Crops require fertile soil, sufficiently rich in organic matter and accessible nutrients. Renewal of fertility today relies on the use of large amounts of mineral fertilizers. The future availability of these is compromised by the scarcity of energy and mineral resources. Ensuring the recycling of nutrients is essential to improve the food resilience of regions. This requires the return to the fields of human excreta (urine and faeces) and the generalized recovery of bio-waste.
State of Affairs
The linearity of the food system
From fertilizers to treatment plants
In essence, agricultural activity consists of exporting food products from cultivated land. In doing so, the nutrients present in the soil and incorporated by the plants are also exported. Cultivated soils are therefore gradually depleted and farmers must compensate for these nutrient losses in order to maintain their fertility . This concerns in particular nitrogen (N), phosphorus (P) and potassium (K), often limiting elements in plant growth. The most common technique for renewing soil fertility is the use of fertilizers , that is, the supply of nutrients from outside the cultivated plot. A distinction is made between traditional organic fertilizers, whose materials used come from living animals or plants (manure, slurry, guano, dried blood and bone, vegetable compost, etc.), mineral fertilizers, which derive from inert mineral matter or are synthesized industrially. These are directly assimilated by plants, and allow high-yield standardized productions. Today produced in very large quantities, mineral fertilizers are a key element of the industrialized agricultural system .
Urea manufacturing plant in Vijaipur (India). Urea is one of the main sources of nitrogen in agriculture. The synthesis of nitrogenous fertilizers uses about 1% of the annual energy consumed by humanity, mainly in the form of natural gas. Crédits : Natfert, CC BY-SA, Wikimedia Commons.
The current use of fertilizers is ineffective. A significant part of the nutrients is washed away by the rains or is transformed into volatile compounds. It is estimated that in France, around 25% of the nitrogen supplied to the field is lost in this way . The waste produced during processing, distribution and cooking also concentrates some of the nutrients. But the essential is found on our plate, then our digestive tract (Figure 35) .
Once used for our physiological needs, these nutrients are rejected by our body: it is excretion. This is done mainly through the production of urine, which alone concentrates about 90% of the nitrogen consumed, 60% of the phosphorus and 75% of the potassium .
Figure 35 : Nitrogen flows associated with the production and consumption of wheat in the Paris urban area. The percentages (indicative values) all relate to the total amount of nitrogen supplied to the fields (mineral fertilizers and biological fixation by legumes). In total, only 3% of the nitrogen supplied is reused during the agricultural spreading of sewage sludge. But the fields that receive this sludge export their production outside the territory and contribute very little to the Parisian food supply, so recycling is virtually nil. Source : Les Greniers d’Abondance, from Esculier et al. (2018).
These nutrients are then evacuated with the flushes and join, in dense areas which concern more than 80% of the French population, a sewer and then a purification station. Wastewater is treated there in order to limit disturbances to aquatic ecosystems. Two thirds of the territory is covered by an obligation to treat nitrogen and phosphorus, responsible for the phenomenon of eutrophication. In this case, nitrogen is volatilized by the action of certain bacteria while the phosphorus precipitates and becomes trapped in the settling sludge. Even when this sludge is used as fertilizer - about two-thirds of it - most of the nutrients that enter the food system through mineral fertilizers are eventually lost (Figure 35). The system is said to be “linear” .
This purification process uses fossil fuels, rejects greenhouse gases and a significant portion of the nutrients still enter the waterways, causing significant pollution locally.
Settling basins at the Néris-les-Bains (Allier) wastewater treatment plant. In France, most wastewater undergoes this type of treatment, during which certain nutrients are concentrated in sewage sludge, while others are volatilized or discharged into the natural environment. Crédits : Toucassé, CC BY-SA, Wikimedia Commons.
Organic waste still too little recovered
The management of organic waste (or bio-waste) produced at each stage of the food chain is also essentially linear.
Companies that produce a large quantity of bio-waste have an obligation to sort it and have it recovered in suitable channels. The thresholds have been gradually lowered: today this concerns professionals producing more than ten tonnes of bio-waste per year. Restaurant owners are struggling to apply this regulation, and this threshold remains high. Organic waste constitutes a quarter of household waste, or about 150 kilograms per year per inhabitant. The vast majority are incinerated or landfilled, and the nutrients they contain are lost. A historic break The current linearity of nutrient flows marks a strong contrast with the situation at the start of the 20th century. Circular management of nutrients had indeed been the subject of growing concern among chemists and agronomists during the nineteenth century, fearing that the rise of cities would lead to a critical depletion of agricultural soils. Most of the cities in France at the time had plants to transform materials collected from cesspools into fertilizer. The installation of the sewer from the end of the 19th century and the spreading of the collected water made it possible to further increase the circularity of the nutrient flows (Figure 36). This recycling then gradually diminished as modern wastewater treatment practices developed.
