Investigation of the agroecological applications of olive mill wastewater fractions from the ultrafiltration-nanofiltration process

Abstract

Agricultural applications of olive mill wastewater (OMW) represent a critical challenge, consistent with waste recycling and the trend towards a more sustainable pattern of agriculture. In this context, an integrated study on the agroecological applications of OMW from the ultrafiltration (UF) - nanofiltration (NF) process was carried out. This process generated three fractions: UF retentate and NF permeate, depleted in salts and phenolic compounds, were studied for their fertilization and irrigation potential, while NF retentate, enriched in these elements, was studied for its potential as a bioherbicide. The phytotoxicity of the NF retentate fraction on two crops (maize and flax) was evaluated on seedlings growth and chloroplast pigments content. In addition, the induced defense responses in maize and flax seedlings were investigated by measuring two parameters: the activity of the detoxification enzyme glutathione-S-transferase (GST) and the concentration of polyphenols, as a component of the antioxidant defense strategy in plants. Biomass, height, and chloroplast pigments content decreased progressively with increasing NF retentate concentration. Conversely, an increase in GST activity and polyphenol concentration was observed. These results highlighted the ability of OMW to induce an oxidative stress on maize and flax seedlings, triggering a defense response through GST and phenolic compounds. On the other hand, in vitro tests on the phytotoxicity of the NF retentate fraction on the common weed Sinapis arvensis were carried out. No germination was observed even with the lowest dilution applied, thus establishing the first data about the selectivity of potential OMW-derived bioherbicides. On the other hand, UF retentate and NF permeate treatments led to a significant increase in maize growth: these fractions could then be considered as a promising organic fertilizer for degraded agricultural soils, as well as an alternative water source for crops irrigation.