First flush analysis using a rainfall simulator on a micro catchment in an arid climate

Evaluation on the performance of a swirling-type hydrodynamic separator using physical and numerical models

Hydrodynamic separators are commonly used to control the total suspended solid concentration in stormwater before being discharged to natural water bodies. The separator studied in this paper, featuring a swirling flow generated by tangential inlet and outlet connections, was analyzed for its sediment removal efficiency in relation to sediment and flow rates. For the separator studied in this paper, the numerical model showed that the flow field was favorable for the sediments to gather at the center and settle. A higher flow rate or a smaller sediment diameter corresponded to a lower removal rate and vice versa. The dimension improvement for increasing the sediment removal rate was also studied. It was found that increasing the diameter of the separator showed a higher sediment removal rate compared with corresponding increase in the height of the separator. A dimensionless parameter J was proposed to assess the sediment removal rate of a separator, which may be used for designing and optimizing such a device. The removal rate is positively correlated with the J value. When the J value reaches 0.5 or above, the sediment removal rate exceeds 80%, which is a good initial target value for designing this type of separator.

Migration and adsorption of naphthalene in road-deposited sediments from stormwater runoff: Impact of the particle size

In this study, we explored the impact of RDS particle size on the migration dynamics of RDS and naphthalene through rigorous wash-off experiments. The results illuminated that smaller RDS particles showed higher mobility in stormwater runoff. On the other hand, RDS particles larger than 150 μm showed migration ratios below 2 %, suggesting that naphthalene adsorbed on larger RDS primarily migrated in dissolved form. Furthermore, we investigated the migration behaviors of RDS and naphthalene under varied conditions, including rainfall intensity, duration, and naphthalene concentrations. Larger rainfall intensity promoted the naphthalene release from RDS, while long rainfall duration (≥10 min) impeded the migration velocities (≤2.91 %/5 min for RDS, and ≤3.32 %/5 min for corresponding naphthalene) of RDS and naphthalene. Additionally, higher naphthalene concentrations in RDS diminished migration ratios of dissolved naphthalene. Significantly, the maximum uptake of naphthalene on RDS was 6.02 mg/g by the adsorption Langmuir isotherm. Importantly, the adsorption process of naphthalene in RDS is primarily governed by the physical adsorption, as demonstrated by the successive desorption experiments, which showed the desorption rate of up to 87.32 %. Moreover, advanced characterizations such as XPS, FTIR and Raman spectra further confirmed the physical nature of the adsorption process. These findings may help the understanding of the migration behavior of other pollutants in urban surface particulates.

New conceptualization and quantification method of first-flush in urban catchments: A modelling study

A major challenge for runoff pollution control lies in the quantification and identification of the first-flush. At present, there is a lack of reasonable theoretical methods to guide engineering practices. To remedy this deficiency, a novel method of cumulative pollutant mass vs. cumulative runoff volume (M(V)) curve simulation is proposed in this study. Subsequently, the first-flush phenomenon was redefined based on the M(V) curve simulation and demonstrate that the first-flush exists until the derivative of the simulated M(V) curve is equal to 1 (F_t' = 1). Consequently, a mathematical model for first-flush quantification was developed. The Root-Mean-Square-Deviation (RMSD) and Pearson's Correlation Coefficient (PCC), as objective functions, were used to evaluate the performance of the model and the Elementary-Effect (EE) method was used to analyze the sensitivity of the parameters. The results indicated the satisfactory accuracy of the M(V) curve simulation and first-flush quantitative mathematical model. The NSE values exceeding 0.8 and 0.938, respectively, were obtained by analyzing 19 rainfall-runoff data for Xi'an, Shaanxi Province, China. The wash-off coefficient "r" was demonstrably the most sensitive factor influencing the model performance. Therefore, interactions between "r" and the other model parameters should be focused on to highlight the overall sensitivities. Overall, this study posits a novel paradigm shift from the traditional dimensionless definition criterion to redefine and quantify first-flush, which has significant implications for urban water environment management.

Insight into the pollution characteristics of road and roof runoff in Changsha, China

