Unearthing digestate with nomad

How digestate could help Europe’s worst-hit soils to recover

Each year, Europe loses around 1 billion metric tonnes of soil to erosion – equivalent to digging a one-metre hole the size of Berlin1, according to the EU. This happens mostly through man-made factors such as industrial farming, urban sprawl, and climate change.

These factors affect the soil in different ways – for example, the increased storms from climate change can erode underlying earth, or pollutants can destroy the well-balanced local ecosystem of bacteria and fungi that help crops to grow. Regaining even one cubic centimetre of topsoil can take centuries.

A Crisis-Point for Soil

Researchers have watched as soils become increasingly weak. ‘’The situation is very bad,’’ says Leonardo Verdi, researcher in agronomy at the Department of Agriculture, Food, Environment and Forestry (DAGRI) at the University of Florence. ‘’Nowadays the average of organic matter soil content is around less than 1 per cent.’’ He says that in comparison, in the 1950s it was common for organic matter to be 3 per cent or more.

Verdi suggests two main causes to increased soil erosion. The first is intense tillage. Constant digging and stirring of the soil causes massive losses of organic matter (for example, decomposing vegetation). Removing vegetation is particularly bad in southern Europe since this exposes the soil to the region’s higher solar radiation and temperatures.

The other main problem, says Verdi, is over-fertilisation. This is when fertilisers in the soil exceed what the crop requires to uptake. This either causes the fertilisers to be oversaturated in the soil or contaminating other soil as runoff. He says that up to 70 per cent of fertiliser can be lost because crops may not be ready to absorb the fertilisers, creating pollution elsewhere.

The Wider Problems

Synthetic fertilisers include nitrogen to increase a plant’s root growth and their absorption of phosphorus. However, misuse of fertiliser leads to it leaving the field as run-off into surface water or leaching into groundwater. The excess nutrients that enter these waters can lead to eutrophication (more weeds and algae) and reduced oxygen levels. This in turn causes biodiversity loss as plants and animals struggle in this new environment.

“nowadays the average of organic matter soil content is around less than 1 per cent, in comparison, in the 1950s it was 3 per cent or more”

There are wider environmental impacts. A 2020 study in Nature2 found that applying nitrogen-based fertilisers to crops has driven up global emissions of nitrous oxide. A greenhouse gas, it can stay in the atmosphere for decades and is more efficient than CO2 in trapping heat.

Antibiotic residues can also make their way from farmyard animals into the soil, creating a hotspot for harmful bacteria resistant to conventional antibiotics. This is a growing issue in Europe; research done in the Netherlands3 has shown that there has been a more than 15-fold increase of individual antibiotic-resistant genes in the country’s soil bacteria between 1940 to 2008. When these soils also have a high amount of nutrients from fertilisers, these antibiotic-resistant bacteria can multiply and transfer their genes quickly and re-enter the food chain through plants and crops.

More Needs to be Done

This erosion is now being felt across the continent, none more so than in southern Europe. A study by the JRC in 20184 found that a third of Italian farmland suffers from extreme erosion, costing farmers EUR 619 million per year.

Unearthing Digestate with NOMAD

Strategies to preserve soil in these regions exist. Verdi points to precision farming – adjusting fertilisation and tillage by synchronising them to the needs of the crops. However, he admits that for now the technology is still too expensive to use on a large scale. ‘’You need to obtain data from the satellite or from some instruments that are not very easy to buy and to be managed by farmers,’’ he said. Another is to use slow-release fertilisers that take time to degrade and to become available to plants.

A third is to replace synthetic mineral fertilisers with something more natural. Countries across Europe are already setting up industrial plants to create biogas from different organic matter. The input for this anaerobic fermentation can come from many areas such as farm slurries or sludge from urban areas.

This organic matter undergoes anaerobic fermentation to produce both biogas and a by-product called digestate – a source of natural, organic fertiliser. This muddy digestate can be separated into a liquid fertiliser and a solid soil amender.

While synthetic mineral fertiliser adds its nutrients to the cycle, digestate reuses the nutrients already available in the ecosystem, creating a complete loop of local nutrients in a circular economy.

“digestate reuses the nutrients already available in the ecosystem, creating a complete loop of local nutrients in a circular economy”

Verdi is researching how to make digestate a viable alternative to farmers. He says that the agriculture sector is in a great position to use digestate, since most digestate used in agriculture nowadays comes from agricultural by-products (e.g. sludge, animal waste, etc). This means that using digestate allows those nutrients to stay in the agricultural production chain.

Still, creating digestate for agriculture nowadays requires moving organic material from the farm for anaerobic digestion, and then for the digestate to be then delivered back to the farm.

Stopping Erosion in its Tracks

Verdi is part of the NOMAD project, which aims to bring all the technology needed for anaerobic digestion onto a truck. That way a farm’s organic matter can be processed on-site to then be reused to improve the soil.

The aim is to have a truck that can collect, pasteurise and separate the digestate fractions and generate biogas. The truck will soon travel across Europe to field trials and is starting in Greece, another area highly susceptible to soil erosion. Like Italy, soils in the country have been used for agriculture for thousands of years and can also be eroded by earthquakes.

In the central region of Western Macedonia soils are threatened by both increased fertiliser use and chemical runoff from the region’s old lignite mines.

‘’In Western Macedonia during the past few years, we are experiencing a ‘post lignite’ era,’’ says Dr Kyriaki G. Sakellariou, a chemical engineer and affiliated researcher in DIADYMA, the official waste management body in the region, also involved in circular economy sector.

