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

Resilience of stormwater biofilters following the deposition of wildfire residues: Implication on downstream water quality management in wildfire-prone regions

Stormwater treatment systems such as biofilters could intercept and remove pollutants from contaminated runoff in wildfire-affected areas, ensuring the protection of water quality downstream. However, the deposition of wildfire residues such as ash and black carbon onto biofilters could potentially impair their stormwater treatment functions. Yet, whether and how wildfire residue deposition could affect biofilter functions is unknown. This study examines the impact of wildfire residue deposition on biofilter infiltration and pollutant removal capacities. Exposure to wildfire residues decreased the infiltration capacity based on the amount of wildfire deposited. Wildfire residues accumulated at the top layer of the biofilter, forming a cake layer, but scraping this layer restored the infiltration capacity. While the deposition of wildfire residues slightly changed the pore water geochemistry, it did not significantly alter the removal of metals and E. coli. Although wildfire residues leached some metals into pore water within the simulated root zone, the leached metals were effectively removed by the compost present in the filter media. Collectively, these results indicate that biofilters downstream of wildfire-prone areas could remain resilient or functional and protect downstream water quality if deposited ash is periodically scraped to restore any loss of infiltration capacity following wildfire residue deposition.

Do drinking water treatment residuals underperform in the presence of compost in stormwater media filters?

Drinking water treatment residuals (WTR), a waste-derived product, are often recommended to use as an amendment in stormwater biofilters to enhance their capacity to remove phosphate and microbial pollutants. However, their efficacy has been assumed to remain high in the presence of compost, one of the most common amendments used in biofilters. This study tests the validity of that assumption by comparing the removal capacities of WTR-amended biofilters with and without the presence of compost. Our results show that amending sand with WTR increased E. coli removal by at least 1-log, but the addition of compost in the sand-WTR media lowered the removal capacity by 13 %. Similarly, the addition of WTR to sand improved phosphate removal to nearly 1177 %, but the removal decreased slightly by 8 % when adding compost to the media. The results confirmed that dissolved organic carbon (DOC) leached from the compost could compete for adsorption sites for bacteria and phosphate, thereby lowering WTR's adsorption capacity based on the amount of DOC adsorbed on WTR. Collectively, these results indicate that the stormwater treatment industry should avoid mixing compost with WTR to get the maximum benefits of WTR for bacterial removal and improve the performance lifetime of WTR-amended biofilters.

Reduction in pollution load to an urban estuary using a sustainable drainage system treatment train

Rapid urbanisation and industrialisation have placed increased pressure on the ecosystem health of urban estuaries. Sustainable drainage systems (SuDS) are globally accepted practices for managing the water quality of stormwater and effluent discharged into urban systems. The Swartkops Estuary in South Africa is a heavily urbanized estuary that has a long history of pollution, specifically trace metal contamination, originating from industrial sources and urban wastewater. Using a novel SuDS treatment train, the physical characteristics (total suspended solids), macronutrients (orthophosphates, nitrate, ammonium), trace metals (As, Cd, Hg, Fe, Pb, Cu), and E. coli concentrations were measured monthly for one year, both before and after the treatment train. The treatment train consisted of five interconnected 500 L plastic tanks for sedimentation, filtration (sand and stone), biodegradation and floating wetlands. Results indicate that the SuDS treatment train provided an efficient method in reducing the pollution load to this urban estuary, by reducing macronutrient concentrations by 76 %, trace elements concentrations by 74 % and faecal bacteria counts (E. coli) by 80 %.

Heavy metals in vegetables: a review of status, human health concerns, and management options

For sustainable global growth, food security is a prime concern issue, both quantitatively and qualitatively. Adverse effects on crop quality from contaminants like heavy metals have affected food security and human health. Vegetables comprise the essential and nutritious part of the human diet as they contain a lot of health-promoting minerals and vitamins. However, the inadvertent excess accumulation of heavy metals (As, Cd, Hg, and Pb) in vegetables and their subsequent intake by humans may affect their physiology and metabolomics and has been associated with diseases like cancer, mental retardation, and immunosuppression. Many known sources of hazardous metals are volcano eruptions, soil erosion, use of chemical fertilizers in agriculture, the use of pesticides and herbicides, and irrigation with wastewater, industrial effluents, etc. that contaminate the vegetables through the soil, air and water. In this review, the problem of heavy metal contamination in vegetables is discussed along with the prospective management strategies like soil amendments, application of bioadsorbents, membrane filtration, bioremediation, and nanoremediation.

Optimisation of urban-rural nature-based solutions for integrated catchment water management

Nature-based solutions (NBS) have co-benefits for water availability, water quality, and flood management. However, searching for optimal integrated urban-rural NBS planning to maximise co-benefits at a catchment scale is still limited by fragmented evaluation. This study develops an integrated urban-rural NBS planning optimisation framework based on the CatchWat-SD model, which is developed to simulate a multi-catchment integrated water cycle in the Norfolk region, UK. Three rural (runoff attenuation features, regenerative farming, floodplain) and two urban (urban green space, constructed wastewater wetlands) NBS interventions are integrated into the model at a range of implementation scales. A many-objective optimisation problem with seven water management objectives to account for flow, quality and cost indicators is formulated, and the NSGAII algorithm is adopted to search for optimal NBS portfolios. Results show that rural NBS have more significant impacts across the catchment, which increase with the scale of implementation. Integrated urban-rural NBS planning can improve water availability, water quality, and flood management simultaneously, though trade-offs exist between different objectives. Runoff attenuation features and floodplains provide the greatest benefits for water availability. Regenerative farming is most effective for water quality and flood management, though it decreases water availability by up to 15% because it retains more water in the soil. Phosphorus levels are best reduced by expansion of urban green space to decrease loading on combined sewer systems, though this trades off against water availability, flood, nitrogen and suspended solids. The proposed framework enables spatial prioritisation of NBS, which may ultimately guide multi-stakeholder decision-making, bridging the urban-rural divide in catchment water management.

