Knowledge Base

A biogas plant is built from a chain of components that receive feedstock, digest it, capture the resulting biogas, and process both the gas and the digestate into usable end-products. On the input side, feeding and storage systems receive and dose liquid and solid feedstock into the process. The digesters, typically insulated and heated concrete tanks fitted with a mixing system and a biogas-storing roof, are where anaerobic digestion takes place. From there, the biogas passes through cleaning and upgrading equipment before being converted into power and heat or biomethane, while the remaining digestate is treated and, in many cases, turned into a biofertilizer. Supporting components such as pumps, filters, flares, and heat exchangers protect the process and improve its overall efficiency.

Feeding systems

Liquid feedstocks such as manure or de-packaged food waste are received in HoSt Liquid Storage® polyester silos, heated or unheated depending on the feedstock and climate, and pumped into the digester. Solid feedstocks such as maize silage or straw are dosed using systems like the HoSt Mixing / Dosing Feeder® or HoSt Moving Floor Feeder®, sometimes combined with the HoSt Biomix Feeder® to pre-mix solid biomass with liquid digestate into a homogeneous, pumpable input.

Digesters

The digester tanks are the core of the plant, where micro-organisms break down organic feedstock in the absence of oxygen to produce biogas. HoSt typically builds these as reinforced concrete tanks, built up layer by layer on site for a sturdy, gas-tight construction, though steel bolted tanks can also be supplied on request. The tanks are insulated and heated and equipped with a manhole and sight glass for maintenance and inspection.

Mixers

Mixing keeps feedstock in suspension, prevents floating layers and sedimentation, and maximizes contact between the material and the digesting flora. HoSt applies several mixer types depending on feedstock and dry matter content: the slow-moving HoSt GiantMix® paddle mixer for high-viscosity liquids and materials prone to floating, such as grass and straw; the HoSt PropMix® propeller mixer for additional mixing efficiency and biological stability; the HoSt TopMix® vertical agitator for tanks with a fixed roof; and the HoSt SpiralMix® for optimal mixing at a lower dry matter content.

Roofs

Most HoSt digesters are fitted with a double membrane roof, which provides biogas storage and buffering, structural rigidity and minimal biogas leakage. HoSt’s patented insulated version of this roof further reduces heat loss, typically by around 50% compared with a standard membrane roof, which matters most in colder climates or when biogas is upgraded to biomethane, since less heat is produced from the upgrading process itself.

Heat recovery and distribution

HoSt collects the available heat of the installation in a central main header, usually placed in a container. Heat from the CHP or boiler and from any heat recovery is gathered here and supplied to external heat exchangers at the digesters. These external heat exchangers work at a higher temperature than internal systems, around 65 to 70°C against a maximum of about 50°C, which gives better heat transfer through continuous pumping. Because there is no internal piping, the leakage risk stays low, maintenance is easier and installation is faster.

Gas cleaning and upgrading

Raw biogas contains H2S, which HoSt removes largely within the digester tank itself through a biological process under the membrane roof, with HoSt Carbon Clean® activated carbon filters providing a polishing step for any residual H2S before upgrading or combustion. The cleaned biogas is then either converted into electricity and heat with a CHP engine or upgraded to biomethane using membrane-based upgrading technology, for grid injection, gas filling stations, or bio-CNG and bio-LNG applications.

Digestate treatment

The digestate leaving the digester is processed depending on its intended use, commonly through mechanical separation with a screw press separator to produce a solid and a liquid fraction, and, where required by the feedstock, pasteurization with the HoSt Hygienisator® to meet regulatory standards before use as a biofertilizer.

Safety and support equipment

Flares such as the HoSt Open BioFlare® and HoSt Closed BioFlare® safely dispose of biogas when it cannot be used, for instance during maintenance of the CHP or biogas upgrading unit or when the gas storage is full. Filters, including the HoSt Filter Basket®, protect pumps and other equipment from larger particles in liquid feedstock, while HoSt’s pumping technology moves biomass through the plant. An automated foam detection and anti-foam dosing system, along with additives such as trace elements and enzymes, help keep the digestion process biologically healthy and stable.