In the city of Berlin combined sewer overflows (CSO) can lead to severe depressions in dissolved oxygen (DO) of receiving urban rivers and hence to acute stress for the local fish fauna. To quantify CSO impacts and optimize sewer management strategies, a model-based planning instrument has been developed. It couples the urban drainage model InfoWorks CS which simulates hydraulics and pollutant transport in the sewer with the river water quality model QSim which simulates hydraulics, mass transport and various biogeochemical processes in the receiving water body. To identify simulated CSO impacts, concentration-durationfrequency-thresholds for DO are applied to river model results via an impact assessment tool. Two kinds of impacts are distinguished: i) suboptimal conditions and ii) critical conditions for which acute fish kills are possible. In the case of Berlin, suboptimal conditions are observed on up to 92 days per year, predominantly during periods of low discharge and high temperatures whereas critical conditions only occur after CSO. For model calibration and validation, continuous measurements in both river and sewer are used. First simulations show good accordance between simulated and measured DO concentration in the river with Nash-Sutcliffe efficiencies between 0.70 and 0.79 for an eight-month time period at three different river monitoring points. However, to assure satisfactory model performance for adverse DO conditions in particular, impact assessment results for measured and simulated data are compared. Regarding suboptimal DO conditions simulated and measured data show good agreement. Nevertheless model representation for critical conditions is poor for some river sections and requires further improvement for CSO conditions. The results underline the importance of combining different validation approaches when dealing with complex systems.

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 oxygen-consuming processes in the hypolimnia of freshwater lakes leading to deep-water anoxia are still not well understood, thereby constraining suitable management concepts. This study presents data obtained from 11 eutrophic lakes and suggests a model describing the consumption of dissolved oxygen (O2) in the hypolimnia of eutrophic lakes as a result of only two fundamental processes: O2 is consumed (i) by settled organic material at the sediment surface and (ii) by reduced substances diffusing from the sediment. Apart from a lake’s productivity, its benthic O2 consumption depends on the O2 concentration in the water overlying the sediment and the molecular O2 diffusion to the sediment. On the basis of observational evidence of long-term monitoring data from 11 eutrophic lakes, we found that the areal hypolimnetic mineralization rate ranging from 0.47 to 1.31 g ofO2 m-2 d-1 (average 0.90 ± 0.30) is a function of (i) a benthic flux of reduced substances (0.37 ± 0.12 g ofO2 m-2 d-1) and (ii) an O2 consumption which linearly increases with the mean hypolimnion thickness (zH)upto ~25 m. This model has important implications for predicting and interpreting the response of lakes and reservoirs to restoration measures.

In agricultural watersheds affected by diffuse pollution, limitation of fertilizer and pesticide application may not be sufficient to achieve good river water quality. After waterworks had to be closed in Brittany due to elevated nitrate concentrations in the river Ic (> 50 mg-NO3 L-1), the project Aquisafe has been initiated. The objective of Aquisafe is to reduce pollutant loads (nitrate and pesticides) from agricultural fields by implementation of near-natural mitigation zones at diffuse pollution hotspots at the head of watersheds. Simple and small solutions have to be designed in order to more efficiently reduce nitrate and pesticide concentrations in receiving rivers. In addition, a planning tool has to be developed to determine optimal locations to construct these systems. Finally, a tool to assess the effectiveness of these reactive zones on watershed water quality will be implemented. In order to reach the first objective, design features are tested on three scales: 1) laboratory scale, 2) technical scale and 3) field scale. 1) In the laboratory, column experiments were conducted with different organic substrates at short hydraulic residence times (HRT). The efficiency for parallel reduction of nitrate and two common herbicides in Europe, Bentazon and Isoproturon, was explored (Krause Camilo, 2012). 2) In technical scale, two parallel swales were filled with the most suitable material determined in (1) for a one year test. The influence of HRT and temperature was investigated. For nitrate, high reduction could be achieved at short HRT; results for herbicides still have to be confirmed. 3) One infiltration ditch and two simple wetlands were constructed in Brittany (France), taking into account experiences from other scales. These systems are now monitored to investigate the effects of upscaling. Site locations were chosen based on a validated and repeatable GIS-based overlay method that prioritises zones of potential contribution to nitrate pollution (Orlikowski et al, 2011). Additionally, a new wetland module is being developed for the Soil and Water Assessment Tool (SWAT). It allows to predict impacts of wetland constructions on nitrate concentrations in receiving rivers; the module is now implemented but still has to be calibrated with in situ monitoring results. The presentation will focus on results of the up-scaling approach, and will show how the tools of Aquisafe can be used for supporting the development of strategies at catchment scale.

This paper presents the results of an evaluation of the environmental footprint of the Braunschweig wastewater scheme with Life Cycle Assessment. All relevant inputs and outputs of the system are quantified in a substance flow model and evaluated with a set of environmental indicators for cumulative energy demand, carbon footprint, acidification, eutrophication, and human and ecotoxicity. The analysis shows that energy demand and carbon footprint of the Braunschweig system are to a large extent offset by credits accounted for valuable products such as electricity from biogas production, nutrients and irrigation water. The eutrophication of surface waters via nutrient emissions is reduced in comparison to a conventional system discharging all effluent directly into the river, because some nutrients are diverted to agriculture. For human and ecotoxicity, a close monitoring of pollutant concentrations in soil is recommended to prevent negative effects on human health and ecosystems. Normalised indicators indicate the importance of the primary function of the wastewater system (= protection of surface waters) before optimisation of secondary environmental impacts such as energy demand and carbon footprint. A further decrease of the energy-related environmental footprint can be reached by applying optimisation measures such as the addition of grass as co-substrate into the digestor, thermal hydrolysis of excess sludge, or nutrient recovery from sludge liquors.

Theoretically the Berlin River Spree could be under pressure from depressions in dissolved oxygen (DO) and high concentration of fish toxic ammonia following overflows of the combined sewer system. However, monitoring results indicate that the Spree is only under pressure from depressions in dissolved oxygen (DO). Consequently, a sewer model, a river water quality model and an impact assessment tool were calibrated and validated for representation of DO depressions. The three elements are joined in a planning tool, which will be used to test the effect of CSO management approaches for the current situation and with altered boundary conditions to account for expected climate change.