Submerged zone and vegetation drive distribution of heavy metal fractions and microbial community structure: Insights into stormwater biofiltration system

Biofiltration system is a widely used stormwater treatment option that is effective in removing heavy metals. The concentration and distribution of heavy metal fractions in biofiltration filter media, as well as the microbiota composition affected by the design parameters, are relatively novel concepts that require further research. A laboratory-scale column study was conducted to investigate the microbial community and the fractionation of heavy metals (Pb, Cu, Cr, and Cd) extracted from filter media samples, subjected to the presence of vegetation, submerged zone (SZ), and major environmental parameters (pH, water content). Sequential extractions revealed that, compared to the three other fractions (exchangeable fraction, reducible fraction, and oxidizable fraction), the residual fraction was the most represented for each metal (41 - 82 %). As a result, vegetation was found to reduce pH value, and significantly decrease the concentration of the exchangeable fraction of Pb in the middle layer, and the oxidizable fraction of Pb, Cu, Cd, and Cr in the middle and bottom layers (p < 0.05). The formation of an anoxic environment by submerged zone settlements resulted in a significant decrease in the concentration of reducible fractions and a significant increase in the concentration of oxidizable fractions for four heavy metals (p < 0.05). In addition, the analysis of the microbiota showed that the diversity and richness of microorganisms increased in the presence of SZ and plants. The dominant phylum in biofiltration was Proteobacteria, followed by Firmicutes, Bacteroidetes, Acidobacteria, and Actinobacteria as major phyla. Heavy metal fractions could regulate the structure of microbial communities in biofiltration. The findings of this study would enrich our understanding of the improvement of multi-metal-contaminated runoff treatment and highlight the impact of design parameters and heavy metal fractionation on microbial community structure in the biofiltration system.

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.

Sponge City Practices in China: From Pilot Exploration to Systemic Demonstration

In recent years, China has been committed to strengthening environmental governance and trying to build a sustainable society in which humans and nature develop in harmony. As a new urban construction concept, sponge city uses natural and ecological methods to retain rainwater, alleviate flooding problems, reduce the damage to the water environment, and gradually restore the hydrological balance of the construction area. The paper presents a review of sponge city construction from its inception to systematic demonstration. In this paper, research gaps are discussed and future efforts are proposed. The main contents include: (1) China’s sponge city construction includes but is not limited to source control or a drainage system design. Sponge city embodies foreign experience and the wisdom of ancient Chinese philosophy. The core of sponge city construction is to combine various specific technologies to alleviate urban water problems such as flooding, water environment pollution, shortage of water resources and deterioration of water ecology; (2) this paper also introduces the sponge city pilot projects in China, and summarizes the achievements obtained and lessons learned, which are valuable for future sponge city implementation; (3) the objectives, corresponding indicators, key contents and needs of sponge city construction at various scales are different. The work at the facility level is dedicated to alleviating urban water problems through reasonable facility scale and layout, while the work at the plot level is mainly to improve the living environment through sponge city construction. The construction of urban and watershed scales is more inclined to ecological restoration and blue-green storage spaces construction. Besides, the paper also describes the due obligations in sponge city construction of various stakeholders.

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.

Biochar-based bioretention systems for removal of chemical and microbial pollutants from stormwater: A critical review

Biochar has been increasingly used as a filter medium in engineered low impact development systems (e.g., bioretention systems) for decontamination of urban stormwater and management of hydrology. This review paper critically analyzes the performance of biochar-based biofiltration systems for removal of chemical and microbial pollutants present in urban runoff. Biochar-amended biofiltration systems efficiently remove diverse pollutants such as total nitrogen (32 - 61%), total phosphorus: (45 - 94%), heavy metals (27 - 100%), organics (54 - 100%) and microbial pollutants (log₁₀ removal: 0.78 - 4.23) from urban runoff. The variation of biofiltration performance is due to changes in biochar characteristics, the abundance of dissolved organic matter and/or stormwater chemistry. The dominant mechanisms responsible for removal of chemical pollutants are sorption, ion exchange and/or biotransformation, whereas filtration/straining is the major mechanism for bacteria removal. The pseudo-second order and Langmuir isotherm are the best models that describe the kinetics and chemical equilibrium of pollutants, respectively. This critical review provides the fundamental scientific knowledge for designing highly efficient biochar-based bioretention systems for removal of diverse pollutants from urban stormwater. The key knowledge gaps that should be addressed in future research include long-term field-scale bioretention study, development of novel methods for filter media regeneration/reuse, and dynamics of filter media microbial communities.

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.