Figure 36 : Agricultural recycling rate of the nitrogen contained in the urine and faeces of the Paris area since 1850. The installation of the sewerage system in the 1890s was accompanied by better agricultural use of excreta, with almost systematic spreading of the collected water. However, this practice gradually regressed during the 20th century in the face of the increasing treatment of wastewater in purification plants. Source : Esculier (2018).
Links to resilience ?
In France, most of the nutrients supplied to agricultural soils come from mineral fertilizers and depend on non-renewable resources . The synthesis of nitrogenous fertilizers consumes large amounts of natural gas. Most other fertilizers (phosphorus, potassium, zinc ...) are made from mineral resources whose exploitation is compromised in the short term by oil supply constraints and by the depletion of better quality deposits. The scarcity of fossil fuels will also restrict the availability of sulfur - an element that is often limiting for the growth of certain crops - and of sulfuric acid used for the synthesis of phosphate fertilizers.
This dependence on non-renewable resources also concerns a probably significant part - but difficult to estimate - of the organic fertilizers used in France. In fact, most of them come from the droppings of farm animals, whose ration comes mainly from crops - or even meadows - fertilized with mineral fertilizers (see path to resilience n ° 10).
A decrease in the availability of mineral fertilizers would quickly lead to lower yields in conventional production systems. Their rise could also cause significant economic difficulties. The collapse of the Soviet bloc, for example, caused a 70% drop in the production and consumption of nitrogenous fertilizers in the regions concerned, causing significant reductions in yield.
In the medium term, in a context of gradual reduction in the supply of new nutrients, the current linear system can only lead to a gradual depletion of agricultural soils and a decrease in overall production .
Unlike many other countries, France has until recently stayed away from "green sanitation" initiatives. Ensuring the territory's autonomy in fertilizer requires moving away from the current linear functioning and seeking to complete the nutrient cycle, so that as much as possible of what is exported from the fields during harvesting, can return there to fertilize subsequent crops.
Two types of resources are to be used: human excretates (urine and faeces), which concentrate the vast majority of nutrients, and organic waste or “bio-waste”.
For excreta, two main treatment routes can be considered:
- Separate collection of urine and feces, known as "source separation", using male and female urinals and / or separate toilets;
- Individual or collective recovery and composting of all materials in dry toilet-type systems. Urine has the advantage of concentrating most of the nutrients, being easily collected and safe. Its agricultural development is therefore particularly easy to implement. However, it represents significant volumes to manage. The urine collected can be concentrated to make different types of fertilizer, liquid or solid, called urino-fertilizers. Faeces, on the other hand, contain some of the interesting nutrients such as phosphorus, and have the advantage of being in the form of carbon-rich organic matter, which can amend the soil. They require composting in order to eliminate pathogens and parasites. However, the potential for energy recovery from human faeces remains very low. Source separation is particularly suitable in urban areas for its simple implementation and the large volumes to manage. For bio-waste, selective collection and the provision of individual or collective composters would allow good recovery of the resource which could then be valued locally, for example in urban or peri-urban agriculture.
From an economic point of view, the agricultural use of excreta makes it possible to reduce the costs of wastewater treatment, as well as the costs of farms. The separate management of bio-waste makes it possible to reduce the volumes and cost of collection. From an environmental point of view, the recycling of nutrients makes it possible to reduce greenhouse gas emissions linked to the production of fertilizers and the treatment of wastewater. It also makes it possible to greatly limit the pollution of aquatic environments, thanks to the reduction in the nutrients released both by mineral agricultural fertilizers, and by partially treated water from purification stations.
The installation of toilets or urinals allowing the recovery of resources represents an investment and requires specific skills. Several professionals are able to support communities to meet these challenges. This type of installation is an opportunity to educate users, who often readily adopt the new system when they understand the stakes. The installation of urine networks requires specific vigilance linked to the risk of blockage of the networks by urine crystals. The OCAPI team has published recommendations on this subject.
The recovery channels are still under construction. Some composting platforms already accept materials from dry toilets. It is also possible to partner specifically with farmers in the community to set up a sector, as the rental of dry toilets for events are already doing.
Regulation of the use of excreta
The use of urine and faeces in agriculture is not specifically regulated by regulations, but the Ministries of Ecology and Health make it possible to rely on existing regulations relating to the spreading of waste matter from the 'sewerage. The use in organic farming is delicate today and would require an evolution of this regulation (necessary also in the paradigm of organic farming which cannot do without fossil mineral fertilizers on a large scale if human fertilizers are prohibited. ). It should be noted that the agricultural recovery of urine and faeces has been widespread in France for centuries through the use of livestock manure as fertilizer. Likewise, two thirds of sludge from wastewater treatment plants is spread. The issues that are sometimes raised about the risks associated with the presence of drug residues or other pollutants in human urine appear relatively minor compared to what is already being done today and compared to the benefits of recycling them.
- Composting rate of bio-waste
- Agricultural recovery rate of nutrients (nitrogen, phosphorus, potassium) contained in the excreta of the population
- Volumes of urine and feces collected