced infiltration materials have little leaching effect on HMs (＜3 μg/L). All the selected enhanced infiltration materials meet the requirements of PPS. The concentration of HMs in the effluent of PPS dropped sharply first, followed rebounded and then maintained at a stable range. Activated carbon PPS (AC), Multi-walled carbon nanotube PPS (MCN), and Multi-layer graphene PPS (MG) could significantly improve the control effect of PPS on nearly all selected HMs. The average removal rates of AC, MCN and MG for six HMs were 75.48%-99.35%, 81.30%-97.59%, and 73.03%-99.33%, respectively. Compared with Traditional PPS (TR), the effluent concentrations of HMs in construction waste brick PPS (CW) and coal gangue PPS (CG) were relatively higher and unstable. AC, CN and MG could adapt to different rainfall conditions and the maximum removal rates of most HMs exceed to 99%. With antecedent dry periods increased, the control effect of HMs was significantly improved. The influences of the antecedent drying period on HMs removal followed as: CW＞CG＞TR＞MG＞CN＞AC. This study provided novel methods to eliminating HMs in runoff and provides implications for the design of PPS.

Real-time control technology for enhancing biofiltration performances and ecosystem functioning of decentralized bioretention cells

Urban stormwater management has become a major issue over the last decades for flood prevention as well as water resource preservation. The development of green infrastructures such as bioretention systems since the 1990s has often been reported as an effective means of runoff mitigation with subsequent conveyed pollutant capture. Nevertheless, climate change involving more frequent extreme weather events as well as the variety of emerging pollutants in urban runoff have put an increasing strain on bioretention processes. Within this context, this mini-review deals with the opportunity of upgrading vegetated bioretention systems with active control technology to enhance their pollutant treatment capacity through proper control of critical bioretention operational variables and relying on improved ecological functioning and resilience. It is envisioned that such nature-based solutions hybridized with real-time control technology would help to improve stormwater reuse for more sustainable urban water management within the nexus of water-energy-food and greenhouse gases in future cities.

Occurrence, concentration, and distribution of 38 organic micropollutants in the filter material of 12 stormwater bioretention facilities

The increased use of bioretention facilities as a low impact development measure for treating stormwater runoff underscores the need to further understand their long-term function. Eventually, bioretention filter media must be (partly) replaced and disposed of at the end of its functional lifespan. While there are several studies of metal accumulation and distributions in bioretention media, less is known about organic pollutant pathways and accumulation in these filters. The present study considers the occurrence and accumulation of 16 polycyclic aromatic hydrocarbons, 7 polychlorinated biphenyls, 13 phthalates, and two alkylphenols throughout 12 older bioretention facilities (7-13 years old) used for stormwater treatment in Michigan and Ohio, USA. These pollutant groups appear to behave similarly, with greater instances of detection and higher concentrations in the upper media layers which decrease with increased depth from the surface. The patterns of detection and concentration in the filter material may be explained by characteristics of the pollutants, such as molecular structures and solubility that affect the removal of the organic pollutants by the filter material. There is also a large variation in concentration magnitudes between the bioretention sites, most likely due to differences in pollutant sources, contributing catchment size and/or land uses.

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.

Phosphorus removal, metals dynamics, and hydraulics in stormwater bioretention systems amended with drinking water treatment residuals

Drinking water treatment residuals (DWTRs) are a promising media amendment for enhancing phosphorus (P) removal in bioretention systems, but substantial removal of dissolved P by DWTRs has not been demonstrated in field bioretention experiments. We investigated the capacity of a non-amended control media (Control) and a DWTR-amended treatment media (DWTR) to remove soluble reactive P (SRP), dissolved organic P (DOP), particulate P (PP), and total P (TP) from stormwater in a two-year roadside bioretention experiment. Significant reductions m SRP, PP and TP concentrations and loads were observed in both the Control and DWTR media. However, the P removal efficiency of the DWTR cells were greater than those of the Control cells for all P species, particularly during the second monitoring season as P sorption complexes likely began to saturate in the Control cells. The difference in P removal efficiency between the Control and DWTR cells was greatest during large storm events, which transported the majority of dissolved P loads in this study. We also investigated the potential for DWTRs to restrict water flow through bioretention media or leach heavy metals. The DWTRs used in this study did not affect the hydraulic performance of the bioretention cells and no significant evidence of heavy metal leaching was observed during the study period. Contrasting these results with past studies highlights the importance of media design in bioretention system performance and suggests that DWTRs can effectively capture and retain P without affecting system hydraulics if properly incorporated into bioretention media.

Natural aging of expanded shale, clay, and slate (ESCS) amendment with heavy metals in stormwater increases its antibacterial properties: Implications on biofilter design

Aging is often expected to decrease the pathogen removal capacity of media because of exhaustion of attachment sites by adsorption of co-contaminants and dissolved organics. In contrast, the adsorption of metals naturally present in stormwater during aging could have a positive impact on pathogen removal. To examine the effect of adsorbed metals on pathogen removal, biofilter media amended with expanded clay, shale, and slate (ESCS) aggregates, a lightweight aggregate, were exposed to metals by intermittently injecting natural stormwater spiked with Cu, Pb, and Zn, and the capacity of aged and unaged media to remove Escherichia coli (E. coli), a pathogen indicator, were compared. Metal adsorption on ESCS media decreased their net negative surface charge and altered the surface properties as confirmed by zeta potential measurement and Fourier-Transform Infrared Spectroscopy (FTIR) analysis. These changes increased the E. coli adsorption capacity of aged media compared with unaged media and decreased overall remobilization of attached E. coli during intermittent infiltration of stormwater. A live-dead analysis confirmed that the adsorbed metals inactivated attached E. coli, thereby replenishing the adsorption capacity. Overall, the results confirmed that natural aging of biofilter media with adsorbed metals could indeed have a net positive effect on E. coli removal in biofilters and therefore should be included in the conceptual model predicting long-term removal of pathogens from stormwater containing mixed pollutants.