Performance of commercially available soil amendments for enhanced Cu attenuation in bioretention media

Bioretention structures such as planter boxes, swales and rain gardens are being increasingly utilized in built landscapes as a strategy to attenuate both stormwater flows and contaminant loads. Copper (Cu) roofing materials contribute significantly higher mass loads of dissolved Cu per unit area than other surfaces such as parking lots and roadways. While a recent study demonstrated that conventional bioretention media can remove greater than 90% of Cu from copper roof runoff, the median Cu concentrations at the point of discharge from bioretention structures (66 μg L^-1) still did not achieve Cu concentrations in stormwater discharges sought in some jurisdictions (for example, < 14 μg L^-1). Consequently, commercially available soil amendments were assessed to improve bioretention Cu removal. The ability of biochar, greensand, and zeolite to improve Cu removal was evaluated in laboratory column studies. Additionally, the performance of zeolite as an underlayer amendment was evaluated in bioretention planter boxes treating stormwater from a picnic shelter with a partitioned copper roof. Cu was measured in the planter box influent and effluent. The field setup included 2 control planter boxes containing only standard bioretention media and 2 amended with the zeolite underlayer. Samples from ten storms were collected with flow-weighted composite sampling. Total Cu in composite samples of the influent waters ranged from 445 to 1683 μg L^-1 and had a median concentration of 934 μg L^-1. Total Cu in the effluent from the control planter boxes ranged from 10 to 64 μg L^-1, with a mean of 29 μg L^-1. Total Cu in effluent from the zeolite amended planter boxes ranged from 4 to 44 μg L^-1 with a mean of 18 μg L^-1. Attenuation in the control planter boxes ranged from 90 to 99% with a median of 93.4% by concentration and ranged from 95 to 99% with a median of 97.5% in the zeolite amended planter boxes.

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).

Modeling of the Suspended Solid Removal of a Granular Media Layer in an Upflow Stormwater Runoff Filtration System

Upflow granular media filtration devices are widely used for stormwater runoff treatment. However, the system performance is not well characterized due to the irregular removal of suspended solid (SS) in the pretreatment (sedimentation) chamber and, hence, its irregular input to the media layer. In this regard, the performance of the granular media layer of an upflow filtration system is investigated herein by the use of various models. Due to the significant variation in the SS concentration of the influent and effluent to and from the media layer, the deep bed filtration model, the k-C* model, and the porous media capture model provide limited descriptions of the system performance. By contrast, the performance is well described using the kinetic model, the modified k-C* model using a specific deposit, and the modified porous media capture model using a specific deposit. The parameters of the latter models are shown to be in good correlation with the filtration velocity, SS removal, and specific deposit. The results suggest that modeling using a specific SS deposit can provide an accurate description of the granular media layer performance under a highly variable influent SS concentration.

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.

Evaluation of solids removal and optimisation of backwashing for an upflow stormwater filtration system utilising novel floating fibrous media

Although filtration devices are already widely used for stormwater runoff treatment, there are much to be improved to ensure the required performance. Additionally, the performance of a device should be verified before on-site installation. In this context, an upflow filtration system using novel high porosity floating fibrous media formed into spherical shape was proposed and evaluated for solid capture and backwashing. At filtration velocities of 20-40 m/h, the maximum head loss was about 2 cm even under a solid load of 30 kg/m², and suspended solid (SS) removal efficiency was >96% throughout 300 min. A considerable amount of SS was removed in the pretreatment chamber, so the load on the media was reduced. Several models were tried to describe the solid capture in the media. The coefficients of solid attachment/detachment showed good correlations with filtration velocity. Other parameters indicated a variation of solid capture and permeability, which is unique to the media in this study. The backwashing with air and water for 1-2 min each showed good head loss recovery under the SS load up to 550-600 kg/m², and the SS discharge was more efficient when the stagnant water was drained before water backwashing. The results in this study suggest the high potential of the combination of fibrous media and upflow filtration system for the efficient control of the nonpoint source pollutants in stormwater runoff.

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

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

The Common Approaches of Nitrogen Removal in Bioretention System

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

Evaluating the relationships between specific drainage area characteristics and soil metal concentrations in long-established bioswales receiving suburban stormwater runoff

Bioswales are used to attenuate stormwater pollution, but their long-term sustainability regarding sequestered metals is relatively unknown, and a clear rationale for prioritizing soil management is lacking. Impervious areas draining into four 14-year-old suburban bioswales were delineated, for which surface soils (top 10 cm; 72 samples) were sampled; soils from 4 adjacent reference sites were also sampled. Total and water soluble metals (Cd, Cu, Pb, Zn) were quantified, and the relationships between metal concentrations and drainage area characteristics evaluated. Annual metal loads were estimated using regional runoff data to simulate current and future metal concentrations; risks to soil biota were assessed by comparing metal concentrations to ecological screening levels. The drainage areas' percent imperviousness (37-71%) and ratios of impervious drainage area to bioswale area (2.0-5.7) varied, owing to differing proportions of rooftops, paved surfaces, lawns, and natural soils. Total Cu and Zn ranged from 10.0 to 43.2 mg/kg dry soil, and 15.6 to 129.5 mg/kg dry soil, respectively. Across all bioswales, total Zn was positively correlated to percent impervious area (r = 0.32, p = 0.0073), the ratio of connected impervious drainage area to infiltration area (r = 0.32, p = 0.0073), and percent drainage area as paved surfaces (r = 0.46, p = 5.6 E-05), but negatively correlated to percent drainage area as lawns (r = -0.48; p = 2.4 E-05). Water soluble metal concentrations were orders of magnitude lower than total metals. Given annual metal loads (0.2-0.4 mg Cu/kg dry soil; 1.5-3.1 mg Zn/kg dry soil), replacing bioswale soils to constrain metal concentrations would be unnecessary for decades. Taken together, this study proposes a transferable approach of estimating, then verifying via sampling and analysis, bioswale soil metal concentrations, such that soil management decisions can be benchmarked to ecological screening levels.

