Environment: From biogas production waste new products for agriculture

ENEA researchers have obtained new products for agriculture, such as fertilizers, soil conditioners and purified water for irrigation from biogas production waste^[1]. as part of the “BIOSOS - BIOgas SOStenibile^[2]” project coordinated by the University of Camerino. An innovative two-stage filtration process was developed that made it possible to extract nutrients (nitrogen, phosphorus and potassium) and organic substances that can be used as fertilizers and agricultural soil conditioners from the liquid fraction of digestate. In addition, according to ENEA researchers, the system would make it possible to recover up to 80 percent of the water exiting the digester, which can be reused in biogasification, for fertilizer dilution or field irrigation. This is especially important considering the increased frequency of droughts linked to climate change.

 “With this innovation we want to contribute to achieving European goals, in particular the Farm to Fork strategy that includes a target to reduce the use of synthetic fertilizers by 20% by 2030; they are energy-intensive to produce, contribute to the depletion of mineral resources such as phosphorus, and generate higher greenhouse gas emissions ^[3]”, explained Gian Paolo Leone at the Circular Bioeconomy Laboratory and ENEA responsible for the contract commissioned by the University of Camerino.

The focus of ENEA's experimentation is on the application of a particular filtration technology that the Sustainable Agrifood Systems Division's has been investigating for years. "It is a liquid separation process where a fluid flows parallel to the surface of a membrane that efficiently separates water and salts from concentrated organic substances. The same technology is already being used to extract molecules from waste streams including food, nutraceutical and cosmetic interest, to recover proteins from dairy wastewater (such as dairy whey) and antioxidant molecules like polyphenols and flavonoids from olive oil vegetation byproducts, and even desalinate brackish and marine waters for water supply purposes," pointed out Daniele Pizzichini, researcher at the ENEA Circular Bioeconomy Laboratory.

Delving into the specifics of the process studied, ENEA researchers utilized a two stages filtration process at the Agro-industrial Processes Technology Hall at the ENEA Research Center at Casaccia. “In the first stage of filtration, achieved through microfiltration or ultrafiltration, bacteria are removed and,  turbidity caused by suspended solids removed, thus lowering the pollutant load for subsequent treatment stages” said Gian Paolo Leone at ENEA. “In the second stage,” he said " nanofiltration or reverse osmosis are used to purified water for crop irrigation and concentrated macronutrients, particularly ammonia nitrogen, presenting different characteristics depending on the organic waste (cattle manure or rainbow trout carcass) but concentrations similar to commercial fertilizers ^[4]”.

The key benefit of the process is ensuring that concentrated fractions are microbiologically safe for potential agronomic use.

Based on testing conducted both in the lab and using rapid methods near the plant,we have been able to verify that the digestion process significantly reduces the microbial load in the digestate.

However, the initial filtration stage achieves the almost total abatement of the bacterial load. This makes it possible to apply the treated fraction directly to soil or crops, without risk to human consumption of agricultural products," said Professor Luca Agostino Vitali of the University of Camerino.

Once the filtration system is developed, ENEA researchers will need to test its performance on a pilot scale, to evaluate how the initial filtration stage membranes handle the high content of organic matter and suspended solids ^[5]. Future developments may regard integrating filtration systems directly into biogas reactors to treat animal waste and digestate water on-site, making the process more efficient and cost-effective. Finally, the energy sustainability of the technological solution will have to be analysed – evaluating its integration with renewable sources such as photovoltaics - and the potential use of biostimulants to support soil microbiota.

The increasing deployment of anaerobic digesters to produce biogas from biomass and organic waste has led to a rise in digestate production, a by-product of the biogasification process. In 2022, the volumes of digestate produced in European plants reached nearly 31 million tons (Mt) of dry matter (with a nitrogen content of 1.7 Mt, phosphorus content of 0.3 Mt and potassium content of 0.2 Mt ^[6]), Forecasts indicate that this volumes will significantly increase, reaching 75 million tons by 2030 and 177 million tons by 2050. In Italy, the annual digestate production is around 3 Mt. Digestate is valued for its nutrient content, particularly nitrogen, phosphorus and potassium, especially when the digestate originates from the digestion of animal manure.

These nutrients, aligning with the principles of the circular economy, can be repurposed as agricultural fertilizers, offering both environmental and economic advantages, given that the global fertilizer market was valued $202 billion in 2023, with projections reaching $257 billion by 2032, driven by increasing food demand from a rising global population.

[1] Digestate from cattle manure and rainbow trout carcasses.

[2] The research conducted by ENEA was commissioned by the University of Camerino included in the activities of the 2014-2020 RDP project of the Marche region called “BIOgas SOStenibile”, which involved, in addition to the University of Camerino, the CIA Marche of Ancona, Sereco Biotest, responsible for the operational tests of the digestion reactor, the Scolastici farm in Macereto di Pieve Torina (MC), in the lead role, and the Cherubini farm in Visso, whose by-products, respectively cattle manure and rainbow trout (Oncorhynchus mykiss) carcasses, were used in the biogasification process.

[3] Replacing 1 ton of artificial fertilizer with digestate saves 1 ton of oil, 108 tons of water and 7 tons of CO2 emissions (source: Anaerobic Digestion and Bioresources Association, mentioned in the Digestate factsheet, EBA).

[4] For example, liquid manure derived from cattle manure digestate, as well as manure from trout carcasses- notably high in ammonia nitrogen, similar to some powdered leaf fertilizers- exhibits chemical characteristics comparable to commercially available products.

[5] At this stage, micro- or ultrafiltration ceramic membranes could be suitable due to their resistance to frequent washing, even with alkaline pH or at medium to high temperatures.

[6] Sustainable use of nutrients.