Sustainable Farming - Organic Fertiliser From Biogas Plant Residue

The most important element in agriculture required for growing crops is nitrogen. Nitrogen is a major component in chlorophyll, which is crucial for photosynthesis. It also is an essential part of plant proteins, giving the crop its structure. It is absorbed through the crop's roots in the form of ammonium or nitrate. Thus, the only way to supply the crops with adequate nutrition is to use nitrogen-rich fertilizers. For this, farmers rely heavily on inorganic options. However, these chemical fertilizers are notorious for causing environmental pollution. Additionally, some farmers use digestate, the main by-product from biogas plants, as an alternative organic fertilizer. Like here in Tuscany, Italy, where an agricultural consortium of local farmers is pushing the use of organic fertilizers for various reasons. Massimo Donati is one of its members. Lately, we've been using chemical fertilizers as little as possible, for economic reasons, because their prices have risen frighteningly in recent years. When we see the plant is stressed, we can intervene punctually, with a minimum amount of fertilizer. Then, of course, the distribution of digestate complements what we have cut down with chemical digestate. This leads to very satisfactory results, because the plant we have seen reacts very well with this form of organic matter of plant effluent. This organic digestate is produced in Italy in large quantities. The country hosts over 1,600 functional biogas plants. If the potential of the biogas industry were fully utilized worldwide, studies project that digestate could replace 5-7% of the synthetic macronutrient fertilizer currently used globally. The challenge here is that high volumes of digestate need frequent transportation. However, digestate consists of 90% water and 10% organic mass, rich in nutrients. If the digestate could be more concentrated, it would gently increase the nutritional value of the fertilizer, especially the nitrogen content, and reduce the need for constant transport. About 20,000 liters of sludge could be converted to 50 kilos of fertilizer. A possible solution is currently being tested with the Nomad Truck, a lab on wheels that has been touring through Malta, Greece and arrived in Italy in December 2022. The demo site of this EU research project was located near Rome. The priority of the scientists on site was the production of biofertilizer from solid residues. Therefore, the solids were filtered out by processes that use membranes with very small pores. These solids then underwent a selective electrodialysis treatment for nutrient recovery. The recovered solid fertilizer was tested at a controlled greenhouse trial at the University of Florence. The plants received five different fertilizers originating from the three Nomad demonstration sites, using different substrates. At the moment, the plants are very close to the harvest moment because we follow the concept to reach the commercial maturity. So now the plants are quite big. In order to test the effect of different fertilizers, plant diameter, height and chlorophyll content were measured weekly. The chlorophyll index provides indirect data on the nitrogen content. This gives the researchers information about the health status of the plant. Their hypothesis? Nutrient content in the Nomad fertilizer is dependent on the raw material processed by anaerobic digestion. Crop waste was a substrate used by the biogas plant at the Italian demonstration site. The substrates used for biogas production in central Italy are mainly grains and plant-based wastes. Often, small farms build consortiums to jointly run a biogas plant. Valentino Smedile is a member of an agricultural consortium with three other farmers in Tuscany. The plant is fed with silages made from maize, winter cereals and sorghum, plus added by-products that are purchased in the market. Currently, these include citrus cake, potatoes and cornmeal, a residue from screening and processing corn, combined with two livestock effluents, cattle slurry and pig slurry, that complete the product mixing process during fermentation. Adding crop residues maximizes the output of the biogas plant. This is because the bacteria in the biogas plants feed on crop substrates to produce higher amounts of biomethane. This plant produces about 8.5 million kilowatts per year and 4,500 cubic meters of digestate every three months. Here, trucks are engaged weekly and used many times a day to transport the liquid sludge to the fields for spraying. If the Nomad truck were to be successful in reducing the volume of the digestate and recovering nutrients, it would drastically reduce the load on storage and transportation and highly increase the nutritional value of the solid fertilizer. The average sizes of the farms in Tuscany is quite small, because to support a biogas plant you need land. Now, the tendency is to reduce the sizes of the biogas plant in order to tailor the dimension of the plant on the production of the farm. Due to the morphology of the country and the high variability in agricultural production, the Nomad project, with the concept of the mobility of the truck that can connect all of the biogas plants, can represent a very interesting point for the farmers and farmers associations in Italy.