Manure-only digestion (Gülle-Monovergärung) means that a biogas plant digests nothing but slurry, sometimes together with solid manure, without energy-rich co-substrates such as maize or grass silage. Its main benefit is climate protection: methane that would otherwise escape from the slurry store is captured in the digester and turned into electricity and heat in a CHP unit. Against this stand low gas yield per cubic metre and high costs per kilowatt. In legal terms it is a special case of the small manure plant under section 44 of Germany’s Renewable Energy Sources Act (EEG).
How manure-only digestion differs from other biogas plants
Most biogas plants in Germany use a mix of livestock manures and energy crops. Maize or grass silage yields several times as much gas per tonne as slurry and makes plants powerful. Manure-only digestion leaves this element out, which has consequences for the technology:
- Simple feeding: liquid slurry is pumped. A solids feeder is only needed if solid manure is digested as well.
- Large volume: because slurry is mostly water, the digester has to be large relative to its output.
- Short distances: ideally slurry flows straight from the barn so that it reaches the digester fresh.
A few technical details matter for manure-only plants. The digester is usually heated so that the microorganisms work steadily. Cattle slurry often carries sand and bedding residues that settle on the bottom or form floating layers. Good mixing and a way of removing sediment extend the intervals between cleanouts. Because slurry can contain a lot of sulphur, the biogas also needs desulphurisation before it reaches the CHP unit. Ask suppliers specifically about these points and about reference plants that have run on slurry alone for several years.
Cutting emissions: methane from the slurry store
In open slurry stores, or stores without a gas-tight cover, microorganisms break down organic matter and produce methane. This is a potent greenhouse gas and, without digestion, it escapes into the air. In manure-only digestion the breakdown takes place in a closed digester. The methane is captured and burnt, and the electricity also replaces power from other sources.
For the climate benefit to materialise, three points matter:
- Fresh slurry: the shorter the storage time before digestion, the less methane forms beforehand.
- Gas-tight secondary digestion or storage: digestate still produces gas, and a gas-tight covered digestate store captures this residual gas.
- A tight plant: leaks at membrane roofs, valves or pressure relief devices can erode the benefit, so regular leak checks are part of operation.
What ultimately counts for the climate is the balance across the whole chain, from barn to digester and storage to spreading. A tight, well-run plant performs much better here than one with an open digestate store or frequent flaring.
Ammonia needs attention: digested slurry contains more ammonium and has a higher pH. Covered stores and low-emission spreading are therefore important to avoid nitrogen losses. The guide to digestate as fertiliser has more on this.
Pros and cons at a glance
| Advantages | Disadvantages |
|---|---|
| uses a residue that arises anyway | low gas yield per cubic metre of slurry |
| no land needed for energy crops | large digester volume per kilowatt |
| lower methane emissions from storage | high investment per kilowatt |
| simpler feeding without silage | full dependence on the farm’s own herd |
| uniform digestate that flows well | gas output falls with grazing or smaller herds |
| often less odour during spreading | sand and foreign matter can settle in the digester |
Be realistic about the dependence on livestock numbers. If cattle graze in summer, there is less slurry in the barn during those months and therefore less gas. A CHP unit running at part load works less efficiently.
How it relates to the small manure plant
The small manure biogas plant under section 44 EEG requires an annual average of at least 80 per cent manure by mass, with up to 20 per cent other inputs allowed. Manure-only digestion does not use this margin and runs practically on slurry alone. It meets the manure rule with certainty but forgoes the extra output that co-substrates bring.
In practice many farms take a middle path: mostly slurry, plus small quantities of feed leftovers, solid manure or clover grass to stabilise gas production. Pure manure-only plants, on the other hand, are often smaller and suit farms that want to use their slurry without tying up land for energy crops. The same payment rules under section 44 EEG apply in both cases.
Economics
The economics of a manure-only plant depend more on investment costs than those of plants with co-substrates. Feedstock costs are low because the slurry arises anyway. On the other hand, each cubic metre of digester volume yields little gas, and costs per kilowatt are high. Existing tanks, good heat use and high, steady CHP utilisation all help. The guide to biogas plant costs gives a realistic cost framework.
A look at the Netherlands
Manure-only digestion is also on the agenda in the Netherlands, where it is known as monomestvergisting. Small farm plants there can receive support through the national SDE++ scheme, which promotes renewable energy and emission reductions. The conditions differ considerably from the German EEG and change regularly. For plants in Germany, only German law applies.
Which farms it suits
Good candidates are farms with a large herd housed all year round, short distances from barn to digester and heat demand on the farm. Less suitable are farms with extensive grazing, strongly fluctuating herd sizes or barns far apart. The type of slurry matters too: cattle and pig slurry differ in dry matter, gas yield and foreign matter, and even within one species the values vary with feeding and water inputs. Having your slurry analysed before planning provides clarity. The biogas calculator gives a first estimate using your slurry volumes. Contractors that build manure-only plants are listed in the biogas plant construction directory.
Frequently asked questions
Is a manure-only plant profitable?
That depends heavily on herd size, existing storage and heat use. Because slurry yields little gas per cubic metre, costs per kilowatt are high. Work through your own case with your actual slurry volumes before asking for quotes.
Why should slurry go into the digester as fresh as possible?
Methane starts forming in the barn and in storage. Gas produced there is lost to the plant and adds to emissions. Fresh slurry yields more biogas and cuts emissions at the same time.
Can co-substrates be added later?
Technically yes, if a solids feeder can be retrofitted. To keep the section 44 EEG tariff you must still use at least 80 per cent manure by mass on annual average. Also check whether the permit covers other feedstocks.
Does digestion increase ammonia emissions?
Digested slurry contains more ammonium and usually has a higher pH. Without countermeasures, more ammonia can therefore escape during storage and spreading. Covered stores and low-emission spreading close to the ground help to prevent this.
How much power and revenue could your livestock deliver? Run the numbers with open assumptions.
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