Non-point source pollution from rainwater runoff presents a serious challenge for urban water management in many cities undergoing urbanization and experiencing climate change. To alleviate water resource conflicts in Changsha, China, this study comprehensively evaluated the pollution characteristics and first flush effect (FFE) of runoff from asphalt roads and colored steel plate roofs under seven rainfall events in April-May 2022. The runoff was collected and purified using bioretention ponds. The results showed that the peak runoff pollutant concentrations occurred within the first 20 min of runoff generation and then decreased to relatively stable levels, with maximum total suspended solids (TSS) concentration and chemical oxygen demand (COD_Cr) reaching 873.5 and 207.32 mg/L, respectively, for road runoff and 162 and 73.31 mg/L for roof runoff, respectively. The main pollutants were TSS and COD_Cr, followed by ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), total phosphorus (TP), and nitrite nitrogen (NO₂^--N). Concentrations of pollutants and FFE for roof runoff were lower than those for road runoff. Road runoff had a more obvious FFE for TP and NH₄⁺-N, whereas the roof runoff showed the presence of TP and NO₃^--N. An important implication is that treating the first 30% of surface runoff from rainfall events with long antecedent dry days or high rainfall amounts is necessary to improve water quality before discharge or utilization. The study also found that road and roof runoff, after treatment with bioretention ponds, exhibit good water quality, thus, allowing their use as reclaimed water or for miscellaneous purposes in urban areas. Overall, this study provides useful information for designing management measures to mitigate runoff pollution and reuse in Changsha.

Discharge of pharmaceuticals from a municipal solid waste transfer station: Overlooked influence on the contamination of pharmaceuticals in surface waters

During the temporal storage of municipal solid wastes (MSWs), pharmaceutically contained in MSWs may percolate into leachates and migrate into receiving waters via surface runoff. However, knowledge of their intra-event variations during the rainfall is quite limited. To fill in this gap, we collected runoff samples in a typical MSW transfer station over the full length of a rainfall event to comprehensively characterize the pharmaceutical contamination profiles. The results showed that 18 pharmaceuticals were detected in the runoff samples with high frequencies and concentrations ranging from below MQL to 18.6 μg/L. During the rainfall event, pharmaceuticals exhibited discrepant leachabilities as a result of different sorption capacities; two concentration peaks of each pharmaceutical were observed, suggesting the leaching effect by rainwater and the potential influence of human-related rinse. A further sampling campaign for one-week-long runoff samples generated by diurnal rinse water was conducted, and the results indicated comparable mass loads of pharmaceuticals in surface runoff receiving rinse water (0.37-8250 μg) to those in rainfall runoff (0.58-1754 μg), suggesting the similar discharge of pharmaceuticals from MSW transfer stations despite the weather. The estimated per capita discharge load of caffeine, one of the typical Pharmaceuticals, from MSW transfer stations was 4383 ng capita^-1 d^-1, higher than that in other emission sources, e.g. municipal wastewater effluent, indicating an overlooked influence of MSW transfer stations on its contamination in the surface waters in Shanghai.

First Flush Stormwater Runoff in Urban Catchments: A Bibliometric and Comprehensive Review

First flush is a phenomenon in stormwater runoff that has been considered a topic of great interest in the field of nonpoint source pollution. Despite several attempts to define the first flush quantitively, the specified characteristics of the phenomenon vary among sources. To address these uncertainties, a bibliometric and comprehensive review on published articles related to first flush was conducted. A corpus of 403 research articles was obtained from the Scopus database, which was then parsed using the CorText Manager for the bibliometric analysis. The study examined quantitative definitions of first flush from various sources; climate and topographic characteristics of monitoring and experimental sites where the studies on first flush were performed; the sample collection methods applied; the first flush values obtained on the studies and how it influenced the nonpoint source pollution in urban watersheds. A network map, two contingency matrices, and a Sankey diagram were created to visualize the relationship of significant keywords related to first flush, as well as their co-occurrences with journals, countries, and years. It was found that the strength of the first flush effect could vary depending on the geographical location of the site, climatic conditions, and the pollutants being analyzed. Therefore, initial rainfall monitoring, runoff sampling, and water quality testing were seen as critical steps in characterizing the first flush in urban catchments. Furthermore, the characterization of first flush was found to be significant to the selection of best management practices and design of low-impact development (LID) technologies for stormwater runoff management and nonpoint source pollution control.

Using multiple isotopes to identify sources and transport of nitrate in urban residential stormwater runoff

Increased nitrogen (N) from urban stormwater runoff aggravates the deterioration of aquatic ecosystems as urbanisation develops. The sources and transport of nitrate (NO₃^-) in urban stormwater runoff were investigated by analysing different forms of N, water isotopes (δD-H₂O and δ¹⁸O-H₂O), and NO₃^- isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) in urban stormwater runoff in a residential area in Hangzhou, China. The results showed that the concentrations of total N and nitrate N in road runoff were higher than those in roof runoff. Moreover, high concentrations of dissolved organic N and particulate N led to high total nitrogen (TN) concentrations in road runoff (mean: 3.76 mg/L). The high δ¹⁸O-NO₃^- values (mean: + 60 ± 13.1‰) indicated that atmospheric deposition was the predominant NO₃^- source in roof runoff, as confirmed by the Bayesian isotope mixing model (SIAR model), contributing 84-98% to NO₃^-. Atmospheric deposition (34-92%) and chemical fertilisers (6.2-54%) were the main NO₃^- sources for the road runoff. The proportional contributions from soil and organic N were small in the road runoff and roof runoff. For the initial period, the NO₃^- contributions from atmospheric deposition and chemical fertilisers were higher and lower, respectively, than those in the middle and late periods in road runoff during storm events 3 and 4, while an opposite trend of road runoff in storm event 7 highlighted the influence of short antecedent dry weather period. Reducing impervious areas and more effective management of fertiliser application in urban green land areas were essential to minimize the presence of N in urban aquatic ecosystems.

