Circular Agri-Food Ecosystems (CAFE)¶
Summary¶
Current "conventional" agricultural practices, notably in Western countries, rely on large amounts of energy and fossil or abiotic resources to produce synthetic fertilizers. As most nutrients extracted during harvests are not brought back to the soil, many farmers depend on these fertilizers. This linear and extractive system has endured at great social and environmental costs around the world.
Because it works as an open circuit, it has dire consequences, with the perspectives of phosphate shortage, and large amounts of energy invested to generate the fertilizers or to process nutrients once they end up in sewage. Furthermore, as they do not replenish the organic content of the soil, conventional practices based on mineral fertilizers lead to significant biodiversity loss, soil erosion and salinisation as well as nutrient leaching. Finally, animal-based diets in the largest economies, notably in the US and Europe, leads to an unnecessary pressure on food systems as feeding farmed animals inflates land and nutrient use.
Source separation and recovery of organic matter (urine, feces, biowaste) is likely to be critical for the long-term sustainability of the agri-food and waste-management systems in an increasingly urban world. Indeed, the transformation of kitchen waste and human excreta can provide invaluable resources such as compost, fertilizers, or energy and eventually remove the need for synthetic fertilizers. This project will therefore develop prospective scenarios for biowaste use, investigating the potential impacts of nutrient recovery and how far it can bring the agri-food system on the path to circularity. To that end, we will assess the possibility of dietary changes (low-meat consumption and plant-based diets) as well as low-fertilizer and agro-ecological practices and compare them with business-as-usual situations. The objective will be to evaluate the technical feasibility of each scenario and quantify how much they can improve agri-food sustainability.
Preliminary work on a python library to quantify organic resources is available here; the work-in-progress packages can also be found on the CAFE repository.
This work is done in collaboration with members of the OCAPI project, which you can also follow on the Fediverse. You can also check my talks on the subject in English or in French.
Some important numbers and rationale¶
The world uses around 100 Mt of nitrogen per year and 17 Mt of phosphorus and 33 Mt of potassium [NutrientUse] (respectively a ten and four times the amounts used in the 50s and 60s). This overconsumption of nutrients, used with very low efficiency, has severe environmental impacts. Recovering human excreta would provide around 30 MtN, 3 MtP, and 5 MtK per year, which is a significant fraction of agricultural needs with more reasonable application rates and efficiency. This is especially crucial for phosphorus, which is a finite resource and is mostly lost at sea with current practices. Yet as Isaac Asimov once warned, “we may be able to substitute nuclear power for coal, and plastics for wood, and yeast for meat, and friendliness for isolation – but for phosphorus there is neither substitute nor replacement".
Wastewater management is a major source of consumption in cities' public services, representing around 20% of the electricity consumption. Around 90% of the energy consumption is associating to the treatment (80%) and final disposal (10%) of the sewage "waste" [Stricker2018]. Recovering human excreta before they end up in sewage has thus some major potential for water and energy savings [AboutNitrogen]. Furthermore, the project gives priority to agricultural (rather than energetic) applications for excreta because, while their energetic potential represents less than 1% of current Western consumption (5 kWh/cap/day on average), they are irreplaceable resources if one wants to grow food on healthy soil.
Preliminary results¶
First steps to evaluate deposits on a metropolis such as Paris show significant variability in the amount and density of these deposits, especially regarding their distribution between the place of residence and the workplace.
The following pictures detail the geospatial properties of the deposits' density and distribution, and the detailed quantity of organic matter and nutrients from the excretions and waste generated by the 6 million people in the area over a week.
Map of Paris and its surroundings showing the nitrogen density from organic matter in each neighborhood or town and the distribution between the place of residence, work, and study. The outer neighborhoods of Paris have the largest deposits and those from the center-west part are almost exclusively work-related as few people live there.¶
Deposits for the different types of organic matter generated each week.¶
Nurients' deposits associated to the organic matter: 73% nitrogen, 8% phosphorus, and 19% potassium.¶
Data was converted from P2O5 to P and K2O to K based on the numbers in OurWorldInData.
Consommation énergétique des filières intensives de traitement des eaux résiduaires urbaines. Journées Information Eaux (23e édition)
In the current system, we pay a lot of energy to convert N2 from air into ammonia to make fertilizer at the beginning of the chain, then pay again a lot of energy to denitrify wastewater by converting the nitrogen back to N2 in the treatment plants, once it's been flushed down our toilets.