**2. Treatment and risk reduction**

from different animals and/or humans, with the addition of plant materials (organic solid wastes), or in special situations waste materials 1 from food or plant processing industries", the origin of the different fractions and their amounts partly defines the risk. Usually the risk is outbalanced by a wide range of benefits that the use of organic fertilizer exerts in agricul‐ ture as nutritional fertilizers and for soil conditioning. It has been further implied as more environmental friendly than the inorganic fertilizers [1] and its effect more tender on biotic components of the ecosystem without much shift in the ecological balance [2]. This is partly reflected by organisms like earthworms which may be negatively affected by inorganic fertilizers but promoted by the use of organic fertilizers and also incorporated as decompos‐

As this chapter deals with the public health aspects and risks involved, we define the organic

**•** Human faecal materials (also sludge from domestic treatment plants and from on-site

**•** Animal manure (some risk differences depending on the species of animals/birds).

**•** Animal urine (often collected/spread separately, but impacted by the animal faeces).

**•** Other types of organic solid wastes (plant materials, domestic, industrial from organic food/

**•** Regrowth of specific bacterial pathogens or opportunistic ones (occurs when the material that, for example should be/are composted, are not well stabilized or broken down. During these circumstances, for example *Escherichia coli, Salmonella* sp., *Listeria* sp. and spore formers

**•** When the collected/stored/kept organic fractions or mixture thereof (see above) function as a breeding site for flies and mosquitoes that serve as vectors of parasitic diseases.

**•** Development of spore-forming thermophilic fungi and Actinomycetes in composting processes, where the spores can cause diseases in both immune-competent and immunecompromised individuals upon inhalation. An example of such an organism is *Aspergillus*

Based on source, the risk will vary to a great extent, depending on the health of the animals/ humans that primarily defines the microbial concentration and partly occurrence of antibiotics and chemical components in the organic wastes (from domestic or animal sludge fractions) that may be conveyed to the agricultural sites and crops fertilized. Additional components may apply if organic industrial wastes are utilized. An indirect organic fertilization may occur through irrigation using wastewater effluent, where the nutrient load serves as an advantage. This is widely applied in developing countries [5]. However, this may result in additional inputs of antibiotics, toxic organic and inorganic compounds and pathogens. All these concepts

ers in aerobic composting processes [3, 4].

344 Organic Fertilizers - From Basic Concepts to Applied Outcomes

sanitation, e.g. pit latrine emptying).

**•** Human urine (if separated).

fodder processing industries).

*fumigatus*.

will regrow in the material if present).

materials utilized by its sources and thus relate to the following:

Additionally, the risk may relate to some storage-specific factors like

The concept of organic fertilization is ever worthwhile, with combined considerations of the public health intricacies that cannot be overemphasized [6]. Several alternative treatment methods can be employed to stabilize the organic fertilizers before use and at the same time reduce the concentrations of potential pathogens, thereby the risks. The efficiency of these will vary based on time, load and different external factors.


**Table 1.** Efficiency of some pathogens' reduction techniques for low-cost sludge treatment strategies.

Low-cost options for pathogen reduction and nutrient recovery from faecal sludge are of special importance to low-income countries. They include settling ponds, planted dewatering drying beds (constructed wetlands), unplanted drying/dewatering beds (for pretreatment), composting (window, thermophilic), pH elevation > 9, anaerobic (mesophilic) and simple storage. They have varying pathogen reduction efficiencies on bacteria, parasitic protozoa and viruses. **Table 1** summarizes the efficiencies of these pathogen reduction techniques with *E. coli* as an example for the bacterial group, helminth's eggs for parasites and some viral examples as stated. The figures serve as examples. Variations can be large based on prevailing local conditions.

Other methods most commonly used in developed countries to treat the sludge include incineration and pasteurization. The former one ensures a total destruction of all pathogenic organisms while the efficiency of the later one depends on time and applied temperature (normally 70°C for at least 1 h). Irradiation with *β*- or γ-rays is an approved method in the USA, and it reduces the pathogenic content to a high extend but is not widely used.

## **2.1. Organic waste stabilization**

Organic wastes can be used as soil amendments or organic fertilizers after an effective stabilization and disinfection. Effective stabilization and disinfection of sewage sludge prior to land application are important not only to protect human health. Currently, some of the most commonly used waste stabilization methods are composting (solid state), aerobic digestion (liquid state), anaerobic digestion, lime stabilization [25, 26] and sludge drying. The aerobic and anaerobic methods of waste stabilization are among the most prominent [27]. Furthermore, there have been growing concerns about the survival of pathogenic microor‐ ganisms in sewage treatment processes, resulting in the release of antibiotic resistant microbial species to the environment [28, 29]. These are further considered below.

#### **2.2. Composting**

Composting is defined as the biological conversion of organic wastes, under controlled conditions, into a hygienic, humus-rich, relatively biostable product that improves land and fertilizes plants [30]. It has the combined effect of pathogen reduction while at the same time stabilizes and converts the organic wastes into product that can be easily handled [31, 32]. The type and concentration of pathogens present in sewage sludge is largely determined by a number of factors including population's state of health, presence of hospitals, abattoirs and factories processing meat [33]. Composting is one of the essential decontamination processes to reduce the load of pathogens in animal wastes. The composting efficiency to ensure inactivation of pathogens depends on allotted time and temperature. Inefficient composting leaves loads of pathogenic bacteria which may be passed on to the end consumers.

