Wet weather discharges from urban catchments are widely recognised as a major cause of unsatisfactory receiving water quality. Among stormwater discharges the impact from combined sewer overflows (CSO) plays a prominent role. The dynamic character of the discharge events lead to particular stress on the water bodies. Legal requirements for CSO follow the precautionary principle and usually set emission standards. Within the Urban Waste Water Treatment Directive 91/271/EEC of May 1991 it is written that “member states shall decide on measures to limit pollution from storm water overflows”. The directive does not give standards but solely proposes that “such measures could be based on dilution rates or capacity in relation to dry weather flow, or could specify a certain acceptable number of overflows per year”. The European Water Framework Directive 2000/60/EC of October 2000 goes beyond and asks for a combined approach to river basin management. On the source side, it requires that all existing technology-driven source-based controls must be implemented as a first step. On the effects side, it provides a new overall objective of good status for all waters, and requires that where the measures taken on the source side are not sufficient to achieve these objectives, additional ones are required. To assess the impact of CSO on the Berlin receiving water the research projects MONITOR and SAM-CSO are carried out in cooperation between Kompetenzzentrum Wasser Berlin, the Berliner Wasserbetriebe and the Senate Department of Environment Berlin. The objective of the projects is to identify and make available receiving water parameters (immission parameters) for the decision making process concerning the optimisation of the urban drainage system. Further on, a method for the evaluation of measures of combined water treatment on the basis of these immission criteria will be defined. The evaluation shall be based on both, available measurement data from the sewer system and the receiving water and simulations with an integrated model for the coupled drainage-river-system. The paper will present the methodology of the project. Special focus is on the description of the processes within the Berlin water bodies (stagnant lowland rivers) and the compilation of relevant physical-chemical and ecological parameters for the assessment of CSO.

The present study “Literature review on impact-based guidelines for stormwater treatment” provides an overview of international guidelines, which evaluate acute impacts of combined sewer overflows (CSO) on receiving surface water bodies. The overview should serve as a basis for the assessment of measured and simulated CSO impacts on Berlin surface waters within the projects “Monitor-1” and “SAM-CSO”, which are currently carried out at the Berlin Centre of Competence for Water. In contrast to the classical approach of sewer emission thresholds, impact-based guidelines focus on possible effects of CSO in the receiving surface water. Impact-based guidelines aim at deriving locally adapted measures to minimize CSO impacts to surface waters. Thanks to this local approach, potential protection measures can be planned dependent on the state of a specific river, reservoir or lake. The following study focuses on acute CSOimpacts, which were identified as relevant for the biocenosis of the River Spree in Berlin within the KWB project ISM: (i) Increased levels of unionised ammonium (NH3) through ammonium input. (ii) Low levels of dissolved oxygen (DO) through the input of degradable organic components, which lead to DO consumption. Guidelines from Germany, Austria, Switzerland, United Kingdom, France and USA are considered along with the approach by Lammersen, which assembles a number of scientific publications. The Austrian guideline (ÖWAV-RB 19) stops at distinguishing whether further investigations are necessary. In the US “CSO control policy” further analysis is delegated mostly to local institutions. The French “Arrêté du 22 juin 2007” also asks to take into consideration the local situation of the receiving water but does not give any limit values. The remaining four approaches provide a detailed evaluation scheme for critical NH3 and DO conditions, using duration-frequency-relationships. These relationships assume that pollution events of a specific duration may only occur in defined recurrence intervals (e.g. Figure 4.1). The Swiss guideline (STORM) is not suitable for dammed lowland river systems such as the Berlin River Spree, since it focuses on fast flowing rivers with salmonid fish populations. As a result there remain three approaches, which are interesting for the Berlin situation: the UPM guideline from the UK, the BWK-M7 guideline from Germany and the Lammersen-approach, which summarizes various scientific results. Apart from the dependency of critical concentrations on event duration and recurrence frequency, influence of temperature, pH and concurrent NH3-concentrations or DO-minima are considered by UPM and the Lammersen-approach. The relationships used by the three approaches for NH3 and DO are similar (see Figures 4.1, 4.3 and 4.4). Nevertheless, their comparability is limited, as the approaches generalize various local situations and cannot be derived strictly scientifically. As a first step we therefore recommend applying the three approaches to existing data from the River Spree and count the respective numbers of critical events. Based on the results it is possible to assess to which extent each approach is applicable for the situation in Berlin. As a second step experts need to evaluate the resulting critical events to distinguish suboptimal from lethal situations. For instance, the Lammersen-approach judges both (i) a two-day period with DO < 5 mg L-1 and (ii) a 30-minutes event with DO < 1.5 mg L-1 as critical. However in the Berlin River Spree (i) occurs basically continuously throughout the summer season and is tolerated by local fish species, whereas (ii) would probably lead to a major fish kill. As a consequence the prevention of (ii) should be given first priority. Based on the experience gained from the assessment of river monitoring data, simulation results can be evaluated in a third step. All the considered guidelines propose numerical simulation of sewer and receiving surface water systems. However only simple model approaches are discussed in detail, while specialized literature is suggested for complex cases. If numerical simulations are used for the planning of concrete measures, model uncertainties must be indicated to avoid feigning accuracy of results that cannot be provided. The Swiss STORM guideline suggests using Monte-Carlo simulations to calculate probabilities of the recurrence of critical events for possible management measures. We suggest a similar approach for the Berlin situation. Thus, decision makers could weigh cost against probability of success for proposed measures.

