# Application of Near-Infrared Spectroscopy for Monitoring and/or Control of Composting Processes
## 5 Use of NIR Spectroscopy to Monitor and/or Control the Composing Process  
As described previously, compost quality is manifested through an assessment of how stable and mature the compost is; these two parameters must be evaluated, and the complexity of composting is measured by the large number of variables that govern it (physical-chemical analysis, microbiological issues, and activity of enzymes). To produce high-quality compost, the process must be well monitored and maintained (as presented for aerobic composting, Figure 2). Composing presents certain challenges, such as (i) determining how to generate high-quality compost at a minimal cost and (ii) how to use compost successfully in organic agriculture. To do this, it is critical to comprehend the quality and chemical components of the compost and raw materials. These qualities are not homogeneous and fluctuate too much for traditional chemical studies to be helpful. Furthermore, no precise definition of compost quality has been developed. Composing materials differ greatly and might include a diverse spectrum of chemical components, even within the same pile. These variations have an impact on the compost's quality. As a result, for quality monitoring of the compost production process, a quick on-site measuring system is necessary. The enormous number of factors that influence it (physical-chemical analysis, microbiological parameters, and enzymatic activities) measure the composting process's complexity, and different chemometric tools should be applied (Figure 4). These techniques are time-consuming, and in most situations, a range of methodologies must be investigated, making it difficult to precisely compare compost quality, hence encouraging the development of fresh approaches for appropriate composting evaluation. Spectroscopic techniques, with specific emphasis on NIR spectroscopy, are regarded as the quick on-site measuring system essential for quality control in the compost manufacturing process. Table 1 contains some experiences of how spectroscopy methods may be used to monitor the composting process. Examples describing different composting processes (in pile or in vessel) efficiently monitored and controlled based on spectroscopy methods and chemometrics techniques available in the literature over the last 20 years are presented. For specific parameters during plant field composting [71], it can be concluded that spectroscopy methods coupled with chemometric techniques can be efficiently used for the monitoring and quantification of predicting ash, carbon, species and fiber ratios, volatile solids, macronutrient size (N, P, and K), and the amounts of feasibly polluting elements (Fe, Cu, Mn, and Zn) as well as for the quantification of odor outputs. Example of NIR spectra measured during composting process are presented in Figure 5.  
[PERSON] et al. [72] used Fourier transform near-infrared (FT-NIR) spectroscopy for monitoring the olive solid waste composting process. The composting process was performed in the form of composting piles, including 70% of olive oil solid waste, 25% of goat manure, and 5% of alfalfa over 35 days. PLS models (as suggested in Figure 4), based on FT-NIR spectra, were developed for the prediction of the compost carbon to nitrogen (C/N) ratio, total nitrogen, inorganic nitrogen, total carbon, pH, and electrical conductivity. Their results show that FT-NIR spectroscopy has the potential to sense olive oil solid waste parameters nondestructively based on the high coefficients of determination for all the analyzed variables. Furthermore, [PERSON] et al. [73] also applied FT-NIR and Fourier transform mid-infrared (FT-MIR) to analyze the chemical changes taking place during torrefraction (at three different temperatures 175, 225, and 275 \({}^{\circ}\)C from 1 to 5 h) of olive pomace compost and to assess the impact of treatment on compost at various stages of maturation.