Kossihouen is the first sanitary landfill in the autonomous district of Abidjan. It is located at the intersection of the North Highway and the unpaved road linking this village to Songon between latitudes 5˚40 and 5˚30N and longitudes 4˚20 and 4˚10W (Figure 1).

The Ivorian climate is tropical. The average annual temperature is 26.6˚C. Abidjan has an annual average rainfall of 1784 mm [7]. According to the 2014 Census, Abidjan had 4,707,000 people [8]. The total landfill site has an area of 3.3 ha, where 1,250,000 tons (fresh matter) of municipal and commercial waste would be deposited [9]. Its capacity of storage is 7 years of operation. According to Waste Management Organization of Abidjan, the produced wastes would be deposited on this site, and the rest would be recycled or composted [9]. On the site, a layer of soil using ramp method as covered wastes [9]. In this study, methane emissions were estimated based on the assumption of a sanitary landfill that has been launched in 2018 and would be closed in 2025. The amount of generated waste was predicted basing on the population growth rate, the rate of waste generation per capita and its changes up to the plan horizon year (2025).

LandGEM is an automated tool for estimating emission rates for total landfill gas, methane, carbon dioxide. LandGEM can use either site-specific data to estimate emissions or default parameters if no site-specific data are available [5] [10]. LandGEM is based on a first-order decomposition rate equation for quantifying emissions from the decomposition of landfilled waste in MSW landfills. The model is presented to estimate annual emissions over a time period based on user specification [5] [11].

The LandGEM (version 3.02) emission methodology can be described mathematically using the following equation [5] :

Q C H 4 = ∑ i = 1 n ∑ j = 0 , 1 1 K L 0 ( M i 10 ) e − K t i j

where: Q C H 4 = annual methane generation in the year of the calculation (m³/year),

i = 1 year time increment,

n = (year of the calculation) − (initial year of waste acceptance),

j = 0.1 year time increment,

K = methane generation rate (/year),

L₀ = potential methane generation capacity (m³/ton),

M_i = mass of waste accepted in the i^th year (ton),

t_ij = age of the j^th section of waste mass Mi accepted in the i^th year (decimal years, e.g., 3.2 years).

According to the equation, the required inputs for the amount estimation of generated methane are the design capacity of the landfill site, the annual acceptance rate, the methane generation constant K, the methane generation potential L₀ and the years of waste acceptance.

In Abidjan, only 65% of generated wastes are collected and transported to the landfill site [6]. Thus, Equation (1) was used to estimate the total quantity of the waste that would be deposited in Kossihouen landfill (M_F) between 2019-2025 [12].

M F ( t ) = 0.65 × M T (1)

M T (tons/yr) is the total mass of waste generated per year

M T ( t ) = P ( t ) × W C × 365 (2)

P ( t ) = P 0 ( 1 + r ) t (3)

P ( t ) is the expected population according to the population growth rate (r) of 4.1%, W C is the waste generation rate, which is of 0.77 kg/capita/day [6].

The electrical energy (kWh/year) that could be obtained from the methane content of collected landfill gas is estimated as [12] :

E P ( LFTGE ) = L H V methane × 0.9 × Q C H 4 × λ × η 3.6 (4)

where LHV methane is the Lower Heating Value of methane and is given as 37.2 MJ/m³ [6] [12] , Q CH 4 = annual methane generation in the year of the calculation (m³/year), 3.6 is the conversion factor from MJ to kWh, where λ is collection efficiency (75%), η is the electrical conversion efficiency for internal combustion engine given as 33%.

Table 1 illustrates the population totals from 2018 to 2025 and the quantity of waste generated during the same period.

These results showed that the quantity of MSW increases with the growth of the population. The values of methane generation constant (K) and the methane generation potential (L₀) have substantial effect on calculation of methane emission [13]. The value of methane generation constant (K) depends on the amount of waste moisture, pH and temperature, and nutrient availability for methanogenic bacteria [14]. Concerning the value of methane generation potential , it strongly depends on the fraction of the landfill waste [15]. According to USEPA guidelines, default values of K and L₀ are 0.5/year and 170 m³/ton, respectively [5]. The determination of these parameters is based on the carbon content of the waste, biodegradable carbon, and a stoichiometric conversion factor [14]. If valid data is available on the quantity and quality of the waste, L₀ was calculated, using several methods used in different references [14]. According to composition of the waste (Table 2) from Abidjan, the values of K and L₀ were determined by Kouadio et al. [6]. Therefore, these values may be appropriate in estimating methane emission of Kossihouen landfill. In this study, default and determined values of K and L₀ were be used to estimate methane emission. Table 3 and Table 4 show the description of the provided input data to run LandGEM (version 3.02). Based on LandGEM instruction, the estimated quantity of methane from landfill should be 50%.

Table 5 shows the quantities solid waste that would be deposited in Kossihouen during 7 years of landfill open years. The deposited waste volume could be estimated to more than 1,000,000 tons during 2018-2020. This quantity could be increased during 2024-2025 to more than 1,300,000 tons. According to Table 5, the total quantity of waste disposed at landfills would be 8,800,117 tons in 2025. This clearly indicates that the growth in MSW in Abidjan would outpace the population growth in future years.

Methane emission was estimated based on the methane generation constant K and the methane generation potential L₀. Figure 2 presents the estimated of methane emission for default and determined values of K and L₀.

MW: Molecular weight for selected gases, Model parameters base on Clean Air (CAA) Regulations.

The estimated quantities of methane would be the highest at least three years after the closure of landfill (2025-2028). The mean values were 7.97E+07 and 6.18E+07 (m³/year) for default and determined values of K and L₀, respectively. However, the estimated quantities of methane for determined values of K and L₀ are greater than those obtained using default values. The estimated quantities of methane gradually decrease until they reach zero. This trend is faster for determined values of K and L₀. This could be explained by the fact that the effect of the half-life duration of organic materials such as food waste, paper, wood and textiles to the overall methane emission is related to the reaction rate (k) of the model through the equation k = t 1 / 2 − 1 ln 2 [16]. It is well known that paper and cardboard identified as an important compound, which could affect the methane emission [6]. According to waste composition (Table 2), paper and cardboard with approximately 14% contribute the large quantity of disposed materials. Half-life for paper estimated around 10 to 15 years, which could be a reason of maximum generation of methane during the years 2025 to 2028 [11]. Moreover, Food waste that ends up in landfills produces a large amount of methane [17]. Food waste contains approximately 45.42% in waste stream to this landfill (Table 2). Figure 3 shows the annually increasing of the methane emission from deposited waste in landfill. In fact, the degradation of MSW in tropical country such as Côte d’Ivoire may be higher due to higher temperature and moisture, which has positive effect on methane emission [11].

In this section, the energy potential assessment of waste to energy from the treatment of MSW in from 2018-2035 is discussed. This assessment is based on the determined values of K and L₀. The energy potential of Landfill Gas to Energy (LFGTE) technology depends largely on the methane content of the landfill gas generated, which in turn depends on the methane generation potential of the waste landfilled [12]. The electrical energy (GWh/year) that could be obtained from the methane content was given in Figure 4. The quantity of electric energy potentially would be available as from 2026 in the Kossihouen landfill would generate 209.15 GWh. In 2035, the recovered landfill methane would be sufficient to generate 54.61 GWh, providing a significant power generation opportunity. In fact, 209.15 GWh would represent the 10% of total electricity consumed in Abidjan. This may be due to the fact that methane has basically the same chemical reactions as anaerobic digestion [12].