Unearthing Digestate with NOMAD

Over several years heavy metals such as cadmium, copper and chromium have leached from the mines into the surrounding soil, making it difficult for plants to survive. Sakellariou is native to the region, and moved back after nearly a decade as a researcher in Thessaloniki to promote and foster more environment-friendly activities within circularity. ‘’Now that all of these mines are closing… we are trying to find ways to restore land in our region,’’ she said.

She says that one method being explored is restoring the land using the digestate-based biofertilisers. She explains that the NOMAD truck uses extra steps to make sure the resulting digestate follows local good agricultural practices and laws. For example, Greek law says that digestate must first be pasteurised before being used in agriculture.

The NOMAD truck can pasteurise digestate on-site, and slowly separate contaminants while keeping the nutrients. It also includes a step to remove any antibiotics from the soil, meaning that the final digestate product is one of the purest possible options to restore the region’s soil.

Getting Results

Sakellariou and Verdi will grow lettuce using NOMAD’s digestate in open fields to get the most realistic conditions for their tests. They will compare the soil quality, as well as the vegetables’ growth, using NOMAD’s digestate and compare it to regular fertiliser. During these tests they will measure qualities of the soil such as pH, electrical conductivity, texture, and the presence of basic elements.  Some extra analysis on heavy metals and greenhouse gas emissions from soil might also be performed.

Verdi is confident that bringing the digestate truck to agricultural sites will be part of a wider trend of providing small-scale, customised solutions to farmers. ‘’I’m pretty sure that in the future, farmers will be paid to adopt specific strategies in order to improve the soil carbon content,’’ he said. It also gives farmers more agency over their land; farmers may choose to sell their digestate as an extra source of income, or as a way to sequester carbon and thus meet their climate goals.

Similarly, Sakellariou says that providing organic fertiliser like digestate to farmers can help the whole Western Macedonia region. She says that people are fearful that shutting down the traditional mining industry will cause many to lose their jobs - but creating a circular, bio-based economy could attract new entrepreneurs to the area and introduce an entirely new economy for the region.

 ‘’It is something like an experiment for all of Europe, to see how things will go on after the closure of the mines,’’ she said. ‘’I want to see things in an optimistic way – [that] people will decide to stay in the region and start something new.’’

Can it Work?

NOMAD project partner Rokiah Yaman, Director at LEAP Micro AD Ltd, answers questions about digestate and how it could solve many of our current soil problems.

Unearthing Digestate with NOMAD

What is digestate and how is it used currently?

Digestate is the liquid bio-fertiliser by-product from anaerobic digestion (AD) that contains all the recovered nutrients and water as well as a small amount of fibre.

What are the benefits of using NOMAD’s by-products over synthetic fertilisers?

NOMAD by-products contain nutrients, fibre and other compounds recovered from organic sources and can therefore be considered renewable. In contrast, synthetic fertilisers are derived from environmentally harmful processes. Nitrogen, a key nutrient is synthesised using the Haber Bosch process, a highly energy intensive method. This alone is responsible for the University of Sheffield’s claim that ammonium nitrate fertiliser accounts for 43% of the greenhouse gas emissions produced during the manufacture of a supermarket loaf, dwarfing all other processes in the supply chain. Meanwhile phosphorus, another key ingredient in synthetic fertilisers, is found in significant quantities only in a few countries. Its mining causing harmful environmental impacts including air pollution, water contamination and the destruction of invaluable wildlife habitats.

AD is a low carbon approach to recovering nitrogen and phosphorus. When processing organic waste feedstocks, NOMAD becomes part of a circular economy approach that significantly decarbonises food production, a sector responsible for over a third of global greenhouse gas emissions.

How will this solution impact future soil health?

Any fertiliser, whether synthetic or organically sourced, can be used in a way that harms the environment if incorrectly applied by causing eutrophication. In addition, using fertiliser without replenishing soil organic matter depletes soil fertility over time, compromising soil structure and its ability to retain water. NOMAD’s by-products include both fertiliser compounds and soil amenders. Using them together will simultaneously boost crop growth and improve soil structure and ecology.

How big is the problem of antibiotic residues in agriculture? Why can it be dangerous for us?

The build-up of antibiotic residues in the food chain leads to the emergence and spread of microorganisms which are resistant to them. Known as antimicrobial resistance, this effect means that current antibiotics become ineffective, posing a serious risk to public health.

How does NOMAD aim to solve the problem of antibiotic residues?

NOMAD will test off-the shelf technology in a new context to removing antibiotic compounds from digestate – one of the more experimental elements of the project. Part of the demonstration trials will involve calibrating this and other processes as well as modifying the sequence of processes to achieve the best results.

Which of NOMAD’s goals is particularly important to you personally, and why?

I’m keen to see how NOMAD could unlock the market for urban micro-scale AD, where networks of AD plants could serve a range of applications, from university campuses to social housing estates. Achieving this would help establish circular bio-economies and generate significant green growth opportunities to improve local economic resilience, with increased social benefits where AD is linked with on-site food production for local consumption.


References

  1. https://ec.europa.eu/commission/presscorner/detail/en/QANDA_21_5917
  2. https://www.nature.com/articles/s41586-020-2780-0
  3. https://pubs.acs.org/doi/full/10.1021/es901221x
  4. https://publications.jrc.ec.europa.eu/repository/handle/JRC105344