Energy and resource recovery from raw municipal wastewater is a pre-requisite for an efficient and sustainable wastewater treatment in the future. This paper evaluates several processes for upgrading existing wastewater treatment plants or new concepts towards energy positive and resource efficient wastewater treatment in their life-cyle impacts on the energy balance. In addition, future challenges for integrating both energy and resource recovery in wastewater treatment schemes are identified and discussed.

This study exemplifies the use of Life Cycle Assessment (LCA) as a tool to quantify the environmental impacts of processes for wastewater treatment. In a case study, the sludge treatment line of a large wastewater treatment plant (WWTP) is analysed in terms of cumulative energy demand and the emission of greenhouse gases (carbon footprint). Sludge treatment consists of anaerobic digestion, dewatering, drying, and disposal of stabilized sludge in mono- or co-incineration in power plants or cement kilns. All relevant forms of energy demand (electricity, heat, chemicals, fossil fuels, transport) and greenhouse gas emissions (fossil CO2,CH4,N2O) are accounted in the assessment, including the treatment of return liquor from dewatering in the WWTP. Results show that the existing process is positive in energy balance (–162 MJ/PECOD * a) and carbon footprint (–11.6 kg CO2-eq/PECOD *a) by supplying secondary products such as electricity from biogas production or mono-incineration and substituting fossil fuels in co-incineration. However, disposal routes for stabilized sludge differ considerably in their energy and greenhouse gas profiles. In total, LCA proves to be a suitable tool to support future investment decisions with information of environmental relevance on the impact of wastewater treatment, but also urban water systems in general.

Risk-based management approaches are more and more used in the water sector and are promoted by the WHO. As a first step towards an overall risk-based management approach of the agricultural wastewater reuse concept of Braunschweig a quantitative microbial risk assessment (QMRA) is conducted. A 1000 trial Monte Carlo Simulation is used for the assessment of microbial risks for fieldworkers and nearby residents. As a tolerable value of risk an additional disease burden of 1 µDALY is set following the current WHO guidelines. Concerning microbial risks risk-based targets are set in terms of additional required pathogen reduction in the STP Steinhof. Based on the model results an additional reduction of 1.5log units is derived for viruses, for which the highest annual risks of infection per person per year (pppy) is calculated in all scenarios.

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 (PECODa). 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/(PECODa) 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.

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/(PECODa) 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/(PECODa)) 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.

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.