**1. Introduction**

High global population growth has led to an excessive increase in solid waste generation. According to the United Nations, at least 7000 and 10,000 million tons of solid waste are collected worldwide yearly [1]. The principal sectors responsible for this amount of solid waste are as follows: (1) construction and demolition (C&D) (34%); (2) municipal solid waste (MSW) (24%); (3) industrial (21%), and (4) commercial (11%) [1].

The MSW is the waste generated by households, mainly composed of an organic fraction, plastic, and paper. Although the organic fraction in MSW varies according to income levels countries (high-income countries ~20–40% and low-income countries ~50–70%), sociocultural patterns, and climatic factors, it could be considered that around half of the MSW would correspond to the organic fraction [2]. It means that about 840–1200 million tons of organic solid waste would be globally generated [1, 3]. It may even be argued that this amount of organic solid waste would be even

higher as it does not consider the agro-industrial sector, whose waste is composed primarily of organic matter. The producers hardly report data from this sector. However, a rough estimate by the International Solid Waste Association (ISWA) concluded that approximately 10,000–20,000 million tons of waste are generated annually by crops, farms, vineyards, dairies, and other agri-food industries [1]. In summary, it could be stated that the amount of organic solid waste generated globally is highly significant, and its leadership in the waste generation field remains a relevant issue.

Globally, in 2018, about 19% of solid wastes were recycled and/or composted, 11% incinerated, 37% disposed of in landfills (with or without gas collection), and 33% disposed of in open dumps (uncontrolled waste disposal) [3]. The high percentage of waste derived from landfilling or open dumps indicates that there are still great opportunities for improvement in the management of organic solid waste. Within the technologies available for recycling and composting, anaerobic digestion (AD) has been recognized as an effective and interesting waste management technology, since it can produce green energy when converting organic matter from waste into biogas [4–6].

Increasing solid waste generation requires cost-effective and environmentally friendly processes, such as AD. With proper control of the AD process, this technique could be adapted to different operating conditions and changes in the feedstock. It would allow the possibility of treating a greater quantity and variability of seasonal waste (e.g. MSW; fruit and vegetable waste (FVW); and juice company waste) in a single solid waste plant, using existing facilities [7]. This advantage could even improve the biogas production and the economic viability of the plants [7, 8]. Because of these reasons, the ability of the AD process to adapt to different changes could be a compelling topic. In this context, variations of parameters have been extensively studied, such as organic loading rate (OLR), hydraulic retention time (HRT), or the operational temperature in the AD process [9–17].

Despite the existing knowledge about the previously cited operational parameters, feedstock type or composition changes in the AD process have been poorly investigated and could affect the process behavior. When an AD process is carried out under fixed operating conditions, there is a steady state in which the system conditions remain constant. However, when a change or disturbance affects the system, e.g. a change in the feedstock type or composition, the existing stability conditions can be lost, resulting in a transitory state. A transitory state could be defined as the period that elapses from when the change is applied to the system until system stabilization is reached. During this period, parameters such as the biogas production and composition or the concentration of volatile fatty acids (VFAs) could change because of biodegradability, pH, organic matter content, and other feedstocks' characteristics. This could directly affect the subsequent AD performance [7].

It has also been reported that the way how changes in conditions are made can cause different results in the system's adaptation [17]. For example, an aggressive change usually results in considerable instability in the system. On the contrary, a gradual change usually entails fewer fluctuations, because the system has more time to adapt to the new conditions. For this reason, the evaluation of the change influence on the development of the transitory state is a fundamental step to realize in the AD process [9].

This chapter aims to provide an insight into the available research on transitory states during the anaerobic digestion process when the feedstock type or composition has suffered a change, to assess the adaptation of the anaerobic digestion process to the system perturbations.

*Anaerobic Digestion of Organic Solid Waste: Challenges Derived from Changes in the Feedstock DOI: http://dx.doi.org/10.5772/intechopen.107121*
