3.1. The Map Design of Suitable Area for the Biogas Plant
We analyze the eight criteria selected under environmental, social, and economic factors to determine the potentially suitable sites. Due to public concerns about odor and health problems [
28], a significant distance between settlements was respected. A minimum distance of 100 m from low land, water wells, and water sources was used, based on the guidelines for biogas plant installation stipulated by the WHO.
The hydrographic network locations less than 150 m from a water source should be removed. The distance from rivers or water bodies should be respected to avoid contamination by the leachate generated by the digester. The points within 70 m of a major, national, or municipal road should be removed. Additionally, for the minimum area, this biogas plant must be installed on land at least 1 km in size. Concerning adequate shape points should be removed (1 ≤ area [km] ≤ 1.5 and compactness < 0.45) or (1.5 < area [km] < 2.5 and compactness < 0.25) (
Figure 3).
A 5–10% slope was considered a gentle slope and given the highest score for suitable site selection (
Figure 4).
The urban development map was used to select biogas plant locations based on the urban growth. Bangui’s land use/land cover (LULC) was created to assess the existing structural plan and provide a vision for the city up to 2030. This projected urban expansion plan was used to conduct a GIS analysis. The process allowed the removal of alternatives located less than 200 m from urban or residential areas (
Figure 5).
Based on the criteria, 593 points were obtained in different locations, with 103 points removed because their area was less than 1 km. Therefore, 490 points met the minimum requirement. Twenty-eight points that did not meet the adequate shape constraints were also removed. Thus, 462 remaining points were appropriate (
Figure 6).
3.3. Determination of Vector Grids as Alternatives
The vector grid was used in the second phase of the site subdivision to achieve a more detailed classification of the 37 points that were not conclusively classified and had areas ranging from 1 km to 3 km. The ET GeoWizards program created quadrangular grids with 3 hectares in this investigation. However, 505 grids were created by overlapping the quadrangular grid with 37 unclassified sites. Therefore, each grid was required to have a minimum 1 km area. This process removed 376 grids and obtained 129 grids, which were reduced to 106 grids by applying the constraint of compactness ≥0.40. Therefore, 23 grids were removed because of the suitability and shape condition for the biogas plant site.
Considering both cases (M1 and M2), the ELECTRE TRI was utilized, with all 106 vector grids considered options. Thirty-three points were considered as not suitable (category 1); twelve points were considered moderately suitable (category 2); four points were considered as most suitable (category 3); thirty-three not-suitable points alternated with the moderately suitable; six points alternated between not suitable and most suitable; and eighteen moderately suitable points alternated with the most suitable.
Based on the minimum size and adequate shape requirement and the automatically created grids by GeoWizards, some of the 37 potential locations did not have grids. As a result, 15 points were considered moderately suitable and most suitable, and they were included in category 2, based on the M2 scenario, and category 3, based on the M1 scenario, respectively, with 8 grids and 7 suitable points. There were five sites that were the most appropriate and ranged from 1 km to 3 km.
These five sites number from 1 to 5 (
Figure 7) are the most suitable locations for the biogas plant construction. They are located in three districts of Bangui, with 55% of the sites in the 8th district, 18% in the 4th district, and 12% in the 7th district, with different geographical coordinates and areas (
Table 6).
3.4. Biogas Plant Site Optimal Selection-Based Factors
The site selection was based on the multicriteria decision and the environmental, social, safety, and economic factors mentioned above for implementing the biogas plant. Geographical knowledge of the study area and the contribution of experts in geology, civil engineering, and agronomy helped to select the most favorable sites.
Site number three (4th district), with a surface area of 2.63325 km2, is the closest to the entire biowaste collection center. Still, it is located close to the urban area and a 590 m high slope known as Bas-Oubangui, thus causing the spread of odor, noise, and olfactory pollution. Its location close to the metropolitan area will cause long-term problems in extending the biogas plant or Bangui, considering the city’s urban growth.
Site 1 (SAKAI I) was selected for establishing the biogas plant. It has the most suitable and more extensive area (3.58623 km
2) among the five most appropriate sites (
Table 7) and could manage the biogas plant’s operation space and equipment. Moreover, its large area can be used in the case of the extension of the plant or the storage of feedstock or fertilizers from the digesters. It is located far from the urban area, practically situated in the border limit of the urban areas of Bangui, and thus avoids any impact of the plant, such as odor and noise, on the population.
Site 1 (SAKAI I) is in the same area as sites 2 (SAKAI II) and 4 (SAKAI IV). Still, it is the closest to the road and the agricultural zone, thereby facilitating crop residue (biowaste) collection for the biogas plant. The site 1 is represented by the green square and the biowaste centers are represented by red points and the distance between them is represented by a line (
Figure 8).
