The goal of this study is to demonstrate the application of Life Cycle Assessment as a tool for systems analysis in wastewater treatment. Therefore, the process for sludge treatment and disposal at the WWTP Berlin-Waßmannsdorf has been analysed with the methodology of Life Cycle Assessment (LCA) to determine the total cumulative energy demand and the carbon footprint of the system as exemplary indicators. In addition to the characterization of the status quo in 2009, several measures for an energetic optimization of the system have been evaluated in their effects on the energy balance and greenhouse gas emissions. The process model of the system encompasses all relevant processes of sludge treatment and disposal, including the supply of electricity and chemicals, transport and incineration of the sludge, and treatment of sludge liquor which is recycled back to the WWTP inlet. Products recovered during sludge treatment (biogas from anaerobic digestion and MAP fertilizer) and disposal in incineration (electricity or substitution of fossil fuels) are accounted by credits for the respective substituted products. Overall, sludge treatment and disposal in Berlin-Waßmannsdorf is an energy-positive process, recovering a net amount of primary energy of 162 MJ (45 kWh) per population equivalent and year (PECOD*a). This is mainly due to the biogas generated in anaerobic digestion and the substitution of fossil fuels in co-incineration. Similarly, the carbon footprint of the process reveals an amount of 11.6 kg CO2-eq/(PECOD*a) as avoided emissions, thus indicating the environmental benefits of energy recovery from sewage sludge. However, process emissions of the powerful greenhouse gases CH4 and N2O are estimated based on generic emission factors from literature, and can have a distinct influence on the overall carbon footprint. This underlines the necessity to support the results of this LCA with primary data from monitoring of emissions on-site. The evaluation of optimization measures shows the benefits of a system-wide analysis: an enhanced recovery of energy is partially offset by increased energy demand, and the carbon footprint does not always correlate with the energy balance. The different routes for sludge disposal differ heavily in their environmental profile and show potentials for optimisation, especially in mono-incineration of sewage sludge. Some measures are beneficial for both energy and carbon footprint (addition of co-substrates into the digestor, utilization of excess heat with an Organic Rankine Cycle process), while others can decrease energy demand but may potentially increase the carbon footprint (treatment of sludge liquor by deammonification, thermal hydrolysis of excess sludge). Overall, the method of Life Cycle Assessment proved to be well suited for a systematic analysis of the environmental footprint of the activities of Berliner Wasserbetriebe. In the future, the existing process model can be extended to include the entire wastewater treatment plant for a comprehensive evaluation of its environmental profile, e.g. for providing information on the environmental consequences of prospective concepts for site development.

Previously, the analysis of energy demand for wastewater treatment was often limited to one-dimensional analyses of electricity demand. How ever, a comprehensive analysis requires the inclusion of all different contributions to energy demand. The Life Cycle Assessment (LCA) methodology defined in ISO 14040/44 is a suitable tool for this task. With it, all different primary and secondary energy demands can be quantified and assessed using consistent indicators, complemented by an assessment of other environmental impacts such as the carbon footprint.

The present study analyses the environmental footprint of the Braunschweig wastewater scheme using the methodology of Life Cycle Assessment. All relevant processes of wastewater treatment and disposal are modelled in a substance flow model based on available full-scale data (year 2010) complemented by literature data to calculate aggregated emissions and resource demand of the system. Products of the system (i.e. electricity from biogas combustion, nutrients, and irrigation water) are accounted with credits for the respective substituted products. Beside the status quo of the Braunschweig system in 2010, a set of optimisation scenarios are assessed in their effects on the environmental footprint which target an enhanced recovery of energy and nutrients. The scenarios include the addition of different co-substrates, thermal hydrolysis of sludge in various configurations, nutrient recovery for nitrogen and phosphorus, and utilization of excess heat via an Organic Rankine Cycle (ORC). The energetic balance of the system is comparatively good, as 79% of the cumulative energy demand can be offset by secondary products, mainly biogas (58%) and fertilizer substitution (14%). The optimisation of nutrient and especially water management offers considerable potential for improving the energy balance, the latter due to the high demand of electricity for pumping the water to the fields. The net carbon footprint of the system amounts to 10 kg CO2-eq/(PECOD*a) and is mainly caused by energy-related processes, augmented by direct emissions of N2O and CH4 in the activated sludge process. Nutrient emissions in surface waters are relatively low (29 g P and 80 g N/(PECOD*a)) due to the transfer of nutrients to agriculture and the polishing effect of the infiltration fields. While effects on human toxicity are small after normalisation to German conditions, Cu and Zn emissions to aquatic and terrestrial ecosystems lead to a substantial impact in ecotoxicity (organic substances not accounted). Normalisation of the environmental footprint reveals the primary function of the wastewater treatment plant, i.e. the protection of surface waters from inorganic and organic pollutants and excessive nutrient input. Whereas the quantitative contribution of the system is high for eutrophication and ecotoxicity, energy consumption and correlated indicators such as carbon footprint, acidification and human toxicity have only a minor share to the total environmental impacts per inhabitants in Germany. Consequently, the optimisation of the latter environmental impacts should only be pursued if the primary function of the sewage treatment and related impacts on surface waters are not compromised by these measures. In scenario analysis, both the addition of co-substrates and the thermal hydrolysis of sludge for improving the anaerobic degradation into biogas have a substantial positive effect on the energy balance and carbon footprint without impairing other environmental impacts. Based on the results of the pilot trials in CoDiGreen, the current energy demand can be reduced up to 80% by a combination of adding ensiled grass into the digestor and hydrolysis of excess sludge (potentials have to be verified in full-scale trials). A twostep digestion process with intermediate dewatering and hydrolysis (DLD configuration with EXELYS™) seems promising in terms of energy benefits and carbon footprint. The recovery of nitrogen or phosphorus from the sludge liquor of dewatering does not result in major benefits in the environmental profile, whereas the implementation of an ORC process for energy recovery from excess heat can be fully recommended from an environmental point of view.

