Dry solid matter feeding vs. liquid feeding - when to take what?
Important elements in all biogas plants: solid matter feeding

When all these points are taken into account, suitable solid matter feeding technology that can feed the biomass cost effectively must be installed in the biogas plant. Liquid waste from agriculture, such as manure and glycerin or residues from grease traps can usually be pumped directly into the digester using pumps. Because this often means waste that includes foreign matter such as stones, metal parts and impurities (e.g. wood, belts, etc.), we recommend the use of macerators and foreign matter separators to reduce the material to a manageable level.

Hygiene

When using input materials that are subject to hygiene regulations, a deep cleaning is required to ensure the destruction of any parasites, viruses or bacteria contained in the material that may pose a threat to the environment. This applies especially to substrates of animal origin such as abattoir waste and food waste. The most common procedure entails heating the input material up to at least 70°C for one hour, and then macerating it to a size of max. 12 mm. Proven systems for this process are contact-based macerators combined with positive displacement pumps. Vogelsang positive displacement pumps often pump viscous and abrasive waste safely, while a 12 mm honeycomb sieve in the RotaCut macerator ensures that all elements are reduced to the maximum permissible size. Special options ensure that no input material gets past the macerator untreated.

Rotary lobe pump of the IQ series and wet cutter RotaCut by Vogelsang — IQ series and RotaCut by Vogelsang

Feeding process for non-flowable biomass

Biogas plants are mainly fed with non-flowable solid matter (also called substrates). The feeding processes can be roughly divided into two categories: Dry solid matter feeding and liquid feeding. For dry solid matter feeding, the input materials are put directly into the digester. Feed screws are the most commonly used method, conveying the solid matter upwards and onto the organic suspension. The main advantage of this technology is the relatively low investment costs. However, the solid matter must be stirred by the mixers in the digester, which is only possible at very high energy levels. Also, this technique is not fully suited for long-fibered substrates (e.g. car grass, straw, or agricultural waste such as dung).

Solid matter feeder PreMix by Vogelsang — PreMix - 4-in-1 solution for liquid feeding

For liquid feeding, the biomass is first mixed with a liquid suspension, the digestate or so-called recirculate from the digester. This process is also called mashing. The resulting feed mash is then conveyed to the fermentation space (this usually means pumped). The advantages of this procedure are that the biomass arrives in the digester in liquid form and mixing requires significantly less energy consumption with a digester content that mixes well with the organic suspension. If the solid matter is also processed simultaneously, the reduced energy requirements are also accompanied by increased gas generation. This also significantly reduces the widespread problem represented by the formation of floating layers.

Alternatively, there are liquid feeding systems that, in addition to renewable materials, can also feed organic waste with extremely different properties. Many biogas plants rely on such systems. This is either because they also want to use crops such as potatoes or beets in addition to classic renewable materials, or because the biomass used has extremely fluctuating properties (e.g. food waste or domestic organic waste). Other biogas plants react very flexibly because they buy up the biomass currently available on the market at favorable terms. Systems such as PreMix are designed for these kinds of biogas plants. They can feed many different types of biomass reliably, easily and well-mashed into the digester, meeting a basic requirement for the economic operation of these biogas plants.