Department Urban Water Management

Stormwater Control Measures (SCMs) contribute to reducing micropollutant emissions from separate sewer systems. SCM planning and design are often performed by looking at the hydrological performance. Assessment of pollutant removal and the ability to comply with discharge concentration limits is often simplified due to a lack of data and limited monitoring resources. This study analyses the impact of using different time resolutions of input stormwater concentrations when assessing the compliance of SCMs against water quality standards. The behaviour of three indicator micropollutants (MP - Copper, Diuron, Benzo[a]pyrene) was assessed in four SCM archetypes, which were defined to represent typical SCM removal processes. High resolution MP data were extrapolated by using high resolution (2 min) measurements of TSS over a long period (343 events). The compliance assessment showed that high resolution input concentrations can result in a different level of compliance with water quality standards, especially when discharged concentrations are close to the limit values. This study underlines the importance of considering the high temporal variability of stormwater micropollutants when planning and designing SCMs to identify the most effective solutions for stormwater pollution management and to ensure a thorough consideration of all the environmental implications.

Combined sewer overflows (CSOs) are an important pathway of organic micropollutants from urban areas to open water bodies. Understanding the temporal dynamics of these micropollutants during overflow events is crucial for applying appropriate sampling methods and implementing effective management strategies. Yet, little is known about the dynamics of micropollutants in CSOs, because most studies report concentrations from single grab samples or event mean concentrations (EMCs). With unique high temporal resolution measurements (3 min), we show the real dynamics of polar organic micropollutants in CSOs of one small (2,700 people: P) and one large (159,000 P) urban catchment, for two micropollutant categories: (i) 33 micropollutants in municipal wastewater and (ii) 13 micropollutants from urban surface runoff. The concentration dynamics depend on the substance source and the catchment size. Indoor substances such as pharmaceuticals show high temporal dynamics with changes of 1 to 2 orders of magnitude within 9 min in the CSO of the small catchment. In contrast, outdoor substances at the small catchment and all substances at the large catchment display considerably lower variation. We tested various time-proportional sampling strategies to assess the range of error when estimating EMCs. We recommend an interval of 3 min to capture the dynamics of indoor substances in CSOs from small catchments. The results highlight that both future monitoring campaigns and the planning and management of urban wet-weather treatment systems will benefit from high temporal sampling resolutions, not only to understand dynamics but also to minimize errors of estimated EMCs.

Urban stormwater was traditionally managed for flood control and property protection. But this view is changing as urban stormwater is increasingly recognized as an underused water resource. As cities implement stormwater capture, treatment, and use practices, there is a growing need to address water quality criteria beyond traditional pollutants such as nutrients, total suspended solids, pathogens, and metals. Current use practice and regulations are lacking on the occurrence of persistent, mobile and toxic (PMT) and very persistent, very mobile (vPvM) organic substances. These substances are poorly removed in conventional stormwater treatment systems, e.g., urban blue-green infrastructure, and, thus, may pose a risk when urban runoff is used to augment drinking water sources or discharged to surface waters. We reviewed 97 stormwater monitoring studies and identified 49 PMT/vPvM substances detected in urban stormwater. Although detection does not equate with risk, this highlights the potential importance of urban stormwater as a source of PMT/vPvM substances released to water resources. There is a lack of surveillance data on PMT/vPvM substances in urban stormwater and their fate in stormwater control measures, which hinders reliable risk assessments of stormwater capture. Unified guidelines are needed to (i) monitor PMT/vPvM substances in urban stormwater runoff, (ii) assess the human and environmental risks PMT/vPvM substances may pose for urban stormwater capture and use, and (iii) establish improved stormwater management and use criteria to ensure safe stormwater capture for water supply.

