Physical-chemical traits, phytotoxicity and pathogen detection in liquid anaerobic digestates

Anaerobic digestates, which are co-products from biogas production, have been recognised as potential biofertilisers for their benefits in nutrient recovery and recycling of different types of organic wastes. Due to the increasing number of different types of organic wastes being used to produce biogas, it is necessary to identify how different types of anaerobic digestates vary in their physical-chemical traits, and how these can impact upon their use as fertilisers. In addition, safe land spreading of anaerobic digestates must be within recommended limits for potentially toxic elements (PTEs) and pathogens. This study analysed physical-chemical traits, phytotoxicity, PTEs and indicator pathogens in a set of eleven different commercial liquid anaerobic digestates from Ireland and the UK, and compared them to the Irish draft standard for digestate. Liquid anaerobic digestates exhibited significant differences (P < 0.001) for most of the physical and chemical traits evaluated, with higher variability found for dry matter (DM) and K (CV = 17.2 and 16.8 respectively), and lower variation for pH and P (CV = 1.78 and 3.55 respectively). PTE concentrations were in general within recommended limits; nevertheless, some digestates showed higher concentrations than the recommended limits for Pb, Zn and Cu. Digestate from wastewater treatment feedstock was shown to be high in PTEs. Anaerobic digestates were found to negatively affect early stages of seed germination, but phytotoxicity effects were decreased by dilution in water. Levels of Salmonella spp. and E. coli were within recommended limits for most of the anaerobic digestates analysed.