Enhanced removal of heavy metals and phosphate in stormwater filtration systems amended with drinking water treatment residual-based granules

To address the clogging issues in stormwater filtration systems, a drinking water treatment residual (DWTR)-based granule (DBG) substrate was developed herein by pyrolyzing and granulating the DWTR with bentonite and corncob. Toxicity characteristic leaching procedure studies indicated that fabricating into DBG stabilized the Al and heavy metals in DWTR and restrained the leaching risk. Then the removal performance of phosphate (PO₄-P) and heavy metal ions by the DWTR and DBG was evaluated in batch and laboratory-scale column experiments. Results from batch tests showed that the amount of Pb(Ⅱ) adsorbed by DBG (18.47 ± 0.56 mg g⁻¹) was approximately 2.3 times of that adsorbed by DWTR (8.05 ± 0.19 mg g⁻¹), whereas the PO₄-P adsorption capacity of DBG (8.63 ± 0.24 mg g⁻¹) was much lower than that of DWTR (25.33 ± 0.81 mg g⁻¹). This could be ascribed to the addition of corncob and bentonite (at a mass ratio of 20% and 40% in DBG, respectively), which provided extremely high cation exchange capacity for the Pb(Ⅱ) adsorption, while no effective PO₄-P adsorption component was involved. Moreover, the pyrolysis process could improve the Pb(Ⅱ) and PO₄-P adsorption capacity of the raw-mixture by 42% and 7%, whereas granulation process decreased those of the pyrolysis-mixture by 15% and 20%, respectively, owing to the reduction of accessible surface area in the DBG. Under various stormwater runoff conditions, the involvement of DBG in stormwater filtration systems exerted consistently fancy performance of Cu(Ⅱ), Pb(Ⅱ), Cd(Ⅱ) and PO₄-P removal, with average removal rates of over 86.20% and desorption rates of less than 3.50%, indicating irreversible and strong complexion between the contaminants and DBG. The DBG column manifested good permeability and stable hydraulic conductivity (2.74-2.52 m d⁻¹) over a 54-day rainfall period, which was beneficial to address the clogging issue of DWTR. Overall, this study provides an alternative pathway to enhance the hydraulic condition and treatment performance of the stormwater filtration systems for urban runoff management.

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.

Biological-based methods for the removal of volatile organic compounds (VOCs) and heavy metals

The current scenario of increased population and industrial advancement leads to the spoliation of freshwater and tapper of the quality of water. These results decrease in freshwater bodies near all of the areas. Besides, organic and inorganic compounds discharged from different sources into the available natural water bodies are the cause of pollution. The occurrence of heavy metals in water and volatile organic compounds (VOCs) in the air is responsible for a vast range of negative impacts on the atmosphere and human health. Nonetheless, high uses of heavy metals for human purposes may alter the biochemical and geochemical equilibrium. The major air contaminants which are released into the surroundings known as VOCs are produced through different kinds of sources, such as petrochemical and pharmaceutical industries. VOCs are known to cause various health hazards. VOCs are a pivotal group of chemicals that evaporate readily at room temperature. To get over this problem, biofiltration technology has been evolved for the treatment of heavy metals using biological entities such as plants, algae, fungi, and bacteria. Biofiltration technology is a beneficial and sustainable method for the elimination of toxic pollutants from the aquatic environment. Various types of biological technologies ranging from biotrickling filters to biofilters have been developed and they are cost-effective, simple to fabricate, and easy to perform. A significant advantage of this process is the pollutant that is transformed into biodegradable trashes which can decompose within an average time period, thus yielding no secondary pollutants. The aim of this article is to scrutinize the role of biofiltration in the removal of heavy metals in wastewater and VOCs and also to analyze the recent bioremediation technologies and methods.

Combined dual-size foam glass media filtration process with micro-flocculation for simultaneous removal of particulate and dissolved contaminants in urban road runoff

In this study, a combined media filtration process with micro-flocculation (CMF) was developed, to simultaneously treat particulate and dissolved contaminants in urban road runoff. Dual-size foam glass media with stone and sand layers were applied and the efficiency of road runoff treatment was investigated according to filtration and micro-flocculation under various experimental conditions (stone/sand layer ratio, linear velocity, and coagulant types). Moreover, the removal efficiencies of suspended solids (SS), phosphorus, organic carbon, and heavy metals (Zn, Cu, Pb, Cd) by CMF were evaluated. The removal rate of SS was maintained to be above 84.1% for 1 h filtration by the dual-size foam glass, regardless of increasing pressure. The removal of phosphorus by micro-flocculation was more suitable in alum than ferric due to a higher initial floc growth rate and an increased particle size. The performance of the CMF was significantly improved over media filtration only process (MF) in removing both particulate and dissolved contaminants. The removal efficiency of all particulate pollutants by CMF was found to be more than 90%, and notably, the dissolved phosphorus, which was mostly not removed by MF, was also removed by 97.4%. Meanwhile, the backwash efficiency of CMF was half that of MF. Physical removal mechanisms, such as internal diffusion, dominated MF, whereas chemical removal mechanisms, such as adsorption and surface precipitation, dominated CMF. These results show the potential of the CMF process for the treatment of urban road runoff and identify the removal mechanisms of the filtration process that use micro-flocculation with dual-size foam glass.