Removal of stormwater dissolved organic nitrogen through biotransformation using activated carbon

Conventional bioretention systems are not effectively designed to remove stormwater dissolved organic nitrogen (DON). Biotransformation study on five organic nitrogenous compounds with different values for adsorption on coal activated carbon (AC) and bioavailability revealed that adsorption is a greater controlling factor for ammonification than bioavailability. This study also showed three apparent benefits: enhancement of the ammonification rate, ammonification of the bio-recalcitrant organic nitrogenous compounds, for example, pyrrole, and bio-regeneration of the adsorbent (coal AC). Low temperature (4°C) did not impact ammonification of leucine at a velocity of 34 cm/h, but negatively affected it at 61 cm/h. It was also observed that bed media height > 30 cm would not appreciably increase ammonification. Under intermittent wetting/draining conditions, the DON removal efficiency was more than 90%, indicating that DON was successfully removed through concurrent adsorption/ammonification, although generated ammonium in the effluent must be properly addressed. PRACTITIONER POINTS: Coal activated carbon appears a better material for DON ammonification compared with charcoal and quartz sand. A temperature as low as 4°C may not adversely impact DON ammonification at a velocity of 34 cm/h or less. A bed media depth of 30 cm is considered as adequate to promote DON ammonification. A larger depth may not be expected to improve ammonification. Ammonification of the bio-recalcitrant organic nitrogenous compounds, for example, pyrrole, and bio-regeneration of the adsorbent, for example, coal activated carbon, may be achieved.

Influence of soil characteristics and metal(loid)s on antibiotic resistance genes in green stormwater infrastructure in Southern California

The synergetic effects of metal(loid)s and soil characteristics on bacterial antibiotic resistance genes (ARGs) in green stormwater infrastructure (GSI) has been relatively understudied. Surface soil samples from six GSIs in Southern California over three time periods were assessed for selected ARGs, class 1 integron-integrase genes (intI1), 16S rRNA genes, and bioavailable and total concentrations of nine metal(loid)s, to investigate the relationships among ARGs, soil characteristics, and co-occurring metal(loid)s. Significant correlations existed among relative gene abundances (sul1, sul2, tetW, and intI1), total metal(loid)s (arsenic, copper, lead, vanadium, and zinc), and bioavailable metal(loid) (arsenic) (r = 0.29-0.61, p_adj < 0.05). Additionally, soil texture, organic matter, and nutrients within GSI appeared to be significantly correlated with relative gene abundances of sul1, sul2, and tetW (r = -0.57 to 0.59, p_adj < 0.05). Multiple regression models significantly improved the estimation of ARGs in GSI when considering multiple effects of soil characteristics and metal(loid)s (r = 0.74, p_adj < 0.001) compared to correlation results. Total arsenic was a significant (positive) correlate in all the regression models of relative gene abundances. This work provides new insights into co-dependencies between GSI ARGs and co-occurring metal(loid)s, indicating the need for risk assessment of metal(loid)-influenced ARG proliferation.

Stormwater treatment for reuse: Current practice and future development - A review

Stormwater harvesting is an effective measure to mitigate flooding risk and pollutant migration in our urban environment with the continuously increasing impermeable faction. Treatment of harvested stormwater also provides the fit-for-purpose water sources as an alternative to potable water supply ensuring the reliability and sustainability of the water management in the living complex. In order to provide the water management decision-maker with a broad range of related technology database and to facilitate the implementation of stormwater harvesting in the future, a comprehensive review was undertaken to understand the corresponding treatment performance, the applicable circumstances of current stormwater treatment and harvesting technologies. Technologies with promising potential for stormwater treatment were also reviewed to investigate the feasibility of being used in an integrated process. The raw stormwater quality and the required quality for different levels of stormwater reuses (irrigation, recreational, and potable) were reviewed and compared. The required level of treatment is defined for different 'fit-for-purpose' uses of harvested stormwater. Stormwater biofilter and constructed wetland as the two most advanced and widely used stormwater harvesting and treatment technologies, their main functionality, treatment performance and adequate scale of the application were reviewed based on published peer-reviewed articles and case studies. Excessive microbial effluent that exists in stormwater treated using these two technologies has restricted the stormwater reuse in most cases. Water disinfection technologies developed for wastewater and surface water treatment but with high potential to be used for stormwater treatment have been reviewed. Their feasibility and limitation for stormwater treatment are presented with respect to different levels of fit-for-purpose reuses. Implications for future implementation of stormwater treatment are made on proposing treatment trains that are suitable for different fit-for-purpose stormwater reuses.

