Theoretical Analysis of Biogas Production from Septic Tanks: The Case of the City of Kinshasa

Ngondo Otshwe Josue¹, Lutete Nzau Wilfrid², Okolonga Djeli Dieumerci³

¹Derpartment of Electrical Engineering, North China Electric Power University, Beijing, China.
²Centre for Geological and Mining Research, University of Liege, Liège, Belgium.
³Derpartment of Electrical Engineering, Mapon University, Kindu, DR Congo.

DOI: 10.4236/ns.2023.154011                

Abstract

As many think that respect for the environment, is not only a question intended for industrialists but has all the sectors of life, in particular sanitary also. In this regard, our article brings alternative management of human waste (excrement) to solve the problems that plague our dear beautiful capital, namely: 1) Lack of latrines that meet the standards; 2) Emptying of septic tanks directly into the gutters and; 3) Water pollution by sewage csompanies. In order to carry out the cartographic analysis of the study area, we used Shapefile data from the OpenStreetMap, Diva-Gis. These different data allowed us, analyzed, to categorize with the software ArArcGIS 0.8.1 to produce different zones according to the cases incurred in the city of Kinshasa. To do this, the analytical method uses the Buswell equation to determine the amount of gas contained in human excrement. Focusing on the analysis of the excrements produced by the population of age superior to 10 years, for 2023, we obtained: 138355.7283 m³/day of CH4 (885476.66 kWh/day or 885.476 MWh/day), which, energy can light: 138,355 lamps of 60 to 100 W for six hours or nearly 70,000 lamps of 60 to 100 W for 12 hours. Considering the last one which offers the lowest access rate, i.e. 3% of the district population to these latrines, we have: a) In Tshangu, we produce: 1618.762 3>/day (10360.07 kWh/day or 10.36 MWh/day) which can light nearly 1600 lamps from 60 to 100 W for six hours or nearly 800 lamps from 60 to 100 W for twelve hours. b) Mont-Amba, we produce 1402.927 3>/day (8978.73 kWh/day or 8.97 MWh/day) which can light nearly 1400 lamps from 60 to 100 W for six hours or nearly 700 lamps from 60 to 100 W for twelve hours; c) In Lukunga, we produce: 946.35 3>/day (6056.66 kWh/day or 6.056 MWh/day) which can light nearly 900 lamps from 60 to 100 W for six hours or nearly 450 lamps from 60 to 100 W for twelve hours. d) Funa, we produce: 182.629 3>/day (1168.83 kWh/day or 1.17 MWh/day) which can light almost 180 lamps from 60 to 100 W for six hours or almost 90 lamps from 60 to 100 W for twelve hours.

Keywords

Biogas, Septic Tanks, Excrement, Electricity, Kinshasa

1. Introduction

For the past few decades, global warming has been a major concern and is at the center of major debates on environmental issues [1]. Among the solutions considered to reduce its harmful impact on the planet earth, it is worth mentioning the respect for the environment through the application of the three Rs (Reduce, Reuse, and Recycle). As many think respect for the environment, is not only a question destined for industrialists but for all sectors of life notably the sanitary one too. For, it plays an important role in the destruction of the environment as shown in Figure 1.

In 2020, nearly 6 billion people had cell phones while 2 billion people still do not have a toilet or latrine. Of these, 673 million defecate in the open, for example in gutters, behind bushes, or in water bodies [2]. This situation creates huge stocks of waste and leads to the appearance of numerous microbes responsible for quite serious diseases. This is especially true in Africa.

Indeed, the transmission of diseases such as cholera, dysentery, hepatitis A, typhoid, polio, and diarrhea occurs as a result of poor sanitation (WHO, June 15, 2019) [3]. The latter alone is responsible for about 8% of all deaths in Africa. Yet these deaths are preventable. Improving water supply, sanitation, and hygiene would, according to WHO, prevent the deaths of 297,000 children under the age of 5 each year.

Faced with this situation, the provincial city of Kinshasa, the largest city in the DRC, is no exception. More than half of the population lives in peri-urban areas with a lack of adequate sanitary facilities.

And the majority of those who have hygienic installations have a problem of regular maintenance. Because, a regular maintenance guarantees the good functioning of a septic tank, something which is very little respected in the city province of Kinshasa and everywhere in the country.

As a workaround, some individuals take advantage of the rain to empty their septic tanks into the gutters or rivers near their place of residence and even some buildings and public toilets located in the commune of Gombe empty directly into the Congo River. Some call on the local drainage company, which uses the procedure shown in Figure 2.

Indeed, the latrines, although neglected, could be a source of valorizable energy, they naturally produce two gases (CO₂ and CH₄), from which the methane (CH₄), otherwise called biogas, can be removed by a simple process and within the reach of all the purses.

As a reminder, 1.33 to 1.87 m³ of biogas is equivalent to 1 L of gasoline. In this study, this energy would be used primarily for cooking and lighting [4].

Figure 1. Degree of environmental impact.

