مقاله سال ۲۰۱۷

Resource scarcity is a topical issue whose solutions should be sought not only from the energy sector, but also from the waste management/ sanitation sector. Currently, urban infrastructures are characterised by centralised treatment plants and long transportation distances, and they have been criticised for high energy and resource usage as well as inadequate resource recycling. The EU has been supporting a circular economy through the Horizon 2020 Research and Innovation programme (Horizon 2020 sections, n.d.). Consequently, there are new technical solutions available, but their testing and implementation are still in the initial stage. The adaptation of technical innovations has been resisted by stable infrastructure regimes, which carry out essential societal functions and are therefore characterised by lock-in and path-dependency processes (Smith and Raven, 2012). In past decades, the centralisation of infrastructures has inevitably provided health and environmental benefits. However, a revival of decentralised urban infrastructures should be considered today to counteract new sustainability challenges.

To understand the present infrastructures and the motivation to change them, technical solutions and resource flows need to be observed critically. At the beginning of the food chain, current agriculture depends on irrigation (Valipour, 2015) and artificial fertilisers produced in an energy-intensive process (nitrogen N) (Brentrup and Palliére, 2008) and mined from scarce reserves (phosphorus P) (Cordell et al., 2009). Agricultural products, and consequently food products, contain high amounts of nutrients that the human body mainly excretes in urine (Spångberg, 2014). In addition, garden and kitchen waste (hereafter referred to as biowaste) contributes to urban nutrient flow (Sokka et al., 2004). In a conventional wastewater-treatment plant, energy and chemicals are used to remove nutrients according to ever stricter environmental requirements. In wastewater treatment, N is converted to atmospheric nitrogen and P is often precipitated into an insoluble form, limiting its reuse. Finally, biowaste and treated sewage sludge are landfilled, incinerated, composted, anaerobically digested (Manfredi and Pant, 2011) and/or recycled into agriculture.

Anaerobic digestion is an attractive treatment technology because it generates renewable energy in the form of biogas, supports nutrient recycling and potentially creates local jobs. Furthermore, anaerobic digestion is suitable for urban areas because the process occurs in enclosed tanks, and emissions are easier to manage than in other treatment methods (Edwards et al., 2015). However, recycling end products from centralised plants to agriculture is marginal (Meers, 2016), so the nutrient loop is not closed. In addition to process limitations, the risk of recycling harmful substances, lack of acceptability (Aubain et al., 2002),

unsupportive or unclear legal frameworks (Hukari et al., 2016), and governance aspects such as poor source-separation or inefficient plant operation (Zabaleta and Rodic-Wiersma, 2015) are hindering the recycling of waste-derived nutrients Source-separating sanitation and decentralised treatment of domestic wastewater have been suggested as an alternative with the potential to improve nutrient recycling and energy efficiency in the sanitation system (Tervahauta et al., 2013). Furthermore, decentralised water systems have the potential to reduce infrastructure costs and support innovations that can be exported to emerging economies (Quezada et al., 2016), whereas distributed energy systems may increase renewable energy production capacity and energy self-sufficiency (Ruggiero et al., 2015); moreover, such systems may enhance sustainability in terms of flexibility, locality and networking (Alanne and Saari, 2006). To promote local resource cycles and renewable energy production, the authors have designed a decentralised circular system (Fig. 2, in Section 2.2) that consists of source-separating low-water toilets, small-scale biogas plants, and the local utilisation of nutrients and produced gas within a residential area (the case city: Tampere, Finland).