**5.2 Disinfection**

Sludge treatment technologies for preparing a valuable fertilizer must meet legislative criteria on sludge hygienization. Numerous technological approaches on SS treatment, which were conducted at ambient temperature or under mesophilic conditions, had a strong effect on biological liquid sludge stabilization and natural dewatering and drying technologies, although disinfection efficiency was unsatisfactory [48–50]. In this respect, further comprehensive research on SS treatment should be focused on a combination of different physical (especially, thermal) and chemical processes, which would convert SS into a qualitative fertilizer with safe microbiological characteristics. **Figure 4** summarizes a broad spectrum of methods for SS disinfection. Several studies have experimented with hybrid methods where two or more technologies can be integrated to increase treatment efficiency and performance [62].
