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