Chemical and biochemical changes in soil after biochar-treated OMW digestate amendment

Anaerobic digestion (AD) of organic wastes is a promising alternative to landfilling for reducing greenhouse gas emission and it is encouraged by current regulation in Europe. AD represents a source of green energy, while the by-product digestate still generates concerns for a safely disposal. The sustainability of AD plants partly depends on the management of digestion residues. Digestate could be used in organic amendment straightaway or after composting to limit possible phytotoxicity effects on crops. This study has been focused on the environmental benefits of digested olive mill wastewater (OMW), recalcitrant agricultural waste. OMW require a complex management due to high production volume in a limited time, fermentative processes occurring during the storage, and toxicity due to phenol compounds. These latter might compromise the AD process affecting microbial metabolism. As biochar is able to adsorb and retain organic and inorganic pollutants, we used biochar as additives during AD to remove phenols, stimulate microbial activity and therefore hydrogen and methane production. The resulted digestates including biochar could be used in order to increase the carbon stock in soil as a valid alternative to other organic amendments.

The aim of this work was to evaluate the effect of solid and liquid digestates, obtained from the AD process of OMW with biochar (30 and 45%), as additive, on soil chemical and biochemical properties in order to validate its use in organic amendment in lab-scale experiment. The liquid and solid digestates were added to soil according to the maximum dose allowed by the Italian nitrates directive concerning non-vulnerable areas (91/676/EEC, DGR 209/2007). Pots containing soil differently amended with liquid and solid digestates were prepared also for the growth of Lactuca sativa L. seedlings.

Thirty days after treatments, positive changes in chemical and biochemical properties in soil pots with biochar-treated digestates, in particular with liquid ones, occurred. Soil organic carbon, microbial biomass carbon and some soil enzymatic activities such as dehydrogenase, phosphomonoesterase, β-glucosidase and fluorescein diacetate hydrolysis significantly improved. Besides an enhancement of lettuce biomass, a significant decrease of nitrate content in plant tissue was registered when pots were amended with biochar-treated digestates.

The assessment of the agronomic quality of liquid and solid digestates, obtained by biochar assisted AD of OMW, as organic soil amendment, demonstrated that also critical biomass such as OMW, if opportunely treated, can entry in a re-use process where biogas and by-products can be part of virtuous circular economy.

This work was part of the project “Mitigation of the environmental impact of olive mill waste water through sustainable bioprocess with energy recovery” funded by the Università degli Studi di Napoli Federico II.