Wet vs. Dry Anaerobic Digestion: There's More to This Than Most People Realise
I'll be honest with you. When I first started digging into anaerobic digestion — the process where microbes munch through your kitchen scraps and farm waste to produce biogas — I assumed it was all pretty straightforwardly good. Waste goes in, clean energy comes out, everyone wins.
Then I stumbled across something that made me put my coffee down.
But let's back up a bit first, because there's a fascinating debate running through the world of biogas that most people outside the industry have never even heard of. It's the wet versus dry digestion question — and buried inside it is a truth about where a surprising amount of our "processed" organic waste actually ends up that I think deserves a wider audience.
Two Very Different Worlds of Digestion
Picture your average home biogas setup — the kind that a growing number of eco-conscious households are experimenting with in back gardens and smallholdings. What you're almost certainly looking at is wet anaerobic digestion. Food slurry, maybe some manure, lots of water, all swirling around in a sealed tank. The material has less than 15% solids, it pumps easily, mixes well, and the bacteria get intimate contact with every scrap of organic matter. The result? A continuous, reliable stream of biogas, plus liquid digestate you can use to feed your garden.
Nobody — genuinely, nobody that we've come across — is running dry anaerobic digestion at home. And when you understand what dry digestion actually involves, that makes perfect sense.
Dry digestion handles the heavier stuff: yard waste, chicken litter, municipal solid waste that hasn't been neatly sorted into nice clean food-waste caddies. The solids content runs between 20% and 40%, which means you're essentially dealing with a material that looks and behaves more like compost than soup. You can't pump it. You stack it into an airtight tunnel, percolate liquid digestate through it to wet it down, wait while it produces biogas, carefully extract the gas, ventilate thoroughly, and then manually remove the processed waste before loading in the next batch.
Some sophisticated commercial systems do run this as a continuous "plug flow" process — essentially shuffling material through in a controlled way — but the manual batch approach is still common.
Here's the significant trade-off that makes engineers pause: wet systems consistently produce more biogas per tonne of material because the mixing is better and the bacteria have more consistent contact with their food. That's a meaningful gap when you're trying to justify the economics of a digestion plant.
Dry digestion does have genuine advantages though. It needs far less water, requires less pre-treatment of incoming waste, and — this is actually quite useful — it's far less bothered by physical contaminants. Bits of plastic, the odd stone that made it into the collection lorry. A wet system hates that stuff. A dry tunnel is largely indifferent.
Now For the Part That Stopped Me Cold
Here's where it gets interesting, and a little uncomfortable.
The first dry digesters weren't built to maximise energy recovery. They were built as a pre-treatment step for mixed municipal solid waste — the big, messy, unsorted heap of everything that used to go straight to landfill or incineration.
Fast forward to today, and in the UK and across the EU, most municipal waste now passes through Material Recycling Facilities (MRFs) — those large sorting centres where the useful stuff gets pulled out. What's left after sorting is the "residual" fraction. Dry digestion is increasingly used on this residual waste to reduce its biological activity — essentially to calm it down so it causes fewer problems.
And then what happens to it?
It goes to landfill.
Let that sink in for a moment. The organic fraction of this municipal solid waste — after it's been through a digestion process, after infrastructure has been built and operated to handle it — ends up in the ground. Why? Because its calorific value is too low to burn efficiently in an incinerator, and it's too contaminated with non-organic material for economical recycling. Digestion, in this context, is essentially a stabilisation step on the road to burial.
I find this genuinely thought-provoking, not as a reason to dismiss dry digestion, but as a reminder that "green technology" exists within systems — and the whole system matters, not just the impressive-sounding process in the middle.
So Which Is Better?
Neither, really — and both, genuinely.
The honest answer is that the right technology depends entirely on what waste stream you're dealing with. Wet digestion is the clear winner for food waste, manure and sewage sludge — high moisture inputs where you want maximum biogas yield. Dry digestion earns its place with agricultural residues and contaminated municipal waste streams where wet processing simply isn't practical.
What's worth holding onto is the bigger picture: both technologies, used in the right contexts, are recognised by authoritative bodies in the UK and across many developed nations as genuinely essential tools for reaching net zero. The biogas they produce displaces fossil fuels. The digestate, where it's clean enough to use, returns nutrients to soil. These aren't fringe solutions — they're part of the infrastructure of a lower-carbon future.
The sobering thought isn't that dry digestion is a waste of time. It's that even our best current tools have limitations baked in — and that understanding those limitations honestly is how we keep improving.
Curious to go deeper on biogas and anaerobic digestion? There's a wealth of further reading out there, and it's a rabbit hole well worth falling into if sustainable energy is your thing. Drop a comment below if you'd like me to explore any aspect of this further.
0 Comments