Urban wet-weather discharges from combined sewer overflows (CSO) and stormwater outlets (SWO) are a potential pathway for micropollutants (trace contaminants) to surface waters, posing a threat to the environment and possible water reuse applications. Despite large efforts to monitor micropollutants in the last decade, the gained information is still limited and scattered. In a metastudy we performed a data-driven analysis of measurements collected at 77 sites (683 events, 297 detected micropollutants) over the last decade to investigate which micropollutants are most relevant in terms of 1) occurrence and 2) potential risk for the aquatic environment, 3) estimate the minimum number of data to be collected in monitoring studies to reliably obtain concentration estimates, and 4) provide recommendations for future monitoring campaigns. We highlight micropollutants to be prioritized due to their high occurrence and critical concentration levels compared to environmental quality standards. These top-listed micropollutants include contaminants from all chemical classes (pesticides, heavy metals, polycyclic aromatic hydrocarbons, personal care products, pharmaceuticals, and industrial and household chemicals). Analysis of over 30,000 event mean concentrations shows a large fraction of measurements (> 50%) were below the limit of quantification, stressing the need for reliable, standard monitoring procedures. High variability was observed among events and sites, with differences between micropollutant classes. The number of events required for a reliable estimate of site mean concentrations (error bandwidth of 1 around the "true" value) depends on the individual micropollutant. The median minimum number of events is 7 for CSO (2 to 31, 80%-interquantile) and 6 for SWO (1 to 25 events, 80%-interquantile). Our analysis indicates the minimum number of sites needed to assess global pollution levels and our data collection and analysis can be used to estimate the required number of sites for an urban catchment. Our data-driven analysis demonstrates how future wet-weather monitoring programs will be more effective if the consequences of high variability inherent in urban wet-weather discharges are considered.

Untreated sewer overflows can contaminate receiving waters with micropollutants. Although concentrations of discharged micropollutants can be ecotoxicologically relevant, only limited data is available to assess occurrence and spatial differences among sewer overflow catchments. Therefore, we present an innovative type of data obtained with passive samplers at 20 combined sewer overflow sites (2-7 events per site; 95 events in total). The data sheds light on concentration ranges for 13 representative polar organic micropollutants and shows that micropollutants in both municipal wastewater and stormwater can be relevant sources of contaminants. We identify indicator micropollutants for further studies: benzotriazole (80% interquantile of time-weighted average concentration: 250–4800 ng/L), carbamazepine (33-910 ng/L), diclofenac (78-1000 ng/L), carbendazim (21-900 ng/L), diazinon (2.1-53 ng/L), diuron (22-1100 ng/L), mecoprop (98–5300 ng/L), metolachlor (6-230 ng/L), and terbutryn (29-810 ng/L). These concentration estimates are assumed to be on the safe side for comparison with environmental quality standards (EQS). A majority of sewer overflow sites (13 of 20) show discharge concentrations above acute EQS for at least one micropollutant and thus would have to rely on dilution by receiving waters to not exceed any EQS. The intersite variability among sewer overflows exceed the within-site variability. Hence, future monitoring studies should cover more sewer overflow sites. No correlation could be found with event durations, specific storage volume or land use data, thus showing the complexity of micropollutant occurrence and indicating that other factors led to the observed high spatial variability. In conclusion, our results clearly show the potential relevance of micropollutants in sewer overflows and the need to assess site-specific measures.

Niederschlags- und Mischabwasserentlastungen aus Siedlungen tragen zu Mikroverunreinigungen in Gewässern bei. Mit Passivsammlern wurden erstmals Konzentrationsbereiche an 20 Standorten für insgesamt 95 Ereignisse bestimmt, was die Analyse von Unterschieden zwischen Standorten sowie die Identifikation relevanter Eintragspfade unterstützt. An 13 Standorten waren bei mindestens einer Substanz die Konzentrationen im Mischabwasser höher als akute Qualitätskriterien.

