Photo-electrochemical oxidation flow system for stormwater herbicides removal: Operational conditions and energy consumption analysis

Photoelectrochemical oxidation (PECO) is a promising advanced technology for treating micropollutants in stormwater. However, it is important to understand its operation prior to practical validation. In this study, we introduced a flow PECO system designed to evaluate its potential for full-scale applications in herbicides degradation, providing valuable insights for future large-scale implementations. The PECO flow reactor demonstrated the ability to treat a larger volume of stormwater (675 mL, approximately 10 times more than previous batch experiments) with effective removal rates of 92 % for diuron and 22 % for atrazine over 6 h of operation at 2 V. To address the large volume issue in stormwater treatment, a multiple module parallel application design is being considered to increase the treatment capacity of the PECO flow reactor. During the flow reactor operations, flow rate was found to have a notable impact on removal performance, particularly for diuron. At a flow rate of 610 mL min-1, approximately 90 % removal of diuron was achieved, while at 29 mL min-1, the removal efficiency decreased to 60 %. While light intensity had minimal effect on diuron degradation (all settings achieved over 90 % removal), it enhanced atrazine degradation from 9 % to 31 % with an increase in intensity from 63 mW cm-2 to 144 mW cm-2. Remarkably, the PECO flow system exhibited excellent removal performance (>90 % removal) for diuron even at extremely high initial pollutant concentrations (240 μg L-1), demonstrating its capacity to handle varying contaminant loads in stormwater. Energy consumption analysis revealed that flow rate as the primary factor influenced the specific energy consumption rate. Higher flow rate (e.g., 610 mL min-1) were preferable in flow reactor due to its well-balanced performance between removal and energy consumption. These findings confirm that the PECO flow system offers an efficient and promising approach for stormwater treatment applications.

Transport and removal of stormwater vehicle-related mobile organic contaminants in geomedia-amended sand columns

Green infrastructure drainage systems are innovative treatment units that capture and treat stormwater. Unfortunately, highly polar contaminants remain challenging to remove in conventional biofilters. To overcome treatment limitations, we assessed the transport and removal of stormwater vehicle-related organic contaminants with persistent, mobile, and toxic (in short: PMTs) properties, such as 1H-benzotriazole, N'N-diphenylguanidine, and hexamethoxymethyl-melamine (PMT precursor), using batch experiments and continuous-flow sand columns amended with pyrogenic carbonaceous materials, like granulated activated carbon (GAC) or wheat-straw derived biochar. Our results indicated that all investigated contaminants were subjected to nonequilibrium interactions in sand-only and geomedia-amended columns, with kinetic effects upon transport. Experimental breakthrough curves could be well described by a one-site kinetic transport model assuming saturation of sorption sites, which we inferred could occur due to dissolved organic matter fouling. Furthermore, from both batch and column experiments, we found that GAC could remove contaminants significantly better than biochar with higher sorption capacity and faster sorption kinetics. Hexamethoxymethylmelamine, with the lowest organic carbon-water partition coefficient (K_OC) and largest molecular volume among target chemicals, exhibited the lowest affinity in both carbonaceous adsorbents based on estimated sorption parameters. Results suggest that sorption of investigated PMTs was likely driven by steric and hydrophobic effects, and coulombic and other weak intermolecular forces (e.g., London-van der Waals, H-bonding). Results from extrapolating our data to a 1-m depth geomedia-amended sand filter suggested that GAC and biochar could enhance the removal of organic contaminants in biofilters and last for more than one decade. Overall, our work is the first to study treatment alternatives for N'N-diphenylguanidine and hexamethoxymethyl-melamine, and contributes to better PMT contaminant removal strategies in environmental applications.

A review of compaction effect on subsurface processes in soil: Implications on stormwater treatment in roadside compacted soil

Sustainable cities require spacious infrastructures such as roadways to serve multiple functions, including transportation and water treatment. This can be achieved by installing stormwater control measures (SCM) such as biofilters and swales on the roadside compacted soil, but compacted soil limits infiltration and other functions of SCM. Understanding the effect of compaction on subsurface processes could help design SCM that could alleviate the negative impacts of compaction. Therefore, we synthesize reported data on compaction effects on subsurface processes, including infiltration rate, plant health, root microbiome, and biochemical processes. The results show that compaction could reduce runoff infiltration rate, but adding sand to roadside soil could alleviate the negative impact of compaction. Compaction could decrease the oxygen diffusion rate in the root zone, thereby affecting plant root activities, vegetation establishment, and microbial functions in SCM. The impacts of compaction on carbon mineralization rate and root biomass vary widely based on soil type, aeration status, plant species, and inherent soil compaction level. As these processes are critical in maintaining the long-term functions of SCM, the analysis would help develop strategies to alleviate the negative impacts of compaction and turn road infrastructure into a water solution in sustainable cities.

