Biogas Feedstocks: Slurry, Manure, Silage and Residues

On a farm biogas plant, slurry and manure form the base feedstock, while silage and agricultural residues raise gas output. Slurry yields little gas per tonne (cattle slurry roughly 20 to 30 cubic metres of biogas per tonne of fresh matter), but it is already on the farm and cheap. Maize silage delivers around 180 to 220 cubic metres per tonne. Operators who want the higher tariff for small manure plants under the German Renewable Energy Sources Act (Erneuerbare-Energien-Gesetz, EEG) must use at least 80 per cent manure by mass on annual average, and poultry manure does not count as manure.

Gas yields of the main feedstocks

The figures below are indicative values from the technical literature for agricultural biogas plants. They show the order of magnitude but do not replace an analysis of your own feedstocks, as dry matter, organic content and freshness vary considerably between farms.

A useful conversion: one cubic metre of methane contains roughly 10 kilowatt hours of energy. One tonne of cattle slurry with 25 cubic metres of biogas at 57 per cent methane therefore holds about 140 kilowatt hours of fuel energy, of which a combined heat and power unit turns roughly one third to two fifths into electricity. The biogas calculator translates livestock numbers and feedstock quantities into an output estimate.

Slurry and manure: the backbone of a farm plant

Slurry is the most important feedstock for farm plants for three reasons. It arises daily and evenly, it has to be stored anyway, and digesting it cuts methane emissions from open slurry stores. Its drawback is the high water content: cattle and pig slurry are more than 90 per cent water, so digesters and digestate stores have to be large relative to the gas produced.

Freshness matters. Slurry that sits in a reception pit for weeks has already lost part of its gas potential. Short routes from the barn to the digester and frequent feeding raise the yield noticeably.

Solid manure from cattle or horses yields far more gas per tonne than slurry. It does, however, need suitable feeding equipment (a solids feeder) and powerful agitators, because long straw tends to form floating layers. Stones, sand and baler twine cause wear and sediment, so many farms shred manure before feeding or blend it in carefully. Whether digesting slurry alone without co-substrates works for your farm is covered in the guide to manure-only digestion.

The 80 per cent manure rule for small plants

The special tariff category for small manure plants under Section 44 EEG requires at least 80 per cent manure by mass on average over the calendar year. Further conditions apply, such as generating electricity at the site of biogas production and a total installed capacity of no more than 150 kilowatts at the site. Tariff levels and capacity bands are explained in the guide to the EEG feed-in tariff for biogas.

Three points matter for feedstock planning:

1. Poultry manure does not count. Poultry manure and dried poultry droppings are expressly excluded from the manure share. They may be used, but they fall within the 20 per cent at most of other feedstocks.
2. Solid manure does count. Solid manure from cattle, pigs or horses is treated as manure under the EU law referenced by the EEG and can raise the manure share.
3. Clover grass as a special case. Perennial clover grass can count towards the manure share up to 10 per cent by mass.

Crucially, the rule is based on mass, not energy. A worked example shows the effect: a farm feeds 4,000 tonnes of cattle slurry and 1,000 tonnes of maize silage a year. The manure share is exactly 80 per cent. At 25 cubic metres per tonne, the slurry produces about 100,000 cubic metres of biogas; at 200 cubic metres per tonne, the maize produces about 200,000 cubic metres. Two thirds of the gas therefore come from one fifth of the mass. Anyone planning close to the 80 per cent threshold should build in a safety margin and keep complete records, because compliance has to be demonstrated to the grid operator. This includes a feedstock log recording the type and quantity of every input.

Silage and energy crops

Maize silage is the most widely used energy crop because of its high and predictable yield. Grass, clover grass and whole crop silages are alternatives that fit many rotations better and are gentler on the soil. On small plants, silage mainly serves to stabilise gas production and make better use of digester capacity.

Quality decides for every silage. Poor fermentation, heating at the clamp face and losses in the silo cost gas before the material ever reaches the digester. Adequate feed-out rates, good compaction and a tight cover are therefore part of the biogas strategy. If you buy in silage, compare the price with the gas yield per tonne of fresh matter and include storage and digestate spreading costs.

Residues and by-products

Agricultural residues can supplement the feedstock base at low cost. Typical examples are feed leftovers, silage face trimmings, beet leaves, chopped straw and landscape management material. Their gas yields vary widely, and some bring contaminants or fibres that are hard to break down. Straw, for example, contains plenty of energy but breaks down only slowly in the digester. Without pre-treatment such as chopping, crushing or mixing with manure, it tends to form floating layers. Before relying on a residue permanently, run a trial with small quantities and watch gas yield, mixing behaviour and wear.

Take care with materials from outside the farm. Biowaste, food waste or by-products from food processing are subject to separate rules, including on sanitisation. They can change the permit situation and affect the status of the digestate as a fertiliser. Before using a new feedstock, check with the permitting authority whether your permit covers it.

Feedstock logistics and feeding

A biogas plant runs most stably when fed evenly. Sudden changes in quantity or composition upset the microbiology and can depress gas output for weeks. Plan storage so that the recipe stays as constant as possible throughout the year.

For bought-in feedstocks, distance is the main cost driver. Because of its water content, slurry can only be hauled economically over short distances, while manure and silage travel better. If you take manure from neighbouring farms, agree quantities, quality, animal health measures and the return of digestate in writing. Specialist haulage and spreading contractors are listed under digestate and feedstock logistics. What happens to the digestate after digestion is explained in the guide to digestate as fertiliser.