In Berlin läuft seit April 2007 das BMBF-Forschungsprojekt SPREE2011, welches zum Ziel hat, eine neuartige Speichertechnologie für Mischwasserüberläufe zu entwickeln sowie Reinigungstechniken für die Mischwasserbehandlung zu erproben, die in Deutschland bisher nur aus anderen Anwendungsfällen bekannt sind. Auf diesem Wege soll ein direkter Beitrag geliefert werden zu den zukünftigen Zielen, die Stoffeinträge aus Misch- und Regenwasser in Gewässer zu minimieren.

The centre of Berlin, Germany, is drained by a combined sewer system. The receiving waters Havel and Spree are characterized by low flow velocities and an increased risk of eutrophication. High demands towards a reduction of the emission loads of combined sewer overflows (CSOs) down to 20 % of the mean annual runoff load of TSS, COD and BOD5 are formulated by the Berlin Water Authority. Therefore a pollution control plan will be carried out until the year 2020 that will lead to a storage enlargement of the combined sewer system by 100 %. To assess if these efforts will lead to the expected water quality of the receiving water regarding the objectives of the European Water Framework Directive, a method will be developed to evaluate in advance the achievable improvement. Starting from the actual status of the water body this model based method should allow for an estimation, if the good status will be achieved after the realization of the measures of storage upgrading in the sewer system. The study currently concentrates on the integrated water quality modelling of the high dynamic processes in the sewer system and the receiving water. The paper focuses on the simulation of oxygen concentration in the receiving water.

The paper describes the planning process for the pilot implementation of a decision support system (dss) for global control of sewage pump stations in Berlin, which is going to be realised by the end of 2006. The objective of the control concept is to systematically manage in-pipe storage capacities in order to reduce cso. The paper presents the preliminary assessment of the system’s control potential based upon the analysis of operational figures and numerical simulations. Furthermore, the structure of the decision support systems procedure is illustrated.

Urban water courses are considerably degraded in terms of their hydrology, riparian and channel morphology, substrate heterogeneity and habitat features as well as water and sediment quality. In addition, the combined sewer overflows and the ecotoxicological impacts of its components lead to a change of the physical-chemical and microbial mass balance affecting the biocenoses of higher trophic levels. Combined sewer overflows are therefore an additional stress to the ecological status of the urban course of the River Spree and of its channels, which is damaged already by both preload and background load of the aquatic environment. With regard to the assessment of the ecological water status, the European Water Framework Directives gives priority to the aquatic biocenoses in their capacity as ecological quality parameters. Against this background, an immission-oriented approach for the assessment of combined sewer overflows has to describe also their impacts on the biocenoses of the macrozoobenthos, the fish fauna, the macrophytes and the phytoplancton. These biocenoses are protected against the harmful impacts resulting from CSO only if the modification of their physical and chemical environment is avoided or reduced to an ecologically tolerable level respectively. In case that unfavourable impacts cannot be completely eliminated, the degree of impairment and the number of damaging CSO discharge events, which appear to be acceptable, should be defined. The present study is based on the bibliographic study „ Impact of urban use on the mass balance and the biocoenosis of lowland rivers under special consideration of combined sewer overflows” and deals with the assessment of CSO impacts on the ecological situation of the urban Spree and the channels (Cyprinid water bodies). In general, the immissionoriented assessment of CSO impact on the biocenoses (macrozoobenthos, fish fauna) requires the observation of the intensity, duration and frequency of occurrence of the individual events based on the assumption that, due to the background pollution, top priority is currently given to the acute CSO impacts. Requirements for the protection of aquatic biocenoses are developed with regard to the target parameters oxygen and ammonium/ammoniac and ecological tolerances of the biocenotic subjects of protection, which are strongest influenced by CSO. Initially, it is discussed to what extent the already existing results from laboratory investigations can be transferred to field situations. Next to the commonly accepted threshold values for oxygen concentrations during continuous persistent loads, particular requirements for the oxygen balance in case of peak loads are formulated.

In Berlin, Germany, the demand for enhanced protection of the environment and the growing economic pressure have led to an increased application of control concepts within the sewage system. A global control strategy to regulate the pumpage of the combined sewage system to the treatment plant was developed and evaluated in a theoretical study. The objective was to reduce CSO. In this paper an extension of the existing control algorithm by information from online rainfall measurement and radar nowcasting is described. The rainfall information is taken into account by two additive terms describing the predicted volume from rainfall runoff. On the basis of numerical simulation the potential of these two complementary forecast terms in the global control algorithm to further reduce CSO is evaluated. The investigations are based on long-time simulations that are conducted with the dynamic flow routing model InfoWorks for three subcatchments of the Berlin drainage system. The results show that at the current Berlin system a CSO reduction of only 0.8% is possible. The effect of the forecast terms is limited by operational constraints. Limits are set to both, the delivery from each individual pump station and the total pumpage to the treatment plant.