Accumulation of heavy metals in stormwater biofiltration systems augmented with zeolite and fly ash

The discharge of high levels of heavy metals into the environment is of concern due to its toxicity to aquatic life and potential human health impacts. Biofiltration systems have been used in urban environments to address nutrient contamination, but there is also evidence that such systems can be effective in reducing heavy metals concentration in stormwater. However, the accumulation pattern of heavy metals and lifespan of such systems, which are important in engineering design, have not been thoroughly explored. This study investigated the accumulation patterns of lead (Pb), copper (Cu), zinc (Zn), manganese (Mn), and iron (Fe), which are common in urban runoff, in non-vegetated filtration columns using three different types of filter media, namely sand (S), and mixtures of sand with fly ash (sand-fly ash mix, SF), and with zeolite (sand-zeolite mix, SZ). The columns were assessed in terms of infiltration rate, the mass of heavy metals accumulation at different depths, and formation of crust layer (schmutzdecke) at the surface. The results show that most of the heavy metals accumulated at the top 5-10 cm of the filter media. However, Zn was found adsorbed to a depth of 15 cm in S and SZ columns, while Mn and Fe were present in column S throughout the entire 30 cm depth of the filter media. The presence especially of Zn, Mn, and Fe in the deeper portions of the filter media before the top 5 cm layer reached its maximum adsorption capacity, hints that transport to the deeper layers is not necessarily dependent on saturation of the upper layers for these heavy metals. SF accumulated heavy metals most at the top 5 cm of the filter media layer, and retained twice the mass of heavy metals in the crust layer, compared to S and SZ columns. SF also yielded the lowest value of infiltration rate of 31 mm/h. Considering both metals accumulation and clogging potential of the filter media, the periodic maintenance of these systems is suggested to be approximately between 1.5 and 3 years.

Trace Organic Contaminant Transfer and Transformation in Bioretention Cells: A Field Tracer Test with Benzotriazole

Bioretention cells can effectively infiltrate stormwater runoff and partly remove conventional water contaminants. A field tracer injection experiment in a conventionally designed bioretention cell was used to investigate the fate of benzotriazole, a model trace organic contaminant, during and between runoff events. Moderate (29%) benzotriazole load reductions were measured during the 6 h long injection experiment. The detection of 1-methyl benzotriazole, hydroxy benzotriazole, and methoxy benzotriazole provided in situ evidence of some rapid benzotriazole microbial transformation during the tracer test and more importantly between the events. The detection of benzotriazole alanine and benzotriazole acetyl alanine also showed fast benzotriazole phytotransformation to amino acid conjugates during the tracer test and suggests further transformation of phytotransformation products between events. These data provide conclusive full-scale evidence of benzotriazole microbial and phytotransformation in bioretention cells. Non-target chemical analysis revealed the presence of a diverse range of trace organic contaminants in urban runoff and exiting the bioretention cell, including pesticides and industrial, household, and pharmaceutical compounds. We have demonstrated the in situ potential of urban green infrastructure such as bioretention cells to eliminate polar trace organic contaminants from stormwater. However, targeted design and operation strategies, for example, hydraulic control and the use of soil amendments, should be incorporated for improved bioretention cell performance for such compounds.

Removal of rubber, bitumen and other microplastic particles from stormwater by a gross pollutant trap - bioretention treatment train

Microplastic particles in stormwater pose significant threats to the environment. This study investigated how effective a stormwater treatment train was at removing rubber, bitumen and other microplastics (incl. fibers, fragments, and paint particles) in the 100-300 µm and >300 µm size fractions from highway runoff. The two treatment trains comprise a gross pollutant trap (GPT) followed by either a vegetated bioretention cell or non-vegetated sand filter. Flow-proportional composite samples were taken from the highway runoff, GPT outflow and the outflow from the two parallel filters during nine rain events to determine overall treatment performance, as well as the performance of individual system components. The identified rubber, bitumen and other microplastic particles mainly represented the 100-300 µm fraction and included high ratios of rubber (30%) and bitumen (60%). Overall, the treatment train efficiently removed rubber, bitumen and other microplastic particles in the 100-300 µm size fraction from the stormwater. The filter cells accounted for a major share of this removal, as the GPT did not reduce microplastic particle concentrations. This observation is likely explained by the fact that the rubber, bitumen and other microplastic particles have a density close to the density of water and thus removal by sedimentation is decreased. This identified an inherent weakness of the system; more specifically, the high microplastic concentrations in the surface water of the GPT means there can be a risk of microplastic release through overflow pits when inflows surpass the system capacity. Despite some differences, both the vegetated bioretention cell and the non-vegetated sand filter removed rubber, bitumen and other microplastic particles to similar extent.

Machine learning approaches for predicting the performance of stormwater biofilters in heavy metal removal and risk mitigation

The increasing amount of data on biofilter treatment performance over the past decade has made it possible to use data-driven approaches to explore the relationships between biofilter performance and a range of input variables. The knowledge gap lies in lack of models to predict the biofilter performance considering both design and operational variables, especially for heavy metals. In this study, we tested three machine learning (ML) approaches, namely multilinear regression (MLR), artificial neural network (NN), and random forest (RF), to predict biofilter outflow concentrations of heavy metals (Cd, Cr, Cu, Fe, Ni, Pb and Zn) using a range of design and operational factors as input variables. The results show that RF performed relatively better than other two models, with median Nash-Sutcliffe Efficiency (NSE) values of 0.995, 0.317, 0.762, 0.636, 0.726, 0.896 and 0.656 for Cd, Cr, Cu, Fe, Ni, Pb and Zn, respectively during model training. However, all the models were less accurate during model validation, with the better performance found for Cd (average NSE=0.964), Zn (0.530) and Ni (0.393) and poorer performance observed for Cu (0.219), Pb (0.058), Fe (-0.054) and Cr (-0.062). Infiltration rate (IR) and inflow concentration (C_in) were sensitive to all pollutants' removal in biofilters. The ratio of system size to catchment size was also found to be important for Zn, Ni and Cd, while ponding depth was an important variable for Cd. Based on thousands of hypothetical design and operational scenarios (generated using raw data), the best ML models were used to predict the biofilter outflow concentrations and estimate the risk quotient (RQ) values with regards to reuse of treated stormwater for various purposes. Results suggest that biofilters were able to reduce health risks associated with heavy metals in stormwater and therefore produce reliable water fit for reuses such as irrigation, swimming, and toilet flushing. Modelling results showed that biofiltration did not meet the requirements for drinking when Cd contamination exists. Explorative analysis also demonstrated how the key operational and design variables can be optimised to further reduce the health risks that can be fit for drinking purposes (i.e., RQ value <1).