Development of a Multicriteria Scheme for the Identification of Strategic Areas for SUDS Implementation: A Case Study from Gijón, Spain

Spain has been pinpointed as one of the European countries at major risk of extreme urban events. Thus, Spanish cities pursue new urban plans to increase their resilience. In this scenario, experiences in the implementation of Sustainable Urban Drainage Systems (SUDS) have increased substantially. Nevertheless, few cities have developed a global urban strategy for SUDS, lacking, in many cases, a method to identify strategic areas to maximize their synergetic benefits. Furthermore, there is still a need for a holistic Multicriteria Decision Analysis (MCDA) framework that considers the four pillars of SUDS design. The city of Gijón, NW Spain, has been selected as a case study due to its environmental and climatic stresses. This research presents the methodology developed for this city, which aims to analyze the need for SUDS implementation throughout the identification of strategic areas. With this aim, a combination of Geographic Information System (GIS) software and the MCDA Analytical Hierarchical Process (AHP) were proposed. The results show the potential for SUDS’ implementation, according to nine criteria related to the SUDS’ design pillars. We found that the areas where the implementation of SUDS would bring the greatest functional, environmental and social benefits are mainly located in consolidated urban areas.

Transport process and source contribution of nitrogen in stormwater runoff from urban catchments

Nitrogen in urban stormwater has been widely studied, and effective management of nitrogen pollution is critical for improving urban stormwater and receiving water quality. This requires an in-depth understanding of the transport process and source contribution to both dissolved and particulate nitrogen in stormwater from urban catchments. In this study, 123 stormwater runoff samples were collected from an urban catchment during different rainfall events. Dissolved and particulate nitrogen concentrations in roof runoff, road runoff, and sewer flow were analyzed. The concentration of dissolved nitrogen was higher in roof runoff than in road runoff and sewer flow. However, the concentration of particulate nitrogen was lower in roof runoff than in road runoff and sewer flow. Isotopic analysis and Bayesian mixing models showed that road runoff was the largest source contributor of both nitrate and particulate organic nitrogen (PON) in sewer flow discharged from the study catchment. In addition, road runoff contributed the majority of PON associated with coarse particles (>105 μm), whereas PON associated with fine particles (<105 μm) was primarily washed-off of sewer sediments. The results provided several suggestions for the management of nitrogen pollution in urban catchments. This study could help to fully understand the transport and sources of nitrogen pollution in urban stormwater and provide recommendations to the government for implementing appropriate stormwater management strategies to minimize stormwater pollution.

Nutrient delivery efficiency of a combined sewer along a lake challenged by incipient eutrophication

Although sewage diversion outside of a lake's watershed is now ordinary practice in the restoration of eutrophic lakes, often the observed recovery is slower than expected and the internal load from the lake anoxic sediments is identified as a possible reason. However, in the case of combined sewer, the quantification of the residual nutrient load discharged from sewer spillways must also be questioned. In this paper, the diversion efficiency of the sewer system along the east coast of Lake Iseo, a prealpine Italian lake where eutrophication effects are still severe, is investigated. To this purpose, a representative part of the sewer system was modelled by PCSWMM and calibrated by using an extensive series of discharge measurements. Water quality monitoring during wet weather periods reveals that the first flush is common in tributary sewers, whereas it is absent along the main collector. Moreover, flow discharges are strongly affected by infiltration waters, which are controlled by the lake water level. The calibrated model, including infiltration modeling, was used to assess the annual overflow volumes and the nutrient load through a continuous 10-year simulation. Simulations were conducted both with regard to the current conditions and to a climate change scenario. Results show that the discharged residual load is at least 7 times larger than the design value, with the water infiltration contributing to 17% to the overflow volume and that non-structural practices could considerably reduce the overall impact of the sewer. This research thus provides important insight into the potential impact of combined sewer overflows on lacustrine environments and addresses effective mitigation measures in similar contexts.