Metals removal from automobile workshop stormwater runoff using rice husk, GAC and gravel filtration

The efficiency of combined filtration media consisting of rice husk (RH), granular activated carbon (GAC) and gravel (GR) for the removal of metals cadmium, copper, lead and iron from stormwater runoff emanating from automobile workshops in Nigeria was investigated. Stormwater runoff samples were collected from five sites over a period of nine (9) weeks and filtered using two filter combinations, GAC - RH, GR - GAC as well as a RH-only filter. All the filters removed metals. Highest average singular metals removals were: approximately 74% copper for GR - GAC; 70% lead for RH, 67% iron for GAC - RH and 46% cadmium for GAC - RH. Average metals removal efficiencies (all metals combined) were GAC - RH 61%, GR - GAC 52% and RH-only 46%. The combined filter materials therefore showed better metals removal efficiencies than the RH-only filter. Further filtration of metals polluted stormwater would be required to lower the average metals concentration to meet local and international discharge standards. Future research into low cost modifications towards optimising the filter materials to improve metals removal efficiencies is recommended.

Sustainable urban drainage systems in established city developments: Modelling the potential for CSO reduction and river impact mitigation

Sustainable urban drainage systems (SUDS) can significantly reduce runoff from urban areas. However, their potential to mitigate acute river impacts of combined sewer overflows (CSO) is largely unknown. To close this gap, a novel coupled model approach was deployed that simulates the effect of realistic SUDS strategies, developed for an established city quarter, on acute oxygen depressions in the receiving river. Results show that for an average rainfall year the SUDS strategies reduce total runoff by 28%-39% and peak runoff by 31%-48%. Resulting relative reduction in total CSO volume ranges from 45%-58%, exceeding annual runoff reduction from SUDS by a factor of 1.5. Negative impacts in the form of fish-critical dissolved oxygen (DO) conditions in the receiving river (<2 mg DO L^-1) can be completely prevented with the SUDS strategies for an average rainfall year. The realistic SUDS strategies were compared with a simpler simulation approach which consists in globally downscaling runoff from all impervious areas. It indicates that such a simple approach does not completely account for the positive effect of SUDS, underestimating CSO volumes for specific rain events by up to 13%. Accordingly, global downscaling is only recommended for preliminary planning purposes.

Valorization of agro-industry residues in the building and environmental sector: A review

Environmental pollution has become a relevant issue as the population rises and resources decrease. Reuse and recycling still have the greatest potential as they turn the waste into a new resource, representing the 'closed-loop' step of a circular economy (CE). Looking for new applications for agro-industry waste represents both an environmental issue, as its incorrect disposal is a cause of pollution, and a chance to exploit zero-cost natural wastes. The present review, with around 200 articles examined, focuses on possible reuses of these residues in (a) building construction, as additives to produce thermal and acoustic insulation panels, and (b) in water treatments, exploited for removal of pollutants. The selected materials (coconut, coffee, corn, cotton and rice) have industry production wastes with suitable applications in both sectors and huge worldwide availability; their reuse may thus represent a new resource, with an impact based on the production rate and the possible replacement of current inorganic materials. Along with possible implementation of the selected materials in the building industry and environmental engineering, a brief description of the production and supply chain are provided.

Framework, Procedure, and Tools for Comprehensive Evaluation of Sustainable Stormwater Management: A Review

To better evaluate and enhance the performance and benefit of sustainable stormwater management (SSWM) in developing countries, this study proposes a comprehensive evaluation framework based on thorough literature review. This framework re-classifies evaluation goals and indicators into four aspects—stormwater system, integrated management, social engagement, and urban development. The purpose of this review is to provide a guideline for decision makers to choose appropriate goals and indicators according to different regional context. Meanwhile, a structured procedure for comprehensive evaluation of SSWM is proposed to guide a well-organised decision-making process. Furthermore, pros and cons of eight decision support tools, as well as their functional focus, are compared, aiming to provide references for SSWM in developing countries. Outcomes presented in this review are expected to support decision makers in the process of screening optimal SSWM strategies and monitoring SSWM projects.

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.

Environmental application of basic oxygen furnace slag for the removal of heavy metals from leachates

Industrial waste is a major environmental concern nowadays, stimulating the thorough study of the minimization and recycling of solid wastes and of the containment and treatment of liquid contaminants. Basic oxygen furnace (BOF) slag, a solid waste from the steel industry, has been found to be effective in the removal of heavy metals. However, this has not been applied so far in low permeability barriers, such as those used as bottom liners in landfills. This work studies the performance of BOF slag in both containment and treatment technologies for toxic leachates. Flow models are developed to assess the transport of metal ions through a permeable reactive barrier and a composite clay barrier. Reactive transport through the slag barrier and adsorption in the clay barrier are coupled for different conditions to find the residence time, the barrier life span and the optimum operative conditions. The results show that the use of BOF slag increases the breakthrough time of the contaminants, enabling improve design of low and high conductivity reactive barriers, and expands the life cycle of the material.

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

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

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

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

Role of metal modified water treatment residual on removal of Escherichia coli from stormwater runoff

Extensive studies have been conducted on bioretention filter media applied in best management practices for stormwater runoff treatment. To date, more reported studies are focused on pollutants elimination such as suspended solids and nutrients. There has been limited research on pathogen removal from stormwater runoff. More focused studies on pathogen removal are therefore required if the intended stormwater is harvested for indirect potable use. In this study, water treatment residuals (WTR), a recycled biofilter media was surface-modified with metals to assess its potential for E. coli removal from stormwater runoff. To achieve this goal, four types of modified WTRs, prepared using iron, copper, platinum, and silver as antibacterial agents, were tested in parallel batch tests. After the cost-effectiveness evaluation among the four modified WTRs for bacterial removal, Fe₂O₃- and CuO-WTRs were shortlisted for further mechanism and stability studies. Stable antibacterial performances (E. coli log removal of 0.58 ± 0.04 and 0.90 ± 0.04, respectively) were achieved using the Fe₂O₃- and CuO-WTRs under intermittent synthetic and natural stormwater runoff conditions. No significant metal leaching was observed over prolonged continuous treatment. The experimental results showed the bio-adsorption onto the surface modified Fe₂O₃- and CuO-WTR was a key mechanism for E. coli removal followed by E. coli inactivation at solid-liquid interface caused by the antibacterial effect of metal coatings (where CuO was reported to have higher biotoxicity than Fe₂O₃). These findings clearly suggested the potential of CuO-modified WTR for pathogen removal in stormwater treatment practices.

