Wastewater treatment plants (WWTPs) are built to treat all wastewater during dry weather. During rainy weather, however, so much more water is discharged into storm and combined sewer systems that the capacities of the ARA are exceeded. To capture heavily polluted stormwater runoff, combined sewer overflows (CSO) have been built since the 1970s to temporarily store diluted combined sewage or discharge it directly into rivers and lakes.
Despite today's technological possibilities, however, these facilities are often not operated optimally for various reasons:
- In some cases, there is a lack of measurement data on the storage and overflow behavior of the plants.
- If measurement data are available, there is often a lack of understanding in the handling of the data. For example, they are logged incorrectly, or are only evaluated for operational aspects, such as alarm messages in the event of malfunctions. For effective enforcement of water protection regulations, however, not only should the authorities know whether plants are functioning well or not, but the data should also be processed and archived for future planning.
- Organizational responsibilities are often not clearly defined, so that ARAs treat a community's wastewater but do not operate the RÜBs and therefore cannot influence RÜB performance.
All of the above problems indicate that in many communities, the infrastructure has been paid for but is being operated in a way that does not fully utilize its potential performance. Moreover, since about 2 billion CHF are to be invested in similar facilities in the next decades, it is necessary to investigate now what organizational and political changes are needed.
The aim of the POLAAR project was therefore to create a better understanding of existing organizational and political conditions and possible challenges in Swiss urban drainage practice. Specifically, we therefore investigated which challenges exist from a socio-technical point of view in relation to the development towards a measurement-based and integrated management of sewer networks and WWTPs. Methodologically, this question was addressed through three separate studies.
The main results
The first study deals with possible challenges regarding the evaluation of measurement data from the sewer network (usually CSO) by cantonal technical offices in Switzerland. The results of a comparative analysis based on interview data show that in most Swiss cantons a vision for measurement-based urban drainage is missing or human resources for the evaluation of measurement data are insufficient. (Status of data: 2017).
In the second study, we developed a conceptual approach to apply socio-technical networks in the context of infrastructure management and describe relevant methodological analysis concepts. The theoretical approach is tested on the example of an WWTP catchment in Switzerland to investigate the interplay between actors and infrastructure elements. Such an empirical socio-technical network can be used to find out, for example, which facilities are already equipped with measurement technology or who can access the measurement data. In this way, further challenges can be identified, such as the lack of access to measurement data by certain actors.
In the third study, the socio-technical network approach was applied to different ARA catchments in the canton of Zurich. Through interviews, document analysis, and an online survey, data were collected from three WWTP catchments in the Canton of Zurich. A statistical network analysis was used to investigate the influence of possible catchment-specific circumstances, such as the form of organization, access to measurement data and progress in terms of measurement-based and integrated management of WWTPs and sewer networks.
In addition, an empirical study on the current status of measurement-based management of sewer networks in Switzerland was conducted in close collaboration with the VSA. Our results (as of 2017, not representative) indicate that a deficit exists in that measurement data are mainly collected for operational optimization and are rarely used as a basis for planning or forwarded to cantonal authorities for performance monitoring. Similar to VSA-DSS, measurement data management should be harmonized today in a similar way. Sewer network operators and cantonal authorities both would like to see concrete targets, either from cantonal technical agencies or from the federal government. Interestingly, this is in line with the conclusions of a master thesis that used an agent-based model to investigate how three different policy instruments affect the use of sensors in stormwater treatment plants in Switzerland. The results suggest that regulatory requirements are an effective strategy.
Concrete recommendations for practice
Based on the results obtained, we recommend 5 concrete strategies or measures.
- The development towards optimal management of urban drainage infrastructure at the level of the WWTP catchment should be promoted. To merge the operation, planning and management of WWTPs and sewer networks at the technical level, stakeholders from the "WWTP side" and the "sewer network" side need to work together.
- In order to use measurement data effectively, the necessary resources should be demanded from the operators, and standardized tools for measurement data management should be provided, for example byauthorities or the VSA, analogous to the VSA-DSS data model (mini).
- The integrated management of ARA and sewer networks requires not only coordinated management of technical infrastructure elements, but also integration at the actor level, so that the relevant actors coordinate with each other. Therefore, policy design should be tailored to the socio-technical characteristics of ARA catchments.
- Measurement-based performance control, or "monitoring and reporting", should already be implemented with simple performance indicators, even if effective, or "correct", performance indicators for sewer networks are still the subject of research today. Internationally, it has been shown that even simple performance indicators, such as days with overflow or the duration of discharges per year, are useful. Possible practical integrated performance indicators for network and ARA are proposed in the (inter)national regulations on integrated management and self-monitoring and self-control.
- In order to learn from mistakes in other countries in the implementation of measurement-based performance monitoring, international cooperation should be strengthened in the next years, when, for example, the European "Urban Wastewater Treatment Directive" will be implemented in neighboring countries. This concerns not only technical aspects, but also improved communication about the effects of the larger transparecies, in particular the operational behavior of RÜB and the intentional periodic discharge of untreated wastewater into the environment. For example, in the UK, there have been very controversial public and political discussions about combined sewer overflows, resulting in extremely stringent legislation on discharges from residential areas when it rains.