Urban stormwater characterization, control, and treatment

This review summarizes over 280 studies published in 2019 related to the characterization, control, and management of urban stormwater runoff. A summary of quantity and quality concerns is provided in the first section of the review, serving as the foundation for the following sections which focus on the control and treatment of stormwater runoff. Finally, the impact of stormwater control devices at the watershed scale is discussed. Each section provides a self-contained overview of the 2019 literature, common themes, and future work. Several themes emerged from the 2019 literature including exploration of substrate amendments for improved water quality effluent from stormwater controls, the continued study of the role of vegetation in green infrastructure practices, and a call to action for the development of new models which generate reliable, computationally efficient results under the physical, chemical, biological, and social complexity of stormwater management. PRACTITIONER POINTS: Over 280 studies were published in 2019 related to the characterization, control, and treatment of urban stormwater. Studies on bioretention and general stormwater characteristics represented the two most common subtopics in 2019. Trends in 2019 included novel substrate amendments, studies on the role of vegetation, and advancements in computational models.

The characteristics of rainfall runoff pollution and its driving factors in Northwest semiarid region of China - A case study of Xi'an

With the effective control of point source pollution, rainfall runoff pollution has become the main source of water pollution in Xi'an. Understanding the characteristics and driving factors of rainfall runoff pollution would provide theoretical foundation for urban rainfall runoff pollution control. In this study, a total of 32 rainwater samples, 604 roof runoff samples and 608 road runoff samples obtained from literature and a total of 35 rainwater samples, 127 roof runoff samples and 70 road runoff samples collected by our group were mixed to analyze the rainfall runoff pollution from 2008 to 2019. The Mann-Kendall and Logarithmic Mean Divisia Index (LMDI) method were used to determine the trend of pollution and the driving factors of water quality changes. The results showed that the major pollutant from rainwater was nitrogen, while the main pollutants from roof and road runoff were COD and SS. The rainwater quality during the study period was "clean to slightly polluted". The roof runoff quality of most years (67%) was "clean to slightly polluted". The road runoff quality was poor: 22% was "moderately polluted", and 45% was "heavily polluted". The concentration of pollutants except COD in rainwater showed a decreasing trend, while the trend of pollutants in roof and road runoff was not completely consistent with that in rainwater. NH₃-N showed strong positive correlation with TN in roof runoff, which indicated common sources of these pollutants. There was a significant correlation between SS and COD in road runoff, and between SS and TP in roof runoff, suggesting SS was an important carrier of COD and TP. Technology innovation was the dominant factor affecting water quality, followed by industrial structure. Economic development and population scale contribute negatively to water quality improvement, and there was a sharp increase in the population scale effect in 2017.

Analysis of Urban Flood Inundation Patterns According to Rainfall Intensity Using a Rainfall Simulator in the Sadang Area of South Korea

An urban flood in the Sadang area located in South Korea was reproduced using a rainfall simulator. The rainfall simulator was developed to be able to demonstrate the rainfall intensity in range of 80–200 mm/h, and the artificial rainfall was created using 42 full cone type nozzles in the urban model. The uniformity coefficient of the rainfall distribution was 89.5%, which indicates the rainfall simulator achieved the high requirements for spatial uniformity. The flood experiments in the 1/200 scale model of the Sadang area were conducted using the rainfall simulator, and the flood patterns were investigated by changing the rainfall intensity. The rainwater mainly accumulated in the lowland of the crossroad where the entrances to the subway station are located. The flow velocity and the inundation depth were sharply increased until the rainfall intensity became 160 mm/h. Furthermore, the unstable human activities based on the moment and the friction instabilities also occurred from 160 mm/h. These results suggest that the study area requires flood damage mitigation facilities considering a rainfall intensity exceeding 160 mm/h.

Development and Calibration of a New Dripper-Based Rainfall Simulator for Large-Scale Sediment Wash-Off Studies

Rainfall simulators are useful tools for controlling the main variables that govern natural rainfall. In this study, a new drop-forming rainfall simulator, which consists of pressure-compensating dripper grids above a horizontal mesh that breaks and distributes raindrops, was developed to be applied in wash-off experiments in a large-scale physical model of 36 m2. The mesh typology and size, and its distance to drippers, were established through a calibration where rain uniformity and distributions of raindrop sizes and velocities were compared with local natural rainfall. Finally, the rain properties of the final solution were measured for the three rain intensities that the rainfall simulator is able to generate (30, 50 and 80 mm/h), obtaining almost uniform rainfalls with uniformity coefficients of 81%, 89% and 91%, respectively. This, together with the very suitable raindrop size distribution obtained, and the raindrop velocities of around 87.5% of the terminal velocity for the mean raindrop diameter, makes the proposed solution optimal for wash-off studies, where rain properties are key in the detachment of particles. In addition, the flexibility seen in controlling rain characteristics increases the value of the proposed design in that it is adaptable to a wide range of studies.