Performance and microbial community of MBBRs under three maintenance strategies for intermittent stormwater treatment

Maintaining microbial activities is a critical problem for biological treatment processes of stormwater runoff because of its intermittent nature. In this study, the suitability of the moving bed biofilm reactor (MBBR) was assessed for stormwater treatment by long-term dry - rainy alternation operation. Three strategies to maintain microbial activities during the dry period, including keeping idle (MBBR_I), introducing river water throughout the period (MBBR_C), and ahead of a rainy day (MBBR_M), were investigated. COD and NH₄⁺-N removal efficiencies declined linearly from 94.2 % and 94.7 % to 51.7 % and 64.6 %, respectively, after the 61-day operation with microbial activity and biomass decreased. Introducing river water adversely affected the process performance as MBBR_C presented the highest declining rates of COD and NH₄⁺-N removal efficiencies. Most genera in MBBRs decayed and their microbial communities developed towards individualization, especially in MBBR_M because of its highest environmental variability. Keeping idle slightly alleviated the performance decline and formed a more stable microbial community structure. However, significantly deteriorating performance in all MBBRs after the long-term operation indicated that MBBRs were unsuitable for treating stormwater independently of intermittent nature.

Corncob-pyrite bioretention system for enhanced dissolved nutrient treatment: Carbon source release and mixotrophic denitrification

Solid biomass waste amendment and substrates modification in bioretention systems have been increasingly used to achieve effective dissolved nutrients pollution control in stormwater runoff. However, the risk of excess chemical oxygen demand (COD) leaching from organic carbon sources is often overlooked on most occasions. Pyrite is an efficient electron donor for autotrophic denitrification, but little is known about the efficacy of autotrophic-heterotrophic synergistic effect between additional carbon source and pyrite in bioretention. Here, four bioretention columns (i.e., corncob column (C), pyrite column (P), the corncob-pyrite layered column (L-CP), and the corncob-pyrite mixed column (M-CP)) were designed and filled with soil, quartz sand, and modified media to reveal the synergistic effects. The results showed that the corncob-pyrite layered bioretention could maintain low COD effluent concentration with high stability and efficiency in treating dissolved nutrients. When the influent nitrogen and phosphorus concentrations were 8.46 mg/L and 0.94 mg/L, the average removal rates of ammonia nitrogen, total inorganic nitrogen, and phosphate were 83.6%, 70.52%, and 76.35%, respectively. The scouring experiment showed that placing the corncob in the mulch layer was beneficial to the sustained release of dissolved organic carbon (DOC). Erosion pits were found in the SEM images of used pyrite, indicating that autotrophic denitrifying bacteria in the bioretention could react with pyrite as an electron donor. The relative abundance of Thiobacillus in the submerged zone of the corncob-pyrite layered bioretention reached 38.39%, indicating that the carbon source in the mulch layer increased the relative abundance of Thiobacillus. Coexisting heterotrophic and autotrophic denitrification in this bioretention created a more abundant microbial community structure in the submerged zone. Overall, the corncob-pyrite layered bioretention is highly promising for stormwater runoff treatment, with effective pollution removal and minimal COD emission.

Use of pilot-scale geomedia-amended biofiltration system for removal of polar trace organic and inorganic contaminants from stormwater runoff

Stormwater runoff capture and groundwater recharge can provide a sustainable means of augmenting the local water resources in water-stressed cities while simultaneously mitigating flood risk, provided that these processes do not compromise groundwater quality. We developed and tested for one year an innovative pilot-scale stormwater treatment train that employs cost-effective engineered geomedia in a continuous-flow unit-process system to remove contaminants from urban runoff during aquifer recharge. The system consisted of an iron-enhanced sand filter for phosphate removal, a woodchip bioreactor for nitrate removal coupled to an aeration step, and columns packed with different configurations of biochar- and manganese oxide-containing sand to remove trace metals and persistent, mobile, and toxic trace organic contaminants. During conditioning with authentic stormwater runoff over an extended period (8 months), the woodchip bioreactor removed 98% of the influent nitrate (9 g-N m^-3 d^-1), while phosphate broke through the iron-enhanced sand filter. During the challenge test (4 months), geomedia removed more than 80% of the mass of metals and trace organic compounds. Column hydraulic performance was stable during the entire study, and the weathered biochar and manganese oxide were effective at removing trace organic contaminants and metals, respectively. Under conditions likely encountered in the field, sustained nutrient removal is probable, but polar organic compounds such as 2,4-D could breakthrough after about a decade for conditions at the study site.

Competitive sorption of Cd, Cr, Cu, Ni, Pb and Zn from stormwater runoff by five low-cost sorbents; Effects of co-contaminants, humic acid, salinity and pH

For a comprehensive estimation of metals removal by sorbents in stormwater systems, it is essential to evaluate the impacts of co-contaminants. However, most studies consider only metals (single or multiple), which may overestimate performance. This study employed a batch method to investigate the performance of five low-cost sorbents - coconut coir fiber (CCF), blast furnace slag (BFS), waste tire crumb rubber (WTCR), biochar (BC), and iron coated biochar (FeBC) - for simultaneous removal of Cd, Cr, Cu, Ni, Pb and Zn from simulated stormwater (SSW) containing other contaminants (nutrients and polycyclic aromatic hydrocarbons). BFS and CCF demonstrated the highest sorption capacity of all metals (> 95% removal) in all systems (single and multi-contaminant). However, the presence of other contaminants in solution reduced metals removal for other sorbents, as follows (highest to lowest removal): single-metal > multi-metal > multi-contaminant solutions, and removal efficiency ranking among metals was generally Cr~Cu~Pb > Ni > Cd > Zn. Humic acid (HA) negatively affected the metal sorption, likely due to the formation of soluble HA-metal complexes; NaCl concentration did not impact removal, but alkaline pH improved removal. These findings indicate that sorbents need to be tested under realistic stormwater solution chemistry including co-contaminants to appropriately characterize performance prior to implementation.

