Methanogenic digestate is a sludge. This is often high in nutrients such as ammoniums and phosphates.
Uses
The primary use of digestate is as a soil conditioner. Acidogenic digestate provides moisture retention and organic content for soils. This organic material can break down further, aerobically in soil. Methanogenic digestate provides nutrients for plant growth. It can also be used to protect soils against erosion. Acidogenic digestate can also be used as an environmentally friendly filler to give structure to composite plastics. Growth trials on digestate originating from mixed waste have showed healthy growth results for crops. Digestate can also be used in intensive greenhouse cultivation of plants, e.g. in Digeponics. Application of digestate has been shown to inhibit plant diseases and induction of resistance. Digestate application has a direct effect on soil-born diseases, and an indirect effect by stimulation of biological activity.
Digestate is technically not compost although it is similar to it in physical and chemical characteristics. Compost is produced by aerobic digestion-decomposition by aerobes. This includes fungi and bacteria which are able to break down the lignin and cellulose to a greater extent. Treatment, e.g. by ultrasonication, has shown to enhance solubilization of digestate as measured by increased levels of soluble Chemical Oxygen Demand, soluble Total Organic Carbon, and soluble Total Nitrogen released into the solution.
Standards for digestate
The standard of digestate produced by anaerobic digestion can be assessed on three criteria, chemical, biological and physical aspects. Chemical quality needs to be considered in terms of heavy metals and other inorganic contaminant, persistent organic compounds and the content of macro-elements such as nitrogen, phosphorus and potassium. Depending on their source, biowastes can contain pathogens, which can lead to the spreading of human, animal or plant diseases if not appropriately managed. The physical standards of composts includes mainly appearance and odour factors. Whilst physical contamination does not present a problem with regards to human, plant or animal health, contamination can cause a negative public perception. Even if the compost is of high quality and all standards are met, a negative public perception of waste-based composts still exists. The presence of visible contaminants reminds users of this. Quality control of the feedstock is the most important way of ensuring a quality end product. The content and quality of waste arriving on-site should be characterised as thoroughly as possible prior to being supplied. In the UK the Publicly Available Specification governs the definition of digestate derived from the anaerobic digestion of source-segregated biodegradable materials. The specification ensures all digested materials are of consistent quality and fit for purpose. If a biogas plant meets the standard, its digestate will be regarded as having been fully recovered and to have ceased to be waste, and it can be sold with the name “Bio- fertiliser”.
