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.

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.

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.

The present study examines the contribution of combined sewer overflows (CSO) to loads and concentrations of trace contaminants in receiving surface water. A simple method to assess the ratio of CSO to wastewater treatment plant (WWTP) effluents was applied to the urban River Spree in Berlin, Germany. The assessment indicated that annual loads are dominated by CSO for substances with removal in WWTP above w95%. Moreover, it showed that substances with high removal in WWTP can lead to concentration peaks in the river during CSO events. The calculated results could be verified based on eight years of monitoring data from the River Spree, collected between 2000 and 2007. Substances that are well removed in WWTP such as NTA (nitrilotriacetic acid) were found to occur in significantly increased concentration during CSO, while the concentration of substances that are poorly removable in WWTP such as EDTA (ethylenediaminetetraacetic acid) decreased in CSO-influenced samples due to dilution effects. The overall results indicate the potential importance of the CSO pathway of well-removable sewage-based trace contaminants to rivers. In particular, high concentrations during CSO events may be relevant for aquatic organisms. Given the results, it is suggested to include well-removable, sewage-based trace contaminants, a substance group often neglected in the past, in future studies on urban rivers in case of combined sewer systems. The presented methodology is suggested for a first assessment, since it is based solely on urban drainage data, which is available in most cities.

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.

A mixed surface and sub-surface flow riparian zone in Brittany (France), which is mainly fed by water from drainage ditches, was monitored for nitrate retention over three years from 2005 to 2007. Results show high time-averaged nitrate retention of >90 % for subsurface and ~70 % for surface passage. However, no retention could be detected during major rain events, which reduced the overall (flow-averaged) retention to ~40 %. Based on the findings, higher nitrate retention can be reached by increasing (i) the water residence time in buffer systems, (ii) the fraction of subsurface passage or (iii) denitrification rates in the system. (i) is only feasible if (active) buffer volume is enlarged, which may be difficult in practice. In the case of Brittany an enlargement can also be reached by extending buffer systems into existing drainage ditches. (ii) is of particular importance in areas with low soil permeability. In such areas, addition of gravel or sand beds can be considered. Regarding (iii), denitrification turns maximal under anaerobic conditions if sufficient carbon sources are available. In straw- and bark-filled column experiments we found high nitrate retention rates of >99 % and ~40 %, respectively, during a comparably low residence time of ~5 hours. As a result, the addition of external carbon sources to buffer systems is suggested. Currently, several pilot sites are constructed in the Ic watershed in Brittany attempting to take into account points (i) to (iii). For the following four buffer types, monitoring will start in February 2010: (a) two short drainage ditches, filled with carbon sources, (b) one drainage ditch and (c) one riparian wetland, each filled with a gravel filter, and optional upstream addition of carbon sources.

When mapping out strategies for an integrated water resource management in urban areas the precipitation-conditioned influences on the quality of waters available as resource are considered in an increasing manner. Amongst water discharges from urban areas, combined sewer overflows (CSO) represent a particular impact on waters due to their dynamic character. To assess CSO impacts, especially for an integrated modelling of sewer system and surface waters, quantity and quality data from the interface combined sewer overflow is needed. A monitoring concept for CSOs in Berlin was developed in the context of the project Monitor-1 by the KompetenzZentrum Wasser Berlin. In 2009, this concept will be realised in cooperation with the Berlin water authority and the utility Berliner Wasserbetriebe. When planning and preparing a monitoring an important aspect is, adjacent from the evaluation of possible locations, the selection of suitable measuring techniques. For this, extensive tests of different online measurement techniques from reputed manufacturers were accomplished at a test facility at the TU Berlin. Apart from questions such as accuracy, response behaviour at suddenly arising load peaks or dilutions and available measuring intervals, particularly aspects of calibration, cleaning and management of the sensors were evaluated. The influence of the calibration was especially examined with the ion-selective sensors (ISE). The question was pursued, how the sensors must be calibrated to offer the greatest possible accuracy for the generally very low concentrations in surface waters and the occurrence of a sudden and precipitous rise of concentration in the case of the start of the CSO. Ammonium and nitrate were also supplemented with chemicals besides the stockpiling with waste water. An important finding was that generally all sensors are applicable for the measurement task.