Woodchips bioretention column for stormwater treatment: Nitrogen removal performance, carbon source and microbial community analysis

This study chose Oak woodchips and gravel as media filter to enhance the denitrification in the bioretention system (saturated zone 7.7 L) treating synthetic stormwater runoff. It revealed that the denitrification process mainly occurred during the drying phase and enlarging volume of saturated zones to retain more stormwater during storm event was the direct method to promote nitrogen removal of the bioretention system. Nevertheless, it was noted that the nitrogen and dissolved organic carbon would be released into the effluent during the wetting period. The denitrification rate with different nitrate nitrogen (NO₃-N) concentrations did not show the obvious change with zero order kinetics constant of 2.91 mg/L∙d on average. Furthermore, it confirmed that woodchips were degraded and converted to volatile fatty acids (VFAs), especially acetic acid as carbon source, further utilized by the denitrifying bacteria, such as Dechloromonas, Acidoborax, Pseudomonas, Denitratisoma and Acinetobacter. In addition, genera of Lachnospiraceae and Lactobacillus, which had the ability to degrade the macromolecular organic components into low molecular VFAs, were observed in the woodchips bioretention system.

Nitrogen process in stormwater bioretention: the impact of alternate drying and rewetting on nitrogen migration and transformation

Nitrogen migration and transformation in the stormwater bioretention system were studied in laboratory experiments, in which the effects of drying-rewetting were particularly investigated. The occurrence and distribution of nitrogen in the plants, the soil, and the pore water were explored under different drying-rewetting cycles. The results clearly showed that bioretention system could remove nitrogen efficiently in all drying-rewetting cycles. The incoming nitrogen could be retained in the topsoil (0-10 cm) and accumulated in the planted layer. However, the overlong dry periods (12 and 22 days) cause an increase in nitrate in the pore water. In addition, nitrogen is mostly stored in the plants' stem tissues. Up to 23.26% of the inflowing nitrogen can be immobilized in plant tissues after a dry period of 22 days. In addition, the relationships between nitrogen reductase activity in the soil and soil nitrogen content were explored. The increase of soil TN content could enhance the activity of nitrate reductase. Meanwhile, the activity of hydroxylamine reductase (HyR) could be enhanced with the increase of soil NO₃^- content. These results provide a reference for the future development of nitrogen transformation mechanism and the construction of stormwater bioretention systems.

Simultaneous removal of multiple polycyclic aromatic hydrocarbons (PAHs) from urban stormwater using low-cost agricultural/industrial byproducts as sorbents

The potential of five low-cost and globally available sorbents, including three raw waste products - waste tire crumb rubber (WTCR), coconut coir fiber (CCF) and blast furnace slag (BFS) - and two modified materials - biochar (BC) and iron coated biochar (FeBC) - were evaluated for removing a mixture of polycyclic aromatic hydrocarbons (PAHs): pyrene (PYR), phenanthrene (PHE), acenaphthylene (ACY) and naphthalene (NAP) from simulated stormwater. The physicochemical characteristics of the sorbents were assessed by BET-N₂ surface area, CHN elemental analysis, FTIR and scanning electron microscope (SEM-EDS). The experimental data were well described by both linear and Freundlich isotherm and pseudo-second order kinetic models. The adsorption rate was mainly controlled by the film diffusion mass transfer mechanism. The magnitude of PAHs partition coefficients (K_d) followed the order of BC > FeBC > WTCR > CCF ≫ BFS, ranging from 80 to 390,000 L/kg. The sorption K_d values were positively correlated with both aromaticity of sorbents and octanol-water partition coefficients (K_ow) of PAHs. Solution ionic strength and pH did not have significant effects on the sorption of PAHs by all sorbents. In contrast, humic acid, as dissolved organic carbon, decreased sorption capacities of WTCR and CCF, and increased sorption efficiency of BFS, which was confirmed with field-collected real stormwater. The hydrophobic π-π interactions were the main mechanism for the sorption of PAHs by various sorbents. These findings are promising for future development of cost-effective sorption filters for removal of hydrophobic organic pollutants from urban stormwater runoff.

