Motivation of this dissertation

This dissertation is based on the potential that the CE principles could have in improving urban food production systems. The promotion of CE principles in UA can help mitigate the environmental impact generated by these systems and move towards a circular agriculture (e.g. Gangnibo et al. 2010; Cao et al. 2011; Trendov 2017; Fan et al. 2018).

According to Ferreira et al. (2018), “agriculture is central to any territorial based circular economy strategy”. Closing the nutrient cycles in UA can produce a regenerative effect on the environment (EMF, 2015a), contributing to utility and value preservation of scarce resources (Bocken et al., 2017). In this sense, the application of CE principles in systems within the city boundaries is strictly related to the concept of urban metabolism, as first conceived by Wolman (1965) as “the metabolism of cities”. Kennedy et al. (2007) defines urban metabolism as “the sum total of the technical and socio-economic processes that occur in cities, resulting in growth, production of energy, and elimination of waste”. In this sense, the application of CE principles in urban systems creates what could be called a circular urban metabolism (Ferrao and Fernandez, 2012), in which waste (outputs) generated in urban areas is transformed into valuable products (inputs) that can be used again within urban limits.

Given the potential of UA as an opportunity to use wastes as resources within city limits (Ferreira et al., 2018; Smit and Nasr, 1992), the application of circular strategies in UA systems is a promising path towards a more circular urban metabolism, as schematized in Figure 1.3. To do so, there is a need to analyze the environmental performance of different kinds of crops in order to identify the environmental hotspots within different forms of UA that follow a linear behavior (Figure 1.3 – Linear UA). In particular, RTG systems remain mostly unexplored although offering additional synergies at a building scale. Once the environmental hotspots are detected, the application of suitable circular strategies can be defined, targeting specific items in the system. Notwithstanding, the application of circular strategies must be strictly monitored in terms of environmental impacts. This analysis will shed light on the alignment or decoupling between CE and environmental sustainability principles, helping to avoid the implementation of contradictory strategies. However, it is essential that the optimization of the circularity or the sustainability of UA does not contradict the ultimate function of these systems: to produce vegetables.

Recovering nutrients in UA systems

Focusing on UA systems, the use of hydroponic cultivation in rooftop farming intrinsically improves the nutrient supply efficiency by allowing for a better control of plant’s nutrition. Moreover, hydroponic cultivation also allows a precise monitoring of the leachates and increases the flexibility of the UA system to manage the residual water and nutrient flows. In this sense, different environmental targets with a linear behavior can be defined with UA at the core. Considering the high demand for fertilizers of

agricultural systems and the depletion of nutrients through the leachates, we need to define strategies to improve the metabolism of nutrient flows in newly implemented UA systems. To this end, finding the best strategy for nutrient recovery in UA systems is a priority, while considering the feasibility and environmental performance of the implemented strategies, among others. From a reuse perspective (Figure 1.3 – Circular UA), this dissertation will assess the real implementation of strategies such as leachates recirculation, which uses the leached flows to irrigate the same crop, or cascade systems,

Figure 1.3⁴ Metabolism of urban agriculture (UA) from a linear perspective to a circular urban metabolsim

which uses the nutrients lost from a donor crop to irrigate a receiving crop. However, these strategies only focus on recovering the nutrients lost in UA systems. Applying CE principles within the urban metabolism framework must go beyond this and exploit possible synergies with other urban systems.

Exploiting the synergy between urban systems through phosphorus recovery

Phosphorus (P) is primarily obtained from phosphate rocks, a non-renewable resource given the slowness of the P cycle. Due to the increasing demand for P to produce fertilizers for agriculture (Figure 4.1) (80% of the available stock of phosphate rocks is being used in the production of fertilizers (Shu et al., 2006)), half of the world’s current phosphate resources will have been used up by the end of the 21^st century (Steen, 1998), although more pessimistic predictions have been recently reported (Li et al., 2016). For this reason, the EU-28 labels P as a critical resource (European Comission, 2014).

In this sense, the European Commission encourages P recovery from local sources by enforcing a shift towards a more circular use of nutrients (European Comission, 2016).

Struvite can contribute to this shift. Magnesium ammonium phosphate (NH⁴MgPO⁴ · 6H²O), commonly called MAP struvite or simply struvite, is a mineral with a low solubility in water (0.018g·100ml^-1 at 25ºC) (Bridger et al., 1961). Due to this parameter, struvite spontaneous precipitation is a regular problem in urban wastewater treatment plants (WWTPs) with a high P load, since it precipitates in ducts and pipes causing operational problems and additional costs (Stratful et al., 2004). However, the development of technologies aimed to avoid this problem by removing P has offered an unexpected opportunity. Intentionally precipitated struvite can be used as a P secondary fertilizer with clear reported benefits. Its low solubility makes struvite a slow-release

Figure 1.4⁵ Annual world production of phosphate rock (t) in the 1990-2015 period. Own elaboration with data from USGS (2015)

fertilizer, improving its supply efficiency, with most experimental tests reporting competitive yields compared to mineral fertilizers (Li et al., 2019). However, two main aspects regarding struvite recovery in urban WWTPs and its application in UA systems remain barely explored.

First, the application of struvite in hydroponics set-ups. Alike soil-based systems, soilless hydroponics allow a precise monitoring of all input and output flows. Apart from analysing its agronomic performance, this dissertation assesses how the application of struvite changes the behaviour of the P flows through experimental tests and analytical methods.

Second, the environmental evaluation of struvite recovery and reuse strategies on a regional scale is still missing from the literature. In this sense, we analyze the environmental performance of struvite recovery and reuse with a regional perspective that treats a metropolitan area like a self-sufficient entity, tapping the full potential of the synergy between urban WWTPs and UA systems (Figure 1.3 – Circular urban metabolism).

The need for a combined assessment

A full system with a maximum circularity is not necessarily a system with an optimized environmental performance. Although the application of circular strategies may entail a reduction in the environmental impacts of a system, it is important not to get the concepts mixed up. The recent position paper by the Life Cycle Initiative calls for precaution: “there is yet no harmonised method to assess whether a specific CE strategy contributes towards sustainable consumption and production” (Peña et al., 2020). To add to this pool of knowledge, this dissertation aims to study all possible circular strategies that could be applied in UA systems considering an urban metabolism perspective. This study needs to include an analysis of the environmental performance of the system, but also an analysis of circularity. The results obtained will help in two different ways. First, to prioritize circular strategies in UA systems. Second, to move towards the combined analysis of sustainability and circularity of production systems through the development of new indicators to define the alignment between the goals of these two big frameworks.