Sinks as Constraints for Urban Development

Modern cities have a high metabolic rate, requiring large amounts of resources which are used on one hand to build up the urban structure (buildings and networks) and on the other hand to power and feed the "urban metabolism". In parallel to the supply of materials, cities also have to dispose of materials such as waste water, off-gases, solid wastes, products of corrosion and weathering of surfaces and others. The main goals of this project are (1) to test by a comprehensive approach the hypothesis if the flows of materials leaving a city are overloading available sinks such as water, air, soil, underground storages now or within the next few centuries, and (2) in case there is a sink limitation to give first indications how to cope with these limits. It is a distinctive characteristic of this project to take a long term point of view, taking into account the building up of urban material stocks as well as the constant deterioration, renewal and disposal of this stock in the future. In order to reach the main objective, the following five steps will be performed: (1) the terms "sinks" and "final sinks" will be defined in a rigid, scientific, and utilitarian manner in view of accommodating emissions from the metabolism of cities; (2) the relevant sinks are substance specifically identified for cities in general, and more particularly for the two case study cities Taipei and Vienna; (3) based on MFA methodology, flows and stocks of the urban metabolism to sinks are determined for Taipei and Vienna for selected indicator substances; (4) the impact of these selected flows on the sinks is assessed for short and long time periods according to existing and new assessment methods such as risk assessment and anthropogenic-geogenic reference methods; (5) the results are used to derive conclusions regarding decision support for designers of urban systems, for urban planners, and for environmental engineers who are responsible for securing metabolic processes in cities while maintaining environmental quality in the city and its disposal-Hinterland. To take advantage of specific scientific strengths, the project will be performed in close cooperation with the group of Prof. Hwong-Wen MA, a long-term partner from National Taiwan University NTU. Methodological issues such as sink definition and assessment methods for flows to sinks are elaborated in close contact to ensure compatibility of results. Both partners use similar methodologies to assess flows and stocks of (the same) materials in Vienna and Taipei. NTU and VUT use different approaches to evaluate sink capacity: While the Taiwanese side uses risk assessment techniques, the Austrian group uses the anthropogenic-geogenic reference method. The project will answer the question if the development of cities might be limited by the availability of sinks. Besides this main result, new definitions and methods to investigate into and evaluate the sink issue will become available. New results about the flows and stocks of selected substances in the two cities will serve urban planners as well as environmental engineers and designers in the two cities and elsewhere. The collaboration of the two research groups will enhance the research cooperation between Taiwan and Austria, and will serve to build up and widen capacity in the field of urban metabolism and industrial ecology.

This project provides knowledge and tools to ensure that the inevitable residues of urban activities do not result in overloading of sinks such as water, air, and soil. The large mass of resources exploited from the earth crust asks for adequate sinks on the back side of the consumption system. Such sinks are required for the disposal of non-recyclable wastes and for emissions. Experience with hazardous (Cd) and non-hazardous (CO2) substances shows that sink overloading is an actual challenge. Hot spots of material turnover are cities: Their growth and maintenance requires large supplies of resources. Is urban development limited by a lack of appropriate sinks? To protect natural sinks, urban off-flows have to be controlled by man-made sinks such as treatment and disposal technologies. The objective of this project is to link sources and sinks, and to explore the hypothesis that the lack of sinks is a major constraint for urban development. In addition, new knowledge and tools are supplied for decision makers in urban planning, resource and waste management, and environmental protection regarding sink constraints. The sharing of expertise between Vienna University of Technology and National Taiwan University proved to be particularly fruitful. The following results have been obtained: (1) A methodology for analysis and evaluation of material flows from sources to sinks on an urban scale, (2) demonstration of the feasibility of the methodology by two case studies in Vienna and Taipei, (3) an indicator for sink constraints that is particularly well suited for communication with stakeholders, discriminating between substance flows that are within sink capacities and those that overload sinks. The collaboration with a comparatively new city like Taipei allowed discerning metabolic differences in urban development: While in Vienna, the substance stocks are already on a high level and require replacement, per capita stocks in Taipei are still growing and causing a large resource demand. In both cities, the fraction of total flows of single metals to inappropriate sinks is little (~1%). Still, due to the very large overall flows, these small flows accumulate in soils, and we recommend improving management of metal flows to urban soils and storm-water overflow. For effective sink management, the following steps are recommended: (i) comprehensive assessment of flows to sinks for determining overloading and priorities; (ii) reduction of critical emissions and waste flows to inappropriate sinks; (iii) the design of sink oriented production and waste management systems, (iv) development of a systematic sink monitoring scheme including a new knowledge base for sink capacities and flows to sinks. This project was highly instrumental to develop the new field of sink research by supplying knowledge (methodology and data) about urban flows to sinks, by developing novel tools for sinks assessment, by educating doctoral students in urban metabolism and sink research, and by increasing sink awareness in general through extended publication and presentation activities.