From value chain to value loop with biogas and mushrooms
Waste is only waste if you cannot put it to good use! EU-funded researchers have developed a circular value chain deriving income sustainably from wet straw, biogas digestate and similar residues, via products such as fertiliser, fuel pellets and mushroom substrate and boosting energy generation in the process.
© gerasimov174 #262946677, source:stock.adobe.com 2020
The EU-funded circular economy project MUBIC focused on the transformation of low-value agricultural waste into high-value products one of which is used to grow mushrooms, whose cultivation the partners set out to revolutionise.
More specifically, this product is a new type of mushroom substrate, the material on which the mushrooms are grown. Along with other outputs, such as fuel pellets, fertiliser, and pre-treated feedstock for faster biogas production, the process shaped in MUBIC also delivers substantial sustainability gains, says project coordinator Peter Damgaard Nielsen of Danish SME Advanced Substrate Technologies (AST).
It notably increases the biogas yield from agricultural wastes like straw or horse bedding by about 30 %, through the prior cutting and grinding of this biomass. The ability to process wet straw, meadow grass and similar material is a particular plus point: the MUBIC process for these challenging types of feedstock also involves pretreatment with nitrogen to break down the lignin in the stalks, Nielsen explains.
Profitable, circular, green
Using this process, you get more biogas and you get it faster, because the bacteria in the biogas plant can also use the lignin as feed, says Nielsen. The nitrogen used for this pre-treatment is recovered from the biogas plant, through the treatment of the digestate the residues of the feedstock.
The nitrogen is extracted from the digestate in facilities designed as annexes to the biogas plants themselves. As of July 2020, operations have begun at the first such add-on plant designed as a commercial facility, according to Nielsen.
In the MUBIC model, the digestate might, in fact, have started out as biomass supplied by the partners. Our lorries take feedstock to the biogas plant and digestate back to the add-on plant, where we dry it, Nielsen continues.
The treated digestate is then converted into a variety of products. Along with the mushroom substrate, these outputs include fuel pellets that are significantly more sustainable than competing products because they are derived from a type of waste and because this waste is a residue of locally sourced biomass.
The MUBIC process delivers a pelletised substrate whose dry matter content, at 85 %, is much higher than that of conventional substrates, which typically average about 30 %. Thus, MUBIC substrate provides far more mushroom feed per cubic metre than competing products, Nielsen explains. Due to its high dry-matter content, it can be stored and does not require cooling during transport over longer distances unlike conventional substrates, which might begin to rot.
MUBICs substrate complements another innovation developed in the project: a system by which mushrooms grown on trays move past pickers on a conveyor belt, as opposed to pickers having to move through the shelves in mushroom farms. This particular innovation was perfected by Dutch SME Panbo Systems, the second partner involved in the project, as a way to reduce harvesting costs.
Rooted in wet straw
MUBIC, which ended in January 2020, was funded via a Horizon 2020 programme specifically designed to help SMEs prepare their innovations for commercialisation. Without this support, the straw-to-substrate concept would have been shelved, says Nielsen, who emphasises that the funding enabled the partners to bring their idea to life.
AST had initially considered selling entire add-on plants, but eventually decided to focus on the products generated throughout the process instead: income is generated at several steps, from raw material that is largely available for free.
Spent substrate from mushroom production is one such residue, and a perfect example of the proposed circular approach. In MUBIC, it is recovered as feedstock for the production of biogas, and the digestate left over from this process is used to make fresh substrate.
Both the new mushroom cultivation system and the various products from MUBICs circular value chain have been met with interest, with new partnerships enabling the project partners to take these developments forward, Nielsen concludes.