Anaerobic digestion to tackle antibiotic resistance

Earlier last year, the Global Leaders Group on Antimicrobial Resistance called on all countries to improve measures to manage and dispose of antibiotic-containing waste, due to its role in the proliferation of antibiotic-resistant organisms. One of the key recommendations was to implement standards to manage antibiotic discharge from animal farms.

This has been known to be a problem in agriculture for a while. A 2015 paper conservatively estimated that the world’s livestock consumed over 63,000 tons of antibiotics. The paper’s authors estimated that this would rise by 67 per cent by 2030, and even double the amount in BRICS countries. And, a large proportion of antibiotics ingested is typically excreted.

Greater use of antibiotics leads to increased chances of antibiotic-resistant bacteria entering the food chain. Traces of antibiotics in animal waste dung can easily make their way into nearby soils and rivers, meaning these too can become breeding grounds for resistant bacteria pathogenic to humans. Genes that allow for antibiotic resistance can be transferred among bacteria at these sites.

The world’s rivers have already been found to be awash with pharmaceutical pollution, as a February PNAS paper states. The study monitored 258 rivers in 104 countries, covering 471 million people. Of those sites over a quarter were found to have ‘active pharmaceutical ingredients’ above safe levels.

Researchers say that antibiotic resistance caused nearly five million deaths in 2019, and in December 2022 the United Nations announced that over half of life-threatening bacterial infections worldwide are now becoming resistant to common treatments. A 2015 review on the rise of antibiotic resistance partly blames agricultural practices.

Europe's digestate market expected to almost double

Antibiotic-heavy faecal matter also poses a problem for the anaerobic digestion industry, which turns waste organic matter from farms into an organic, nutrient-rich crop fertiliser and biogas. But several scientific reports highlight that conventional mesophilic anaerobic digestion is ineffective at removing antibiotics. Antibiotic residues can survive the anaerobic digestion process, meaning that digestate used as fertiliser can introduce antibiotics or antibiotic-resistant genes into the soil and the wider ecosystem.

This comes at a time when Europe's digestate market is expected to almost double this decade, from EUR 1.72 billion in 2021 to EUR 3.2 billion by 2028. As Europe attempts to cut down on greenhouse gas emissions as part of its Green Deal, switching to digestate could drastically reduce emissions, argues the European Biogas Association. The Haber-Bosch process used for industrially-manufactured fertilisers accounts for three to five per cent of the world’s gas consumption. By comparison, anaerobic digestion treats the organic waste while generating biogas and digestate, which can minimise the consumption of artificial fertilisers produced by the Haber-Bosch process. 

With ample opportunity for digestate-based fertilisers to be used in agriculture, overcoming the antibiotics issue is a priority.

“The best way to remove antibiotics from agriculture is not to use them at all,” says Dr Francisco Rubio Rincón, who studies wastewater treatment at IHE Delft. “But the benefits of using them at the moment are too great.”

Removing antibiotics from digestate

Researchers are, therefore, experimenting with multiple methods to remove antibiotics from digestate, ranging from using electrodialysis to degrade antibiotics to using biochar to adsorb them to a membrane.

One such project is the EU-funded NOMAD project, which was built to make anaerobic digestion more affordable and available to small-scale farmers. “So instead we put it in two trucks and we make it mobile,” says Rubio Rincón. The trucks collect, pasteurise and separate digestate fractions to remove antibiotics and recover nutrients.

Rincón and colleagues at NOMAD have experimented with different methods of removing antibiotics during the digestate process, which must be effective within the confines of the mobile digestate plant and maintain their effectiveness after a long time of inactivity (when the trucks are not being used, for example).

After experimenting with different methods such as reverse osmosis, biochar, and advance oxidation they settled on a two-step process of using both hydrogen peroxide and two UV light sources to degrade antibiotics. The 2 UV wavelengths allow oxygen to first be transformed into ozone and later on into hydroxyl radicals. “These radicals are able to remove any type of chemical that can be oxidised, including antibiotics,” says Rincón.

The project follows on from previous projects that also investigated ways to remove pharmaceuticals from wastewater, such as using algae to break down the antibiotic compounds, though NOMAD is currently the only major EU project focusing on removing antibiotics through anaerobic digestion.

The project is now in its closing phase, where the truck will be applied in real-use cases in Italy, the UK, Greece and Malta. Rincón and his colleagues will monitor these cases to evaluate the efficiency of the process on removing antibiotics and how that is affected by the different physico-chemical matrix of digestates treated.

Even with urgency coming from the United Nations and his team’s new technologies, Rubio Rincón knows there is no quick fix, as farmers using digestate also need to be consulted. “As an engineer, to have a good campaign we need to help people to understand the benefits of the new process,” he said.

“We need to listen to their problems and whether the solution that we're proposing is the best for them. That takes time and conversations with different stakeholders, but it’s what’s needed to build up support.”