## 1 Introduction
### Wastewater Treatment Plants and the Need for Cost-Effective Preventive Maintenance Strategies  
Wastewater treatment is essential to protect human health and ensure the environmental sustainability of the planet [8]. Besides the relevance of stabilization and treatment of effluents for human consumption, wastewater treatment-related processes represent a fundamental task in a wide variety of industries such as livestock, chemical, and mining [12]. In response to the industrial and government requirements and protocols, wastewater treatment can be carried out through a wide variety of physical, chemical, and biological processes [13]. In this regard, biological processes, including anaerobic digestion processes, have demonstrated economic advantages over other treatment processes in terms of invested capital and related operational costs [12]. Biosolids that are generated from the primary and secondary wastewater treatment processes are denominated sludge. Generally, municipal sludge contains a wide variety of organic and inorganic materials, including biomass, oils, heavy metals, synthetic materials, and pathogens; for this reason, these solids must be necessarily pretreated before their disposal [13]. Within the wastewater treatment process, the treatment and use of sewage sludge have become an international waste management problem. In this sense, the digestion process in sludge treatment has become the first step in the reuse chain, representing up to 50% of the operational costs in sewage treatment plants [13; 14]. Additionally, the sewage treatment process establishes a valuable source of raw materials, especially for obtaining nutrients, energy, and water for human consumption [9]. Within the municipal sludge treatment systems, anaerobic digestion processes have demonstrated wide application and positive externalities as a sustainable technology [15; 16]. Anaerobic digestion consists of a series of biological processes in which several microorganisms decompose biodegradable materials in the absence of oxygen, differing in this regard from aerobic treatment systems such as, e.g., activated sludge processes [13; 17]. During the anaerobic digestion process, microorganisms convert the organic material into mainly two products: digestate and biogas. The latter is mainly composed of carbon dioxide and methane, which can be used to generate electricity and heat, starting from the decomposition of biodegradable organic materials [18]. By the implementation of digestion processes, the formation of odors is eliminated and pathogens are destroyed, also generating a stabilization of the organic product [19]. For these reasons, anaerobic digestion processes have received increasing attention in several countries and organizations during the past decades [17]. Anaerobic digesters or anaerobic reactors can be used both for sewage pretreatment and sludge stabilization. Upflow anaerobic sludge reactors (UASB) closely resemble those for sewage treatment, except that: (a) Recirculation of the sludge and its mechanical agitation are kept to a minimum or omitted; (b) the reactor is equipped in the upper part with a system for separation of solid-gas phases [20]. This type of digester has been successfully implemented in recent years for the treatment of municipal wastewater, achieving high efficiency in the removal of suspended solids and biodegradable organic matter in countries with warmer climates [21]. In addition, sludge from high-strength wastewaters, for example, are preferably treated by anaerobic processes, thus providing potential for energy generation while producing low-surplus sludge [12]. Although anaerobic digestion processes, in general, have demonstrated a compact design and positive performance, they have evidenced certain difficulties, e.g., in providing complete stabilization in the case of high resistance water treatment.