Unearthing the gold in soil

As WC Lowdermilk wisely stated, ‘Civilization rests on the soil.'”

I’ve started with this quote mainly because I think Walter Clay Lowdermilk has such a brilliant name, I wanted to shoehorn it into a blog. More importantly, he was an agronomist who wrote long, dense pamphlets about how ancient societies ultimately collapsed because they failed to look after their soil. Which brings us nicely to today’s biogas topic: the unglamorous issue of what primarily comes out of your anaerobic digestion (AD) (apart from gas), and that, of course, is digestate.

At first glance, it looks like dark, black, wet soil that smells faintly of ammonia. I t gets under your fingernails, the smell seeps into your clothes, and the last time I visited a biogas plant, the woman beside me on the train home audibly tutted, raised her eyebrows, and moved to another seat.

But if you spend a long time visiting AD plants, you come to the almost religious realisation that this dark, smelly stuff isn’t just a byproduct; it’s a signal – a pungent, nutrient-rich reminder that the energy world needs to change not just in the skies, but in the soil.

WHAT IS DIGESTATE, REALLY?

Let’s start with the basic physics. The process of converting any form of organic product into biogas (CH4 and Co2) in an AD plant doesn’t significantly reduce the mass, so you get pretty much the same weight out as what you put into the tank. If anything, the mass goes up because you have to add water to create viscosity so the stuff can be pumped around the plant.

Europe has over 10,000 AD plants, which generate more than 120 million tonnes of digestate per year. That’s enough to fill over 48,000 Olympic swimming pools or, if you’re a logistics nerd, around 6 million 20-foot shipping containers.

And what’s in it? On average, every tonne of digestate contains:

• 4–7 kg of nitrogen (N)
• 2–5 kg of phosphorus (P₂O₅)
• 5–8 kg of potassium (K₂O)
• And, obviously, a lot of water

The non-watery bit makes it a competitive alternative to synthetic fertilisers, especially in Europe, which has been scrambling for fertiliser security after supply chain shocks recently caused prices to surge by 300%. Last year, the UK imported about 2.8 million tonnes of fertiliser and a lot of this used to come from Russia hence the post-Ukrainian price surge. Nowadays, we look to buy it from the Egyptians, the Americans and Trinidad and Tobago, which creates a large carbon footprint and adds massively to the carbon intensity of European farming (as per our previous blog). And this stuff isn’t cheap. A tonne of fertiliser costs about £350 which will only cover maybe 15 acres depending on what you are trying to grow, and the UK alone spends £ 1.2 billion buying it.

THE PROBLEM ISN’T THE PRODUCT—IT’S THE PERCEPTION

Although one tonne of digestate can offset half to one-third of a farmer’s annual nitrogen application per hectare, less than 30% of European digestate is processed beyond basic storage or local field spreading.

Digestate isn’t traded on global markets like oil or lithium. Its value is in my newly made-up word – hyperlocal, and that’s both the catch and the potential upside. A cubic metre of raw liquid digestate might only be worth £3–£5, depending on nutrient content. Most farmers won’t pay you to take it, and some will even charge the AD plant to take it away because it’s mainly water. One biogas plant owner I know recently told me with tearful frustration that he pays over £1 million per annum for a contractor to basically truck away tankers of dirty, brown water every day.  But run this liquid through a separator, recover the ammonium nitrate, and dry the solid fraction? Suddenly, you have a competing product for that Egyptian fertilizer, and it’s produced an awful lot nearer to home.

Let’s be honest with ourselves: well-meaning Europeans shipping millions of tonnes of fertilizer halfway around the planet while agonizing over plastic straws and tote bags is the definition of missing the wood for the trees, or in this case, the soil for the sludge.

BUT HERE’S THE CATCH: FINANCE STILL HASN’T CAUGHT UP

Investors still flock to solar and wind where the output is clean, metered, and indexed. But they shy away from investing in digestate dewatering infrastructure, the prosaic technology of dewatering, drying, and pelletisers, because there’s no PPA, no offtake guarantee, and it doesn’t really feel like project finance

That’s the short-term view.

The capital cost of dewatering and pelletising this stuff is £1.5 M – £2 million, depending on throughput and technical complexity. But with fertiliser prices high and carbon intensity scores becoming mainstream, this is beginning to look more like an attractive idea.

THE ROAD AHEAD: IT’S DIRTY, BUT MAYBE IT IS PAVED WITH GOLD

No surprises here: the Germans are leading the charge. With over 9,500 biogas plants, they’re already turning digestate from a messy headache into a shiny revenue stream. After all, these are the same people who invented the Mercedes car and now somehow manage to reserve every poolside lounger in Europe by 6 am, efficiency and first-mover advantage run deep in the German psyche.

The slightly more relaxed French, Dutch, and English are scrambling to catch up, nudged along by a heady mix of policy carrots (rewarding nutrient recovery) and regulatory sticks (penalties on spraying untreated slurry on fields). The EU’s Nitrates Directive and Farm to Fork Strategy are forcing everyone to take a hard look at what’s lurking in those tanks. Where farmers once saw a stinky liability, they’re now seeing £ signs. And investors usually allergic to things not made of steel and concrete are starting to circle too. That dirty water is starting to look like liquid gold.

Some plants are going even further, turning to what might be the ultimate unsung hero of the bio world: algae. Yes, that slippery green film that clogs up your pond and takes over your fish tank the moment you turn your back. Give it nutrients, CO₂, and sunlight, and it grows faster than a spotty teenage boy on a diet of PlayStation, pizza and full-fat coke. Nutrients and Co2 are of course produced by AD plants, so farmers and biologists are starting to join the metaphorical dots and build algae factories near the plants.

To give a sense of the opportunity, the slightly ominously named algal strain Chlorella vulgaris yields about 50% pure protein if we feed it the dirty water distilled from the digestate produced by pig manure. This protein can then be mixed into other feeds and fed back to new pigs. It’s probably best if we don’t explain to the second of group of pigs the relationship between their tasty dinner and the first group of pigs but let’s just call it the circular economy in action.

FINAL THOUGHTS: BEYOND THE SHINY THINGS

Investors and magpies share two qualities: they’re not universally loved, and they both love shiny things.

Solar panels glinting in the sun, towering wind turbines, sleek EV chargers, these are the poster children of the energy transition, the stock shots of 1,000 investment prospectuses. But behind the drearily predictable photos of attractive people with great teeth in fields of solar panels lies a darker, less appealing truth: a key part of the energy transition jigsaw might just be the sludge lurking in the black lagoons behind biogas plants.

We began with WC Lowdermilk partly because he has a name that sounds like a jazz pianist, but mostly because he understood that everything ultimately rests on the soil. Fast forward 100 years: the woman beside me on the train who didn’t like how I smelled was missing the point that digestate is probably the most unglamorous, undervalued, yet vital commodity in the climate economy. Maybe if she knew that she wouldn’t have moved seats. Maybe.