Multi-objective analysis of nutrient use and recycling under consideration of economic conditions and regional structures

Abstract

This thesis explores sustainable nutrient reuse and recovery strategies in various economic and environmental contexts, focusing on both smallholder farms in the Sio-Malaba-Malakisi RiverBasin in Kenya and Uganda and municipal wastewater in Austria. It underscores the necessity of tailored nutrient reuse and recovery strategies to address distinct challenges posed by different regional conditions and economic capacities.The most important results for the Sio-Malaba-Malakisi catchment area show that agriculturall and suffers from considerable nutrient deficits due to its high susceptibility to erosion, low nutrient input and high nutrient output as a result of the constantly increasing population pressure. Local resource utilization, like urine collection and vermicomposting, could enhancenutrient recovery and agricultural productivity potentially increasing maize yields by 8-40%.However, even for the comparatively small area of analysis, differences in the effectiveness ofproposed nutrient management strategies depending on the area’s environmental and societalpreconditions are revealed. The involvement of agricultural extension workers will be crucial for the successful implementation of region-specific technologies tailored to the diverse agroecological and socio-economic conditions.In contrast to Sub-Saharan Africa, European countries like Austria set a strong focus on Precovery from wastewater, driven by economic dependencies on mineral phosphorus-supplying countries rather than a low availability of nutrients. This alternative can decrease the dependency on the global phosphate rock market but may influence environmental impacts from fertilizer production. This thesis first evaluated 18 phosphorus recovery technologies using life cycle analysis to assess cumulative energy demand, global warming potential, and acidification potential,alongside their recovery potential, heavy metal and micropollutant decontamination, and fertilizerefficiency. Results indicate a broad range of potential emissions and energy demand changes due to different phosphorus recovery technologies. Recovery from the liquid phase has minimal emissions but low efficiency, where as recovery from sewage sludge, particularly ash, offers the highest phosphorus recycling potential despite varying environmental trade-offs. This part of the thesis revealed that future research should consider local infrastructure and legal frameworks to show both environmental and economic outcomes.As a basis for further analysis of optimal P recovery strategies, a survey of Austrian wastewater treatment plant infrastructure, focusing on phosphorus removal, sludge treatment, and disposal was commenced in 2017. The analysis covers information on the construction and dimensioning of wastewater treatment plants, P removal and sludge stabilization, sewage sludge production, watercontent and quality of sewage sludge, as well as on sludge disposal. There by, the datacollection revealed useful insights on the current state of Austrian wastewater treatment as a basis for evaluating the implementation of P recovery from wastewater.Next to resource recovery, under the EU Green Deal and Circular Economy Action Plans,European wastewater and sludge treatment also faces high expectations for greenhouse gasmitigation, energy demand reduction and micropollutant removal 2. This thesis further utilized the aforementioned datacollection on Austrian wastewater treatment plants, coupled with substance flow analysis, energy flow analysis, life cycle assessment, and cost estimation to provide amore fine-grained picture of current environmental and economic costs of the system. The analysis highlighted the importance of electricity demand for overall energy use, and shows large inefficiencies in the use of wastewater resources and sludge management systems. For example,the widespread composting of sludge and use for landscaping was identified as environmentally unfavorable in comparison to other sludge disposal methods.At last, this thesis employed a scenario-based analysis to evaluate the environmental andeconomic impacts of various national phosphorus recovery strategies within Austria’s detailed wastewater infrastructure context. Combining material flow analysis, life cycle assessment,and life cycle costing, this part of the thesis examined phosphorus recycling and utilizationrates, heavy metal removal, global warming potential, cumulative energy demand, acidification potential, freight transport volume, and annual costs under different scenarios. Key conclusions include the high potential of phosphorus recovery from ash, the necessity of decontamination technologies to prevent agricultural soil pollution, and the cost-effectiveness of different recovery strategies. Optimal phosphorus recovery can reduce greenhouse gas emissions if nitrous oxideemissions from sludge incineration are minimized and heat utilization is efficient. The thesis emphasizes the trade-offs between centralized and decentralized mono-incineration strategies,providing a foundation for informed policy and infrastructure decisions to enhance sustainable phosphorus recovery.In summary, this thesis offers valuable insights into the multifaceted challenges and potential solutions for sustainable nutrient management and phosphorus recovery. By addressing the unique needs of different regions, these strategies can contribute to improved agricultural productivity,environmental sustainability, and economic resilience.2EU (2024). Directive (eu) 2024/3019 of the European Parliament and of the Council of 27 November 2024 concerning urban wastewater treatment (recast).URL http://data.europa.eu/eli/dir/2024/3019/oj