The urban stretch of the River Spree is a regulated lowland-river, which is affected by a number of anthropogenic pressures, most notably impacts from combined sewer overflows (CSO) of the Berlin sewer system. Collected data show that occurrence of CSO can be detected in the river through a combination of continuous monitoring data, such as specific conductivity, ammonium (NH4), chemical oxygen demand and dissolved oxygen (DO). Comparison with stormwater guidelines indicates that drops in DO from CSO lead to regular problematic conditions for the fish fauna. In contrast, observed NH4 peaks never reach fish-toxic levels. Mitigation measures are currently implemented to reduce these negative impacts during storm events. The effect of past and potential future CSO measures can be studied with a model tool, which has been tested and is currently calibrated based on the above monitoring data.

The effect of combined sewer overflow (CSO) control measures should be validated during operation based on monitoring of CSO activity and subsequent comparison with (legal) requirements. However, most CSO monitoring programs have been started only recently and therefore no long-term data is available for reliable efficiency control. A method is proposed that focuses on rainfall data for evaluating the effectiveness of CSO control measures. It is applicable if a sufficient time-series of rainfall data and a limited set of data on CSO discharges are available. The method is demonstrated for four catchments of the Berlin combined sewer system. The analysis of the 2000–2007 data shows the effect of CSO control measures, such as activation of in-pipe storage capacities within the Berlin system. The catchment, where measures are fully implemented shows less than 40% of the CSO activity of those catchments, where measures have not yet or not yet completely been realised.

During periods of heavy rainfall storm sewage volumes can exceed the capacity of combined sewer systems and overflow to surface water bodies. Combined sewer overflows (CSO) cause significant impacts on the water quality and their identification is crucial to plan CSO control programs or to fulfil legal requirements. This paper proposes and demonstrates six different methods to identify the occurrence of CSO based on information on the sewer system alone (methods 1 and 2), in combination with rain data (methods 3 and 4) or in combination with water quality data of the receiving surface water (methods 5 and 6). The methods provide different information on CSO, from occurrence to pollution load and impacts in receiving surface water. The methods introduced have all been applied to the Berlin urban water system. Based on these experiences they are compared considering the effort needed for their application, the required data and the obtained output. It is concluded that certainty of CSO identification can be increased by combining some of the presented methods.

In the city of Berlin regular combined sewer overflows (CSO) lead to acute stress of aquatic organisms in the receiving River Spree and its side channels. Of most concern are oxygen depressions, following the inflow of degradable organic matter via ~180 CSO outlets, along a river stretch of 16 km. For the assessment of the severity of these oxygen depressions, an existing impact-based approach suggested by Lammersen (1997) was combined with information on the local fish fauna. Application of this locally adapted assessment method to seven years of oxygen measurements at a CSO hotspot in the river yielded an annual average of 14 periods with suboptimal conditions for which adverse effects on the fish fauna are expected and 20 periods with critical conditions for which acute fish kills are possible. Further investigation on rain and sewer management data proved that such critical conditions only occurred as a direct result of CSO events, whereas suboptimal conditions are also possible at dry weather and may last up to 32 days (Riechel et al. 2010).

During its passage through the City of Berlin (Germany), the quality of the River Spree is strongly influenced by combined sewer overflows (CSO), which lead to critical oxygen concentrations (DO) every year and to occasional larger fish kills. A continuous integrated monitoring concept, using state-of-the-art online sensors, was planned and started in spring 2010. It combines (i) continuous measurements of the quality and flow rates of CSO at one main CSO outlet downstream of the overflow structure and (ii) continuous measurements of water quality parameters at five sites within the urban stretch of the receiving River Spree. The first monitoring results show that continuous water quality measurements in CSO outlets and at downstream river stations are possible at high accuracy, even for comparably complex parameters such as chemical oygen demand (COD). Analysis of measured data confirms the significance of CSO discharges on receiving waters and underlines the value of continuous measurements in describing the local dynamics of the CSO and their impacts on water bodies.