Considerable pollutant loads can enter surface waters during rain events. Three factors challenge quantification of these pollutant fluxes using traditional sampling methods: (i) concentration fluctuations; (ii) unknown event duration; and (iii) placement, operation, and maintenance of equipment. Passive samplers offer the advantage of sampling in a continuous mode without power supply. However, variable uptake rates due to environmental factors and desorption in the case of fluctuating concentrations can affect the accuracy of time-weighted average (TWA) concentration estimates. While uncertainties related to environmental factors could be accounted for with additional effort, we can neither control nor quantify the concentration variability. We present measured and modelled concentration profiles at high temporal resolution and provide a systematic approach to assessing deviations from true TWA concentration due to fluctuating concentration profiles. We evaluate sampling of sewer overflows (0.3–14 h) with Chemcatcher and 1-week sampling in rivers. The uncertainty due to fluctuating concentrations is small, and other factors such as chemical analyses and sampler calibration have a similar or higher impact. The uncertainty due to fluctuations clearly increases with the sampling duration, particularly when exceeding the half-life of equilibrium. We conclude that passive sampling can also be used in wastewater systems with potentially high concentration variations.

Contaminants in sewer overflows can contribute to exceedances of environmental quality standards, thus the quantification of contaminants during rainfall events is of relevance. However, monitoring is challenged by i) high spatiotemporal variability of contaminants in events of hard-to-predict durations, and ii) a large number of remote sites, which would imply enormous efforts with traditional sampling equipment. Therefore, we evaluate the applicability of passive samplers (Empore styrene-divinylbenzene reverse phase sulfonated (SDB-RPS)) to monitor a set of 13 polar organic contaminants. We present calibration experiments at high temporal resolution to assess the rate limiting accumulation mechanisms for short events (<36 h), report parameters for typical sewer conditions and compare passive samplers with composite water samples in a field study (three locations, total 10 events). With sampling rates of 0.35–3.5 L/d for 1 h reference time, our calibration results indicate a high sensitivity of passive samplers to sample short, highly variable sewer overflows. The contaminant uptake kinetic shows a fast initial accumulation, which is not well represented with the typical first-order model. Our results indicate that mass transfer to passive samplers is either controlled by the water boundary layer and the sorbent, or by the sorbent alone. Overall, passive sampler concentration estimates are within a factor 0.4 to 3.1 in comparison to composite water samples in the field study. We conclude that passive samplers are a promising approach to monitor a large number of discharge sites although it cannot replace traditional stormwater quality sampling in some cases (e.g. exact load estimates, high temporal resolution). Passive samplers facilitate identifying and prioritizing locations that may require more detailed investigations.

Motivation. Our surface water is impacted by a wide variety of contaminants released through human activities. One contaminant group of concern is micropollutants, which can have eco-toxicological effects in trace concentrations. Micropollutants, for example, include plant protection products and biocides, which are specifically designed to harm organisms. An important input pathway of micropollutants to surface water can be untreated sewer overflows (combined sewer overflows and stormwater outlets). However, the understanding of factors influencing the occurrence and levels of micropollutants is limited and challenged by: i) high spatial differences among sites, ii) a large number of discharge sites and iii) high temporal fluctuating discharge events.

Research objective. This thesis intends to enhance our understanding of micropollutants in sewer overflows. It aims to investigate efficient methods to prioritize sewer overflow sites, which can be potentially harmful for receiving waters. The presented work focuses on the development of model prediction and efficient monitoring approaches and includes the following: i) a Swiss-wide model to assess how many discharge sites may be critical (Chapter 2), ii) fundamentals to develop passive sampling as an alternative monitoring method for short duration sewer overflows (Chapter 3 and 4) and iii) an extended monitoring study covering 20 combined sewer overflows (CSOs) to explore spatial differences among sites (Chapter 5). The main findings of this thesis are presented in the following.

Model-based screening. A dynamic substance flow modeling approach was applied to 2,500 Swiss municipalities, considering micropollutant accumulation and wash-off on urban surfaces. The approach is based on the simplification, that there is one CSO and one stormwater outlet per municipality. The model allows for an estimation of the micropollutant concentration fluctuations in sewer overflows in 10-min resolution. The results show the importance of untreated discharges from sewer overflows as emission into receiving waters. The load contribution to sewer overflows of micropollutants in stormwater (e.g. plant protection products and biocides) is estimated to be considerably higher than of micropollutants in municipal wastewater (e.g. pharmaceuticals and other household chemicals). In addition, the results indicate that up to 83% of all urban catchments have sewer overflows that need to rely on upstream dilution in order to not exceed substance-specific environmental quality standards (EQS). Thus, this nationwide screening approach clearly highlights the need for a more detailed assessment of micropollutants in sewer overflows, and especially, the need for more measured data to allow the development of validated micropollutant prediction models.

