## 1 Introduction
According to EU Directive 2018/851 [7], \"waste management in the Union should be improved and transformed into sustainable materials management in order to safeguard, protect and improve the quality of the environment, protect human health, ensure the prudent, efficient and rational use of natural resources, promote the principles of the circular economy, intensify the use of renewable energies, increase energy efficiency, reduce the Union's dependence on imported resources, provide new economic opportunities and contribute to long-term competitiveness. The Italian legislation, in addition to the definition of waste, identifies the conditions under which a substance or object is not to be considered waste, introducing the concept of by-product, which is described in Article 183-bis of the Legislative Decree n. 152/06 [8] as \"the substance or object originates from a production process, of which it is an integral part, and whose primary purpose is not the production of such substance or object; it is certain that the substance or object will be used, during the same or a subsequent production or use process, by the producer or third parties; the substance or object can be used directly without any further treatment other than normal industrial practice; the further use is legal, i.e. the substance or object fulfils, for the specific use, all relevant product and health and environmental protection requirements and will not lead to overall negative impacts on the environment or human health\". As matter of fact, olive oil by-products contain a high amount of bioactive compounds, namely phenols (as reported in Table 1). The most of the phenolic fraction present in olives is found in OMMWW (up to 53%) and OP (approximately 45%), with only 2% of the initial content remaining in virgin olive oil [9]. The phenolic compounds present in OMMW are hydroxytyrosol, tyrosol, verbascoside, acids (such as caffe, gallic, vanillic, and syringe) and polymeric substances [10; 11]. Recently, the use of OMMW has been successfully proposed for different applications, and many studies have focused on obtaining compounds with high added value, i.e., phenolic extracts, through different approaches, including enzymatic and chromatographic techniques, solvent extraction methods, and membrane processes, such as microfiltration, ultrafiltration, nanofiltration, and reverse osmosis [6]. Therefore, OMMW could be considered as a potential low-cost starting matrix for extraction of antioxidants to be applied in several fields, including the food industry, where they could be used for both fortifying and prolonging the shelf life of final products [12; 13; 14]. In the present work, a literature survey was carried out taking into account a fixed timeline, between 1996 and 2020, and the keywords \"olive mill wastewater\". Searching on ScienceDirect, 794 records were found. Most of them fell within the scope of environmental science, such as chemical engineering, energy fuels, and agriculture, with quite constant increasing numbers in recent years, from two papers published in 1992 to 54 papers in the last five years (Figure 1a). To confirm the increasing interest in biotechnological approaches to OMMW treatment, 298 records were identified in the field of biotechnology and microbiology (Figure 1b) [15]. The aim of this review is to provide a summary of updated information on research that has been conducted using OMMW as a renewable raw material to generate high added-value ingredients/products for agro-food industries, including the functional food sector. Figure 1: _Cont_.