**3. Energy recovery technologies**

In 2016, the total investment in biomass and WTE technologies was 6.8 billion USD which was a decline from 19.9 billion USD in 2011, 14.9 billion in 2012, 12.4 billion in 2013, 10.8 billion in 2014 but an increase from 6.7 billion in 2015 [14]. Despite the increase, it is evident that interest in WTE is not growing across the globe and yet it is successful in European countries. Energy from waste can either be heat, power, or a combination of heat and power and/or secondary energy carriers of gas, liquid or solid. The choice is usually dependent on the energy requirements of the country or region [6].

#### **3.1 Landfill with gas recovery**

Landfills are semi-natural terrestrial ecosystems remodelled on lands that were formerly used for disposing of waste. Landfills exist in various regions and are commonly defined by their age, the composition of waste, design, and ecological operation. They are usually disposal for MSW and sometimes for hazardous solid wastes when they are secure [15]. The landfills are designed to make sure the waste is separate from the surrounding environment [16]. Also, two design structures are feasible for a landfill that is landfilling (where waste in packed in an unwanted hole) or land raising (where waste is directly dumped on the ground) [17]. The average landfill occupies 600 acres [18].

The by-products of landfills are landfill leachate produced when rainwater penetrates and channels through the decaying waste, and landfill gas produced through bacterial degradation under anaerobic conditions [15]. These products can be hazardous in the following ways. Firstly, when acids from degrading waste mix with other components waste, it could cause the leachate to become toxic. Secondly, landfill gas is a source of GHG emissions comprising methane and it is highly flammable its leakage poses a risk of explosions to the surrounding environment [17]. Since the by-products can escape or diffuse through cracks in the deposited material, landfills are designed to minimise their movement to protect the environment [15, 19]. Liners and a leachate collection system are installed to prevent leachate from moving to surrounding soil and water while a gas collection system or a landfill cap is installed to hinder the gas from escaping to the air. The system consists of vertical or horizontal wells used to access the gas which can be collected for 7–10 years. Also, its average efficiency is reported to be 70–85% [19]. **Figure 2** shows a typical landfill gas system.

The landfill gas produced contains 45–55% methane and is collected through a system of gas pipes and through combustion it produces electric power by running a gas engine and/or turbine. Also, the gas can be used for cooking in nearby communities, or boiler fuel for district heating and industrial purposes and this is demonstrated in **Figure 3** [6, 19]. The energy potential of landfills across regions ranges from 5 to 40 L/ kg of waste depending on the organic composition and has a CV of about 4500 kcal/ m3 . Also, it is key that the landfill gas is purified to remove any hazardous chemicals [16, 19]. **Figure 4** shows an example of a landfill recovery site in the UK managed by Viridor. Viridor operates 32 landfill sites in the UK which generate a total of 86 MW of power to supply 50,000 homes with power all year [23].

When landfills reach the maximum capacity, they are closed for replenishment through appropriate engineering designs and the older type is normally deserted [15]. Many disposal sites are poorly operated and stay as open dumps which pose a risk to the environment both in the short- and long-term [13]. Landfills must be closely monitored by the respective municipalities to prevent leakage of the by-products [15].

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

**Figure 2.** *Schematic of landfill treatment set-up [20].*

**Figure 3.** *Set-up showing the use of gas recovered from a landfill [21].*