Measuring. Difficulties of monitoring sewer overflows with traditional methods are i) high concentration fluctuations with unknown event durations which require a high temporal sampling resolution, ii) many discharge sites and iii) placement, operation, and maintenance of equipment. Hence, passive samplers for polar organic micropollutants are analyzed systematically as a viable alternative to monitor short duration sewer overflows. The main advantage of passive samplers is the continuous accumulation of contaminants from the water phase, with no need for external energy sources nor for immense water sample storage volume. In a first step, the impact of fluctuating concentrations on deviations from true time-weighted average (TWA) concentrations was studied. The results highlight that the deviations induced by fluctuating concentrations are similar or smaller than uncertainties arising from chemical analysis and environmental factors. Understanding the uptake mechanism of micropollutants is one of the most important issues that needs to be resolved to reduce uncertainties in passive sampling data. The uptake experiments in sewer conditions for short duration events (<36h) indicate that the mass transfer is either dominated by the sorbent, or, by a multi-step mechanism. Therefore, a new semi-empirical mixed rate control model is proposed, which can be directly used for future studies under similar conditions. A field validation (3 locations, 10 events) shows that TWA concentration estimates measured by passive samplers are within a factor of 0.4 to 3.1 compared to composite water samples. Hence, the use of upper limit TWA concentration estimates is recommended for compliance checking with EQS. The findings clearly show that passive samplers are suitable to estimate TWA concentration ranges in sewer overflows and, thus facilitate identification of potentially critical discharge sites.

Occurrence of micropollutants. Unique monitoring data from 20 CSO sites was collected with passive samplers. In total 13 polar organic micropollutants were analysed in 95 events (2-7 events per site). The results highlight indicator micropollutants, which were often found in the monitored CSOs and could be used in future monitoring studies: diclofenac, benzotriazole, carbamazepine, diazinon, diuron, carbendazim, mecoprop, metolachlor and terbutryn. Further, our results reveal that the spatial differences among CSO sites are bigger than the inter-event differences within a site for all studied micropollutants. Nevertheless, no significant correlation with land use data could be identified. The results, therefore, suggest that additional factors, most likely the contaminant usage pattern, have a considerable influence on the observed spatial differences. Both municipal wastewater and stormwater contributed to concentrations above the EQS in CSOs (not in the receiving water). In addition, the monitoring with passive samplers indicates that at least 13 out of 20 CSOs show concentrations above EQS in CSOs, and would rely on dilution by receiving waters to not exceed EQS.

Outlook. Overall, the thesis shows the relevance of sewer overflows as a source of micropollutants entering receiving water bodies. The findings can serve as a basis for the development of a systematic and stepwise approach to efficiently identify critical sites. The development of such an approach would require joint efforts by eco-toxicologists, analytical chemists, regulators and wastewater engineers. This approach could be based on a simplified modeling approach as a first indicator, followed by passive sampling to assess micropollutant levels. To enable the widespread application of passive samplers in monitoring programs, regulators’ and operators’ confidence in passive sampling needs to be strengthened with more field validation studies and common guidelines.

