First flush stormwater pollution in urban catchments: A review of its characterization and quantification towards optimization of control measures

High-frequency monitoring during rainstorm events reveals nitrogen sources and transport in a rural catchment

Rural areas lacking essential sewage treatment facilities and collection systems often experience eutrophication due to elevated nutrient loads. Understanding nitrogen (N) sources and transport mechanisms in rural catchments is crucial for improving water quality and mitigating downstream export loads, particularly during storm events. To further elucidate the sources, pathways, and transport mechanisms of N from a rural catchment with intensive agricultural activities during storm events, we conducted an analysis of 21 events through continuous sampling over two rainy seasons in a small rural catchment from the lower reaches of the Yangtze River. The results revealed that ammonia-N (NH4+-N) and nitrate-N (NO3--N) exhibited distinct behaviors during rainstorm events, with NO3--N accounting for the primary nitrogen loss, its load being approximately forty times greater than that of NH4+-N. Through examinations of the concentration-discharge (c-Q) relationships, the findings revealed that, particularly in prolonged rainstorms, NH4+-N exhibited source limited pattern (b = -0.13, P < 0.01), while NO3--N displayed transport limited pattern (b = -0.21, P < 0.01). The figure-eight hysteresis pattern was prevalent for both NH4+-N and NO3--N (38.1% and 52.0%, respectively), arising from intricate interactions among diverse sources and pathways. For NO3--N, the hysteresis pattern shifted from clockwise under short-duration rainstorms to counter-clockwise under long-duration rainstorms, whereas hysteresis remained consistently clockwise for NH4+-N. The hysteresis analysis further suggests that the duration of rainstorms modifies hydrological connectivity, thereby influencing the transport processes of N. These insights provide valuable information for the development of targeted management strategies to reduce storm nutrient export in rural catchments.

New optimization strategies for SWMM modeling of stormwater quality applications in urban area

Build-up/wash-off models were originally developed for small-scale laboratory facilities with uniform properties. The effective translation of these models to catchment scale necessitates the meticulous calibration of model parameters. The present study combines the Mat-SWMM tool with a genetic algorithm (GA) to improve the calibration of build-up and wash-off parameters. For this purpose, Mat-SWMM was modified to equip it with the capacity to provide comprehensive water quality analysis outcomes. Additionally, this research also conducts a comparative examination of two distinct types of objective functions in the optimization. Rather than depending on previous literature, this study undertook a numerical campaign to ascertain an appropriate range for the relevant parameters within the case study, thereby ensuring the optimization algorithm's efficient functionality. This research also implements an integrated event calibration approach, i.e., a novel method that calibrates all rainfall events collectively, thus improving systemic interaction representation and model robustness. The findings indicate that employing this methodology significantly enhances the reliability of the outcomes, thereby establishing a more robust procedure. The first objective function (TSS instantaneous less squared difference function, OF 1), which is widely employed in the literature, was designed to minimize the difference between observed and predicted instantaneous Total Suspended Solids (TSS) concentrations. In contrast, the second function (mass and mass peak consistency function, OF 2) considers integral model outputs, i.e., the overall mass balance, the time of the peak mass flow rate, and its intensity. The analysis of the outputs revealed that both objective functions demonstrated sufficient performance. OF 1 provided slightly better performance in predicting the TSS concentrations, whereas OF 2 demonstrated superior ability in capturing global event characteristics. Notably, the optimal parameter set identified through OF 2 aligned with the physically plausible ranges traditionally recommended in technical manuals for urban catchments. In contrast, OF 1's optimal set necessitated an expansion in the acceptable parameter ranges. Finally, from a computational burden viewpoint, OF 1 demanded a significantly higher number of function evaluations, thus implying an escalating computational cost as the range expands. Conversely, OF 2 necessitated fewer evaluations to converge toward the optimal solution.