Figure 2. Emptying procedure in Kinshasa.

For this purpose, 1 m³ of biogas is equivalent to:

• Light a 60 - 100 Watt bulb for 6 hours.

• Cook 3 dishes for a family of 5 - 6 people.

• 0.7 kg of oïl.

It can generate 1.25 kWh of electricity.

In this regard, our article provides an alternative management of human waste (excrement) to solve the problems that plague our dear beautiful capital, namely:

- Lack of latrines that meet the standards;

- Emptying of septic tanks directly into the gutters and;

- Water pollution by sewage companies.

2. Presentation of the city

As shown Figure 3, the city of Kinshasa (/kin.ʃa.sa/; Lingala: Kisásá), known as Léopoldville (Dutch: Leopoldstad) from 1881 to 1966, is the capital and largest city of the Democratic Republic of Congo (DRC) and covers an area of 9965 km². With an estimated population of 15,628,085 in its metropolitan area in 2022, it is the third largest metropolitan area in Africa after Cairo and Lagos, and is the largest French-speaking metropolitan area in the world, having surpassed Paris in the 2010s, and is one of the most populous metropolitan areas in the world.

Located on the southern bank of the Congo River, at the Malebo Pool, it faces the capital of the Republic of Congo, Brazzaville (Figure 4). The city limits are very large, and more than 90% of its area is rural or forested (especially in the commune of Maluku); the urbanized parts are located in the west of the territory. Kinshasa has the administrative status of a city and is one of the country’s 26 provinces. The city is composed of [5]:

- 24 municipalities;

- 370 quarters;

- 49,950 avenues and

- 1,240,220 parcels.

2.1. Urbanization of the City

The urban population of the Democratic Republic of Congo is growing rapidly. Estimated at 42 percent in 2015, the proportion of the population living in urban areas in the Democratic Republic of Congo

Figure 3. The city of Kinshasa (Source Wilfrid_Lutete 2023).

Figure 4. The special distribution of the different districts of the city of Kinshasa.

is the third largest in sub-Saharan Africa, after South Africa and Nigeria [6].

Much of this population growth is attributable to factors in the source localities (i.e., conflict and inadequate rural services) rather than to incentives in the cities (including better work and living opportunities). With an estimated population of 12 million in 2016, Kinshasa represents the densest and fastest growing urban system in Central Africa. At its current rate of growth, the city will be home to nearly 24 million people within ten years and will be the most populous city in Africa, ahead of Cairo and Lagos. This prospect constitutes an opportunity, but also a risk that the living conditions of the people in Kinshasa will become even more precarious and that the city will become the largest slum in Africa (Figure 5) if urbanization is not properly managed and the trend of exclusive urbanization and marginalization is not reversed [7].

Rapid population growth brings with it many challenges. It increases the demand for:

- Social services and infrastructure.

- Education, health and basic services.

- To make cities livable.

At the same time, significant investments are needed to ensure that capital, infrastructure and businesses are productive. The city is made up of several large squares through which many people pass and which need to be improved (sanitary facilities, stops, garbage garbage cans, etc.).

2.2. Population

Between 1984 and 2010, the city’s annual population growth rate averaged 5.1%, compared to 4.1%

Figure 5. Map of the city showing major slums.

nationally [5]. Given this population density, the city of Kinshasa will become the largest megacity in Africa by 2030. Table 1 represents the population evolution for the period from 2010 to 2035.

3. Biogas from latrines

Table 2 and Table 3 show us how Biogas is a solution to fight deforestation. The production of this gas is done by fermenting in a tank (called digester) buried. Human and animal wastes, excrements and slurry of pigs or cattle for example are used as raw materials. This process of biological degradation, called methanization and due to the biological fermentation of fermentable organic matter in an anaerobic environment, i.e., without oxygen, is the same as that which occurs in certain circumstances in swampy areas, sludge from sewage plants or in uncontrolled landfills. This simple and natural process allows for better treatment of animal excrements and dejecta and for their valorization through the use of the gas produced for lighting and cooking.

Table 1. Evolution of the population of Kinshasa [8].

Table 2. Contents of human excrement [9].

Table 3. Amount of biogas produced [4].

In Africa in general and in DRC in particular, most rural and mountain areas are isolated and have no other source of energy than wood. The latter, widely used for cooking, contributes to deforestation.

3.1. Benefits of Biogas

This technology offers several advantages [10] namely:

• Free fuel used (for cooking and lighting) and especially less polluting.

• Use of the residues as natural fertilizer and finally.

• Improvement of the hygiene of houses and waterways.

• The investment cost is low despite the need for skilled help, especially in construction.

• This is a boon for poor and poorly urbanized regions.

• The construction can also be done with local materials and with little land, the tank being built underground in most cases.

• Improving the living conditions of urban, peri-urban and rural populations, and more particularly the living conditions of women by reducing the time they spend collecting wood (for peri-urban and rural areas). While eliminating respiratory diseases caused by the prolonged inhalation of harmful fumes from the burning of wood, coal or in some regions dried cow dung.