3.5. Biogas Production and Valorization of This Site
3.5.1. Biogas Plant Operation
The biogas plant site is located near a slaughterhouse and agricultural area with a large capacity of feedstock. It can be built on approximately 9000 m
2 to 1500 m
2 with a digester capacity of 1600 m
3 to 1800 m
3 of biogas/day. With a current installed capacity of 657 kW, the plant can inject between 6500 and 6600 kWh daily into the electricity network, with possible extensions. A total of 2,303,100.23 kW of electricity can be produced and sold annually (
Table 8).
The digestate is subdivided into three products: liquid fertilizer, solid biofertilizer, and organic soil amendment. The operating income is generated by the sale of electricity and digestate in the form of biofertilizers. The revenue generated by the electricity production could be doubled, and a high demand for fertilizers could be observed because of the country’s scarcity and the cost of chemical fertilizers.
The electricity and emissions saved by biogas cogeneration were evaluated for producing electricity and heat using natural gas [
27]. We consider electricity production with fossil resources using a boiler with 50% efficiency to calculate the final energy saved [
26]. The electricity production of
8.3 TJ.
3.5.2. Environmental Benefits
The environmental benefits of the study are mainly related to the recovery of biowaste from households, toilets, livestock, hospitals, hotels, supermarkets, factories, and slaughterhouses in Bangui to produce electricity and biofertilizer. This process could help to solve waste management and reduce GHG emissions related to waste proliferation by reducing the use of polluting fossil fuels and enabling the development of renewable energies in the country or throughout the region.
We estimate the avoided emissions if producing 1 GJ of electricity from natural gas emits 57 kg of CO2. With the cogeneration of biogas, the emission of 946,200 kg of CO2 is avoided per year if we transform all the biowaste produced by the population into energy and use it for the electricity supply in Bangui. In that case, the CO2 emissions caused by electricity production using fossil resources could be reduced, reaching 0% emissions in 2030.
3.6. Discussion
The study model combines spatial and nonspatial data using the GIS-based method and thus provides extensive understanding and a comprehensive perspective of the biogas plant implementation in urban and rural areas while respecting the environmental, social, and economic factors. This study presents methods that innovate the design phase as the iterative application of the ELECTRE TRI to accomplish the MCDA by defining the alternatives evaluated as contrary to the MC-SDSS process that is developed based on the usual design of the spatial multicriteria decision analysis.
In previous studies, Nas, B. et al. [
29] used MCDA–GIS methods in land use assessment. The MCDA method has been associated with GIS to solve a variety of problems, including ecology [
30], unfavorable location [
31], energy such as solar farm location [
32], biogas site location [
33], and hybrid renewable energy systems [
34], but all were primarily based on the general process for MC-SDSS [
15]. In the previous study by Silva, S., et al. [
17], the advanced MCDA–GIS and the ELECTRE TRI method used for the biogas plant construction was limited to the resolution of suitable sites; otherwise, it was not based on the estimation of biowaste and the determination of the distance between the biowaste centers and the main biogas production sites.
In this study, we estimated the quantity of biowastes, such as sewage sludge, food waste, livestock, and crop residues produced by the city, in order to ensure the large-scale production and long-term operation of the biogas plant. We geolocated these biowaste sources using the ArcGIS Distance toolset-enabled Euclidean (straight-line) distance to calculate the distance between the suitable sites and the biowaste location. Therefore, this method allowed the selection of a nearby area for the biogas plant construction, considering the distance from the potential biowaste centers in order to reduce the transportation fee of the feedstock. Moreover, the electricity production capacity, the biogas plant site operation system, and the environmental benefits were the determiners that made estimating the economic viability of the biogas plant possible.
The estimation of the volume and collection system of the household and animal biowaste sources (fecal sludge) as feedstock is limited in this study. A collection chain can be set up along the route of the seven potential biowaste centers. However, the strategic selection of biowaste collection centers, their treatment methods by cogeneration, and the conversion of heat into electricity, considering the area’s conditions provide a strategic advantage for this biogas plant’s operation and economic management.
Based on the quantity of the biowaste that can be recovered and the annual biogas production, the biogas plant was classified according to the NY/T 667 rules for the biogas standard system in China, considering an average of 1300 m
3/day; our biogas plant is classified according to NY/T 667–2003 and NY/T 667–2011 as a large biogas plant with daily biogas yields of Q ≥ 300 and 5000 > Q ≥ 500, respectively [
35]. This biogas plant has a larger production capacity than that proposed in the previous studies [
29], which have a similar method but do not present production estimates or study the evolution of the plant over time and according to population growth.
This study is more structured on the plan of environmental impact analysis, economic studies, security investigation, and the theoretical and technical planning of the production of biogas than previous studies [
36]; the yields and the production of electricity are reasonable and can be estimated in the future based on the population growth and the biowaste production per year (
Figure 9).