To support decision makers in planning effective combined sewer overflow (CSO) management strategies an integrated modelling and impact assessment approach has been developed and applied for a large urban area in Berlin, Germany. It consists of an urban drainage model, a river water quality model and a tool for the quantification of adverse dissolved oxygen (DO) conditions in the river, one of the main stressors for urban lowland rivers. The coupled model was calibrated successfully with average Nash- Sutcliffe-efficiencies for DO in the river of 0.61 and 0.70 for two validation years. Moreover, the whole range of observed DO concentrations after CSO down to 0 mg L-1 is simulated by the model. A local sensitivity analysis revealed that in the absence of CSO dissolved oxygen principally depends on phytoplankton dynamics. Regarding CSO impacts, it was shown that 97% of the observed DO deficit can be explained by the three processes (i) mixing of river water with CSO spill water poor in DO, (ii) reduced phytoplankton activity due to CSO-induced turbidity and (iii) degradation of organic matter by heterotrophic bacteria. As expected, process (iii) turned out to be the most important one. However depending on the time lag after CSO the other processes can become dominant. Given the different involved processes, we found that different mitigation schemes tested in a scenario analysis can reduce the occurrence of critical DO deficits in the river by 30-70%. Overall, the study demonstrates that integrated sewer-river-models can be set up to represent CSO impacts under complex urban conditions. However, a significant effort in monitoring and modelling is a requisite for achieving reliable results.

Das Berliner Wissenschaftszentrum sorgt für Qualität und Kontinuität in der Beforschung wasserwirtschaftlicher Sachverhalte.

The presented work studies the influence of the sampling strategy on the quality of locally calibrated UV-VIS probe measurements in combined sewer overflows (CSO) and the receiving river. Results indicate that UV-VIS spectrometers are not able to provide reliable measurements of water quality in urban stormwater without being calibrated to local conditions with laboratory analyses of water samples. The use of the global calibration (supplied by the manufacturer) led to errors of at least 30% and 45% for CSO load and river concentration of chemical oxygen demand (COD), respectively. Even with reliable local calibration, COD loads contained significant uncertainties close to 20%. Uncertainties in COD load and concentration decrease below 30% if more than 15-20 samples (i.e. 3-4 stormwater events) are sampled for local calibration. The effort and associated sampling costs to gain more than 15-20 samples are much less effective, since load and concentration uncertainties remain relatively stable with an increasing number of samples used for the calibration. The presented analysis aims at supporting practitioners in the planning, operation and calibration of UV-VIS spectrometer probes.

The present study aims to explore the relationship between rainfall variables and water quality/quantity characteristics of combined sewer overflows (CSO), by the use of multivariate statistical methods and online measurements at a principal CSO outlet in Berlin (Germany). Canonical correlation results showed that the maximum and average rainfall intensities are the most influential variables to describe CSO water quantity and pollutant loads whereas the duration of the rainfall event and the rain depth seem to be the most influential variables to describe CSO pollutant concentrations. The analysis of Partial Least Squares (PLS) regression models confirms the findings of the canonical correlation and highlights three main influences of rainfall on CSO characteristics: (i) CSO water quantity characteristics are mainly influenced by the maximal rainfall intensities, (ii) CSO pollutants concentrations were found to be mostly associated with duration of the rainfall and (iii) pollutants loads seemed to be principally influenced by dry weather duration before the rainfall event. The prediction quality of PLS models is rather low (R² < 0.6) but results can be useful to explore qualitatively the influence of rainfall on CSO characteristics.

Data play an important role in water-related research. In the field of limnology, monitoring data are needed to assess the ecological status of water bodies and understand the bio-geochemical processes that affect this status. In wastewater management, measured or simulated data are the basis for planning and control of sewer networks. Given the importance of data in water-related research makes them a valuable resource, which should be handled in an adequate way. Based on experiences in data collection and data processing in water-related research this paper proposes – both from a computer scientist’s and an environmental engineer’s point of view – a set of rules for data handling: Rule 1: Protect raw data. Rule 2: Save metadata. Rule 3: Use databases. Rule 4: Separate data from processing. Rule 5: Use programming. Rule 6: Avoid redundancy. Rule 7: Be transparent. Rule 8: Use standards and naming conventions. Applying these rules (i) increases the quality of data and results, (ii) allows to prepare data for long-term usage and make data accessible to different people, (iii) makes data processing transparent and results reproducible, and (iv) saves – at least in the long run – time and effort. With this contribution the authors would like to start a discussion about best data handling practices and present a first checklist of data handling and data processing for practitioners and researchers working in the water sector.

