Figure 3. Generation of sewage sludge in the sewage treatment
plant
Each treatment plant generates sludge with different physicochemical
properties whose common features include: high water content (from 99%
in the case of raw sewage sludge, 55%–80% for dewatered sludge and
below 10% for thermally dried sludge), high organic components (from
75% d.m. in raw sludge to 45%–55% d.m. in stabilized sludge), high
content of nitrogen compounds (2%–7% d.m.) and lower content of
phosphorus and potassium (Yu et al., 2019).
Heavy metals, polychlorinated biphenyls (PCBs), polycyclic aromatic
hydrocarbons (PAHs), adsorbable organic halides (AOXs), polychlorinated
dibenzodioxins (PCDDs), and polychlorinated dibenzofurans are examples
of potentially harmful pollutants present in the treated sewage
(Saravanan et al., 2021). Due to the existence of many harmful
organisms, sewage sludge that has not been treated in the hygiene
procedures might provide an epidemiological risk. Ascaris
lumbricoides, Trichuris sp. , and Toxocara sp. are the
most common parasite eggs recovered from sewage sludge (Kacprzak et al.,
2015; Reimers et al., 1986). Escherichia coli, Salmonella spp.,
Shigella spp., Bacillus anthracis, Pseudomonas aeruginosa, Listeria
monocytogenes, Vibrio cholerae, Proteus vulgaris, Clostridium
perfringens, Mycobacterium tuberculosis, and Streptococcus
pyogenes are among the microorganisms found in sewage. Viruses,
particularly polyviruses that cause poliomyelitis, rotaviruses, and HIV
and HCV viruses, are among the many harmful microbes. Fungi, such as
Penicillium, Verticillium, Mortierelta, Fusarium, Aspergillus, Mucor,
Geotrichum, and Trichoderma, are the most often isolated microbes from
sewage sludge (Pepper et al., 2006; Amin, 1988). Different
physicochemical components and
parameters of a municipal sewage
sludge is shown in Table 4.
Table 4. Characteristics of municipal sewage sludge