The sewer system of the city shows predominantly low gradients and partly high inline sewer capacities. A historically founded system of 63 pump stations is used for the delivery and distribution of combined water and wastewater from the collection systems via long force mains to six wwtps. Simultaneously, in case of rainfall events the pumps act as variable throttles on the outflow of the combined sewerage and activate the inline sewer capacities. High demands are formulated by the water authority to the emissions out of the waste water system into the sensible water bodies. Five of six wwtps of the city are situated in the surrounding area of Berlin. Due to the long distances between the pump stations in the inner city and the wwtps, the time until the dilution effect of the stormwater will be noticed at the inlet of the wwtp may last several hours. Due to the increased delivery rate at the pump stations of the combined sewer system during stormwater runoff (twice the dry weather peak flow), the pollution load at the wwtp increases immediately in the same amount. Due to the enlargement of storage within the combined sewer systems until the year 2020 to meet higher demands of the water authority, the total duration of a raised inflow to the wwtp during and after rain events will increase. To furthermore keep the processes at the wwtp stable (especially the nitrogen removal) the construction of a storage tank at the inlet of the wwtp (=outlet of the pressure main) as an option shall be taken under research. The volume of the storage tank is not only determined by the quantity but also by the quality of the inflow To provide evidence, that the new version of InfoWorks 9.5 CS® is able to calculate the flow and pollution processes in the pressure main network, a diploma thesis is carried out at the Berlin Centre of Competence for Water with the participation of the Berliner Wasserbetriebe

With increased computer performance and data-processing functionalities, there has been a tendency in the last few years to apply detailed hydrodynamic sewer modelling for long-term simulations, with long time series of rainfall. Although this is now fairly realistic for small networks, there is still a clear limit as to what can be done in the case of running bigger models for a long time, which need a lot more computational effort. Therefore, the idea has grown to investigate the possibilities of hybrid sewer modelling, a combination of conceptual and mechanistic modelling approaches to combine the advantages of both models, the speed of conceptual models and the accuracy of mechanistic models. Suggestions for hybrid model simplifications are presented in this paper within their application for two case studies.

Combined sewer overflows (CSO) can have a strong impact on the quality of surface waters. A common measure to reduce CSO is the construction of storage tanks. The objective of this study was to determine the required volume of a storage tank by means of a numerical long-term simulation and to assess uncertain input data. Particularly, the influence of the considered rain series’ length on the calculated storage volume was investigated. Engineering standards usually recommend the use of at least 10 to 15 years of rain series. Here, the hydraulic behaviour of the studied sewer system was simulated in a 30 year hydrodynamic simulation. Special effort was made to calibrate an available model by use of currently measured data. The quality of calibration was evaluated by means of the Nash-Sutcliffe model efficiency coefficient. The analysis of input data uncertainty revealed that applying a 10 year series results in tank volumes that differ between -12 % and +19 %, respectively from the dimensioning result achieved by applying the 30 year rain series.

The application of hydrodynamic sewer modelling allows for detailed description of complex hydraulic situations. However, for large systems long-term calculations with hydrodynamic models still require high computation times. This paper shows a possibility to overcome this problem by using a hybrid sewer model, which is a conjunction of conceptual and mechanistic modelling approaches to combine the calculating speed of conceptual models and the accuracy of mechanistic models in one model. The implementation of a hybrid sewer model was performed and tested in two case studies, in Berlin (Germany) for 6 representative catchments and in Herent (Flanders, Belgium) for one sewer system, using the hydrodynamic modelling software InfoWorks CS. Besides the motivation of the case studies on the sewer systems in Berlin and Herent this paper presents the methodologies developed for a hybrid simplification of the sewer network model, considering the calibration of the simplified network as well as the evaluation of the simplification performance. The use of a hybrid model for both case studies is then evaluated and the transferability of the methodologies is discussed.

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 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.

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.