Pilot and Field Studies of Modular Bioretention Tree System with Talipariti tiliaceum and Engineered Soil Filter Media in the Tropics

Stormwater runoff management is challenging in a highly urbanised tropical environment due to the unique space constraints and tropical climate conditions. A modular bioretention tree (MBT) with a small footprint and a reduced on-site installation time was explored for application in a tropical environment. Tree species used in the pilot studies were Talipariti tiliaceum (TT1) and Sterculia macrophylla (TT2). Both of the MBTs could effectively remove total suspended solids (TSS), total phosphorus (TP), zinc, copper, cadmium, and lead with removal efficiencies of greater than 90%. Total nitrogen (TN) removal was noted to be significantly higher in the wet period compared to the dry period (p < 0.05). Variation in TN removal between TT1 and TT2 were attributed to the nitrogen uptake and the root formation of the trees species. A field study MBT using Talipariti tiliaceum had a very clean effluent quality, with average TSS, TP, and TN effluent EMC of 4.8 mg/L, 0.04 mg/L, and 0.27 mg/L, respectively. Key environmental factors were also investigated to study their impact on the performance of BMT. It was found that the initial pollutant concentration, the dissolved fraction of influent pollutants, and soil moisture affect the performance of the MBT. Based on the results from this study, the MBT demonstrates good capability in the improvement of stormwater runoff quality.

Balancing Hydraulic Control and Phosphorus Removal in Bioretention Media Amended with Drinking Water Treatment Residuals

Green stormwater infrastructure like bioretention can reduce stormwater runoff volumes and trap sediments and pollutants. However, bioretention soil media can be both a sink and source of phosphorus (P). We investigated the potential tradeoff between hydraulic conductivity and P sorption capacity in drinking water treatment residuals (DWTRs), with implications for bioretention media design. Batch isotherm and flow-through column experiments were used to quantify the maximum P sorption capacity (S_max) and rate of P sorption for three DWTR sources. S_max values varied greatly among DWTR sources and methodologies, which has implications for regulatory standards. We also conducted a large column experiment to determine the hydraulic and P removal effects of amending bioretention media with solid and mixed layers of DWTRs. When applied to bioretention media, the impact of DWTRs on hydraulic conductivity and P removal depended on layering strategy. Although DWTR addition in solid and mixed layer designs improved P removal, the solid layer restricted water flow and exhibited incomplete P removal, while the mixed layer had no effect on flow and removed ~100% of P inputs. We recommend that DWTRs be mixed with sand in bioretention media to simultaneously achieve stormwater drainage and P reduction goals in green stormwater infrastructure.

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.

Nature-Based Solutions and Real-Time Control: Challenges and Opportunities

Nature-based solutions (NBS) as green infrastructures to urban drainage are an effective mitigation strategy both in terms of quantity and quality of runoff. Real-time control (RTC) can complement both flood mitigation and improvement of water quality by controlling elements of the drainage and sewage system. This study assessed the improvement opportunities with RTC of three NBS-related techniques commonly applied in urban drainage with different spatial scales: green roof, bioretention and detention basin and the remaining challenges to integrate both methods. Additionally, our investigations showed that the main difficulties reported involve the planning and monitoring stages of the RTC system. All of the studied devices can benefit from RTC. It is possible to observe that, despite the good results reported in the literature, the application of RTC to NBS studies on urban drainage are very recent. There are several opportunities that can be explored to optimize the performance.

The impact of stormwater biofilter design and operational variables on nutrient removal - a statistical modelling approach

Biofiltration systems can help mitigate the impact of urban runoff as they can treat, retain and attenuate stormwater. It is important to select the optimal design characteristics of biofilters (e.g., vegetation, filter media depth) to ensure high treatment performance. Operational conditions (e.g., infiltration rate) can also lead to significant changes in biofilter treatment performance over time. The impact of specific operational conditions on water quality treatment performance of stormwater biofilters is still not well understood. Furthermore, despite the importance of design characteristics and operational conditions on biofilter treatment performance, there is a lack of models that can be used to determine the optimal design and operation. In this paper, we developed a series of statistical models to predict the Total Phosphorus (TP) and Total Nitrogen (TN) removal performance of stormwater biofilters using various numbers of design characteristics and operational conditions. These statistical models were tested using data collected from four extensive laboratory-scale biofilter column studies. It was found that all models performed relatively well with a Nash-Sutcliffe Efficiency (NSE) of 0.42 - 0.61 for TP and 0.37 - 0.63 for TN. The most important design characteristics were filter media type and depth for TP treatment, and vegetation type and submerged zone depth for TN treatment. In addition, infiltration rate and inflow concentrations were the operational conditions that greatly influence outflow TP and TN concentrations from stormwater biofilters. As such, these variables need to be carefully considered when designing and operating stormwater biofilters. Sensitivity analysis results indicate that the model was quite sensitive to all regression coefficients and intercepts. Additional modelling exercises show that the model could be further simplified by reducing the number of cross-correlated parameters. These models can be used by practitioners for not just optimising the design, but also operating biofilters using real-time monitoring and control to achieve optimum performance.

