Wastewater treatment sludge composting

In recent years, the amount of sewage sludge generated by wastewater treatment plants (WWTPs) has increased due to worldwide population growth and to efficiency of biological treatment processes [1,2]. Sludge is an important source of secondary pollution to aquatic environments and a potential risk to human health; moreover, it represents one of the most important cost items in the functioning of water treatment plants [3–5]. About 60% of the operating costs of secondary wastewater treatment plants in Europe can be associated with the treatment and disposal of products [6]. For this reason, proper sludge management becomes increasingly important, at both national and international level, and it becomes necessary to find effective measures to limit the environmental impacts and to reuse sludge as a resource, within a circular economy vision [2,7]. Current methods of utilization of sewage sludge include agricultural application, landfilling, incineration, drying, and composting and/or vermicomposting. Composting is a widely used cost-effective and socially acceptable method for treating solid or semisolid biodegradable waste [8]. In agriculture sewage sludge is used for rehabilitation of degraded soils, reclamation, or adaptation of land to specific needs [9]. The above consideration comes from several studies showing that the application of sludges to agricultural land can improve soil fertility and, therefore, crop productivity [10–12]. This field of use is also possible due to its composition; in fact, it is rich in organic matter, nitrogen, phosphorus, calcium, magnesium, sulfur, and other microelements needed by plants and living native organisms in the soil. However, sewage sludge may contain a wide range of harmful toxic substances such as heavy metals, polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzo-p-dioxins and dibenzo-p-furans, polychlorinated biphenyls, di(2-ethylhexyl) phthalate, polybrominated diphenyl ethers, detergent and drug residues, pharmaceutical and personal care products (PPCPs), endogenous hormones, synthetic steroids and pathogenic organisms [13,14], which can cause harm to the environment and humans. Due to the presence of those toxic elements, stabilization of sewage sludge is necessary to avoid any environmental risk [15]. Stabilization of sewage sludge is defined as “biological, chemical or thermal treatment, long-term storage or any other appropriate process aimed at reducing its fermentability and the health hazards arising from its use” [16]. This definition is found in Council Directive 86/278/EEC, which was issued to regulate the use of sludge in agriculture, the primary objective of which is the environment, in particular the soil, and the protection of human health. European Directive 86/278/EEC was implemented in Italy by Legislative Decree 99/1992 [17]. Both the European Directive and the Italian legislative decree can be considered obsolete, this is why the European Union is moving towards amending them to reflect the new needs of the sector and to keep up with technological innovations. Currently, there are several processes for sludge stabilization, including composting, which is one of the most widely used methods for stabilizing organic matter in general, reducing the number of pathogenic microorganisms and the amount of toxic elements [18]. This is possible because during the composting process the organic compounds present in the biomass to be composted are converted into chemically recalcitrant, that is, stabilized, humic substances, while pathogens are eliminated due to the heat generated during the process thermophilic phase [19,20]. During the composting of sludges, the addition of bulking agents is needed, as they ameliorate the composting performance by providing structural support that improves aeration and regulates moisture content and C/N ratio of composting mass [21,22]. Sludge composting, however, has to be focused on limiting some secondary causes of pollution related to the process itself, such as greenhouse gas (GHG) emissions and heavy metal contamination [23]. Indeed, in the last decades, the handling of sewage sludge with traditional methods has led to the release of an enormous amount of greenhouse gases. The choice of an appropriate bulking agent is, therefore, fundamental to limit the emission of climate-altering gases, and, at the same time, to increase the microbial activity thus improving the quality of the compost [24,25]. This chapter aims (1) to give an overview of the national and international legislation on sludge management and reuse, (2) to analyze the composting process and the state of the art regarding sludge composting to understand the limitations at large-scale application, and (3) to discuss the technological innovations in the field and highlight future perspectives.