Removal of metals from mine drainage waters by in situ mineral sorbent-based pilot filter systems

Discharge of metal-containing wastewater streams into the environment is an environmental concern because these pollutants do not degrade and tend to bioaccumulate. A number of laboratory-based investigations on the effectiveness of a wide range of filter materials for metal removal from diluted wastewater streams have been reported. However, only a few pilot or full-scale investigations have been conducted. Therefore, this study investigated the metal retention capabilities of mineral-based filter materials (commercially available mineral product (5-15 mm), recycled mineral material (2-4 mm) and slag by-product (2-4 and 4-16 mm)) when used in pilot-scale filter systems under continuous operation in a closed mining area in North Ostrobothnia, Finland, between June and October 2017. The influence of material particle size on system function and on metal retention efficiency was also evaluated. The results revealed that system performance was dependent on material composition and particle size (smaller particle size being more effective). The highest metal removal efficiencies (Zn, Ni, Cd, Cu and Pb) and largest amount of water treated (per volume of material applied) were achieved by an aluminium oxide-based recycled mineral material (2-4 mm). While smaller-grained materials performed better in terms of removal efficiency, the removal rates achieved by coarser-grained, commercially available mineral product (5-15 mm) were comparable to those achieved by small-grained slag (2-4 mm). Full-scale systems using the recycled mineral product (2-4 mm) would have an approximately two-fold longer material replacement time than systems using the slag (2-4 mm). Replacement time for the larger-grained materials tested could not be determined, due to problems with freezing. Overall, the recycled mineral material tested can be recommended for full-scale tests, especially when high zinc removal rates are required.

Performance Assessment of a Laboratory Scale Prototype Biofiltration System in Tropical Region

Biofiltration systems, as one of the best management practices, have good potentials to improve stormwater quality and hydrology of urban catchments. While biofiltration systems are well-studied in developed countries, the majority of those studies are conducted for temperate climate and there is a lack of lab-scale and field-scale studies on such systems under tropical conditions. This paper focuses on the performance of a lab-scale prototype biofiltration systems in stormwater retention efficiency as well as pollutants removal (including heavy metals and nutrients) from synthetic stormwater reproducing tropical rainfall events. A three-layer sand-based filter media with two different native plants including Pedilanthus tithymaloides and Cyperus alternifolius was selected for this study. Results showed that the system with Cyperus has a better stormwater retention capacity compared to the one with Pedilanthus. In addition, the observed infiltration rate in Cyperus and Pedilanthus were 338 mm/h and 267 mm/h, respectively. The better hydraulic performance in the system with Cyperus was attributed to the deeper and more extensive root penetration of this plant (as deep as 800 mm) compared to Pedilanthus (as deep as 250 mm). While both systems failed to perform well in removing total nitrogen, they performed significantly better in removing total phosphorus (Cyperus and Pedilanthus removed 67.3% and 62.5% of total phosphorus, respectively). The statistical analysis of results showed that the top 100 mm layer of filter media is the main contributor to total phosphorus removal. However, no major differences were observed between the two systems in phosphorus removal. Moreover, both systems were also capable of removing the available heavy metals (i.e., Fe, Cu, Mn, Ni, Pb, and Zn) as the removal efficiencies exceeded 90%, except for Fe (76%). Similar to phosphorus, it was concluded that the top layer is the major contributor to the heavy metals removal. Overall, the biofiltration system using Cyperus was found to be a successful system for operating under tropical conditions.

Retention and transport processes of particulate and dissolved micropollutants in stormwater biofilters treating road runoff

Road runoff is contaminated by various micropollutants and may be treated using low impact development techniques, such as stormwater biofilters. Better understanding the processes, such as filtration, sorption and leaching, which affect pollutants in these systems is essential to reliably predicting treatment performance and optimizing system design. Field data from an in situ monitoring campaign, wherein dissolved and particulate concentrations of a wide range of micropollutants (trace metals, polycyclic aromatic hydrocarbons, bisphenol-A, alkylphenols and phthalates) were characterized in untreated road runoff and biofilter outlets for 19 rain events, are used to explore transport and retention processes. Although retention of the particulate phase of pollutants was generally quite effective, unusually high particle concentrations were observed at biofilter outlets for three winter events. Particle characterization in road runoff and outlet waters revealed that this degraded performance was due to poor filtration rather than particle erosion, which was attributed to the relative abundance of small (<10 μm) particles during this period, along with possible preferential flows. Dissolved pollutants were less effectively removed in general. To better understand this behavior, field results were combined with laboratory sorption and leaching tests. Dissolved concentrations of trace metals were shown to be influenced by organic carbon; leaching from road-originated particles may also influence their transport. Removal of the dissolved phase of organic micropollutants was limited by the contamination of the filter media, either before installation or during the first period of operation, due to emissions from construction materials.