Effects of plant species and traits on metal treatment and phytoextraction in stormwater bioretention

To study effects of plant species selection on total and dissolved metal treatment performance of bioretention systems (BRS), 12 sets of columns were prepared, each planted with one of 12 species that are either widely used in BRS or have potentially important traits for metal removal (ability to hyperaccumulate metals, C4 photosynthesis, or ability to form mycorrhiza). Artificial stormwater was applied to half of the columns during all of a 31-week test period, while treatment of the others included a 5-week long dry period to test interactive effects of drying and plant traits on BRS metal treatment in more realistic alternating wet and dry conditions. Concentrations of metals (dissolved and total) in the effluent significantly differed between most columns with different plants, and the differences in concentrations of dissolved metals after the dry period were particularly important. Mean dissolved Cd concentrations exceeded Swedish reference values in effluents from BRS with two of the plant species, while mean dissolved Zn concentrations exceeded them in effluents from BRS with three of the species (and non-vegetated controls). Dissolved Cu leaching was observed in effluents from BRS with five of the plant species after the dry period, and mean concentrations exceeded Swedish reference values in effluents from all the BRS (including the constantly watered systems). Some support in terms of metal concentrations in shoots and shoot/soil ratios was obtained for using hyperaccumulators in BRS to remove metals from filter material. For example, Armeria maritima (a hyperaccumulator with the lowest shoot biomass) and Miscanthus sinsenis (a C4 plant with the highest biomass production) took up similar amounts of metals despite large differences in biomass. However, no significant correlations between effluent metal concentrations and plants' metal uptake were found, possibly because of the short duration of the experiment. The results indicate that root biomass affected effluent metal concentrations more strongly. Root biomass was often positively correlated with total and (particularly) dissolved effluent metal concentrations. Further experiments with different soil metal concentrations, organic matter analyses and stronger focus on root characteristics are recommended, including additional tests of effects of hyperaccumulators and mycorrhiza on metal treatment and phytoextraction.

Water treatment residual: A critical review of its applications on pollutant removal from stormwater runoff and future perspectives

In recent years, many studies have been conducted on using different filter media in bioretention systems for stormwater runoff treatment. This critical review paper provides a comprehensive review on the current state of water treatment residual (WTR), a recycled material that can be used as bioretention filter media for removals of key stormwater runoff pollutants (especially phosphorus) and future perspectives with innovative modification on WTR applied for pathogen removal from stormwater runoff. This review paper comprised (i) a brief summary of the reported WTR characteristics, (ii) a thorough evaluation of WTR performance on major pollutants removal from stormwater runoff (iii) a discussion on phosphorus removal mechanisms by WTR applied in the stormwater runoff treatment, and (iv) a review of the future perspectives of WTR for pathogen removal and other potential practical application in the field of stormwater treatment. As outlined in this review, WTR in stormwater runoff treatment has yet to be fully explored. The possible enhancements, especially metal surface modification on WTR are reviewed to bring about the widespread use of WTR in stormwater reuse practices.

Dissolved metal adsorption capacities and fractionation in filter materials for use in stormwater bioretention facilities

The dissolved metal adsorption and association was determined for ten different filter materials recommended and/or implemented in bioretention facilities. Batch adsorption and batch kinetic experiments were performed at lab-scale using both single and multi-metal solutions. Metal strengths and association were determined by sequential extraction analysis. All materials adsorbed metals and 90% of adsorption occurred within 1 h. However, as metal solutions became more complex, adsorption behavior changed. Generally, filter materials classified as sand with a naturally high pH, relatively low organic matter (OM) content and large specific surface area seem to be good choices for removing dissolved metals. Additionally, a chalk additive might improve metal adsorption whereas biochar did not significantly improve metal retention and may be an unwanted (due to degradation over time) extra source of OM. Regardless of filter material, metals primarily adsorbed to the exchangeable form which indicates that metal adsorption might not be permanent, but rather substantially reversible in some cases. More research is needed to assess whether dissolved metals adsorbed in filter materials of bioretention systems pose a delayed threat instead of an immediate threat. Finally, the authors strongly recommend filter materials intended for stormwater bioretention facilities to be tested prior to implementation.

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.

Redox potential as a method to evaluate the performance of retention soil filters treating combined sewer overflows

Retention soil filters (RSFs) protect water bodies from pollutant loads originating from combined sewer overflows (CSOs) by filtering the wastewater through a filter layer having a depth of 0.75 to 1 m. The microbiological processes in the filter material are influenced by the redox potential (Eh). This potential is a strong indicator of the prevailing environmental conditions and the possible type of microbial activity. Previous investigations of filter bodies have been confined to constructed wetlands (CWs) with regular intermittent wastewater inflow. Compared to CWs, RSFs are characterized by higher oxygen availability due to alternating operating and dry periods. This study aimed to determine the Eh in RSFs and investigate its influence on the removal efficiency for different substances. We established a conceptual model for the standard Eh curve following a loading event, and the variations to this standard in two depths and between treatments. Correlations were determined with a canonical correlation analysis between the pollutant removal of COD, ammonium, phosphorous, E. coli, somatic coliphages and diclofenac and the Eh. Although the removal efficiency is influenced by several additional operating factors such as the preceding dry period, filter age and the respective inflow concentrations, our results show that the Eh is an adequate approach to assess the removal efficiency of RSFs for these substances.