The impacts of the unintended release of engineered nanomaterials (ENMs) and the resulting exposure to the ecosystems are still poorly understood and intensively debated. In current mass flow models, a global transfer coefficient is used to account for ENMs discharged from sewer systems to surface waters during rain events.
However, different types of sewer systems and specific ENM applications likely require a more elaborate approach to quantify ENM loads entering surface waters during rain events. Therefore, we evaluated the impact of distinguishing between different application areas of ENMs (indoor/outdoor) and different types of sewer systems (combined/separate) on the loads of selected ENMs passing through wastewater systems and reaching the surface water. We applied a dynamic mass flow analysis to eight substances (TiO₂, ZnO, Ag, SiO₂, FeO_x, CuO, CeO₂ and carbon black) for six different sewer system configurations. To estimate the runoff from streets and facades, we used a hydrological model developed for dissolved substances. For areas with separate sewer systems only and ENMs predominantly used in and released from outdoor applications (e.g. CuO, carbon black), up to ~80% of the annual loads entering the sewer systems never reach a sewage treatment plant. Those ENMs are directly discharged to surface waters via stormwater outlets. For combined sewer systems and ENMs mainly applied indoors (e.g. Ag, ZnO), the percentage of ENM bypassing sewage treatment plants remains below 2%. Although the model is setup for Switzerland and other geographical areas will show for example different rain characteristics, the relative importance of the different release pathways will broadly remain the same and the results should, therefore, be transferrable to other countries.

The promise of collecting and utilising large amounts of data has never been greater in the history of urban water management (UWM). This paper reviews several data-driven approaches which play a key role in bringing forward a sea change. It critically investigates whether data-driven UWM offers a promising foundation for addressing current challenges and supporting fundamental changes in UWM. We discuss the examples of better rain-data management, urban pluvial flood-risk management and forecasting, drinking water and sewer network operation and management, integrated design and management, increasing water productivity, wastewater-based epidemiology and on-site water and wastewater treatment. The accumulated evidence assembled from research documented in the literature points towards a future UWM that offers significant potential benefits thanks to increased collection and utilisation of data. The findings show that data-driven UWM allows us to develop and apply novel methods, to optimize the efficiency of the current network-based approach, and an extended functionality of today’s systems. However, generic challenges related to data-driven approaches (e.g. data processing, data availability, data quality, data costs) and the specific challenges of data-driven UWM need to be addressed, namely data access and ownership, current engineering practices and the difficulty of assessing the cost benefits of data-driven UWM.

High spatio-temporal variability characterizes micropollutants discharged from separate and combined sewers. This challenges monitoring and prediction. We propose passive samplers (Chemcatcher_® for polar, organic substances) as an alternative to active water sampling. Our results indicate sufficient sensitivity of passive samplers for short exposures so that typical rain event durations (<30 h) can be sampled integratively. Although time-weighted average concentrations estimated via passive sampling are higherthan in composite water samples -on average by a factor 2.7 - we conclude that passive sampling is a cost-effective approach to identify locations with critical concentration ranges.

Wet-weather discharges contribute to anthropogenic micropollutant loads entering the aquatic environment. Thousands of wet-weather discharges exist in Swiss sewer systems, and we do not have the capacity to monitor them all. We consequently propose a model-based approach designed to identify critical discharge points in order to support effective monitoring.
We applied a dynamic substance flow model to four substances representing different entry routes: indoor (Triclosan, Mecoprop, Copper) as well as rainfall-mobilized (Glyphosate, Mecoprop, Copper) inputs. The accumulation on different urban land-use surfaces in dry weather and subsequent substance-specific wash-off is taken into account. For evaluation, we use a conservative screening approach to detect critical discharge points. This approach considers only local dilution generated onsite from natural, unpolluted areas, i.e. excluding upstream dilution.
Despite our conservative assumptions, we find that the environmental quality standards for Glyphosate and Mecoprop are not exceeded during any 10-min time interval over a representative one-year simulation period for all 2500 Swiss municipalities. In contrast, the environmental quality standard is exceeded during at least 20% of the discharge time at 83% of all modelled discharge points for Copper and at 71% for Triclosan. For Copper, this corresponds to a total median duration of approximately 19 days per year. For Triclosan, discharged only via combined sewer overflows, this means a median duration of approximately 10 days per year. In general, stormwater outlets contribute more to the calculated effect than combined sewer overflows for rainfall-mobilized substances. We further evaluate the Urban Index (A_{urban,impervious}/A_natural) as a proxy for critical discharge points: catchments where Triclosan and Copper exceed the corresponding environmental quality standard often have an Urban Index >0.03.
A dynamic substance flow analysis allows us to identify the most critical discharge points to be prioritized for more detailed analyses and monitoring. This forms a basis for the efficient mitigation of pollution.