Combined sewer overflows (CSO) after heavy rainfall can cause acute depletions of dissolved oxygen (DO) in the Berlin River Spree. Further aggravation of ecological deficits can be expected from global climate change. A planning instrument for CSO impact assessment under different sewer management and climate conditions has been developed at Kompetenzzentrum Wasser Berlin. It couples the sewer model InfoWorks CS, the river water quality model Hydrax/QSim and an impact assessment tool. The planning instrument was validated for the years 2010 and 2011. Simulation results for the critical parameters discharge and DO concentrations in the Berlin River Spree agree well with measurements. Although not all observed DO deficits can be simulated accurately, the very good representation of processes related to the oxygen budget allows assessing relative changes in boundary conditions, e.g. from climate change or different CSO control strategies. The conducted scenario analysis indicates that the coupled sewer-rivermodel reacts sensitively to changes in boundary conditions (temperature, rainfall, storage volume and other CSO control strategies, etc.). Based on the simulation year 2007 – representing an extreme year with regards to CSO volume and critical conditions in the river – sewer rehabilitation measures planned to be implemented until 2020 are predicted to reduce total CSO volumes by 17% and discharged pollutant loads by 21 - 31%. The frequency of critical DO conditions for the most sensitive local fish species will decrease by one third. For a further improvement of water quality after the year 2020, the reduction of impervious surfaces emerges as a very effective management strategy where feasible. A reduction of the impervious connected area by 20% results in a decrease in the frequency of critical DO conditions by another third. The studied increase in surface air and water temperature as part of the climate change scenarios leads to a significant aggravation of DO stress due to background pollution in the Berlin River Spree, while acute DO depletions after CSO are barely affected. However, changes in rain intensity have a considerable effect on CSO volumes, pollutant loads and the frequency of critical DO concentrations. A general reduction of discharged pollutant loads by 60% based on the sewer status 2020 can prevent critical DO conditions in the Berlin River Spree, even for the exceptionally rain intense year 2007. A detailed analysis of river processes after CSO, has shown that the biodegradation of organic carbon compounds is the most important contributor to acute DO depletions in the Berlin River Spree. An additional impairment of DO conditions is caused by the inflow of oxygen free CSO spill water and suspended solids into the Berlin River Spree. In this report, CSO impacts under different management strategies or climate change conditions are assessed only for a part of the Berlin combined sewer system (although the main part) and for one exemplary year. An extension of the planning instrument to the entire combined sewer system would enable to evaluate the full impact of measures. For a robust prediction of future CSO impacts it is also recommended to test different simulation periods or conduct long-term simulations.

The present study aims at demonstrating the possibilities of on-line sensors for describing CSO emissions and river impacts. A continuous integrated monitoring, using state-of-the-art on-line sensors, was started in Berlin in 2010. It combines (i) continuous measurements of water quality and flow rates of combined sewer overflows (CSO) at one main CSO outlet and (ii) continuous measurements of water quality parameters at four sites within the urban stretch of the receiving river. UV-VIS probes provide continuous measurements of parameters such as chemical oxygen demand (COD) with relatively low uncertainties (10-30%). However, experience shows that on-line UV-VIS probes are not able to provide accurate measurements of water quality without being calibrated to local conditions. Several methodologies to analyze on-line CSO and river measurements are presented and illustrated with an exemplary event. Results show that reliable information such as the CSO load, the proportion of wastewater in CSO, the contribution of wastewater to CSO load, the first flush effect and the intensity of river impacts can be gained at high precision and temporal resolution. Given the broad range of high quality information from CSO impacts in the river to the characterization of CSO emissions, the study suggests the use of continuous integrated monitoring programs to support decisions on CSO management.

A continuous monitoring, using UV-VIS spectrometers, was carried out in Berlin from 2010 to 2012. It combined (i) continuous measurements of the quality and flow rates of combined sewer overflows (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. Locally, the collection of data aims at (i) characterizing CSO emissions, (ii) assessing the local dynamics and intensity of CSO impacts on the river and (iii) calibrating sewer and river water quality models being part of a planning tool for future CSO management in Berlin (Riechel et al., 2011). UV-VIS spectrometers are in-situ probes, which measure absorbance spectra ranging from UV to visual wavelengths. Concentrations, such as chemical oxygen demand (COD), are calculated from these spectra. Due to the varying composition of waste and river water a local calibration is required to enhance the measurement quality. According to Gamerith et al. (2011), manufacturer global calibration can lead to systematic error up to 50% for COD measurements.

Das vorgestellte modellbasierte Werkzeug bildet Mischwasserüberläufe aus dem Berliner Mischkanalsystem und deren kurzfristige Auswirkungen im Gewässer ab. Es soll für die Massnahmenplanung und die Berechnung von Zukunftsszenarien verwendet werden. Das Werkzeug zeigt eine gute Übereinstimmung mit Messungen bezüglich des Verlaufes der Sauerstoffkonzentration im Gewässer und des Auftretens kritischer Bedingungen für die Fischfauna.