Small Manure Biogas Plants in Germany: Rules and Herd Size

A small manure biogas plant (Gülle-Kleinanlage) is a farm-scale anaerobic digestion plant whose electricity is paid a specific tariff under section 44 of Germany’s Renewable Energy Sources Act (Erneuerbare-Energien-Gesetz, EEG). To qualify, the plant must digest an annual average of at least 80 per cent manure by mass, generate its power at the biogas site and have no more than 150 kW of installed capacity at that site in total. The statutory reference value is 22 ct/kWh up to 75 kW rated output and 19 ct/kWh up to 150 kW rated output, with a small annual reduction for new plants.

What German law means by a small manure plant

The term Gülle-Kleinanlage does not appear in the Act itself but is the usual name for plants under section 44 EEG. The provision sets the payment for electricity from biogas produced by anaerobic digestion of biomass with a high share of manure. You can read the wording in section 44 EEG on gesetze-im-internet.de (German only).

Three conditions have to be met together:

1. Generation on site: the biogas is turned into power where it is produced. A satellite CHP unit in the village, fed through a long gas pipeline, does not fit this model.
2. No more than 150 kW installed capacity: this is the total installed electrical capacity at the site, not that of a single CHP unit.
3. At least 80 per cent manure by mass on annual average: poultry manure and dried poultry droppings do not count as manure. Perennial clover grass can be credited up to 10 per cent by mass.

The remaining 20 per cent or less can be other feedstocks such as feed leftovers, grass silage, clover grass or maize. Which ones make sense depends on the technology and the land available. Inputs with a high dry matter content, for example, need a separate solids feeder, which adds to the investment.

Output limits: rated output and installed capacity

Two capacity terms are used for these plants and they are easily confused:

An example: a plant with 100 kW installed that generates 657,000 kWh a year has a rated output of exactly 75 kW (657,000 kWh divided by 8,760 hours). All of its output therefore receives the higher first-band value. If the rated output is higher, section 23c EEG requires a pro rata calculation: electricity up to 75 kW gets the first-band value and the rest the lower second-band value.

In practice it matters that a fixed feed-in tariff is only available up to 100 kW installed capacity. Above that, the electricity must be sold through direct marketing with a market premium. The guide to the EEG feed-in tariff for biogas explains the details.

The 80 per cent manure rule in daily operation

The manure rule is an average over the calendar year and refers to the mass of the inputs, not their gas yield. This is important: manure is mostly water and delivers relatively little gas per tonne. Even small quantities of energy-rich co-substrates can therefore provide a large share of the gas without exceeding the 20 per cent limit.

Operators keep a feedstock log recording the type, quantity and origin of every input. If manure drops below 80 per cent in a year, the section 44 value is lost for that year. Plan a margin, for example for years with smaller herds or barn conversions.

How many animals does a small manure plant need?

The herd size required can only be given as an order of magnitude. Gas yield, dry matter content of the slurry and CHP efficiency vary from farm to farm. This back-of-the-envelope calculation shows the method:

1. Electricity: 75 kW rated output equals 657,000 kWh of electricity per year.
2. Methane: with an electrical CHP efficiency of roughly 38 to 40 per cent and about 10 kWh of energy per cubic metre of methane, the plant needs roughly 165,000 to 175,000 m³ of methane a year.
3. Biogas: at 55 to 60 per cent methane content, that means roughly 275,000 to 315,000 m³ of biogas.
4. Slurry: one cubic metre of cattle slurry yields roughly 20 to 30 m³ of biogas. Using a mid value of 25 m³ gives a requirement of roughly 11,000 to 12,500 m³ of slurry a year.
5. Herd: a dairy cow produces roughly 15 to 22 m³ of slurry a year, high-yielding cows towards the upper end. For a 75 kW plant on cattle slurry alone that means roughly 500 to 700 cows, or more if each animal produces less slurry.

With 20 per cent energy-rich co-substrates by mass, the requirement falls considerably. Grass silage or maize yields several times as much gas per tonne as slurry, so depending on the co-substrate the slurry volume needed can roughly halve. Treat all of these figures as indicative. You can test your own numbers in the biogas calculator.

Which farms are a good fit

A small manure plant suits farms that already handle large volumes of slurry and have to store them anyway. Typical candidates are dairy and beef farms, and in some cases pig farms. The benefits are the use of a residue, lower methane emissions from open slurry stores and a fairly uniform digestate.

Limits come from economics and workload. Investment per kilowatt is high for small plants, and daily operation takes time and skill. The guide to biogas plant costs gives a realistic overview. If you want to digest manure only, without co-substrates, see the guide to manure-only digestion.

Heat and digestate

Besides electricity, a CHP unit produces roughly as much usable heat or more. Part of it is needed to keep the digester at temperature, especially in winter. The rest can heat the farmhouse, barns, milking parlour or neighbouring buildings. Every kilowatt hour of heat that replaces heating oil or gas improves the economics, but heat is not paid for under the EEG.

Digestate also belongs in the plan. It arises in almost the same quantity as the slurry fed in and must be stored and spread in line with fertiliser law. An existing slurry tank can often continue in use as a digestate store if it is technically suitable.

Next steps

Before requesting quotes, record your slurry volumes over a full year, check possible co-substrates and estimate the heat demand on the farm. After that it is worth talking to specialist contractors who supply small manure plants as complete systems. You will find suitable companies in the directory of biogas plant construction firms. Clarify planning and permit questions and the grid connection early as well, because these often shape the timeline more than the construction itself.