Performance of wood waste biochar and food waste compost in a pilot-scale sustainable drainage system for stormwater treatment

Sustainable drainage system (SuDS) for stormwater reclamation has the potential to alleviate the water scarcity and environmental pollution issues. Laboratory studies have demonstrated that the capacity of SuDS to treat stormwater can be improved by integrating biochar and compost in the filter media, whereas their performance in scaled-up applications is less reported. This study examines the effectiveness of a pilot-scale SuDS, bioswale followed by bioretention, amended with wood waste biochar (1, 2, and 4 wt.%) and food waste compost (2 and 4 wt.%) to simultaneously remove multiple pollutants including nutrients, heavy metals, and trace organics from the simulated stormwater. Our results confirmed that SuDS modified with both biochar (2 wt.%) and compost (2 wt.%) displayed superior water quality improvement. The system exhibited high removal efficiency (> 70%) for total phosphorus and major metal species including Ni, Pb, Cd, Cr, Cu, and Zn. Total suspended solids concentration was approaching the detection limit in the effluent, thereby confirming its capability to reduce turbidity and particle-associated pollutants from stormwater. Co-application of biochar and compost also moderately immobilized trace organic contaminants such as 2,4-dichlorophenoxyacetic acid, diuron, and atrazine at field-relevant concentrations. Moreover, the soil amendments amplified the activities of enzymes including β-D-cellobiosidase and urease, suggesting that the improved soil conditions and health of microbial communities could possibly increase phyto and bioremediation of contaminants accumulated in the filter media. Overall, our pilot-scale demonstration confirmed that the co-application of biochar and compost in SuDS can provide a variety of benefits for soil/plant health and water quality.

Characterization of road-deposited sediment wash-off and accurate splitting of initial runoff pollution in heterogeneous urban spaces

Particulate materials arising from road-deposited sediments (RDS) are an essential target for the control and management of surface runoff pollution. However, the heterogeneity of urban spaces hinders the identification and quantification of particulate pollution, which is challenging when formulating precise control measures. To elucidate the factors that drive particulate pollution in heterogeneous urban spaces, the accumulation of RDS on dry days and the total suspended solids during six natural rainfall events were investigated across three urban-rural spatial units (central urban, central suburban, and remote suburban). The underlying surface type (asphalt or cement roads) and particle size composition jointly determined the spatial heterogeneity in the static accumulation and dynamic output loads of RDS during rainfall. These two factors explained 59.6% and 18.9% of the spatial heterogeneity, respectively, according to principal component analysis. A novel CPSI exponential wash-off equation that incorporates particle size composition and underlying surface type was applied. It precisely described the spatial heterogeneity of RDS wash-off loads, the estimated values exhibiting event mean concentration errors of 10.8-18.2%. When coupled with the M(V) curve, this CPSI exponential wash-off equation more precisely split the initial volume of runoff: a lower total volume (17.6-38.0%) was shown to carry a higher proportion of the load (70.0-93.7%) compared to the traditional coupled exponential wash-off equation (volume: 31.6-49.0%, load: 37-90%). This study provides a new approach to characterizing RDS wash-off processes and splitting initial runoff in heterogeneous spaces.

Evolution of water quality in rainwater harvesting systems during long-term storage in non-rainy seasons

The development of rainwater utilization strategies has relied on rainwater harvesting (RWH) systems for centuries to alleviate the pressure on water resources. However, there are still significant knowledge gaps regarding the changes in water quality in RWH systems during long-term storage in non-rainy seasons. This study evaluated the water quality processes in RWH systems through static rainwater storage experiments for approximately 60 days. The results revealed that nutrients in rainwater accumulated in sediment during storage. Disturbance and redox conditions at the rainwater-sediment interface contribute to the release of sedimentary facies materials. The rainwater showed distinct DO stratification, with the biochemical reactions of sedimentary facies being the primary factor driving oxygen consumption. ORP and turbidity showed positive correlations with COD (r = 0.582; 0.572), TOC (r = 0.678; 0.681), TN (r = 0.452; 0.439), and NH4+-N (r = 0.502; 0.553) (P < 0.05). The regulation of water quality and extension of the usage cycle were identified as critical factors influenced by DO. In addition, bacteria share similar ecological niche preferences. These findings provide scientific evidence for the high-quality reuse of rainwater in decentralized RWH systems during long-term storage in non-rainy seasons.

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.