Predicting long term removal of heavy metals from porous pavements for stormwater treatment

Porous pavements are commonly used stormwater management systems. However, the understanding of their long-term capacity to retain heavy metals is limited. This study aims to investigate the long-term removal of heavy metals in three different porous pavements - Porous Asphalt (PA), Hydrapave (HP) and Permapave (PP) over accelerated laboratory experiments representing 26 years with varying hydrological conditions (drying/wetting periods and flow rates). A treatment model that simulates adsorption and desorption processes was developed for the first time to predict the long-term heavy metal removal by porous pavements. Unsurprisingly, all tested porous pavements performed better in removing metals that tend to attach to solid particles (e.g. Pb, Al, Fe) than more soluble ones (e.g. Cu, Zn, and Mn). There was a general increase of heavy metal concentrations at the outlet of the pavements over time as a result of a decrease in adsorption capacity of the systems, especially after the occurrence of clogging; the soluble heavy metals removal decreased with a reduction in flow rates which was speculated to be due to more time being available for desorption of metals and breakdown of accumulated sediments. The proposed model simulated the trend, fluctuations and peaks of heavy metal concentrations reasonably well, achieving the Nash-Sutcliffe coefficient (NSE) values of 0.53-0.68 during model calibration. The model was most promising in predicting Al and Cu release from porous pavements (50%-91% of the observed data within the 90% uncertainty bands, NSE = 0.44-0.74), followed by Fe and Pb (27-77% observations within the bands, NSE = 0.20-0.69). Further improvements of the model are needed for it to be applicable for Zn and Mn.

Stormwater disinfection using electrochemical oxidation: A feasibility investigation

Electrochemical oxidation (ECO) has shown good potential for disinfection of wastewater discharges but has not been tested for stormwater. Due to far lower salinity and chloride levels present in stormwater than in wastewaters, the knowledge so far on the ECO disinfection performance cannot simply be used for stormwater applications. This paper presents the first study on the feasibility of ECO technology for disinfection of pre-treated stormwater. Disinfection performance of E. coli was tested using a dimensional stable anode (DSA) in a series of batch experiments with synthetic stormwater of 'typical' chemical and microbial composition. The results showed that effective disinfection could be achieved with very low energy consumption; e.g. the current density of 1.74 mA/cm² achieved total disinfection in 1.3 min, using only 0.018 kWh per ton of stormwater treatment. Chlorination was found to be the key disinfection mechanism, despite the synthetic stormwater containing only 9 mg/L of chloride. Real stormwater collected from three stormwater treatment systems in Melbourne was then used to validate the findings for indigenous microbe species. Disinfection below the detection limit was achieved for stormwater from the two sites where chloride levels were 9 and 200 mg/l, respectively, but not for the third site where stormwater contained only 2 mg/L chloride. Unfortunately, deterioration of the DSA anode was observed after only 8-10 h of its cumulative operation time, very likely due to high voltage that had to be applied to low saline stormwater to achieve the required current density. In conclusion, ECO was found to be a very promising low energy disinfection technology for stormwater, but far more work is needed to optimise the technology for unique stormwater conditions.

Predictive Power of Clean Bed Filtration Theory for Fecal Indicator Bacteria Removal in Stormwater Biofilters

Green infrastructure (also referred to as low impact development, or LID) has the potential to transform urban stormwater runoff from an environmental threat to a valuable water resource. In this paper we focus on the removal of fecal indicator bacteria (FIB, a pollutant responsible for runoff-associated inland and coastal beach closures) in stormwater biofilters (a common type of green infrastructure). Drawing on a combination of previously published and new laboratory studies of FIB removal in biofilters, we find that 66% of the variance in FIB removal rates can be explained by clean bed filtration theory (CBFT, 31%), antecedent dry period (14%), study effect (8%), biofilter age (7%), and the presence or absence of shrubs (6%). Our analysis suggests that, with the exception of shrubs, plants affect FIB removal indirectly by changing the infiltration rate, not directly by changing the FIB removal mechanisms or altering filtration rates in ways not already accounted for by CBFT. The analysis presented here represents a significant step forward in our understanding of how physicochemical theories (such as CBFT) can be melded with hydrology, engineering design, and ecology to improve the water quality benefits of green infrastructure.

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.

Assessment of metal retention in newly constructed highway embankments

Newly constructed embankments should provide both a specific bearing capacity to enable trafficability in emergency cases and a sufficient pollutant retention capacity to protect the groundwater. A number of lysimeters were installed along the A115 highway to determine total and dissolved metal concentrations in road runoff and in the soil solution of newly constructed embankments. Dissolved concentrations in soil solution of the embankments did not exceed the trigger values of the German legislation. Depending on the metal, total concentrations in soil solution were more than twice as high as dissolved concentrations. The high infiltration rates lead to increased groundwater recharge beneath the embankments (up to 4100 mm a^-1). Although metal concentrations were not problematic from the legislators' point of view, the elevated infiltration rates beside the road facilitated the transfer of high metal loads into deeper soil layers and potentially into the groundwater as well.

Highly recoverable TiO2-GO nanocomposites for stormwater disinfection

A highly recoverable titanium dioxide-graphene oxide (TiO2-GO) composite was developed by a facile method of ultrasonic treatment of GO nanosheets and TiO2 nanoparticles, which should overcome the separation problem of nanosized TiO2 from treated water. Separability of the prepared samples was systematically investigated by gravity settling experiments. The samples' photocatalytic activity for stormwater disinfection was also studied under the irradiation of a solar simulator. The results demonstrated that TiO2-GO showed high efficient separability due to its accelerated settling behaviour. Zeta-potential analysis showed that the accelerated sedimentation of the catalyst was attributed to the aggregation of TiO2-GO resulting from the electrostatic attraction between TiO2 and GO. The TiO2-GO composite with a mass ratio of 100:2 (TiO2-2%GO) achieved both higher separability and good photocatalytic activity for stormwater disinfection. Its suspension became clear (turbidity < 50 NTU) after 8 h of sedimentation, while 99.5% of E.coli were deactivated in 90 min. The TiO2-GO composite exhibited excellent durability; no apparent change in the separability of TiO2-2%GO was observed after 10 treatment cycles (15 h in total), while only slight decrease in the photocatalytic activity was noted. In conclusion, the developed TiO2-GO composite showed promising results for stormwater disinfection.