Integration of machine learning classifiers and higher order tensors for screening the optimal recipe of filter media in stormwater treatment

Filter media have oftentimes been used in fixed-bed column tests to examine their removal efficiencies for various pollutants, such as nutrients in stormwater runoff. With limited data sets from column studies, a response surface method (RSM), such as the Box-Behnken Design (BBD), and machine learning methods, can be used to transition from discrete mode assessment to continuous mode optimization, from which the key ingredients of filter media can be better synergized. In this study, similarly to drug discovery via chemometrics, RSM is used to generate meta-models and identify the optimum ratio between clay and iron-filings contents in Iron-filings-based Green Environmental Media (IFGEM) for nutrient removal in stormwater treatment. To achieve the continuous mode optimization, artificial neural network (ANN), deep belief network (DBN), and extreme learning machine (ELM) were selected as machine learning models to compare with BBD to explore the limited column data sets and improve the data science. While separate RSM can help realize the removal efficiencies of total nitrogen (TN), total phosphorus (TP), and ammonia based on varying ratios of clay and iron-filings contents in IFGEM, heterogeneous and inconsistent response surfaces generated from the four learners or classifiers (ANN, ELM, DBN, and BBD) complicate the selection of the final optimal recipe. The power of higher order singular value decomposition (HOSVD) helps synergize the optimal clay and iron filings matrixes of IFGEM in the context of continuous mode optimization via ANN, ELM, DBN, and BBD. With the aid of HOSVD, the optimal recipe for a holistic nutrient removal of TN, TP, and ammonia was determined to be 5% clay, 10% iron filings, 10% tire crumb, and 75% sand.

Stormwater herbicides removal with a solar-driven advanced oxidation process: A feasibility investigation

The solar driven advanced oxidation process (AOP) has the potential to be developed as a passive stormwater post-treatment method. Despite its widespread studies in wastewater treatment, the applicability of the process for micropollutant removal in stormwater (which has very different chemical properties from wastewater) is still unknown. This paper investigated the feasibility of three different AOP processes for the degradation of two herbicides (diuron and atrazine) in pre-treated stormwater: (i) photoelectrochemical oxidation (PECO), (ii) electrochemical oxidation (ECO), and (iii) photocatalytic oxidation (PCO). The durability of different anode materials, the effects of catalyst loading, and solar photo- and thermal impacts under different applied voltages were studied. Boron-doped diamond (BDD) was found to be the most durable anode material compared to carbon fiber and titanium foil for long-term operation. Due to the very low electroconductivity of stormwater, a high voltage was required, causing severe oxidation of the carbon fiber material. PECO achieved the best degradation results compared to ECO and PCO, with over 90% degradation of both herbicides in 2 h under 5 V, following a first-order decay process (with a half-life value of 0.40 h for diuron and 0.58 h for atrazine). The voltage increase had a positive impact on the oxidation processes, with 5 V found to be the optimal applied voltage, while catalyst loading had a negligible effect. Interestingly, the solar thermal effect plays a dominant role in enhancing the performance of the PECO process, which indicates the potential of integrating a photovoltaic chamber with a PECO system to harness both the light and heat of solar energy for stormwater treatment.

Size-dependent biochar breaking under compaction: Implications on clogging and pathogen removal in biofilters

Breaking of biochar during compaction of amended soil in roadside biofilters or landfill cover can affect infiltration and pollutant removal capacity. It is unknown how the initial biochar size affects the biochar breaking, clogging potential, and contaminant removal capacity of the biochar-amended soil. We compacted a mixture of coarse sand and biochar with sizes smaller than, similar to, or larger than the sand in columns and applied stormwater contaminated with E. coli. Packing columns with biochar pre-coated with a dye and analyzing the dye concentration in the broken biochar particles eluted from the columns, we proved that biochar predominantly breaks under compaction by disintegration or splitting, not by abrasion. Increases in biochar size decrease the likelihood of biochar breaking. We attribute this result to the effective dissipation of compaction energy through a greater number of contact points between a large biochar particle and the adjacent particles. Most of the broken biochar particles are deposited in the pore spaces of the background geomedia, resulting in an exponential decrease in hydraulic conductivity of amended sand with an increase in suspended sediment loading. The clogging rate was higher in the columns with small biochar. The columns with small biochar also exhibited high E. coli removal capacity, partly because of an increase in bacterial straining at reduced pore size after compaction. These results are useful in selecting appropriate biochar size for its application in soils and roadside biofilters for stormwater treatment.

The interaction of dissolved organic nitrogen removal and microbial abundance in iron-filings based green environmental media for stormwater treatment

Nonpoint sources pollution from agricultural crop fields and urbanized regions oftentimes have elevated concentrations of dissolved organic nitrogen (DON) in stormwater runoff, which are difficult for microbial communities to decompose. The impact of elevated DON can be circumvented through the use of green sorption media, such as Biosorption Activated Media (BAM) and Iron-Filing Green Environmental Media (IFGEM), which, as integral parts of microbial ecology, can contribute to the decomposition of DON. To compare the fate, transport, and transformation of DON in green sorption media relative to natural soil (control), a series of fixed-bed columns, which contain natural soil, BAM, and two types of IFGEM, respectively, were constructed to compare nutrient removal efficiency under three distinct stormwater influent conditions containing nitrogen and phosphorus. The interactions among six microbial species, including ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, complete ammonia oxidation (comammox) bacteria, anaerobic ammonium oxidation (anammox) bacteria, dissimilatory nitrate reduction to ammonium bacteria, and iron-reducing bacteria, were further analyzed from microbial ecology perspectives to determine the DON impact on nutrient removal in BAM and IFGEM. Natural soil was only able to achieve adequate DON transformation at the influent condition of lower nutrient concentration. However, the two types of IFGEM showed satisfactory nutrient removals and achieved greater transformation of DON relative to BAM when treating stormwater in all three influent conditions.

