Closing wastewater loops solves multiple problemsWhen a waste product can be transformed into useful resources, it is possible to solve many environmental problems at once. Multiple reductions in ecological footprints are also achieved. In Stockholm, transforming sewage into biogas and organic rest products is aimed at preventing or solving several major environmental problems: climate emissions, air pollution, landfill, food security, etc. Multiple benefits make wastewater treatment more economically sustainable, and so more transferable to poorer countries.
Keywords: wastewater, water treatment, biogas, anaerobic digestion, nutrient recycling, sludge
Stockholm’s water treatment strategy is aimed at turning waste products into useful resources: sewage produces biogas, fertilizer, and even heat. The primary technique is the multiple generation of energy (for heat, electricity, cooking gas, vehicle fuel) through anaerobic digestion of biological wastes. When implemented fully, this could solve an associated range of environmental problems, thereby reducing ecological footprints – and increasing the economic sustainability of water treatment (see also Singapore).
- Reduction of greenhouse gas emissions: biogas partly replaces other fuels (particularly fossil fuels) in sewage plant operation, vehicle operation, the heating sector, electricity production, and even fertilizer production. Biogas itself produces no net CO2 emissions.
- Reduction of energy demand: the sewage plants become net-energy producers by producing and using biogas. Part of this is realised by reduced energy use (estimated at 5 GWh/year), but also via increased gas production from organic material.
- Water/sewage treatment: this is a key urban function that a large proportion of the world’s cities cannot afford. But it becomes more economically and environmentally feasible when water/sewage treatment's resources are utilised (e.g. biogas-energy, sludge-organic products).
- Reduction of air pollution: Lower emissions of particulates, nitrogen oxides, and other carcinogenic substances when biogas replaces the use of fossil fuels.
- Transition to renewable energy sources: biogas is a renewable energy source.
- Recycling of nutrients: Phosphorus, a crucial and non-renewable input for all agriculture, can be recovered at high percentages along with nitrogen. Sweden has a national goal for at least 60% of phosphorus in wastewater to be recovered for agriculture (see also Hyderabad).
- Food security: Stockholm’s approach boosts food security in multiple ways. Along with the recycling of non-renewable phosphorus, a necessary input for all plant-growth, the approach produces large amounts of plant fertiliser. Also, it provides a more sustainable energy source than biofuels, whose crop requirements compete with food for agricultural land. Biogas from sewage does not require additional agricultural land.
- Reduction of landfill flows: by finding useful end products for sewage sludge, it is possible to reduce the flow to landfills and their potential pollution of air, water, etc.
- District heating: The temperature of the treated water averages 7-20 C, enabling the recycling of heat into district heating for the city. District heating has been one of the most important ways that Sweden has reduced its climate emissions in the recent two decades (see also Danish towns).
- Ecological restoration: Sludge from the water treatment is also used for the restoration of land around mining sites.
Additional economic benefits are the low operating costs of the technology, and the boost given to the development of new technologies (e.g. based on biogas). These add to making the wastewater treatment approach economically sustainable, and therefore more transferable to even poorer countries.
Pekka Lahti, Enrique Calderón, Phillip Jones, Michiel Rijsberman, Jan Stuip (editors), 2006, Toward Sustainable Urban Infrastructure: Assessment, Tools and Good Practice – Results of COST Action 8 “Best Practice in Sustainable Urban Infrastructure”, European Commission programme for Research and Innovation COST programme and the European Science Foundation
Swedish Environmental Protection Agency, 2009, “Better air quality in Stockholm thanks to sewage sludge”, http://www.swedishepa.se/Documents/publikationer/978-91-620-8403-5.pdf
Stockholm Vatten, ”Restprodukter”, ”Biogas”, “Fordonsbränsle”, ”Slam” (2009-10), http://www.stockholmvatten.se/sv/Vattnets-vag/Restprodukter/
Key data are retrieved from the UN World Urbanization Prospects: The 2011 Revision, http://esa.un.org/unup/unup/index_panel2.html