The Saclay plateau is a small peri-urban area located about ten kilometers south of Paris. Despite its increasing urbanization, agriculture still occupies a prominent place, with around 3,500 hectares. Mainly conventional, it is based on the significant use of synthetic or mineral fertilizers. The main crops are wheat, rapeseed, corn and barley, and require significant inputs of nitrogen (N) and phosphate (P2O5) fertilizers.
Nitrogen fertilizers are produced using a very energy-intensive process, in the order of 1 to 2% of world consumption. Phosphorus is extracted from mines with limited reserves. A peak in production could occur within a few decades.
Once consumed by humans, the nutrients contained in food are mainly excreted via the urine and are found in wastewater.
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Very limited nutrient recycling
On the Saclay plateau, the wastewater of some 200,000 inhabitants is now directed to the treatment plants of the interdepartmental syndicate for the sanitation of the Paris agglomeration (SIAAP). The nutrients (from the point of view of agriculture) or pollutants (from the point of view of sanitation) that they contain are thus treated.
After adding reagent, the phosphorus precipitates about 80% in the sewage sludge (the rest is discharged into the Seine). Conversely, nitrogen is mainly eliminated by denitrification (53%) or discharged into treated water (38%), a minimal part ending up in sewage sludge (9%). The spreading of part of these in agriculture, therefore, allows only limited recycling of phosphorus and especially nitrogen contained in wastewater, estimated at 43% and 4% respectively on the scale of the Parisian agglomeration.
The current management of wastewater on the Saclay plateau, therefore, does not allow nutrient recycling while the agriculture of the plateau is an important consumer. It is also associated with other impacts: significant greenhouse gas emissions during water treatment in wastewater treatment plants, discharge to the Seine, spreading of undesirable elements during the recycling of sewage sludge in agriculture, etc.
Urine, to reconnect the city and the fields
Among wastewater, urine represents about 80% of nitrogen and 50% of phosphorus, all concentrated in a small volume compared to the overall volume of wastewater. The urine is little contaminated with metals and pathogens, unlike other inputs such as certain mineral fertilizers and sewage sludge. However, questions remain about drug residues present in low concentrations in the urine.
Different techniques are possible to separate urine at the source and thus avoid its dilution in wastewater and its contamination by other discharges: separating toilet, dry urinal (male or female).
Once the urine has been separated from the other components of the wastewater, it is possible to perform a multitude of treatments for different purposes.
The first option is to stabilize the nitrogen in order to reduce ammoniacal volatilization (which pollutes the air and leads to a loss of the fertilizing value of urine) and to reduce odors. The second possibility is to reduce the volume applied, which varies from that of liquid manure to that of a mineral fertilizer for the most concentrated urine fertilizers. Finally, the third treatment consists of restricting contamination with pathogenic microorganisms or pharmaceutical residues.
These different treatments result in various fertilizers based on urine or “urinofertilizers” with various characteristics (levels of nutrients, contaminants, a form of nutrients, etc.).
A privileged area for experimentation
For several decades, the Saclay plateau has been subject to increasing urbanization but remains an important place of agricultural production located only about ten kilometers from Paris. This dynamic context allows the installation of innovative devices on new constructions.
Encouraged by public authorities and associations, several projects are currently underway for the installation of devices to collect urine in establishments open to the public. Due to the gradual establishment of various research and higher education organizations on the Saclay plateau, interactions between farmers and research are also frequent.
For example, the LEADER program on the Saclay plateau has for some years been funding part of the experiments on recycling organic matter in agriculture carried out by INRAE and the Ile-de-France chamber of agriculture. The materials studied until now were more traditional: composts, livestock effluents, anaerobic digestion, etc.
Good fertilizing efficiency
Farmers’ interest in recycling urine has been studied through several surveys. This practice, even if it is little known, is generally welcomed. But farmers have pointed out the need for experimentation with these new products, both in terms of their fertilizing value and their contamination with medicinal residues.
In order to partially respond to this interest of farmers and to study in more detail the various possible urinofertilizers, agronomic trials were set up on the Saclay plateau as part of the AGROCAPI research project (INRAE, AgroParisTech, École des Ponts). The fertilizing efficiency of these treatments has been tested on different crops (wheat, rapeseed and grain corn) for two years.
According to the results, it is close to mineral fertilizers and superior to conventional organic fertilizers such as cattle manure. One kilogram of nitrogen contained in a urine fertilizer has the same effect as a kilogram of nitrogen from mineral fertilizers, unlike nitrogen from organic fertilizers (slurry), which is less effective in the short term. In addition to the study of the fertilizing value, greenhouse gas emissions, ammoniacal volatilization and drug residues are being analyzed.
There is still some way to go before generalizing the recycling of nitrogen and phosphorus on the plateau. This fall, however, bread was produced with wheat harvested from plots fertilized with urine.
On the Saclay plateau, the urine of the inhabitants represents almost twice the needs of farmers for fertilization. The case of the Paris agglomeration is particularly relevant because it brings together more than 10 million people and is surrounded by cereal plains which are in high demand for fertilizers. Ile-de-France mineral fertilizer needs could be covered with the urine of the Paris conurbation.