Removal of metals and hydrocarbons from stormwater using coagulation and flocculation

As the understanding of how stormwater pollutants are fractioned and need for mitigation has increased, so has the investigation into more advanced treatment techniques. The present study investigated the treatment efficiency of coagulation/flocculation and sedimentation in semi-synthetic stormwater. Five coagulants were evaluated in terms of reducing particle content, organic carbon, total and dissolved metals, hydrocarbon oil index, and polycyclic aromatic hydrocarbons (PAHs). Changes in the resulting particle size distribution as a consequence of the coagulation treatment were also investigated. The pollutants in the semi-synthetic stormwater were predominantly in the particulate phase. The medium and longer chained hydrocarbons dominated the hydrocarbon oil index, while medium to high molecular weight PAHs were most abundant. Iron chloride was the only coagulant that affected particle size distribution post-treatment, shifting the distribution toward larger particles. In terms of total metal removal, the performance of the coagulants was similar, with over 90% removal on average. Concentration of zdissolved copper, one of the metals found in the dissolved phase, was reduced by 40% via coagulation treatment. The iron chloride coagulant increased dissolved Zn, a change attributed to a considerable drop in pH resulting in higher ion mobility. Similarly, the reduction in organic content (total organic carbon, oil, and PAHs) was over 90% for most coagulants.

Copper impact on enzymatic cascade and extracellular sequestration via distinctive pathways of nitrogen removal in green sorption media at varying stormwater field conditions

Nutrient removal efficiency in green sorption media such as biosorption activated media (BAM) for treating stormwater runoff can be heavily influenced either on a short- or long-term basis by varying field conditions of linear ditches due to the presence of copper in stormwater runoff. It is also noticeable that the linear ditch undergoes physical or mechanical impacts from the traffic compaction, chemical impact of carbon sources from the nearby farmland, and biological impact from potential animal activities (such as gopher tortoises, moles, and ants). In the nitrogen cycle, two denitrification pathways, the dissimilatory nitrate reduction to ammonia and common denitrification, are deemed critical for such assessment. A fixed-bed column study was set up to mimic different linear ditch field conditions for BAM applications and measure the effect of short-and long-term copper addition on microbial dynamics given the varying decomposition of dissolved organic nitrogen (DON). The findings confirm that, as the denitrifiers (in the second pathway) were the dominant species, their population continued to grow and maintain small-sized cells for extracellular sequestration under long-term copper impact. Furthermore, the study indicated that the ammonia oxidizer comammox was found in higher quantities than ammonia oxidizing bacteria or archaea. An enormous amount of DON was released during this process to bind the copper ion and reduce its toxicity as the enzymatic cascade effect appeared. In addition, the long-term copper exposure posed salient inhibitory effects on the microbial community regardless of varying field conditions in BAM. Short-term copper toxicity exerted an important but varying role in the enzymatic cascade effect over different linear ditch field conditions in BAM.

Waste-derived compost and biochar amendments for stormwater treatment in bioretention column: Co-transport of metals and colloids

Bioretention systems, as one of the most practical management operations for low impact development of water recovery, utilize different soil amendments to remove contaminants from stormwater. For the sake of urban sustainability, the utilization of amendments derived from waste materials has a potential to reduce waste disposal at landfill while improving the quality of stormwater discharge. This study investigated the efficiency of food waste compost and wood waste biochar for metal removal from synthetic stormwater runoff under intermittent flow and co-presence of colloids. Throughout intermittent infiltration of 84 pore volumes of stormwater, columns amended with compost and biochar removed more than 50-70% of influent metals, whereas iron-oxide coated sand was much less effective. Only a small portion of metals adsorbed on the compost (< 0.74%) was reactivated during the drainage of urban pipelines that do not flow frequently, owing to abundant oxygen-containing functional groups in compost. In comparison, co-existing kaolinite enhanced metal removal by biochar owing to the abundance of active sites, whereas co-existing humic acid facilitated mobilization via metal-humate complexation. The results suggest that both waste-derived compost and biochar show promising potential for stormwater harvesting, while biochar is expected to be more recalcitrant and desirable in field-scale bioretention systems.

