Biogas digestate is the material left-over after the anaerobic digestion of an organic (carbon based material) feedstock. The anaerobic digestion process produces two primary outputs. These are biogas digestate (digestate) and biogas. Digestate is produced both at the start of the reaction, when inititial breakdown of the complex chemicals in each cell when alive is broken up by acidogenesis, and methanogenesis (the biogas/ methane making stage) and each has different characteristics.
Digestate is technically not the same as compost. This confuses many people, because it certainly resembles it both in its chemical and physical qualities. Compost is produced by aerobic digestion in which decomposition occurs by the action of aerobes. Aerobes include fungi and bacteria (germs!) which have the ability to break down the woody materials containing a high proprtion of lignin and cellulose, to a higher extent.
So, digestate, in whatever form you see it, is the solid residue from the initial input (feed) material that went into the to the biogas digesters which the “germs” are unable to utilize. In 2 stage biogas digester systems, various types of digestate come from different digestion tanks, but in this discussion we will only discuss the two already mentioned above.
Digestate usually contains materials, such as lignin, that can not be broken down by the anaerobic micro-organisms. The digestate will usually contain a high level of ammonia that is phytotoxic (toxic to growing plants). In such cases, digestate would hinder the growth of plants if it was used as a soil improving product, or worse.
From consideration of these 2 factors, a maturation or composting phase is normally applied to digestate after digestion and before it is used. In a composting pile lignin and other products are readily available for destruction by aerobic bacteria, such as fungi. So, not only is the ammonia concentration reduced by subsequent composting but composting is also instrumental in reducing the remaining volume of the digestate material.
Throughout this maturation period, the ammonia will be oxidized into nitrates, enhancing the fertility of the product and making it a better soil improver. In dry anaerobic digestion processes the composting stage may be the longest, when compared with other AD systems.
The biogas digester produces a solid and a liquid digestate in the slurry which flows out of the digester. the production of digestate is unavoidable so the procedure would not be sustainable unless an ecologically safe method of digestate disposal was available. Clearly, the aim of a biogas plant manager will always be to dispose of the output from his or her digesters in a way that meets environmental standard whilst also providing a “product” which can be sold (or at lest given away) for a beneficial use.
Thankfully, the application of digestate to land as a soil conditioner, has been shown, in studies and by experience, to have a remarkable ability to prevent plant illness and even to induce resistance to infection, where none existed before. In this way, the application of digestate to land has a direct positive impact on soil-borne diseases, and an indirect impact because it also stimulates biological activity, as shown by scientific studies.
Acidogenic digestate, when applied to land and mixed into the soils, provides better wetness retention and a higher organic content for soils.
The liquid biogas digestate (methanogenic digestate) is rich in nutrients, leading to its obvious use as a fertiliser. Just how rich in nutrients a digestate is will always depend on the quality of the material being digested. Levels of possibly hazardous materialss (PTEs) need to be chemically examined. The quality of the liquid digestate will be closely related to the quality of the initial feedstock. In most cases, the levels of PTEs will certainly be low. In the case of wastes originating from the general market, the levels of PTEs may be greater and will certainly have to be taken into account when determining an appropriate end-use for the product.
The qualities of any digestate produced in a biogas digester can be evaluated on three criteria, as follows:
- biological and
- physical aspects.
Chemical quality has to be thought about in terms of heavy metals and other inorganic contaminants, consistent natural substances and the material of macro elements such as Nitrogen, Phosphourous and Potassium. Depending upon their source, bio-wastes can also contain pathogens, which can result in the spreading of human, animal or plant diseases, if the risk of the presence of pathogens is not appropriately managed.
In the UK there is such a thing as a “Publicly Available Specification” (called PAS110) which governs the definition of digestate originated from the anaerobic digestion of source segregated eco-friendly products. The specification ensures all digested materials are of constant quality and “fit for purpose”. If a biogas plant meets the requirement, its digestate will certainly be regarded as having actually been completely recovered and to have stopped being waste, and once that happens it can be offered for sale with the name “Bio fertiliser”.
Many proprietory biogas digester systems incorporate both anaerobic digestion and composting. This might comprise of either taking the full anaerobic digestion phase, followed by the maturation (composting) of the digestate.
In some, so called “dry biogas digester” process designs, a partial anaerobic digestion phase can be caused through water that is percolated through the raw waste, dissolving the easily available sugars. This is then followed very often with the material being sent out to a windrow composting center.