The quality of digestate as organic fertilizer could be described by the four main groups of indicators, namely nutrients content, phase state, stability and impurities content (Figure 1).
Figure 1 – Digestate quality indicators
The content of nutrients, humic substances and trace elements in the digestate determines its fertilizing value, and therefore its possible market price. The over-content of certain types of pollutions impairs its commercial value, limits the list of applications, or makes it impossible to use it further without additional processing.The moisture content and density of the digestate based products have a significant impact on the costs of transporting and applying to the fields. Finally, the digestate stability level affects convenience and safety in use and trade.
The production and recycling of digestate as fertiliser requires quality management and quality control throughout the whole closed cycle of AD, from the production of the AD feedstock until the final utilisation of digestate as fertiliser. Quality management implies the use of controlled high quality feedstock, pre-processing of specific feedstock types, close control of the AD process and of process parameters affecting digestate quality, digestate processing, declaration, optimal storage and application as fertiliser. The overall matrix for digestate quality management could be presented as in Figure 2.
There are two main ways how to manage digestate quality, namely prevention and processing.
The rule of thumb is that if efficient pollutant removal cannot be guaranteed either by pre-treatment or through the AD process, the respective material must not be used as feedstock in biogas plants where digestate is used as fertiliser or for other agricultural purposes.
Figure 2 – Digestate quality management matrix
Author: Petro Kucheruk – expert of Bioenergy Association of Ukraine