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.

Das Forschungsprojekt CoDiGreen (2010-2012) zielt auf eine Optimierung der Rückgewinnung von Energie und Nährstoffen in der Abwasserbehandlung in Braunschweig und Berlin. Dafür werden in Pilotversuchen die Auswirkungen einer Zugabe von Co-Substraten (Grassilage, Topinambur) und einer thermischen Druckhydrolyse des Überschussschlamms auf den Biogasertrag der Faulung untersucht. Zusätzlich wird die Co-Vergärung von Grassilage im großtechnischen Maßstab in einem Faulturm des Klärwerks Braunschweig-Steinhof getestet. Neben dem experimentellen Teil wird über eine Ökobilanz der ökologische Fußabdruck des Abwassersystems in Braunschweig und der Schlammbehandlung im Klärwerk Berlin-Wassmannsdorf analysiert, um Optimierungspotential zu erfassen und anhand ausgewählter Szenarien zu bewerten. Abschließend werden vergleichbare Konzepte der landwirtschaftlichen Wiederverwendung von Klarwasser und Schlamm in einer Marktstudie ermittelt und über eine Risikobewertung potentielle Gefahren dieses Systems identifiziert. Die Pilotversuche zeigen, dass sowohl die Zugabe von Co-Substraten als auch die thermische Hydrolyse einen substantiellen Gewinn an Biogasmenge und –qualität (CH4Gehalt) in einer mesophilen Faulung (Verweilzeit: 20d) ermöglichen kann. Die Methanerträge können um 10%, 9% und 13% durch thermische Hydrolyse von Überschussschlamm, Zugabe von Grassilage (+10% FS) und eine Kombination beider Maßnahmen gesteigert werden (sofern der Methanertrag lediglich auf den oTR des zugeführten Schlamms bezogen wird, betrug die Steigerung 10%, 31% und 38%). Eine zweistufige Faulung mit zwischengeschalteter Hydrolyse („DLD“) erbringt +19% CH4. Für anorganische und organische Schadstoffe werden dabei vorgeschriebene Grenzwerte der aktuellen Klärschlammverordnung nicht überschritten. Weiter zeigen Laboranalysen einen positiven Effekt auf die Entwässerbarkeit des Schlamms und den Bedarf an Polymeren. Leider können die vielversprechenden Ergebnisse der Co-Vergärung mit Gras in der Großtechnik nicht bestätigt werden. Für eine großtechnische Realisierung einer Co-Vergärung lässt sich abschätzen, dass für 100.000 EW ca. 30 ha extensiv bewirtschafteter Fläche erforderlich sind, um 10% oTR an Gras in Bezug zum oTR des Rohschlamms zu erzeugen. Leider können die vielversprechenden Ergebnisse der Co-Vergärung mit Gras in der Großtechnik nicht bestätigt werden, in der nur -8% Biogasertrag gemessen werden (+2% wenn der Methanertrag lediglich auf den oTR des zugeführten Schlamms bezogen wird). Obwohl die technische Machbarkeit der Graszugabe gezeigt werden kann, scheinen betriebliche Probleme (Größe der Fasern, hydraulische Durchmischung, niedrige Verweilzeit) die Umsetzung des maximalen Potentials der Graszugabe in der Großtechnik zu verhindern. Die Bewertung der Umweltwirkungen der Systeme in Berlin und Braunschweig zeigt eine hohe Eigenenergieerzeugung in beiden Systemen, so dass dadurch der Treibhauseffekt und andere relevante Umweltwirkungen vermindert werden. Dennoch kann noch Optimierungspotential bei der Energie- und Nährstoffrückgewinnung aufgezeigt werden, zu dessen Erschließung auf der Grundlage einer Szenarienanalyse Empfehlungen formuliert werden. Die Umweltvorteile der Wiederverwendung in Braunschweig zeigen sich vor allem in einer verminderten Emission von Nähr- und Schadstoffen in die Gewässer. Die Normalisierung der Umweltwirkungen unterstreicht die Bedeutung der Primärfunktion der Kläranlage (= Schutz der Oberflächengewässer), die durch Optimierung von Energiebedarf und Treibhausgasemissionen nicht eingeschränkt werden sollte. Die Risikobewertung der Braunschweiger Systems folgt dem HACCP-Konzept und quantifiziert Risiken für die menschliche Gesundheit durch Krankheitserreger und Schwermetalle in der Landwirtschaft und ökologische Risiken durch Schwermetalle. Potentielle Risiken der Wiederverwendung werden auf Grundlage quantitativer Modelle von Umweltverhalten und Exposition identifiziert (Viren, Cadmium für Menschen, Zink für Ökosystem) und sollten durch entsprechende Messprogramme überwacht werden. Schließlich werden basierend auf den Projektergebnissen Empfehlungen zur Optimierung der Energie- und Nährstoffrückgewinnung in der Abwasserbehandlung in Berlin und Braunschweig formuliert, um letztlich die negativen Umweltwirkungen zu minimieren und potentielle Risiken im Betrieb zu vermeiden.

