# Application of Near-Infrared Spectroscopy for Monitoring and/or Control of Composting Processes
## 3 Composing: General Information  
The growth of the world's population, especially in developed countries in the last fifty years, has had an exponential effect on the amount of produced solid waste [1; 2; 3]. Sustainable waste management of any kind, including food waste, is seen as an approach in which the primary goal is to reduce waste in any part of the production cycle and, if possible, recycle and/or separate those components that are still usable in a sustainable way as possible and thus, reduce the related impacts, starting from resources, production, sale, consumption to use, and final disposal--thus covering the entire life cycle [4; 5], which encourages the professional and scientific community to develop and implement new, greener strategies in solid waste management as soon as possible [6]. This increase in waste generation has put a pressure on the various components of the environmental system. As a result, the introduction of a suitable and environmentally favorable solid waste disposal plan has been identified as a vital requirement [6]. Considering that a major section of compact waste is organic matter [7], composting has attracted attention as an alternative method of organic waste recycling. Composing can divert waste from  
Figure 1: Number of publications according to Web of Science (WoS) in fields of (i) NIR spectroscopy and composting and (ii) composting and chemometrics. landfill, mitigate groundwater contamination, reduce air pollution and greenhouse gas emissions, and generate usable goods [8]. As described by [PERSON] et al. [9], composting can be categorized into two types according to the characteristics of the microorganisms responsible for the decomposition of organic wastes, namely aerobic (Equation (1)) and anaerobic (Equation (2)) [10]:  
\[\mathrm{C_{a}H_{b}O_{c}N_{d}}+\bigg{(}\frac{4a+b-2c+3d}{4}\bigg{)}\mathrm{O_{2} }\rightarrow\mathrm{aCO_{2}}+\bigg{(}\frac{b-3d}{2}\bigg{)}\mathrm{H_{2}O}+ \mathrm{dNH_{3}} \tag{1}\]  
\[\mathrm{C_{a}H_{b}O_{c}N_{d}}+\bigg{(}\frac{4a-b-2c+3d}{4}\bigg{)}\mathrm{H_{2 }O}\rightarrow\bigg{(}\frac{4a+b-2c-3d}{8}\bigg{)}\mathrm{CH_{4}}+\bigg{(} \frac{4a-b+2c+3d}{8}\bigg{)}\mathrm{CO_{2}}+\mathrm{dNH_{3}} \tag{2}\]  
As a number of studies indicate, the creation of the main byproducts (thermal energy, H2O, and CO2) is the result of the aerobic process of the biodegradation of organic matter [1,11] (Figure 2). In addition to the aerobic effect, during the decomposition of organic waste, there is also an anaerobic segment, and as stated in the term itself, it takes place without the presence of oxygen, so microorganisms that work in such conditions (anaerobic microorganisms, i.e., anaerobes) dominate the process of biogas production (e.g., methane) but also a strong-smelling byproduct called digestate (slurry mixture) [12,13]. Although considered a robust technology with numerous advantages, there are still some challenges when performing composite. According to [PERSON] et al. [14], challenges in composting can be assigned to (i) composting material composition, (ii) odors produced during composting, (iii) process variable monitoring (temperature, oxygen supply, moisture content, pH, C/N ratio, and particle size), and (iv) composting mixture condition. During the controlled process of composting under aerobic conditions, in addition to aeration, it is necessary to control the following extremely important parameters: (i) temperature, (ii) humidity, (iii) degree of acidity, and (iv) the ratio of carbon and nitrogen. A prerequisite for good quality products during composting is the control of the aforementioned conditions [6]. An indication of microbial activities during the composting process is the change in temperature, which is justifiably considered a parameter that indicates the status of the process [15, 16, 17]. When the favorable conditions for the implementation of the composting process have been achieved, it will take place in several main stages, which we can follow according to the microorganisms that prevail in a certain stage [6].