Combination of Lagrangian Discrete Phase Model and sediment physico-chemical characteristics for the prediction of the distribution of trace metal contamination in a stormwater detention basin

Elevated trace metal concentrations in sediments pose a major problem for the management of stormwater detention basins. These basins provide a nature-based solution to remove particulate pollutants through settling, but the resuspension of these contaminated deposits may impact the quality of both surface and groundwater. A better understanding of trace metal distribution will help to improve basin design and sediment management. This study aims to predict the distribution of trace metal contamination in a stormwater detention basin through (i) investigation of the correlation between metal content in sediments and their settling velocity, and (ii) the coupling of such correlation with a Lagrangian Discrete Phase Model (LDPM). The correlation between Fe, Cr, Cu, Ni, Pb contents and the settling velocity is firstly investigated, based on the sediments collected from 6 sites (inlet and 5 traps at the bottom of a detention basin situated in Chassieu, France) during 5 campaigns in 2017. Results show that Fe is strongly correlated to settling velocity and can be considered as a good indicator of trace metal contents. The derived correlation is then combined with a LDPM for the prediction of trace metal distribution, producing results consistent with in situ measurements. The proposed methodology can be applied for other stormwater basins (dry or wet). As described in this article, the interactions between hydrodynamics and sediment physico-chemical characteristics is crucial for the design and management of stormwater detention basins, allowing managers to target the highest contaminated sediments.

A dynamic life cycle assessment of green infrastructures

As stormwater and its associated nutrients continue to impair our nation's waterways, green infrastructures (GIs) are increasingly applied in urban and suburban communities as a means to control combined sewer system overflows and stormwater related pollutants. Although GIs have been widely studied for their life cycle impacts and benefits, most of these studies adopt a static approach which prevents that information from being scaled or transferred to different spatial and temporal settings. To overcome this limitation, this research utilizes a dynamic life cycle assessment (LCA) approach to evaluate seven different GIs by integrating a traditional LCA with a system dynamics model which simulates the daily loadings and treatments of nutrients by the GIs across a 30-year life span. A base model was first developed, calibrated, and validated for seven GIs that are currently installed on the campus of the University of New Hampshire. The base model was then expanded to assess different scenarios in terms of geographic locations, land uses, GI design sizes, and climate changes. Our results show these aforementioned factors have significant influences on GIs' life cycle performances, with life cycle nitrogen reductions varying -100.90 to 512.09kgNeq. and life cycle phosphorous reductions varying from -23.77 to 63.43kg P eq. Furthermore, nutrient loading thresholds exist for certain GIs to offset nutrient emissions from their construction and maintenance activities. Accordingly, an optimal GI design size can be estimated for a given spatial and temporal setting. Such thresholds and optimal sizes are important to be identified to inform the decision-making and future planning of GIs.

Evaluation of pilot-scale biochar-amended woodchip bioreactors to remove nitrate, metals, and trace organic contaminants from urban stormwater runoff

Stormwater is increasingly being valued as a freshwater resource in arid regions and can provide opportunities for beneficial reuse via aquifer recharge if adequate pollutant removal can be achieved. We envision a multi-unit operation approach to capture, treat, and recharge (CTR) stormwater using low energy, cost-effective technologies appropriate for larger magnitude, less frequent events. Herein, we tested nutrient, metal, and trace organic contaminant removal of a pilot-scale CTR system in the laboratory using biochar-amended woodchip bioreactors following eight months of aging under field conditions with exposure to real stormwater. Replicate columns with woodchips and biochar (33% by weight), woodchips and straw, or woodchips only were operated with continuous, saturated flow for eight months using water from a watershed that drained an urban area consisting of residential housing and parks in Sonoma, California. After aging, columns were challenged for five months by continuous exposure to synthetic stormwater amended with 50 μg L^-1 of six trace organic contaminants (i.e., fipronil, diuron, 1H-benzotriazole, atrazine, 2,4-D, and TCEP) and five metals (Cd, Cu, Ni, Pb, Zn) frequently detected in stormwater in order to replicate the treatment unit operation of a CTR system. Throughout the eight-month aging and five-month challenge experiment, nitrate concentrations were below the detection limit after treatment (i.e., <0.05 mg N L^-1). The removal efficiencies for metals in all treatments were >80% for Ni, Cu, Cd, and Pb. For Zn, about 50% removal occurred in the woodchip-biochar systems while the other systems achieved about 20% removal. No breakthrough of the trace organic compounds was observed in any biochar-containing columns. Woodchip columns without biochar removed approximately 99% of influent atrazine and 90% of influent fipronil, but exhibited relatively rapid breakthrough of TCEP, 2,4-D, 1H-benzotriazole, and diuron. The addition of straw to the woodchip columns provided no significant benefit compared to woodchips alone. Due to the lack of breakthrough of trace organics in the biochar-woodchip columns, we estimated column breakthrough with a diffusion-limited sorption model. Results of the model indicate breakthrough for the trace organics would occur between 10,000 and 32,000 pore volumes. Under ideal conditions this could be equivalent to decades of service, assuming failure by other processes (e.g., clogging, biofouling) does not occur. These results indicate that multiple contaminants can be removed in woodchip-biochar reactors employed in stormwater treatment systems with suitable flow control and that the removal of trace organic contaminants is enhanced significantly by addition of biochar.