The research project CoDiGreen (2010-2012) targets the optimisation of energy and nutrient recovery in the wastewater treatment schemes of Braunschweig and Berlin. Therefore, pilot experiments are conducted to test the effect of addition of co-substrates (grass silage, topinambur) and the thermal hydrolysis of excess sludge on the biogas yield of anaerobic digestion. In addition, co-digestion of grass silage is also tested in a full-scale digestor of the wastewater treatment plant (WWTP) Braunschweig-Steinhof. Beside the experimental part, the environmental footprint of the wastewater treatment scheme in Braunschweig and the sludge treatment line in WWTP Berlin-Waßmannsdorf is analysed with Life Cycle Assessment (LCA) to identify potentials for optimisation and assess selected technical options in their effects on the environmental profile. Finally, a market review of the concept of agricultural reuse of effluent and sludge in Braunschweig is conducted to get an overview of the market situation, and a risk assessment is initiated to identify potential risks associated with this practice. The results of the pilot experiments show that both the addition of co-substrates and thermal hydrolysis can substantially increase the biogas yield and quality (CH4 content) during mesophilic digestion (HRT = 20d). Methane yields can be increased by 10%, 9% and 13% for thermal hydrolysis of excess sludge, addition of grass silage (+10% TS), and the combination of both (if the methane yield is only related to the VS of the sludge, the increase was 10%, 31% and 38%). A two-step digestion with intermediate hydrolysis (“DLD”) yields +19% CH4. No exceedance of legal requirements for inorganic and organic pollutants can be detected, whereas lab-analysis indicate positive impacts on sludge dewaterability and polymer demand for dewatering. For a full scale realisation of co-digestion it can be estimated that a 100.000 PE WWTP would require approximately 30 ha of extensively cultivated area to add +10% VS of grass substrate. However, the promising results of co-digestion with grass cannot be confirmed in full-scale trials, where only -8% of biogas yield can be measured (+2% if related to the VS of the sludge only). Even though the technical feasibility of grass addition can be shown, operational difficulties (fibre size, hydraulic mixing, low HRT) seem to prevent the realisation of the maximum potential of grass addition in full-scale. The environmental assessment of the systems in Berlin and Braunschweig reveals a high degree of energy production in both systems, lowering associated impacts of carbon footprint and other environmental impacts. However, potentials for optimisation are identified in terms of energy production and nutrient recovery, and recommendations for the future testing of technical options are given based on the scenario analysis within the LCA. Environmental benefits of the reuse approach in Braunschweig are quantified and relate mostly to the lower discharge of nutrients and other pollutants into surface waters. The normalised environmental profile underlines the primary functions of wastewater treatment (= protection of surface waters), which should not be compromised while optimising energy demand and carbon footprint.