Effect of a Submerged Zone and Carbon Source on Nutrient and Metal Removal for Stormwater by Bioretention Cells

A bioretention system is a low-impact and sustainable treatment facility for treating urban stormwater runoff. To meet or maintain a consistently satisfactory performance, especially in terms of increasing nitrogen removal efficiency, the introduction of a submerged (anoxic) zone (SZ) combined with a module-based carbon source (C) has been recommended. This study investigated the removal of nitrogen (N), phosphorus (P) and heavy metals with a retrofitted bioretention system. A significant (p < 0.05) removal enhancement of N as well as total phosphorus (TP) was observed, in the mesocosms with additions of exogenous carbon as opposed to those without such condition. However, even in the mesocosm with SZ alone (without exogenous C), TP removal showed significant enhancement. With regard to the effects of SZ depth on nutrient removal, the results showed that the removal of both N and P in module with a shallow SZ (200 mm) showed significant enhancement compared to that in module with a deep SZ (300 mm). Removal efficiencies greater than 93% were observed for all three heavy metals tested (Cu, Pb, and Zn) in all mesocosms, even in the bioretention module without an SZ or plants, and it indicated that adsorption by the filtration media itself is probably the most important removal mechanism. Only Cu (but not Pb or Zn) showed significantly enhanced removal in module with an SZ as compared to those without an SZ. Carbon source played a minor role in metal removal as no significant (p > 0.05) improvement was observed in module with C as compared to that without C. Based on these results, the incorporation of SZ with C in stormwater biofilters is recommended.

Reduction and Accumulative Characteristics of Dissolved Heavy Metals in Modified Bioretention Media

Twelve bioretention filter columns with different media were designed to study the effects of media on dissolved heavy metals in bioretention systems by changing three test conditions (inflow concentration, discharge ratio, and recurrence interval). The results showed that the average load reduction efficiency of the bioretention soil media (BSM)+10%water treatment residue, BSM+10%green zeolite, and BSM+10%medicinal stone for Cu and Zn was larger than 80%. The highest volume reduction efficiency is 39.25% by BSM+coconut bran. Among the three factors selected in tests, inflow concentration had the biggest degree of influence, followed by discharge ratio and recurrence interval. The media of the upper, middle, and lower layers of each filter column were detected before and after the treatment to study the accumulative characteristics of heavy metals in the bioretention system. The accumulation of Cu, Zn, and Cd in the media of BSM+medicinal stone, BSM+fly ash, BSM+vermiculite, and BSM+turfy soil was relatively low. The contents of the three metals were positively correlated with urease and negatively correlated with protease in the media, but no obvious rule was showed in the accumulation of dissolved heavy metals with depth.

Simultaneous Adsorption of Heavy Metals from Roadway Stormwater Runoff Using Different Filter Media in Column Studies

Stormwater runoff from roadways often contains a variety of contaminants such as heavy metals, which can adversely impact receiving waters. The filter media in stormwater filtration/infiltration systems play a significant role in the simultaneous removal of multiple pollutants. In this study, the capacity of five filter media—natural quartz sand (QS), sandy soil (SS) and three mineral-based technical filter media (TF-I, TF-II and TF-III)—to adsorb heavy metals (Cu, Pb and Zn) frequently detected in stormwater, as well as remobilization due to de-icing salt (NaCl), were evaluated in column experiments. The column breakthrough data were used to predict lifespan of the filter media. Column experiment operated under high hydraulic load showed that all technical filters and sandy soil achieved >97%, 94% and >80% of Pb, Cu and Zn load removals, respectively, while natural quartz sand (QS) showed very poor performance. Furthermore, treatment of synthetic stormwater by the soil and technical filter media met the requirements of the Austrian regulation regarding maximum effluent concentrations and minimum removal efficiencies for groundwater protection. The results showed that application of NaCl had only a minor impact on the remobilization of heavy metals from the soil and technical filter media, while the largest release of metals was observed from the QS column. Breakthrough analysis indicated that load removal efficiencies at column exhaustion (SS, TF-I, TF-II and TF-III) were >95% for Cu and Pb and 80–97% for Zn. Based on the adsorption capacities, filtration systems could be sized to 0.4 to 1% (TF-I, TF-II and TF-III) and 3.5% (SS) of their impervious catchment area and predicated lifespan of each filter media was at least 35, 36, 41 and 29 years for SS, TF-I, TF-II and TF-III, respectively. The findings of this study demonstrate that soil—based and technical filter media are effective in removing heavy metals and can be utilized in full-stormwater filtration systems.

Co-transport and remobilization of Cu and Pb in quartz column by carbon dots

Carbon nanoparticles such graphene, carbon nanotubes, and carbon dots offer the potential to improve environmental treatment technologies due to their unique properties such as low toxicity and high metal sorption capacity. However, there are no studies on facilitated transport and remobilization of pre-sorbed metals by carbon dot (CD) nanoparticles in quartz sand columns. Here, we investigate the effects of solution ionic strength (IS; 1, 100 and 200 mM NaCl) and pH (Chen et al., 2017; Chen et al., 2010; Cornell and Schwertmann, 2006), initial CD concentration (200, 400, 600 and 800 mg L^-1), and clay content (10, 20 and 30%w kaolinite) in quartz sand columns on the transport, retention and remobilization of Cu and Pb in saturated (upward flow) quartz porous media. Batch sorption experiments were employed to underpin the findings of the column transport experiments. Both CD and quartz adsorbed Cu and Pb from water, but adsorption was higher on CD than quartz. Co-transport experiment demonstrated the CD-facilitated transport of Cu and Pb. Sequential transport experiments (first three phases) demonstrated the retention of Cu and Pb in the quartz column, with higher retention of Pb compared to Cu. The Cu and Pb retention was attributed to their sorption on the quartz grains and precipitation under the experimental conditions investigated in this study. Cu retention increased with the increase in ionic strength, pH and clay content. Pb was nearly totally retained in the quartz column at all experimental conditions. The subsequent injection of CD resulted in Cu and Pb remobilization to different extents, except in the presence of high kaolinite concentration. CD is most efficient in remobilizing Cu and Pb at 400 mg L^-1 CD concentration and under low ionic strength (ca. 1-100 mM), low pH (ca. 6) and in the absence of clays. Deviation from these conditions results in reduced remobilization of Cu and Pb.

