Technical Report

Quality management of digestate from biogas plants used as fertiliser

May 2012

The increasing global demands for food dictate higher yields per hectare which can be achieved, inter alia, through an increase in the use of fertilisers. The traditional use of mineral fertilisers has important limits and requires new, sustainable alternatives. The main limits concern the decreasing worldwide natural reserves of mineral fertilisers and the negative environmental impact caused by the use of fossil fuels for their production. Digestate from biogas plants is rich in plant nutrients and has excellent fertiliser qualities and has great potential worldwide as a sustainable alternative to mineral fertilisers. Despite its potential, the use of digestate as fertiliser is limited in many countries due to unfamiliarity of the product and insufficient confidence in its quality and safety. Quality assurance is therefore an important condition for increased market confidence in digestate and for its enhanced use as fertiliser. Digestate quality management is implemented through various means: standards of digestate quality, digestate certification systems, nutrient regulations and legislative frameworks, and most important through on-going quality control practices along the whole digestate production cycle.

This brochure is focused on quality management of liquid digestate from biogas plants where animal manures and slurries, crop residues, organic wastes and residues from agri-food processing industries and from other industrial processes are the principal feedstocks. The aim is to provide guidance on best practices for the production of high quality digestate, which is suitable for application as a crop fertiliser and with a positive environmental impact and a high degree of safety for human and animal health. The information contained in this brochure should be of interest to biogas and digestate producers, to farmers who use digestate as fertiliser, to industries which supply organic wastes to biogas plants as well as to policy makers, regulators and consumers.

The biogas process, usually called anaerobic digestion (AD), occurs naturally in different environments (Figure 1): the stomach of ruminants, landfills, volcanic hot springs, submerse rice fields, etc. The main difference between naturally occurring AD and biogas plants is that in a biogas plant the AD process is deliberately controlled to achieve maximum methane production. In controlled AD processes organic matter breaks down in the same way as in nature, in the absence of molecular oxygen. This results in two valuable products: renewable methane and digestate.

The biogas that is produced this way is a very useful source of renewable energy, whilst digestate is a highly valuable biofertiliser. IEA Bioenergy Task 37 has a number of publications on different aspects of biogas production and on utilisation of digestate as biofertiliser. These can be accessed and downloaded at: www.ieabiogas.net/publications.

Use of digestate as fertiliser requires that rigorous attention is paid to the quality of digestate and the feedstock supplied to biogas plants where digestate is intended for use as fertiliser. This is the only way to achieve maximum ecological and economic benefits, while at the same time ensuring sustainability and environmental safety. Quality management of digestate used as fertiliser should be integrated into overall national environmental protection and nutrient management policies. Good examples of this can be found in countries like Austria, Canada (Ontario), Denmark, Germany, Netherlands, Sweden, Switzerland and the United Kingdom. National regulatory frameworks for digestate quality management and certification for use enhance its use as fertiliser in a safe and sustainable way.