**3.2 Anaerobic digestion**

Anaerobic digestion involves a sequence of biological processes through which biodegradable waste is digested by microorganisms in the absence of free oxygen to generate biogas [6]. The most suitable waste for this technology is organic waste that includes food and agricultural wastes like animal slurries and that the main end product of anaerobic digestion is biogas usually containing 60 and 40% of methane and carbon dioxide, respectively [24]. The composition of waste is comparable to

**Figure 4.**

*Viridor Dunbar, a potential landfill energy recovery facility [22].*

what is reported by [25] that is 55–70% for methane and 30–35% for carbon dioxide. Anaerobic digestion has relatively long digestion days ranging from 20 to 40 days [24] and because of this decomposition needs to happen by the action of an enzyme [11]. Other factors that affect the process are pH, temperature (35–38°C), loading rate, mixing rate, and toxic compounds [19]. When food waste is added to the process, the methane quantities will increase, and the process of methane production will speed up [24]. The typical amount of biogas produced usually ranges from 50 to 150 m3 / tonne of wastes and this also depends on the composition of the waste [19]. The plants emit residue gases which comprise nitrous oxides, hydrogen chloride, carbon monoxide, and the total organic carbon [25].

The biogas is used to produce electricity and/or heat with a biogas CHP gas engine. Also, the biogas can be used as a renewable natural gas or in the transportation sector as a fuel. The other product of the process is a nutrient-enriched digestate used as a soil fertiliser [19, 24]. **Figure 5** presents a typical illustration of a biogas plant.

This technology is considered to be environmentally friendly and solves the problem of disposing of the bio-degradable waste [11]. Also, anaerobic digestion is usually used to pre-treat the organic component of waste to reduce its weight, and reduce the

**Figure 5.** *Schematic of biogas plant [26].*

### *Evaluating Waste-to-Energy Technologies as a Waste Management Solution for Uganda DOI: http://dx.doi.org/10.5772/intechopen.101904*

methane and leachate emissions [27]. **Figure 6** shows a biogas plant established in Nakuru, Kenya which uses the crop residues of a farm to generate 2.2 MW of electricity used to cultivate 1740 acres of vegetables and flowers, supplies electricity to up to 6000 rural homes and sells surplus power to the Kenya National Grid [29]. **Figure 7** demonstrates small scale applications of anaerobic digestion in households in India.

#### **3.3 Incineration**

Incineration is the regulated burning of solid waste with sufficient oxygen under anaerobic conditions at high temperatures above 850°C to release heat. The process also leads to a high-temperature combustion flue gas consisting of CO2 and water and bottom ash which consists of minimal amounts of leftover carbon [6, 11].

The waste burned can either be in a raw form that is waste immediately after the first three stages of the waste hierarchy or in a pre-treated for like RDF and for each case the plant configuration changes depending on the feedstock. The energy content of raw residue typically ranges from 8 to 11 MJ/kg and the energy content of the pre-treated feed is typically between 12 and 17 MJ/kg [6]. The higher energy content in RDF is because water, recyclable (metals and glass), and inert materials (stones) are removed leaving the waste with the higher energy content [6, 19]. The other advantage of RDF over raw residue is it provides an opportunity to remove most of the hazardous material that could be harmful when burned [19].

The major importance of incineration is to get rid of problematic waste [31]. Incineration decreases the initial quantity and weight of waste by 90% and by 75%, respectively [31] and this makes it suitable for disposing of waste especially in countries

#### **Figure 6.**

*Gorge Farm Energy Park, Nakuru Kenya [28].*

**Figure 7.** *A family in Maharashtra, India cooking using biogas [30].*

that are facing disposal management problems [32]. Typically, 65–80% of the organic waste energy content retrieved as energy. This process uses the combustion heat through a boiler to generate steam. The steam is either applied in steam turbines to produce power and/or used in the heat exchanger technologies to meet heating requirements of an industry or community [6, 19, 24, 33]. A CHP plant that generates heat and/or electricity and is reported to be the most efficient way of recovering energy using a steam boiler [6]. When the incinerator produces heat only, or electricity only, or a combination of both, the efficiency of the plants ranges from 70–80%, 20–25%, and 50–60%, respectively [34]. The choice on whether to produce heat or electricity or both will depend on the needs of the country. The residue bottom ash can be discarded in a landfill or applied as construction material [11]. **Figure 8** demonstrate a CHP incineration plant.

Even though incineration is efficient, the long-term consequences of pollution become evident and suggests the need to improve the fuel compositions, reduce the moisture in the fuel, reduce the sizes of the waste fuel particles, and modify incinerator designs to reduce pollution [24]. The importance of cleaning the flue gas before letting it out in the environment by placing pollution control devices (electrostatic precipitators), or placing an appropriate furnace configuration, or by controlling the combustion process [11]. This flue gas can also be retrieved in the form of energy to generate electricity [24].

Incineration is considered very expensive in terms of capital, and O&M. The process is reported to be more expensive than controlled landfilling and that for the project to economically feasible the energy recovered must be sold. Also, technology is not efficient when the waste composition has low calorific values [33]. **Figure 9** shows a CHP incineration plant in Sweden that handles 700,000 tpa, produces 2174 GWh of heat used in district heating, and 197 GWh of electricity, yearly [36].
