# Food Waste as Feedstock for Anaerobic Mono-Digestion Process
## 1 Introduction  
It has been well known for some time, based on multiple lines of evidence, that globally, about one third of the food produced for humans (1.3. billion tonnes) is lost or wasted every year [1, 2, 3]. Roughly speaking, food waste (FW) is collected from numerous sources. For example, in 2021, in the European Union, FW from households represented more than 31 million tonnes of fresh mass [4] (Figure 1). Other sectors contributing to FW included food processing (above 12 million tonnes of fresh mass), primary production (5 million tonnes of fresh mass), restaurants and food services (more than 5 million tonnes of fresh mass), and retail and food distribution (4 million tonnes of fresh mass). It should be pointed out that while the FW issue is currently on the rise, the Food and Agriculture Organization of the United Nations reported that due to economic and population growth, food production will increase by 60% by 2050 [5]. FW is defined as food that has not been ultimately consumed by humans and, as a result, is discarded or recycled [6]. It has been widely documented that FW, without an efficient treatment process, is a potential contaminant of the environment and a source of greenhouse gas emissions, contributing to both global warming and climate change [7, 8, 9, 10, 11]. Based on the literature review, it was noted that there is a growing recognition that FW comprises a significant amount of unused energy (Figure 2). Hence, there is no doubt that the development of sustainable and circular system solutions for FW are expected [12]. Currently, it is well known that in order to achieve sustainable environmental development, the reuse of FW by a conversion processes is essential. It is highly recommended since, as recognized in the literature, FW is a promising carbon source characterized by high methanogenic potential, biodegradability, and a high concentration of nutrient contents [13; 14; 15; 16]. Moreover, it was highlighted by [PERSON] et al. [17] that FW is characterized by higher biogas and methane yields compared to corn stover (CS) and chicken manure (CM). At present, the main FW disposal method is anaerobic digestion (AD). AD is a low-cost and environmentally friendly biochemical process widely used for the production of methane (CH\({}_{4}\))-rich biogas and liquid digestate via the conversion of organic materials in the absence of oxygen. It is important to note that FW ensures a higher biogas yield than most substrates. By way of example, according to [18], FW is characterized by higher methane potential than both animal manure (AM) and municipal sewage sludge (SS). Likewise, [PERSON] and [PERSON] [19] pointed out that the AD yield obtained with the use of FW leads to the production of 15 times more biogas per tonne than farm waste. The fundamentals of the AD process, including its unique advantages as well as current trends and future perspectives in biogas production, have been presented and thoroughly discussed in several recently published review articles [20; 21; 22; 23; 24; 25; 26; 27]. The AD process may be performed in one or two/a few stages [27]. Briefly, in a single process, usually, only a continuously stirred tank reactor (CSTR) is used. Accordingly, all steps of the AD process occur in a single reactor [28]. On the other hand, a two-stage system applies two digesters and ensures the separation of acid fermentation and methanogenesis [29]. It is very essential to mention that in references [29; 30; 31], it was indicated that a two-stage AD process is characterized by more advantages than a single-reactor system. On the other hand, numerous studies have argued that the single-stage anaerobic system provides (i) higher sludge stabilization, (ii) less advanced control and operation, and (iii) minimal engineering costs [28; 32]. Indeed, [PERSON] et al. [33] highlighted that this strategy appears as a reliable alternative due to simplicity and economic viability. It  
Figure 1: Sources of FW in the European Union in 2021. Based on [4].