Evaluating emerging organic contaminant removal in an engineered hyporheic zone using high resolution mass spectrometry

The hyporheic zone (HZ), located at the interface of surface and groundwater, is a natural bioreactor for attenuation of chemical contaminants. Engineered HZs can be incorporated into stream restoration projects to enhance hyporheic exchange, with flowpaths optimized to promote biological habitat, water quantity, and water quality improvements. Designing HZs for in-stream treatment of stormwater, a significant source of flow and contaminant loads to urban creeks, requires assessment of both the hydrology and biogeochemical capacity for water quality improvement. Here, we applied tracer tests and high resolution mass spectrometry (HRMS) to characterize an engineered hyporheic zone unit process, called a hyporheic design element (HDE), in the Thornton Creek Watershed in Seattle, WA. Dye, NaCl, and bromide were used to hydrologically link downwelling and upwelling zones and estimate the hydraulic retention time (HRT) of hyporheic flowpaths. We then compared water quality improvements across hydrologically-linked surface and hyporheic flowpaths (3-5 m length; ∼30 min to >3 h) during baseflow and stormflow conditions. We evaluated fate outcomes for 83 identified contaminants during stormflow, including those correlated with an urban runoff mortality syndrome in coho salmon. Non-target HRMS analysis was used to assess holistic water quality improvements and evaluate attenuation mechanisms. The data indicated substantial water quality improvement in hyporheic flowpaths relative to surface flow and improved contaminant removal with longer hyporheic HRT (for ∼1900 non-target compounds detected during stormflow, <17% were attenuated >50% via surface flow vs. 59% and 78% via short and long hyporheic residence times, respectively), and strong contributions of hydrophobic sorption towards observed contaminant attenuation.

Adsorption of myo-inositol hexakisphosphate in water using recycled water treatment residual

Dissolved organic phosphorus (DOP) in rainwater runoff or other contaminated waters can cause or aggravate eutrophication of water bodies. Water treatment residual (WTR) containing spent coagulant has been shown to provide excellent adsorption capacity for inorganic phosphorus such as orthophosphate, but little information has been available on adsorption of DOPs by WTR. In this study, the adsorption characteristics of myo-inositol-1,2,3,4,5,6-hexakisphosphate (IHP), a prototype DOP in soil and stormwater, by WTR were investigated through batch adsorption equilibrium and kinetic experiments. The influences of pH and various size fractions of WTR on the adsorption capacity were tested and analyzed, and the adsorption mechanism was elucidated based on Fourier-transform infrared spectroscopy (FTIR) analysis. The experimental results showed that WTR can effectively adsorb IHP from simulated rainwater, and the IHP uptake was favored under neutral and acidic conditions. Moreover, the 1.0-2.0-mm fraction of the WTR particles was most suitable for practical application because of the well-balanced adsorption rate and capacity. The classical Langmuir isotherm model well described the equilibrium adsorption data and the pseudo-second-order kinetic model adequately interpreted the rate data. Thermodynamic analysis revealed that the adsorption is a spontaneous, endothermic, and entropy-driven reaction. The FTIR analysis indicated that adsorption of IHP on WTR is associated with the formation of ≡Al-PO₃^- groups and the release of -OH from WTR. A comparison of the adsorption capacities of orthophosphate and IHP on WTR suggested that binding one IHP may take two times more sites than for orthophosphate, indicating that two of the six phosphate groups in IHP were bound to WTR. This work shows that recycled WTR may be used as a low-cost adsorbent for effective removal of organic phosphate in gray water and wastewater.

Heavy metal removal mechanisms of sorptive filter materials for road runoff treatment and remobilization under de-icing salt applications

The objective of this research study was to elucidate the removal and remobilization behaviors of five heavy metals (i.e., Cd, Cu, Ni, Pb, and Zn) that had been fixed onto sorptive filter materials used in decentralized stormwater treatment systems receiving traffic area runoff. Six filter materials (i.e., granular activated carbon, a mixture of granular activated alumina and porous concrete, granular activated lignite, half-burnt dolomite, and two granular ferric hydroxides) were evaluated in column experiments. First, a simultaneous preloading with the heavy metals was performed for each filter material. Subsequently, the remobilization effect was tested by three de-icing salt experiments in duplicate using pure NaCl, a mixture of NaCl and CaCl2, and a mixture of NaCl and MgCl2. Three layers of each column were separated to specify the attenuation of heavy metals as a function of depth. Cu and Pb were retained best by most of the selected filter materials, and Cu was often released the least of all metals by the three de-icing salts. The mixture of NaCl and CaCl2 resulted in a stronger effect upon remobilization than the other two de-icing salts. For the material with the highest retention, the effect of the preloading level upon remobilization was measured. The removal mechanisms of all filter materials were determined by advanced laboratory methods. For example, the different intrusions of heavy metals into the particles were determined. Findings of this study can result in improved filter materials used in decentralized stormwater treatment systems.

