**2.3. Aerobic digestion**

This occurs in engineered ecosystems where biomass consisting of a mixed microbial com‐ munity and other solids are constantly maintained in a suspension in an aerobic basin supported by mixing [50]. This is usually used in stabilizing sewage and wastewater, produc‐ ing high-quality treated effluent through the metabolic reactions of the microbial community [51]. The sanitation efficiency of this system depends largely on time, temperature and loading rates [28, 50]. This process still yields poorly stabilized organic matter with a fluid product, having little or no volume reduction and pathogen reduction efficiency is usually low [28].

Moreover, using them as organic fertilizers in an inefficiently sanitized stage can further result in direct microbial contamination of surface water or via runoff from lands amended with such organic waste [28] in addition to their direct exposure effects and effects through crops. Most aerobic sewage sludge treatment plants operate at mesophilic temperatures (30–35°C). Within this temperature range, the stabilization processes are inefficient in the removal of viruses, bacteria and Parasite's eggs [28].

### **2.4. Anaerobic digestion**

0.1400 D 131700000 / 10 *<sup>t</sup>* =

<1/25 g Nil 0/50 g <103

MPN/ g

**Organisms US New Zealand UK New South Wales EU Class A Class B Class A Class A** *Escherichia coli* N/A <100 MPN/g 1000 CFU/g N/A 0/50 g

Faecal coliforms <1000 MPN/g <2,000,000 MPN/g N/A <1000 MPN/g

Enteric viruses <1 PFU/4 g <1 PFU/4 g <1 PFU/4 g Helminth ova <1/4 g 1/4 g <1/4 g

been suggested as an indicator of properly sterilized compost [35].

MPCN, most probable cytophatic number; MPN, most probable number; PFU, plaque-forming unit.

**Table 3.** Standards for maximum concentrations of pathogens in biosolids and composts used as organic fertilizers [49,

In a properly ventilated composting pile, the temperature usually reaches between 55 and 68°C. This temperature level can last for a few days to months depending on the size of the system and the composition of the ingredients [45–47] and is the determinant for the sanitiza‐ tion effectiveness. The average time required for killing specific pathogen is exemplified below (**Table 2**). *Salmonella* spp. and *E. coli* have been known as pathogen indicator bacteria in organic fertilizer, supplemented with soil-transmitted helminths [48] and enteric viruses when a broader spectrum of organisms needs to be assessed. Several national and international standards/guidelines have been established to ensure public health safety when using these organic fertilizers (**Table 3**). Due to high heat resistance of some bacteriophages, they have

This occurs in engineered ecosystems where biomass consisting of a mixed microbial com‐ munity and other solids are constantly maintained in a suspension in an aerobic basin supported by mixing [50]. This is usually used in stabilizing sewage and wastewater, produc‐ ing high-quality treated effluent through the metabolic reactions of the microbial community [51]. The sanitation efficiency of this system depends largely on time, temperature and loading rates [28, 50]. This process still yields poorly stabilized organic matter with a fluid product, having little or no volume reduction and pathogen reduction efficiency is usually low [28].

Moreover, using them as organic fertilizers in an inefficiently sanitized stage can further result in direct microbial contamination of surface water or via runoff from lands amended with such organic waste [28] in addition to their direct exposure effects and effects through crops. Most aerobic sewage sludge treatment plants operate at mesophilic temperatures (30–35°C). Within

where D is time in days and t is temperature (°C).

348 Organic Fertilizers - From Basic Concepts to Applied Outcomes

*Salmonella* spp. <3 MPN/4 g

**2.3. Aerobic digestion**

52, 53].

total solids

Anaerobic digestion involves the breakdown of complex organic material into simple mono‐ merics or fraction and production of biogas (bioenergy) in closed system through the activity of anaerobic microorganisms [54]. Anaerobic digestion can be carried out either at mesophilic (30–38°C) or at thermophilic (50–55°C) temperatures. Compared to composting, there is lesser heat generation during anaerobic decomposition, which reduces the sanitizing effect of the process on organic waste [37]. Digesting organics at high temperatures reduces the time required for bacterial inactivation, which eventually results in faster bacterial kill during thermophilic digestion compared to mesophilic [55]. Bacterial spores including *Bacillus cereus* and *Clostridium perfringens* are normally resistant to temperature inactivation at both mesophilic and thermophilic ranges [55–57]. Chauret et al. [58] also noted the resistance of *Cryptosporidium* sp. oocysts and *Giardia* sp. cysts to anaerobic sludge digestion. This finding is of importance since *Cryptosporidium* sp. oocysts can persist in soil amended with sludge for at least 30 days [59].

#### **2.5. Lime (alkaline) stabilization**

Lime stabilization is a preferred alternative compared to anaerobic and aerobic stabilization processes due to its cost efficiency and enhanced sanitizing effect [25, 60]. It effectively reduces the concentration of pathogens in sludge (**Table 4**), heavy metal availability and enhances its agricultural uses [25]. Free calcium ions resulting from the lime solution form complexes with odorous sulphur species and organic mercaptans; moreover, the high pH precipitates metals from the sludge thereby reducing their solubility and availability. Alkaline stabilization involves the addition of lime slurry in the form of Ca(OH)2 or CaO to the liquid sludge in order to raise its pH to about 12 or higher [60]. Apart from the high pH, the addition of quicklime to the liquid sludge can result in thermophilic temperature (up till 70°C) which inactivates the viruses, bacteria and other microorganisms [61,62]. In a study by Farzadkia and Bazrafshan [25], addition of lime slurry to sewage sludge resulted in a reduction of faecal coliforms with more than 99.99% in stabilized sludge. Arthurson [26] noted that there is a need for further investigation on the potential of alkaline stabilization methods since this process is an effective sewage sludge sanitization method but some contradictory results exist.



**Table 4.** Pathogen-reduction performance of the different treatments of sludge [28, 63, 64].
