# Reuse of Spent Coffee Grounds: Alternative Applications, Challenges, and Prospects--A Review
## 1 Introduction  
Coffee is an agricultural and food product widely consumed as a beverage, and it is the second most traded commodity in the world after oil and its derivatives [1]. There are two primary species of coffee: Arabica and Robusta. In 2023, global coffee exports comprised 62.7% Arabica beans and 37.3% Robusta [2]. The International Coffee Organization reported that 80% of the world's coffee is produced in ten countries: Mexico, Indonesia, Honduras, Brazil, Colombia, Vietnam, Ethiopia, Guatemala, Uganda, and India. Brazil is the leading coffee-exporting country, producing approximately 2,859,000 tons annually [3]. It is estimated that 500 billion cups of coffee are consumed daily worldwide. According to the literature, the highest coffee consumption in Europe occurs in Scandinavia, averaging about 10 kg of coffee per capita annually. In Finland, this figure is 12 kg per capita per year, 9.9 kg in Norway, 9 kg in Iceland, 8.7 kg in Denmark, and 8.2 kg in Sweden. In other European countries, annual coffee consumption per capita is lower: 8.4 kg in the Netherlands, 7.9 kg in Switzerland, 6.8 kg in Belgium, and 6.5 kg in Luxembourg. In Poland, the average annual coffee consumption per capita ranges from 2.2 to 3 kg [4]. Coffee beans have a complex chemical composition that depends on factors such as their origin, cultivation methods and location, soil quality, cleaning methods, and roasting degree. The coffee cherries from which coffee is produced can be processed using either a dry or a wet method. The dry processing method involves drying the whole harvested cherries,while the wet method involves initial pulping, washing, and final fermentation [5]. There are four types of coffee waste: pulp, silverskin, parchment, and grounds. Spent coffee grounds (SCGs) are the sediment that results after brewing and are the most abundantly generated waste. For every ton of coffee, approximately 650 kg of grounds are produced. Coffee-producing companies generate over 2000 million tons of waste and byproducts annually worldwide [6]. While it might seem that SCGs deposited in landfills do not have harmful environmental effects, there is a significant risk of ignition of dried SCGs (similar to other organic wastes). Additionally, methane and carbon dioxide, gases responsible for global warming, may be produced [7; 8]. Furthermore, the high content of tannins, caffeine, chlorogenic acid, and phenols in SCGs can lead to environmental pollution. However, SCGs are rich in biologically active chemical compounds, which can have various applications [9]. From the perspective of a circular economy, SCGs are a valuable resource across various industrial sectors. The bioactive substances found in SCGs can be utilized in the pharmaceutical, cosmetics, and fuel industries. Meanwhile, the residual material remaining after the recovery of valuable active substances from SCGs can be used as fertilizer, fuel additive, or for producing biodegradable materials [10]. The aim of this paper is to present the possibilities for alternative uses of SCGs. This article aims to highlight the diversity of SCG applications and to expose key drawbacks, difficulties, and limitations related to the use of this material. An important element is also outlining potential directions for future research that may contribute to more efficient and sustainable use of SCGs. Unlike other review papers, which often focus on a general description of the reuse possibilities of different coffee components [11; 12], such as coffee extracts or coffee husks, or limit themselves to a single specific field [13; 14], this article presents a broader perspective. This article emphasizes various areas of SCGs application--from the construction industry, through cosmetics, to environmental purification technologies. This approach enables a better understanding of the relationships between different methods of SCG utilization and their limitations.