Metals such as zinc, copper, cadmium, lead, arsenic, chromium, mercury, vanadium and nickel are usually of great concern [34] when sludge from industrial effluent are used as feed stock for composting both from a health perspective and in the degradation of the productivity of land. Industrial sludge may contain elevated heavy metal concentration which makes them unsafe for garden use. Despite the fact that copper and zinc are important micronutrients, the possibility of bioaccumulation to phytotoxic or deleterious level for human consumption still makes them a concern.

Low-cost options for pathogen reduction and nutrient recovery from faecal sludge are of special importance to low-income countries. They include settling ponds, planted dewatering drying beds (constructed wetlands), unplanted drying/dewatering beds (for pretreatment), composting (window, thermophilic), pH elevation > 9, anaerobic (mesophilic) and simple storage. They have varying pathogen reduction efficiencies on bacteria, parasitic protozoa and viruses. **Table 1** summarizes the efficiencies of these pathogen reduction techniques with *E. coli* as an example for the bacterial group, helminth's eggs for parasites and some viral examples as stated. The figures serve as examples. Variations can be large based on prevailing

Other methods most commonly used in developed countries to treat the sludge include incineration and pasteurization. The former one ensures a total destruction of all pathogenic organisms while the efficiency of the later one depends on time and applied temperature (normally 70°C for at least 1 h). Irradiation with *β*- or γ-rays is an approved method in the

Organic wastes can be used as soil amendments or organic fertilizers after an effective stabilization and disinfection. Effective stabilization and disinfection of sewage sludge prior to land application are important not only to protect human health. Currently, some of the most commonly used waste stabilization methods are composting (solid state), aerobic digestion (liquid state), anaerobic digestion, lime stabilization [25, 26] and sludge drying. The aerobic and anaerobic methods of waste stabilization are among the most prominent [27]. Furthermore, there have been growing concerns about the survival of pathogenic microor‐ ganisms in sewage treatment processes, resulting in the release of antibiotic resistant microbial

Composting is defined as the biological conversion of organic wastes, under controlled conditions, into a hygienic, humus-rich, relatively biostable product that improves land and fertilizes plants [30]. It has the combined effect of pathogen reduction while at the same time stabilizes and converts the organic wastes into product that can be easily handled [31, 32]. The type and concentration of pathogens present in sewage sludge is largely determined by a number of factors including population's state of health, presence of hospitals, abattoirs and factories processing meat [33]. Composting is one of the essential decontamination processes to reduce the load of pathogens in animal wastes. The composting efficiency to ensure inactivation of pathogens depends on allotted time and temperature. Inefficient composting

leaves loads of pathogenic bacteria which may be passed on to the end consumers.

Metals such as zinc, copper, cadmium, lead, arsenic, chromium, mercury, vanadium and nickel are usually of great concern [34] when sludge from industrial effluent are used as feed stock for composting both from a health perspective and in the degradation of the productivity of land. Industrial sludge may contain elevated heavy metal concentration which makes them

USA, and it reduces the pathogenic content to a high extend but is not widely used.

species to the environment [28, 29]. These are further considered below.

local conditions.

**2.2. Composting**

**2.1. Organic waste stabilization**

346 Organic Fertilizers - From Basic Concepts to Applied Outcomes

Zoonoses are among the public health concerns associated with improperly sanitized organic fertilizer. Zoonotic diseases and emerging zoonoses that could be associated with organic fertilizer includes salmonellosis, entrohaemorrhagic *E. coli* (EHEC), anthrax and Newcastle diseases just to mention a few [35]. *Thermoactinomyces vulgaris* is another organism of impor‐ tance. It produces heat-resistant endospores that can survive high temperature during composting. This organism is the causative agent of "farmer's lung" which is an allergic disease of the respiratory system of agricultural workers. The pathogens present in soil amendments are directly related to the organic waste source. The reduction or removal of pathogens in a compost will depend on the composting temperature and the process used [36]. This implies that improperly carried out composting leaves the organic matter poorly sanitized with the compost becoming a source of recontamination with pathogenic or parasitic organisms [37]. *E. coli, Salmonella* sp. and a few others possess advantage for regrowth in compost [38, 39]. Also, due to rich nutrient composition, contaminating *E. coli* grows very rapidly in presanitized organic fertilizers [40–43] that is not properly composted or stabilized. *Salmonella* spp. equally grow in composted sewage sludge if the carbon/nitrogen ratio is >15 and the manure content 0.2 index.


**Table 2.** Temperature-time relationship required for killing specific pathogens [35, 36, 49].

There is therefore need to ensure that the mature compost does not contain plant and human pathogens. In composting, the thermophilic temperature is the effective determinant of destroying the pathogen and the efficiency further related to the exposure time. The required time at a given temperature for efficient pathogen inactivation, according to USEPA [44] can be estimated using a time-temperature formula:

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


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

**Table 3.** Standards for maximum concentrations of pathogens in biosolids and composts used as organic fertilizers [49, 52, 53].

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