## 1 Introduction  
Population growth and increased domestic material consumption have led to a global rise in waste production. As a result, waste management has gained social and political importance, and policies aimed at promoting recycling processes and discouraging direct landfill disposal have been implemented in recent years [1, 2]. Large amounts of sewage sludge are generated in wastewater treatment plants (WWTPs), and the amounts produced are expected to increase even further in the near future [1]. Sewage sludge is a challenging type of waste to treat/manage because it contains several types of pollutants and also pathogenic microorganisms. Unfortunately, less than 25% of the material is effectively composed or recycled, while 50% of sewage sludge is simply dumped on agricultural or forest soils ([[http://ec.europa.eu/eurostat](http://ec.europa.eu/eurostat)]([http://ec.europa.eu/eurostat](http://ec.europa.eu/eurostat)), accessed on 18 July 2024) [1, 2]. An option to circumvent the high load of contaminants found in sewage sludge from WWTPs located in big cities and industrial areas is to process sewage sludge from small WWTPs belonging to medium-small cities that usually have significantly lower levels of heavy metals and other contaminants [3, 4]. Vermicomposting offers an eco-friendly solution for managing sewage sludge. This process involves the combined action of earthworms and microorganisms to modify and expedite the decomposition process. It consists of an active phase and a maturation phase. The active phase consists of the transit of the substrate through the earthworm gut (referredto as gut-associated processes or GAPs), during which microbial communities are greatly modified and excreted as earthworm casts [2; 5]. The maturation phase consists of the ageing of casts (cast-associated processes, CAPs), during which microbial succession is the main process involved [2; 5]. Some research has shown that vermicomposting can effectively reduce the presence of human bacterial pathogens (HBPs) [6; 7; 8; 9], environmental pollutants, such as antibiotic resistance genes, pharmaceuticals and personal care products, and also the bioavailability of heavy metals of sewage sludge [10; 11; 12; 13]. Furthermore, several studies suggest that at appropriate doses, vermicompost can be used as a soil conditioner and fertilizer due to its physical, chemical and biological characteristics [14; 15; 16; 17]. One major gap in knowledge about vermicomposting is understanding the composition of microbial communities and their changes throughout the process [2; 10; 18]. As mentioned previously, this change in composition happens in two phases: first, earthworm gut digestion, followed by the maturation of earthworm cast (GAPs and CAPs). Most studies focusing on changes in bacterial communities during vermicomposting have shown that the bacterial communities of vermicompost differ significantly at the phylum level from those of sewage sludge [2; 5; 14; 15; 16; 17]. Thus, bacterial communities of sewage sludge are commonly dominated by Campylobacterota and Proteobacteria [2] or Actinobacteriota and Bacteroidetes [19]. Once earthworms have digested the sludge (i.e., after GAPs), the main bacterial phyla change to Bacteroidota and Verrucomicrobiota [2]. After CAPs, the main bacterial phyla of vermicompost are Actinobacteriota, Proteobacteria, Bacteroidetes, Chlorofexi, Firmicutes and Verrucomicrobiota [2; 10; 19; 20; 21; 22]. Fungal communities have been far less studied in comparison to bacteria during vermicomposting [2; 19; 23; 24]. Vermicomposting significantly altered the fungal communities of sewage sludge, typically dominated by the phyla Basidiomycota [2], Ascomycota [19] or a combination of both [23]. GAPs promoted the rise of Ascomycota, Basidiomycota and Mortierellomycota [2], while CAPs enhanced Mortierellomycota and Basidiomycota [2], Basidiomycota and Zygomycota [19], Rozellomycota and Ascomycota [23] or Mucoromycota, Ascomycota, and Cryptomycota [24].