Fertile cities: Nutrient flows from new sanitation to urban agriculture

Developments in urban agriculture and new sanitation systems bring about new narratives to the status quo of both food production and human excreta management, and reintroduce the opportunity to partially close nutrient cycles at the urban scale. Urban agriculture is the production of food in and around (peri-urban) a city. New sanitation systems collect, transport and treat streams containing human excreta and aim to recover valuable resources from those streams. The recognition of the mutual benefit for nutrient exchange between urban agriculture and new sanitation has increased. In this regard, urban agriculture has a demand for nutrients and new sanitation a supply of nutrients, which if matched, can facilitate nutrient recycling and thereby minimize nutrient losses. Nevertheless, numerous challenges remain to match nutrient flows between urban agriculture and new sanitation. Not only do the quantities and qualities of nutrient demand and supply need to be matched – taking into account parameters for plant requirements, as well as human hygiene and environmental safety (e.g. pathogens, heavy metals) – but also spatial and temporal dynamics of demand and supply (e.g. when and where fertilizers are needed and when and where nutrients are excreted) need to be optimized for coupling of nutrient flows. This thesis contributes to uncovering the potential of integrating urban agriculture and new sanitation so as to establish nutrient recirculation between the two. Specific objectives include (i) an analysis of nutrient demand and supply, (ii) an evaluation of spatial and temporal aspects of supply and demand matching, and (iii) a reflection on trade-offs for improved nutrient recycling within the urban environment. This thesis primarily focuses on the three macronutrients, nitrogen (N), phosphorus (P) and potassium (K), as well as organic matter (OM), although, other macro- and micronutrients are tangentially discussed.

Chapter 2 presents a first exploration of closing cycles between urban agriculture and new sanitation using the Urban Harvest Approach, and shows the achievable nutrient self-sufficiency for phosphorus and partially for nitrogen and organic matter in the city of Rotterdam. It was the indication in this first study of nutrient over-fertilization at two urban farms which gave rise to the larger study on fertilizer inputs in urban agriculture in Chapter 3, including interviews at 25 urban farms across the Netherlands. This study serves as a quantitative benchmark to understand nutrient management practices in urban agriculture and urban nutrient demand. Research on nutrient supply is presented in Chapter 4, which includes an expansive option space of recovery pathways that exist to recover nutrients from streams containing human excreta. The chapter further identifies broad patterns and trends across the field as a whole, highlighting the focus on individual recovery technologies and on the targeted recovery of phosphorus. In addition, the review is meant to serve as a basis for organizing and categorizing information on nutrient recovery pathways for more effective sharing and consolidation. Chapter 5 moves the research towards including a spatial dimension to nutrient flows, recognizing the importance to understand where resource consumption and waste production take place, and where losses occur. Such understanding, as well as spatially explicit data on the scale at which practitioners work, can increase capacity building for planning and decision-making with regards to intervention strategies for improved resource management. The study used geographic information system (GIS) analysis to identify locations with high nutrient excretion (supply) at building and neighborhood scale. Chapter 6 builds on this first spatial study by matching the supply with nearby demand and optimizing transportation distances between the two. The next chapter, Chapter 7, looks to the subject at hand from a theoretical lens, and presents a plea for reframing human excreta management as part of food and farming systems. Finally, Chapter 8 places the results within a broader perspective on nutrient management and finishes by indicating areas for further research.