Enhanced roadside drainage system for environmentally sensitive areas

Stormwater runoff from roadways that encroach upon environmentally sensitive areas (ESAs) is one of the leading causes of degradation in urbanizing watersheds around the world. This is due to toxicity of the pollutant cocktail commonly found in roadway runoff, including heavy metals and sediments, as well as road salts from winter maintenance operations. This paper presents a novel design of an enhanced roadside drainage system (ERDS); an improved roadside drainage system that is intended to protect groundwater recharge zones and sensitive aquatic species in ESAs. The methods highlighted in this paper can be used to select soil amendments and size filter media for ERDS based on a combination of anticipated roadway pollutants and loads, treatment media efficacy and capacity, and consideration of applicable regulatory guidelines. The design of the ERDS must ensure compliance with the regulatory guidelines related to the protection of groundwater recharge zones as well as the receiving streams to protect priority species living therein. The performance monitoring results from a pilot-scale ERDS are presented to provide guidance for the key novel aspects of the design.

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.

Cotransport of graphene oxide and Cu(II) through saturated porous media

This study examines the cotransport of graphene oxide (GO) and Cu in porous media. The impacts of GO concentration and ion strength (IS) on Cu transport in laboratory packed columns were investigated. The results indicated that GO had fairly high mobility at a IS of 1mM, and could serve as an effective carrier of Cu(II). The facilitated transport was found to increase with increasing concentration of GO (CGO). The peak effluent concentration (C/C0)max of Cu was 0.57 at CGO of 120mg/L and IS=1mM and 0.13 at 40mg/L and IS=1mM. The Cu appears to be irreversibly adsorbed by the sand because no Cu appeared in the effluent in the absence of GO. However, the GO-facilitated Cu transport was reduced as the IS increased from 1 to 1000mM. In fact, the facilitated transport was zero percent at an IS of 1000mM. Particle size analysis, Zeta potential measurements and DLVO calculations demonstrated that higher IS values made the GO became unstable and it flocculated and attached to the sand. We also fed GO into the column pre-equilibrated by Cu as sequential elution experiments and found that the later introduced GO can complex the pre-adsorbed Cu from the sand surface because GO has a higher adsorption affinity for Cu. An advection-dispersion-retention numerical model was able to describe the Cu and GO transport in the column. Our work provides useful insights into fate, transport and risk assessment of heavy metal contaminants in the presence of engineered nanoparticles.

Cadmium removal from urban stormwater runoff via bioretention technology and effluent risk assessment for discharge to surface water

Bioretention technology, a low-impact development stormwater management measure, was evaluated for its ability to remove heavy metals (specifically cadmium, Cd) from urban stormwater runoff. Fine sand, zeolite, sand and quartz sand were selected as composite bioretention media. The effects of these materials on the removal efficiency, chemical forms, and accumulation and migration characteristics of Cd were examined in laboratory scale bioretention columns. Heretofore, few studies have examined the removal of Cd by bioretention. A five-step sequential extraction method, a single-contamination index method, and an empirical migration equation were used in the experiments. The average Cd removal efficiency of quartz sand approached 99%, and removal by the other media all exceeded 90%. The media types markedly affected the forms of Cd found in the columns as well as its vertical migration rate. The Cd accumulated in the four media was mainly in residual form; moreover, accumulation of Cd occurred mainly in the surface layer of the bioretention column. The migration depth of Cd in the four media increased with elapsed time, in the following sequence: zeolite>quartz sand>fine sand>sand. In contrast, the migration rate decreased with elapsed time, and the migration rate of Cd was lowest in sand (0.015 m per annum over the first ten years). The comprehensive risk index analysis indicated that the risk arising from Cd discharge to surface water was "intermediate", and that the degree of risk was lowest in sand, then quartz sand, zeolite, and fine sand in sequence. These results indicate that the adsorption and accumulation of Cd in the four media are more significant than the migration of Cd. In addition, the results of Cd risk assessment for the effluent indicate that each of the four media can serve as long-term adsorption material in a bioretention facility for purifying stormwater runoff.

Stormwater quality management in rail transportation--past, present and future

Railways currently play an important role in sustainable transportation systems, owing to their substantial carrying capacity, environmental friendliness and land-saving advantages. Although total pollutant emissions from railway systems are far less than that of automobile vehicles, the pollution from railway operations should not be underestimated. To date, both scientific and practical papers dealing with stormwater management for rail tracks have solely focused on its drainage function. Unlike roadway transport, the potential of stormwater pollution from railway operations is currently mishandled. There have been very few studies into the impact of its operations on water quality. Hence, upon the realisation on the significance of nonpoint source pollution, stormwater management priorities should have been re-evaluated. This paper provides an examination of past and current practices of stormwater management in the railway industry, potential sources of stormwater pollution, obstacles faced in stormwater management and concludes with strategies for future management directions.