# Application of Near-Infrared Spectroscopy for Monitoring and/or Control of Composting Processes
## 3 Composing: General Information
The first phase is the so-called mesophilic, and it is dominated by fungi and bacteria that start with the decomposition of easily degradable macronutrients (proteins and simpler carbohydrates, such as sugars), and due to the microbial activity, the temperature rises evenly (25 to 45 \({}^{\circ}\)C), and the heat increases, which enables the transition to the next, thermophilic phase [6]. The thermophilic temperatures (above 55 \({}^{\circ}\)C) are desirable because they kill pathogens [18]. Figure 2: General scheme of aerobic composting process (adapted from [PERSON] et al. [9]). During the thermophilic phase, high-energy compounds (sugars and proteins) are broken down, and the supplies are depleted, which results in a decrease in temperature and the redominance of mesophilic microorganisms. The last phase is characterized by the humification of the material and presents the maturation phase [6; 18]. Moisture is a variable closely related to microorganisms and should be in appropriate amounts through the composting cycle [19]. The most favorable moisture content at the beginning of the composting process depends on the physical state and size of the particles of the material. A moisture content around 50-60% should be satisfactory [13]. A low moisture content (less than 30%) can lead to dehydration of the compost, while excessive moisture content will limit the movement of oxygen; thus, anaerobic conditions will reduce the rate of composting [18; 20]. The most favorable pH range for the composting process is between 5.5 and 8.0. Bacteria favor practically neutral pH, and fungi develop in acidic environments [21; 22]. In the first stage of composting, the pH may decrease due to the organic acids which are produced during the degradation of organic compounds. Further mineralization leads to an increase in pH, and at the end of the process, it is around 8.0-8.5 [19; 22]. Food waste can present a challenge for applying the composting method due to low pH, which is a consequence of the presence of short-chain organic acids. Therefore, several studies have investigated the use of alkaline materials (sodium acetate, sodium hydroxide) as pH control amendments in order to provide an improvement in the composting process [23]. The ratio of carbon to nitrogen (C/N ratio) indicates the degree of decomposition of organic matter and its identification; an optimal ratio is of great significance to obtain favorable composting results and highly depends on the properties of the composting material [8]. The ultimate for the active composting of most types of wastes is 25-30:1, and during the composting process, it decreases constantly due to mineralization of the carbon compounds [24; 25]. Aeration is important for microbial growth and gas emissions [8]. Composing is an aerobic process where oxygen is consumed, and carbon dioxide is released [18; 26]. Studies show that an optimal level of aeration will provide enough oxygen that is needed to evaporate excess moisture from the substrate due to the oxidation of organic material [27], while inappropriate aeration can lead to anaerobic conditions (in the case of insufficient aeration) or can lead to cooling and prevent the thermophilic conditions necessary for optimal decomposition rates (with excessive aeration) [28]. A well-executed composting process results in compost that is suitable for improving soil characteristics and will enable increased infiltration and water retention, and its extremely sustainable feature is that it will also provide a number of nutrients necessary for plant growth [29]. Ensuring microbial diversity is considered a key factor for healthy soil as it enables the implementation of biological processes and the circulation of nutrients, where enzymatic activities also have an important role in biochemical reactions. Enzymatic activities can be enhanced using organic fertilizers [6]. The final use of compost defines which requirements will be set for the quality of the compost.