Validation of stormwater biofilters using in-situ columns

Stormwater harvesting biofilters need to be validated if the treatment is to be relied upon. Currently, full-scale challenge tests (FCTs), performed in the field, are required for their validation. This is impractical for stormwater biofilters because of their size and flow capacity. Hence, for these natural treatment systems, new tools are required as alternatives to FCT. This study describes a novel in-situ method that consists of a thin stainless steel column which can be inserted into constructed biofilters in a non-destructive manner. The in-situ columns (ISCs) were tested using a controlled field-scale biofilter where FCT is possible. Fluorescein was initially used for testing through a series of continuous applications. The results from the ISC were compared to FCT conducted under similar operational conditions. Excellent agreement was obtained for the series of continuous fluorescein experiments, demonstrating that the ISC was able to reproduce FCT results even after extended drying periods (Nash-Sutcliffe coefficient between the two data sets was 0.83-0.88), with similar plateaus, flush peaks, slopes and treatment capacities. The ISCs were then tested for three herbicides: atrazine, simazine and prometryn. While the ISC herbicide data and the FCT data typically matched well, some differences observed were linked to the different climatic conditions during the ISC (winter) and FCT tests (summer). The work showed that ISC is a promising tool to study the field performance of biofilters and could be a potential alternative to full scale challenge tests for validation of stormwater biofilters when taking into account the same inherent boundary conditions.

Effect of weathering on mobilization of biochar particles and bacterial removal in a stormwater biofilter

To improve bacterial removal, a traditional stormwater biofilter can be augmented with biochar, but it is unknown whether bacterial removal remains consistent as the biochar weathers during intermittent exposure to stormwater under dry-wet and freeze-thaw cycles. To examine the effect of weathering on bacterial removal capacity of biochar, we subjected biochar-augmented sand filters (or simplified biofilters) to multiple freeze-thaw or dry-wet cycles for a month and then compared their bacterial removal capacity with the removal capacity of unweathered biofilters. To isolate the effect of physical and chemical weathering processes from that of biological processes, the biofilters were operated without any developed biofilm. Biochar particles were mobilized during intermittent infiltration of stormwater, but the mobilization depended on temperature and antecedent conditions. During stormwater infiltration without intermediate drying, exposure to natural organic matter (NOM) in the stormwater decreased the bacterial removal capacity of biochar, partly due to exhaustion of attachment sites by NOM adsorption. In contrast, exposure to the same amount of NOM during stormwater infiltration with intermediate drying resulted in higher bacterial removal. This result suggests that dry-wet cycles may enhance recovery of the previously exhausted attachment sites, possibly due to diffusion of NOM from biochar surfaces into intraparticle pores during intermediate drying periods. Overall, these results indicate that physical weathering has net positive effect on bacterial removal by biochar-augmented biofilters.

Salt tolerant plants increase nitrogen removal from biofiltration systems affected by saline stormwater

Biofiltration systems are used in urban areas to reduce the concentration and load of nutrient pollutants and heavy metals entering waterways through stormwater runoff. Biofilters can, however be exposed to salt water, through intrusion of seawater in coastal areas which could decrease their ability to intercept and retain pollutants. We measured the effect of adding saline stormwater on pollutant removal by six monocotyledonous species with different levels of salt-tolerance. Carex appressa, Carex bichenoviana, Ficinia nodosa, Gahnia filum, Juncus kraussii and Juncus usitatus were exposed to six concentrations of saline stormwater, equivalent to electrical conductivity readings of: 0.09, 2.3, 5.5, 10.4, 20.0 and 37.6 mS cm(-1). Salt-sensitive species: C. appressa, C. bichenoviana and J. usitatus did not survive ≥10.4 mS cm(-1), removing their ability to take up nitrogen (N). Salt-tolerant species, such as F. nodosa and J. kraussii, maintained N-removal even at the highest salt concentration. However, their levels of water stress and stomatal conductance suggest that N-removal would not be sustained at concentrations ≥10.4 mS cm(-1). Increasing salt concentration indirectly increased phosphorus (P) removal, by converting dissolved forms of P to particulate forms which were retained by filter media. Salt concentrations ≥10 mS cm(-1) also reduced removal efficiency of zinc, manganese and cadmium, but increased removal of iron and lead, regardless of plant species. Our results suggest that biofiltration systems exposed to saline stormwater ≤10 mS cm(-1) can only maintain N-removal when planted with salt-tolerant species, while P removal and immobilisation of heavy metals is less affected by species selection.

