Approximately 1 million tons of waste cooking oils (WCO) are generated in European Union per year. Oftentimes, these oily wastes are discharged through public sewerage systems, increasing the water treatment costs in wastewater treatment plants. The oleaginous yeast Yarrowia lipolytica W29 is used to simultaneously degrade WCO and to produce lipase (market demand is increasing due to its application in the field of bioenergy). A statistical experimental design based on Taguchi method is employed to assess the effect of initial medium pH, WCO, and arabic gum concentration on lipase production. Initial medium pH is found to be the most significant parameter and the interaction between WCO and arabic gum concentration had the highest influence for lipase production. Additionally, the effect of oxygen mass transfer is studied in batch cultures in a stirred tank bioreactor and the maximum lipase activity (12 000 U · L−1) is obtained at kLa of 16 h−1. Simultaneously with lipase production, also lipid‐rich biomass (48% of lipids mass per dry cellular mass), enriched in unsaturated fatty acids (oleic and linoleic acids) is obtained. Practical Applications: In this work it was demonstrated the possibility of replacing an expensive edible oil (olive oil) by a non‐edible oil waste (WCO) for lipase production. Moreover, no more wastes are produced, since whole cells of Yarrowia lipolytica W29 (lipid‐rich biomass), are a potential source for food supplements. Thus, the bioprocess described fulfill the requisites of circular economy: a low cost substrate is used as raw material, added‐value products (lipase and lipid‐rich biomass) are obtained and no further wastes are generated (take‐make‐dispose concept). Waste cooking oils (WCO) are generated from vegetable oils used at high temperatures in food frying. The biological degradation of WCO by the oleaginous yeast Yarrowia lipolytica W29, with concomitant production of added‐value compounds (lipase and lipid‐rich biomass), allows the reduction of pollutant burden of these oily wastes.

Waste cooking oils (WCO) are vegetable oils discarded after food frying and great amounts are produced worldwide. Its management is a challenge, due to the environmental risk of illegally disposal into rivers and landfills. The main approaches for WCO valorization included their incorporation as component of animal feed and biodiesel manufacturing. Yet, the development of new feasible approaches is attractive from an economic and ecological standpoint. Due to their composition in triglycerides, untreated WCO can be used as feedstock for microbial growth (several species are able to use them as carbon source) and production of added-value compounds. In this way, microbial valorization of WCO is a sustainable biotechnological approach to upgrade a waste into a renewable feedstock for bio-based industry, favoring the circular economy concept. The objective of this review is to highlight the potential use of WCO in bioprocesses as an alternative to other physicochemical treatments. Firstly, an introduction to WCO problematic is presented, describing most common applications used currently. Then, an extensive review on the use of WCO by microorganisms is shown, focusing on bacterial and fungi species and its exploitation for bioprocesses development to produce metabolites of industrial interest, such as biopolymers, biosurfactants, lipases and microbial lipids. KEYWORDS Added-value compounds; microbial conversion; waste cooking oils 1. Waste cooking oils: General overview Waste cooking oils (WCO) are generated from vegetable oils (coconut, sunflower, soybean, palm tree, cottonseed, rapeseed, olive, etc.) employed to fry foods in household and HORECA (Hotels, Restaurants and Catering) segments and are no longer suitable for human consumption. Specifically, in HORECA sector, fast food restaurants (particularly those of chicken and

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