Im Forum Chriesbach der Eawag ist ein NoMix-System installiert, das den Urin sammelt und getrennt vom restlichen Abwasser abführt. Da der Urin einen Grossteil an Nährstoffen wie Stickstoff enthält, kann er separat aufbereitet und anschliessend als landwirtschaftlicher Dünger genutzt werden. Im Leitungssystem wird der Harnstoff, welcher die grösste Stickstoffkomponente im Urin bildet, durch Bakterien abgebaut. Für ein besseres Verständnis der Harnstoffhydrolyse muss bekannt sein, wie lange der Urin im Leitungssystem verweilt. Um die Aufenthaltszeit von Urin zu quantifizieren wurden in dieser Arbeit Tracerversuche mit Bromid durchgeführt. Dabei wurden neben einer mittleren Aufenthaltszeit auch der Einfluss der Benutzerfrequenz und der Distanz, also dem Stockwerk aus dem der Urin kommt, untersucht. Zusätzlich wurde die Aufenthaltszeit mit Hilfe einer Urinbilanz berechnet, bei der die summierten Input und Output Kurven des Urins aufgezeichnet wurden. Die zeitliche Verschiebung der beiden Kurven beschreibt eine mittlere Verweilzeit.
Die Tracerversuche zeigten, dass sich die Aufenthaltszeit des Bromids und des eigentlichen Urins deutlich unterscheiden. Der Tracer repräsentiert somit nicht wie ursprünglich angenommen den Grundstrom. Wie aus der Urinbilanz hervorging hat der Urin eine mittlere Verweilzeit von 31 Minuten mit einer Standartabweichung von 12 Minuten. Die mittlere Aufenthaltszeit des Bromids ist gemäss den Tracerversuchen etwa viermal grösser. Das unterschiedliche Verhalten ist dadurch zu erklären, dass das Bromid im Leitungsrohr in den Biofilm diffundiert und deshalb mehr Zeit als der Urin benötigt. Mit zunehmender Benutzerfrequenz gleichen sich die beiden Aufenthaltszeiten an, da mehr Urin vorhanden ist, in welches das Bromid zurückdiffundieren kann. Das Stockwerk beeinflusst die Aufenthaltszeit des Urins und des Bromids. Der Einfluss auf das Bromid ist jedoch grösser, da die Diffusion in den Biofilm in den vertikalen Leitungen grösser ist als in den horizontalen.
Auf die Verweilzeit des Harnstoffs konnte aus den Ergebnissen auf Grund der Diffusion nicht geschlossen werden, da weder der Urin noch das Bromid repräsentativ sind. Es kann aber mit grosser Wahrscheinlichkeit gesagt werden, dass auch der Harnstoff und weitere gelöste Stoffe in den Biofilm diffundieren und damit eine grössere Aufenthaltszeit als der Urin besitzen.

Only little is known about the micropollutants from urban areas discharged via combined sewer overflows during rain events. In urban catchments various sources of micropollution
occur that change over time and with different kind of land uses. In Switzerland different rain regions and various land use types have to be considered to characterise the large variability of urban drainage systems. Thus a geographical information system based analysis of the urban areas in Switzerland combined with the modelling of the amount of pollutants released via combined sewer over flows during rain events is feasible to asses the dominant sources. Therefore a model was developed to determine the combined sewer over flow emissions loads of micropollutants for Switzerland. This model accounts for the main urban sources of micropollutants and the variation in the urban area of Switzerland.
The overall model results show, that combined sewer over flow emissions of urban areas are an important contribution to the pollutant load in surface water. Especially for pollutants
washed-off impervious areas, such as roofs or traffic surfaces, the load discharged via combined sewer over flow was estimated to be in the same range as the load discharged to the wastewater treatment plant. Thus the discharge via combined sewer over flow can result in acute effects on the environment. The result also emphasizes the importance of mitigation measures at the pollutant source rather then end-of-pipe measures.