From biogas plants fertilisers

Biogas plants can produce fertilisers and soil improvers suitable for arable fields as by-products of renewable energy. This was the finding of the newly completed BIOVIRTA joint research project, the results of which show that the end products of biogas plants do not adversely affect plants to a significant degree.

Because soil humus and organic matter is declining, it is important to recycle organic matter into the soil also via biogas plants. The organic compounds not broken down in the biogas process increase the carbon content of farmland and improve the biological activity of the soil as they break down naturally.

In some part better than manure

For some of the end products, plant bioassay results were even better than with manure, to which the obtained results were compared. Results obtained with chemical fertiliser were also comparable to those for the end products of biogas plants. The highest risk of a harmful response was with solid processing residues from the biogas plant.

The results showed that alongside chemical analysis, plant bioassays are well suited to determining the quality of processing residues.

Health and environmental risks must be acknowledged

To use end products of biogas plants safely, their health and environmental risks must be managed. A further requirement is applicable analysis and testing methods and quality criteria for different purposes.

Bioassays, which can be used to determine the actual effect of a compound directly on a living organism, such as a plant, bacterium or protozoan, are increasingly used to estimate environmental impacts and in risk assessment. Bioassays are also suitable for assessing combined effects of chemical compounds, such as harmful organic substances.

However, when interpreting bioassay results it must be noted that the result may be affected by a number of factors. As an example, plants react easily to changes in growing conditions as well as sample properties; e.g. the ammonium nitrogen, acetic acid and volatile fatty acids (VFA) contained in the sample can influence the results. The choice of plant species also affected the results. For example, Chinese cabbage as test plant was clearly more sensitive than barley and yielded more distinct differences between the products. Determining the root length of cress proved to be useful as a test of end product quality, as it demonstrated the effects of the products well.

The project was coordinated by Natural Resources Institute Finland (Luke) and carried out jointly with the Finnish Food Safety Authority Evira, Technical Research Centre of Finland VTT, University of Jyväskylä and the companies.

BIOVIRTA project results (in Finnish), abtract in English.

The project results have also been reported in a peer-reviewed scientific journal
Maunuksela, L., Herranen, M., Torniainen, M.
Quality Assessment of Biogas Plant End Products by Plant Bioassays.
International Journal of Environmental Science and Development 2012, vol. 3, No. 3, pp. 305-310.