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 environmentalfootprint 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.

Bisherige Analysen des Energieverbrauchs in der Abwasserreinigung beschränken sich oft auf die naheliegende Erfassung des Stromverbrauchs. Im Sinne einer ganzheitlichen Betrachtung sollten aber auch andere Formen der Energie erfasst werden, wie zum Beispiel für die Herstellung von benötigten Chemikalien wie Flockungs- und Flockungshilfsmittel, beim Transport des zu entsorgenden Schlamms oder für zusätzliche Brennstoffe bei der Klärschlammtrocknung. Dafür ist die Erweiterung der Grenzen des zu betrachtenden Systems auf vor- und nachgelagerte Prozesse notwendig, um alle relevanten Beiträge zum Energieverbrauch zu berücksichtigen. Zudem können so auch die verschiedenen Sekundärprodukte der Abwasserreinigung erfasst werden: die Stromproduktion aus Faulgas, die Rückführung von Nährstoffen und Wasser in die Landwirtschaft oder die Substitution von fossilen Brennstoffen in der thermischen Klärschlammentsorgung. Ein geeignetes Instrument für diese Betrachtungsweise ist die Methodik der Ökobilanz nach ISO 14040/44. Mit dieser Methodik lassen sich alle unterschiedlichen Energieformen und Sekundärfunktionen abbilden und in einheitlichen Indikatoren darstellen, ergänzt durch weitere Umweltwirkungen wie den Treibhauseffekt.

This study presents the use of Life Cycle Assessment as a tool to quantify the environmental impacts of processes for wastewater treatment. In a case study, the sludge treatment line of a large sewage treatment plant is analysed in energy demand and the emission of greenhouse gases. Results show that the existing process is positive in energy balance (+166 MJ/PECOD*a) and GHG emissions (+19 kg CO2-eq/PECOD*a) by supplying secondary products such as electricity from biogas production and substituting fossil fuels in incineration. However, disposal routes for stabilised sludge differ considerably in their environmental impacts. In total, LCA proves to be a suitable tool to support future investment decisions with information of environmental relevance on the impact of WWTPs, but also larger urban water systems.

Triggered by climate change, local freshwater scarcity and rising public awareness towards ecological issues, environmental aspects are becoming key decision criteria for planning of urban water management infrastructure. Simultaneously, the implementation of measures according to the EU Water Framework Directive requires huge investments in the coming years for both upgrading of existing infrastructure and the construction of sewer networks or treatment plants. Among existing tools for environmental impact assessment, LCA offers the most accepted and comprehensive method to support decision makers with information on the environmental profile of new investments or upgrading of existing infrastructure. This paper describes on-going case studies using LCA for systems of urban water management and raises potential difficulties while applying LCA in the water sector.

The goal of this study is the identification of ecological advantages and disadvantages of alternative sanitation systems in comparison to conventional wastewater treatment. The methodology of Life Cycle Assessment (LCA) is adopted as an evaluation tool for the ecological assessment of various sanitation scenarios for a hypothetical middle-sized settlement in Germany (ca 5000 inhabitants). The scenarios include a reference system with conventional drainage and treatment in an activated sludge plant with anaerobic sludge digestion and sewage gas production. In the alternative scenarios, urine is source-separated in the toilet, collected and applied as fertilizer. Faeces are either collected by gravity drainage and composted together with biowaste or collected by a vacuum system and co-digested with biowaste to gain biogas for energy production. The remaining greywater is treated in a soil filter or in a technical plant (Sequencing batch reactor). All relevant processes of the investigated scenarios are modelled in detail for the Life Cycle Inventory, based on data from pilot plants and literature. This implies the processing of the different waste fractions, transport and energy supply, mineral fertilizer substitution, and sludge incineration. Beside the operational expenditures, the construction phase is included with material and energy demands. The resulting substance flow model is evaluated with a set of environmental indicators relating to the demand of energy, non-renewable resources, climate change, eutrophication, acidification, and various toxicity potentials. As a result, the alternative scenarios cause less environmental burden in almost all impact categories. The source-separation of human excreta disburdens the wastewater treatment process and lowers nutrient emissions into surface waters. The secondary fertilizer from urine and faeces has lower heavy metal content than an average mineral fertilizer. Depending on the system configuration, alternative sanitation systems can have a lower demand for fossil fuels and subsequently cause fewer emissions of climate-active gases. Only the increased emission of acidifying gases represents a considerable drawback compared to the conventional system. A normalisation of all indicators to the average environmental burden of a single person in Germany reveals that the decisive categories for the overall comparison are related to eutrophication, acidification, and terrestrial ecotoxicity. Energy-related indicators have a smaller contribution, but they can be important in terms of world-wide scarce fossil resources and climate change. The advantages of alternative sanitation systems can only be realized if the secondary functions of mineral fertilizer substitution and energy supply are fully utilized. Important key parameters for future LCA studies of alternative sanitation systems are identified, which may simplify the data acquisition. The construction phase has only a minor relevance for the ecological assessment and may therefore be neglected in future studies. In all, the data quality of this LCA study can be further improved, because many processes of alternative systems have not yet been investigated or realized in full-scale. Hence, the development of a universal decision support method could not be realized in a reasonable way due to the lack of adequate long-term process data and the high influence of case-specific boundary conditions on the technical implementation. However, this LCA study gives a first assessment of potential ecological benefits and drawbacks of alternative sanitation systems.