## 1 Introduction  
Coffee is of the world's most popular beverages and functional food commodities [1, 2]. Spent coffee grounds (SCG) are a solid waste generated during the brewing of ground coffee beans and industrial processes such as instant coffee production [3]. SCG generation is estimated at around 650 kg of per ton of green coffee, and around 2 kg of wet SCG per kg of soluble coffee [4]. Considering the constant increase in global coffee consumption [5] and annual global coffee production reaching around 10 million tonnes [6], substantial amountsof waste SCG (around six million tonnes) are generated annually around the world [7]. Such high amounts of SCG waste may pose a great hazard to the environment if disposed of incorrectly [8]. The growing amounts of wastes generated in food processing and pollution of the environment led to the creation of the circular economy concept (CEC) [9]. CEC proposes a closed-loop of material flows and focuses on several paths for extending agro-industrial product life cycles and recycling plant waste materials, thus minimizing the environmental impact [10]. SCG waste is abundant in many valuable compounds, including phenolics, alkaloids (e.g., caffeine) and melanoidins, which are only partially extracted from crushed coffee beans during the brewing process [11]. These compounds are thought to be valuable due to a variety of bioactive effects, mainly antioxidant properties, and have substantial health benefits. Additionally, these compounds are valued ingredients in the functional food sector for enhancing the organoleptic properties of many plant-based foods or extending shelf-life when used as natural preservatives [12; 13]. Many studies in recent years have concentrated on the extraction of bioactive compounds from spent coffee grounds, and the isolation of antioxidant components with biological activity from coffee by-products constantly attracts attention in scientific communities around the world [14; 15; 16; 17; 18; 19]. Extractions can be performed by many varied methods. These techniques are divided into conventional (e.g., liquid-liquid, solid-liquid extraction) and non-conventional extraction techniques [20; 21]. Traditional solid-liquid extraction techniques utilize a solid matrix and the extracting power of various solvents (pure solvents, mixtures, aqueous solutions), and they remain a commonly employed method for recovery of antioxidant compounds in the food industry. The major difficulties of conventional methods are the long extraction times, requirements for costly high-purity solvents, evaporation of the large amounts of extractant, low extraction selectivity, and thermal degradation of thermolabile compounds. To overcome these limitations of conventional extraction techniques, certain innovative non-conventional extraction techniques, such as ultrasound-assisted extraction, have been developed. Ultrasound-assisted extraction is based on a special type of soundwave which passes through the solution and generates cavitation [22; 23]. The final yield of the extraction process is dependent on a number of factors, such as the solvent to matrix ratio, organic solvent concentration, length of the extraction process, procedure temperature and many more [24]. Conventionally, the effectiveness of the process can be optimized by conducting numerous experiments, which ultimately requires more time and financial costs [25]. To overcome these constraints, multivariate statistical optimization techniques such as factorial design or response surface methodology were introduced [26]. These techniques allow researchers to determine the optimal levels of process variables that maximize the effect levels and minimize the number of experimental runs needed [27]. For these reasons, the main objective of this study was to optimize the SCG extraction process for maximization of antioxidant activity and bioactive compound recovery by means of response surface methodology.