(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