To gain better understanding of the impact of combined sewer overflows (CSO) on the chemical and ecological status of lowland rivers and to evaluate the effect of CSO control measures a planning instrument for impact-based CSO management is being developed in Berlin, Germany. After completion the model-based planning instrument will be used by the Berlin water and wastewater utility and the water authority for scenario analysis of CSO management strategies. To adapt the planning instrument to their respective needs and to guarantee an efficient transfer of the results a specific project structure was established. Through direct participation in project management, technical and scientific work as well as demonstration the end-users can influence the development and provide technical input on local issues. First project results show the relevance of CSO impacts compared to the background condition of the Berlin river system and the need for additional measurements to provide data for model adaptation, calibration and validation.

The effect of combined sewer overflow (CSO) control measures should be validated during operation based on monitoring of CSO activity and subsequent comparison with (legal) requirements. However, most CSO monitoring programs have been started only recently and therefore no long-term data is available for reliable efficiency control. A method is proposed that focuses on rainfall data for evaluating the effectiveness of CSO control measures. It is applicable if a sufficient time-series of rainfall data and a limited set of data on CSO discharges are available. The method is demonstrated for four catchments of the Berlin combined sewer system. The analysis of the 2000-2007 data shows the effect of CSO control measures, such as activation of in-pipe storage capacities within the Berlin system. The catchment, where measures are fully implemented shows less than 40 % of the CSO activity of those catchments, where measures have not yet or not yet completely been realised.

The quality of the River Spree during its passage through the city of Berlin is mainly influenced by the discharge of treated effluent from waste water treatment plants and by combined sewer overflows (CSO). CSO are discharged diffusely and during short periods of time leading to acute impacts like oxygen depletion and locally increased ammonia concentration in the river. They are dominant stress factors to Berlin’s lowland River Spree and its biocenosis. In order to improve the water quality of the River Spree, measures limiting the emissions of CSO are envisaged such as utilization of in-pipe storage capacities, implementation of weirs for real-time control, construction of additional stormwater tanks. In order to build an efficient and immission oriented strategy with the different available solutions and to be able to cope with future challenges the Berlin Centre of Competence for Water (KWB), Berliner Wasserbetriebe, Veolia Water and the Berlin Senate of Environment are conducting two projects, the EU project PREPARED and the MIA-CSO project. An impact-based CSO management instrument is being developed with the aim to evaluate measures of CSO control. It consists in (i) a river water quality/ecosystem model that will be used to simulate water quality processes in the receiving water and (ii) a methodology to identify critical water quality situations occurring in the Spree River. For model adaptation, calibration and validation an integrated monitoring is conducted. The monitoring consists in (i) continuously measuring the quality and flow of CSO discharges at one representative network location and (ii) in parallel, continuously monitoring water quality parameters at 5 sites within the impacted stretch of the Spree River. The concept of the integrated monitoring, i.e. definition of monitoring sites as well as monitoring strategy and design will be presented during the M3 Workshop.

Stormwater impact guidelines for dissolved oxygen (DO) were applied to the Berlin River Spree, which (a) receives the effluents of more than 100 combined sewer discharge points and (b) is subject to significant anthropogenic background pollution. Discrimination of DO depressions, which are the direct result of combined sewer overflows (CSO) from DO depressions which are not related to CSO was achieved by combining stormwater impact guidelines with the analysis of data for: (i) rain events before critical DO depressions, (ii) water temperature (T) and conductivity as indicators for CSO impact in the river and (iii) T and DO before critical DO depressions to assess the effect of background pollution. Results indicate that the River Spree is in a critical state regarding DO for two main reasons: (a) upstream of the stretch with CSO discharge points because of background pollution and (b) downstream of the stretch because of CSO. Highly critical situations with DO < 2 mg L-1 only occurred under CSO influence. Nevertheless, the analysis underlines the importance of measures to reduce both CSO and background pollution in urban rivers.