Digestate Isn't Just Liquid Fertiliser — And That Matters When Comparing It With Compost

I noticed an interesting piece of anaerobic digestion news this week which deserves more attention than it will probably receive.

A newly published market study forecasts that the global market for digestate-based fertilisers will grow from about $390.5 million in 2026 to $570.4 million by 2033.

That represents projected annual growth of 5.6%, with Europe already accounting for a substantial share of the market.

At the same time, another reminder of the vulnerability of fossil-fuel markets has arrived from the Middle East. Oil prices have again been above $100 per barrel amid disruption and uncertainty surrounding Gulf exports.

There is an important connection here.

Manufactured nitrogen fertiliser is closely connected to energy markets, particularly through the natural gas required for ammonia manufacture. Digestate, meanwhile, contains nutrients that have already entered our food and agricultural system.

Anaerobic digestion gives us an opportunity to recover those nutrients and put them back onto agricultural land.

But there is something about digestate which I think is still too often misunderstood.

There Is No Single Product Called "Digestate"

Digestate is frequently discussed as though it were one uniform liquid fertiliser.

It isn't.

The material leaving an anaerobic digester may be used as whole digestate, or it can be separated into liquid and solid fractions.

Those three forms can behave quite differently when transported, stored and applied to agricultural land.

This matters when people ask whether digestate is "better than compost".

Before answering that question, we really need to ask:

Which digestate?

Whole Digestate

Whole digestate is essentially the material leaving the digestion process before mechanical separation.

It contains both liquid and suspended or fibrous solids and retains much of the nutrient content of the original feedstock.

Where there is suitable agricultural land close to the AD plant, using whole digestate may be perfectly logical.

Its disadvantage is equally obvious: it contains a great deal of water.

Transporting water any significant distance costs money, so the geography of the AD plant and the surrounding agricultural land can be just as important as the theoretical fertiliser value.

Liquid Digestate

Separating digestate produces a liquid fraction containing much of the readily available nitrogen, particularly ammonium nitrogen.

That makes liquid digestate potentially valuable as a crop nutrient source.

It also means that it should be treated as a fertiliser rather than simply as something which needs disposing of.

Application rate, crop requirement, soil condition, timing and nutrient-management rules all matter.

Applying nutrients when the crop cannot use them isn't efficient recycling.

Solid Digestate Fibre

The separated solid fraction is different again.

It contains more of the fibrous material and tends to retain a greater proportion of the phosphorus and organic matter.

That makes its characteristics closer in some respects to those we associate with soil conditioners and composted materials.

It can also be easier to transport than large quantities of dilute liquid digestate.

Further processing — including composting, drying, pelletising or other treatment — can potentially turn digestate fractions into more convenient and higher-value products.

So Is Digestate Better Than Compost?

I don't think that is really the right question.

Compost and digestate do different things well.

Compost is generally valued particularly for stable organic matter, soil structure and longer-term soil improvement.

Digestate can offer more readily available plant nutrients, particularly nitrogen, while also recycling phosphorus, potassium and organic matter.

And, of course, anaerobic digestion has already extracted renewable energy from the organic material before the remaining nutrients are returned to land.

That is an important distinction.

Rather than asking which material "wins", farmers and AD operators should be asking which material — or combination of materials — best meets the requirements of a particular soil and crop.

Why Digestate May Become More Valuable

The new market forecast is interesting because it suggests that digestate is increasingly being regarded as a product rather than merely an AD by-product.

That change in thinking is overdue.

Energy and fertiliser markets remain vulnerable to international events. At the same time, agriculture is under pressure to recycle nutrients, improve soil management and reduce unnecessary dependence on externally manufactured inputs.

Digestate cannot replace every mineral fertiliser in every situation, and its nutrient composition varies according to feedstock and processing.

Quality matters too. Poor feedstock control can introduce plastics and other contaminants, which is one reason I have repeatedly argued for good-quality feedstock preparation and contamination control.

But properly produced, analysed and applied digestate represents something increasingly valuable: nutrients we already possess being returned to productive use.

A More Detailed Digestate vs Compost Guide

I recently published a much more detailed examination of this subject, including the differences between whole, liquid and solid digestate and how each compares with compost:

Digestate vs Compost as Agricultural Biofertiliser: Value and Application

For readers wanting to understand digestate as part of the complete AD process, my broader practical guide also follows anaerobic digestion from feedstock selection through biogas production to digestate management:

Anaerobic Digestion: A Practical Guide to Feedstocks, Biogas, Digestate and Successful AD Plants

The more I look at where anaerobic digestion is heading, the more I think we should stop viewing digestate as simply the material left over after producing biogas.

Biogas is one product of anaerobic digestion. Digestate is the other.

And if fertiliser and energy security continue to become more important, that second product may receive considerably more attention in the years ahead.

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Sources: Grand View Research, Digestate-based Fertilizer Market Size, Share & Trends Analysis, 2026–2033, September 2026; UK Parliamentary Office of Science and Technology, Upscaling Anaerobic Digestion, 2026.