Nutrient release and ammonium sorption by poultry litter and wood biochars in stormwater treatment

The feasibility of using biochar as a filter medium in stormwater treatment facilities was evaluated with a focus on ammonium retention. Successive batch extractions and batch ammonium sorption experiments were conducted in both deionized (DI) water and artificial stormwater using poultry litter (PL) and hardwood (HW) biochars pyrolyzed at 400°C and 500°C. No measureable nitrogen leached from HW biochars except 0.07 μmol/g of org-N from 400°C HW biochar. PL biochar pyrolyzed at 400°C leached 120-127 μmol/g of nitrogen but only 7.1-8.6 μmol/g of nitrogen when pyrolyzed at 500°C. Ammonium sorption was significant for all biochars. At a typical ammonium concentration of 2mg/L in stormwater, the maximum sorption was 150 mg/kg for PL biochar pryolyzed at 400°C. In stormwater, ion competition (e.g. Ca(2+)) suppressed ammonium sorption compared to DI water. Surprisingly, ammonium sorption was negatively correlated to the BET surface area of the tested biochars, but increased linearly with cation exchange capacity. Cation exchange capacity was the primary mechanism controlling ammonium sorption and was enhanced by pyrolysis at 400°C, while BET surface area was enhanced by pyrolysis at 500°C. The optimal properties (BET surface area, CEC, etc.) of biochar as a sorbent are not fixed but depend on the target pollutant. Stormwater infiltration column experiments in sand with 10% biochar removed over 90% of ammonium with influent ammonium concentration of 2mg/L, compared to only 1.7% removal in a sand-only column, indicating that kinetic limitations on sorption were minor for the storm conditions studied. Hardwood and poultry litter biochar pyrolyzed at 500°C and presumably higher temperature may be viable filter media for stormwater treatment facilities, as they showed limited release of organic and inorganic nutrients and acceptable ammonium sorption.

Optimizing low impact development (LID) for stormwater runoff treatment in urban area, Korea: Experimental and modeling approach

Currently, continued urbanization and development result in an increase of impervious areas and surface runoff including pollutants. Also one of the greatest issues in pollutant emissions is the first flush effect (FFE), which implies a greater discharge rate of pollutant mass in the early part in the storm. Low impact development (LID) practices have been mentioned as a promising strategy to control urban stormwater runoff and pollution in the urban ecosystem. However, this requires many experimental and modeling efforts to test LID characteristics and propose an adequate guideline for optimizing LID management. In this study, we propose a novel methodology to optimize the sizes of different types of LID by conducting intensive stormwater monitoring and numerical modeling in a commercial site in Korea. The methodology proposed optimizes LID size in an attempt to moderate FFE on a receiving waterbody. Thereby, the main objective of the optimization is to minimize mass first flush (MFF), which is an indicator for quantifying FFE. The optimal sizes of 6 different LIDs ranged from 1.2 mm to 3.0 mm in terms of runoff depths, which significantly moderate the FFE. We hope that the new proposed methodology can be instructive for establishing LID strategies to mitigate FFE.

Particle size distribution variance in untreated urban runoff and its implication on treatment selection

Understanding the particle size distribution (PSD) of sediment in urban runoff assists in the selection of appropriate treatment systems for sediment removal as systems vary in their ability to remove sediment across different particle size fractions. Variation in PSD in runoff from individual urban surfaces both during and across multiple rain events is not well understood and it may lead to performance uncertainty in treatment systems. Runoff PSDs in international literature were compiled to provide a comparative summary of PSDs from different urban surfaces. To further assess both intra-event and inter-event PSD variation, untreated runoff was collected from road, concrete roof, copper roof, and galvanized roof surfaces within an urban catchment exposed to the same rainfall conditions and analysed for PSD and total suspended solids (TSS). Road runoff had the highest TSS concentrations, while copper roofs had high initial TSS that reduced to very low levels under steady state conditions. Despite variation in TSS concentrations, the median particle diameter of the TSS was comparable across the surfaces. Intra-event variation was generally not significant, but substantial inter-event variation was observed, particularly for coarser road and concrete roof surfaces. PSD variation for each surface contributed to a wide range in predicted treatment performance and suggests that short-retention treatment devices carry a high performance risk of not being able to achieve adequate TSS removal across all rain events.

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.

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.

Performance characterisation of a stormwater treatment bioretention basin

Treatment performance of bioretention basins closely depends on hydrologic and hydraulic factors such as rainfall characteristics and inflow and outflow discharges. An in-depth understanding of the influence of these factors on water quality treatment performance can provide important guidance for effective bioretention basin design. In this paper, hydraulic and hydrologic factors impacting pollutant removal by a bioretention basin were assessed under field conditions. Outcomes of the study confirmed that the antecedent dry period plays an important role in influencing treatment performance. A relatively long antecedent dry period reduces nitrite and ammonium concentrations while increasing the nitrate concentration, which confirms that nitrification occurs within the bioretention basin. Additionally, pollutant leaching influences bioretention basin treatment performance, reducing the nutrients removal efficiency, which was lower for high rainfall events. These outcomes will contribute to a greater understanding of the treatment performance of bioretention basins, assisting in the design, operation and maintenance of these systems.