(Source: EUROSTAT, 2022)
Figure 2 . Disposal of sewage sludge from urban wastewater
treatment
Sludge Characteristics
The origin and quantity of flushing water (public toilet, private
toilet), its collection type (on-site, off-site), and subsequent
treatment degree, such as digesting, all influence the properties of
sludge. Raw and processed sewage sludge have different characteristics,
as seen in Table 3. (Kacprzak et al., 2017). Fresh, untreated sludge has
many pathogens, contains a large amount of water, has a high biochemical
oxygen demand (BOD), and is typically rotten and odorous. Once
stabilized, the organic carbon in the sludge can be used as a soil
conditioner, improving soil structure for plant roots, or converted into
energy by bio-digestion or cremation. Because sewage may obtain
dangerous contaminants (e.g., heavy metals, pharmaceuticals) from
industry and other activities that might build in its sludge, sludge
collected from on-site systems is usually regarded safer for reuse
unless homeowners utilize their toilets for general waste disposal.
Table 3 . Comparison between raw and digested sludge