Stabilization of Stormwater Biofilters: Impacts of Wetting and Drying Phases and the Addition of Organic Matter to Filter Media

Ripening period refers to a phase of stabilization in sand filters in water treatment systems that follow a new installation or cleaning of the filter. Intermittent wetting and drying, a unique property of stormwater biofilters, would similarly be subjected to a phase of stabilization. Suspended solids are an important parameter that is often used to monitor the stabilization of sand filters in water treatment systems. Stormwater biofilters, however, contain organic material that is added to the filter layer to enhance nitrate removal, the dynamics of which is seldom analyzed in stabilization of stormwater biofilters. Therefore, in this study of stormwater biofiltration in addition to suspended solids (turbidity), organic matter (TOC, DOC, TN, and TKN) was also monitored as a parameter for stabilization of the stormwater biofilter. One Perspex bioretention column (94 mm internal diameter) was fabricated with filter layer that contained 8 % organic material and fed with tapwater with different antecedent dry days (0-40 day) at 100 mL/min. Samples were collected from the outflow at different time intervals between 2 and 150 min and were tested for total organic carbon, dissolved organic carbon, total nitrogen, total Kjeldhal nitrogen, and turbidity. The column was observed to experience two phases of stabilization, one at the beginning of each event that lasted for 30 min, while the other phase was observed across subsequent events that are related to the age of filter.

Comparison of filter media materials for heavy metal removal from urban stormwater runoff using biofiltration systems

The filter media in biofiltration systems play an important role in removing potentially harmful pollutants from urban stormwater runoff. This study compares the heavy metal removal potential (Cu, Zn, Cd, Pb) of five materials (potting soil, compost, coconut coir, sludge and a commercial mix) using laboratory columns. Total/dissolved organic carbon (TOC/DOC) was also analysed because some of the test materials had high carbon content which affects heavy metal uptake/release. Potting soil and the commercial mix offered the best metal uptake when dosed with low (Cu: 44.78 μg/L, Zn: 436.4 μg/L, Cd, 1.82 μg/L, Pb: 51.32 μg/L) and high concentrations of heavy metals (Cu: 241 μg/L, Zn: 1127 μg/L, Cd: 4.57 μg/L, Pb: 90.25 μg/L). Compost and sludge also had high removal efficiencies (>90%). Heavy metal leaching from these materials was negligible. A one-month dry period between dosing experiments did not affect metal removal efficiencies. TOC concentrations from all materials increased after the dry period. Heavy metal removal was not affected by filter media depth (600 mm vs. 300 mm). Heavy metals tended to accumulate at the upper 5 cm of the filter media although potting soil showed bottom-enriched concentrations. We recommend using potting soil as the principal media mixed with compost or sludge since these materials perform well and are readily available. The use of renewable materials commonly found in Singapore supports a sustainable approach to urban water management.

A review of ion and metal pollutants in urban green water infrastructures

In urban environments, the breakdown of chemicals and pollutants, especially ions and metal compounds, can be favoured by green water infrastructures (GWIs). The overall aim of this review is to set the basis to model GWIs using deterministic approaches in contrast to empirical ones. If a better picture of chemicals and pollutant input and an improved understanding of hydrological and biogeochemical processes affecting these pollutants were known, GWIs could be designed to efficiently retain these pollutants for site-specific meteorological patterns and pollutant load. To this end, we surveyed the existing literature to retrieve a comprehensive dataset of anions and cations, and alkaline and transition metal pollutants incoming to urban environments. Based on this survey, we assessed the pollution load and ecological risk indexes for metals. The existing literature was then surveyed to review the metal retention efficiency of GWIs, and possible biogeochemical processes related to inorganic metal compounds were proposed that could be integrated in biogeochemical models of GWIs.

Biofilters for stormwater harvesting: understanding the treatment performance of key metals that pose a risk for water use

A large-scale stormwater biofilter column study was conducted to evaluate the impact of design configurations and operating conditions on metal removal for stormwater harvesting and protection of aquatic ecosystems. The following factors were tested over 8 months of operation: vegetation selection (plant species), filter media type, filter media depth, inflow volume (loading rate), and inflow pollutant concentrations. Operational time was also integrated to evaluate treatment performance over time. Vegetation and filter type were found to be significant factors for treatment of metals. A larger filter media depth resulted in increased outflow concentrations of iron, aluminum, chromium, zinc, and lead, likely due to leaching and mobilization of metals within the media. Treatment of all metals except aluminum and iron was generally satisfactory with respect to drinking water quality standards, while all metals met standards for irrigation. However, it was shown that biofilters could be optimized for removal of iron to meet the required drinking water standards. Biofilters were generally shown to be resilient to variations in operating conditions and demonstrated satisfactory removal of metals for stormwater-harvesting purposes.

Dissolved nutrients and atrazine removal by column-scale monophasic and biphasic rain garden model systems

Rain gardens are bioretention systems that have the potential to reduce peak runoff flow and improve water quality in a natural and aesthetically pleasing manner. We compared hydraulic performance and removal efficiencies of nutrients and atrazine in a monophasic rain garden design versus a biphasic design at a column-scale using simulated runoff. The biphasic rain garden was designed to increase retention time and removal efficiency of runoff pollutants by creating a sequence of water saturated to unsaturated conditions. We also evaluated the effect of C substrate availability on pollutant removal efficiency in the biphasic rain garden. Five simulated runoff events with various concentrations of runoff pollutants (i.e. nitrate, phosphate, and atrazine) were applied to the monophasic and biphasic rain gardens once every 5d. Hydraulic performance was consistent over the five simulated runoff events. Peak flow was reduced by approximately 56% for the monophasic design and 80% for the biphasic design. Both rain garden systems showed excellent removal efficiency of phosphate (89-100%) and atrazine (84-100%). However, significantly (p<0.001) higher removal of nitrate was observed in the biphasic (42-63%) compared to the monophasic rain garden (29-39%). Addition of C substrate in the form of glucose increased removal efficiency of nitrate significantly (p<0.001), achieving up to 87% removal at a treatment C/N ratio of 2.0. This study demonstrates the importance of retention time, environmental conditions (i.e. saturated/unsaturated conditions), and availability of C substrate for bioremediation of pollutants, especially nitrates, in rain gardens.