Olive mill waste sludge: From permanent pollution to a highly beneficial organic biofertilizer: A critical review and future perspectives.

Olive mill wastewater sludge (OMWS) is a by-product of the olive extraction process that is attracting substantial attention due to its extremely hazardous effects on aquatic and terrestrial ecosystems. OMWS is a product of the common disposal method of olive oil mill wastewater (OMWW) that accumulates in evaporation ponds. It is estimated that approximately 10 × 10 m of OMWS is generated worldwide each year. OMWS is characterized by its significantly variable physicochemical properties and organic pollutant constituents, such as phenols and lipids, which are dependent upon the environmental features of the receiving ponds. Nonetheless, many related studies have recognized the biofertilizer potential of this sludge owing to its high mineral nutrient and organic matter load. OMWS exhibits promising valorization potential in several fields, including agriculture and energy production. Compared to those of OMWW, studies of OMWS are still lacking concerning its composition and characteristics, which are necessary for the future implementation of efficient valorization strategies. The main purpose of this review paper is to fill the gap that exists in the literature by providing a critical analysis of the available data on OMWS production, distribution, characteristics, and properties. Additionally, this work sheds light on important factors affecting OMWS properties, including the variability of the indigenous microbial communities regarding bioremediation. Finally, this review addresses the current and future valorization routes, from detoxification to the development of promising applications in agriculture, energy, and the environment, which could have significant socioeconomic implications for low-income Mediterranean countries.