• Sensitization of the populations to alternative energies in order to remedy the serious problem of deforestation caused, among other things, by the use of wood for cooking, whose uncontrolled use can lead to the desertification of many regions of the world.

• Finally, the construction of biogas tanks meets the criteria of sustainable development, as the technique of methanization consists of producing clean energy from organic matter such as manure and its use is accompanied by a transfer of technology to the beneficiary communities, the training of personnel, both for the construction of the system and for its maintenance, and the creation of management committees.

3.2. The Process

The process consists of concentrating and treating animal excrement and waste in an anaerobic tank or digester where, in the absence of oxygen (anaerobic), micro-organisms multiply and derive the energy necessary for their development from organic substances which they decompose into gas with a high proportion of methane and with a high caloric and energy potential.

The biogas tank is buried and directly connected to a family (or public) latrine built on its roof. For insulation reasons (digestion by anaerobic bacteria is optimal at 37˚ and constant temperature).

The performance of the system is improved by:

• Direct connection of the latrine to the digester.

• Association of a small animal yard (mainly pigs).

• On the roof of the tank (improvement of the sanitary situation and provision of additional insulation for better gas production.

4. Methodology

In order to carry out the cartographic analysis of the study area, we used Shapefile data from the OpenStreetMap, Diva-Gis. These different data allowed us, analyzed, to categorize with the software ArcGis 10.8.1 to produce different zone according to the cases incurred in the city of Kinshasa.

To carry out this study we use the analytical method using the BUSWELL equation. This equation was developed by BUSWELL and MULLER in 1952 [11]. It allows to predict the quantity and the theoretical composition of biogas produced during the anaerobic biodegradation of a substrate whose elementary composition is known.

Formula:

${\text{C}}_a{\text{H}}_b{\text{O}}_c+\left(a-\frac{b}{4}-\frac{c}{2}\right){\text{H}}_{\text{2}}\text{O}\Rightarrow \left(\frac{a}{2}-\frac{b}{8}+\frac{c}{4}\right){\text{CO}}_2+\left(\frac{a}{2}+\frac{b}{8}-\frac{c}{4}\right){\text{CH}}_4$ (1)

The Equation (1) was completed by BOYLE in 1976 by integrating sulfur and nitrogen, becoming:

$\begin{array}{l}{\text{C}}_a{\text{H}}_b{\text{O}}_c{\text{N}}_d{\text{S}}_e+\left(a-\frac{b}{4}-\frac{c}{2}+\frac{3d}{4}+\frac{e}{2}\right){\text{H}}_2\\ \Rightarrow \left(\frac{a}{2}-\frac{b}{8}+\frac{c}{4}+\frac{3d}{8}+\frac{e}{4}\right){\text{CO}}_{\text{2}}+\left(\frac{a}{2}+\frac{b}{8}-\frac{c}{4}-\frac{3d}{8}-\frac{e}{4}\right){\text{CH}}_{\text{4}}+d{\text{NH}}_{\text{3}}+e{\text{H}}_{\text{2}}\text{S}\end{array}$ (2)

We use this expression for the production of biogas. However, this method is not the only one.

The expression used in practice is the following:

$\begin{array}{l}{\text{C}}_{450}{\text{H}}_{2050}{\text{O}}_{950}{\text{N}}_{12}{\text{S}}_1+\left(450-512.5-475+9+0.5\right){\text{H}}_{\text{2}}\text{O}\\ \Rightarrow \left(225-256.25+237.5+4.5+0.25\right){\text{CO}}_2\\ +\left(225+256.25-237.5-4.5-0.25\right){\text{CH}}_4+12{\text{NH}}_3+{\text{H}}_{\text{2}}\text{S}\end{array}$ (3)

This gives:

$C_{450}{H}_{2050}{O}_{950}{N}_{12}{S}_1+\left(-528.5\right)H_{2}O\Rightarrow 211C{O}_2+239C{H}_4+12N{H}_3+H_{2}S$ (4)

Assumptions

In order to make our calculations possible, certain assumptions are essential, namely:

1) We will consider that 20% of the population under >10 years of age

2) Each household is composed of at least 6 people

3) In order to determine the quantity of gas contained in the fecal matter we will make the calculation for 10,000 people

4) Either the biodegradable carbon content is 60% or

5) We will analyze the following biogas production scenarios:

- 30% of the population (Pop) in the district access public toilets for high need.

- 20% of the district’s population accesses public toilets for high need.

- 10% of the district’s population accesses public toilets on a high need basis.

- 3% of the district’s population accesses public toilets for high need.

6) The distribution of the population of Kinshasa by district is as follows

- Tshangu: 39%;

- Mount Amba: 33%;

- Lukunga: 22.8% and

- Funa: 4.4.

5. Results and interpretation

5.1. Biogas Calculation

From Equation (4), we determine the following:

$\text{Minof}{\text{C}}_{450}{\text{H}}_{2050}{\text{O}}_{950}{\text{N}}_{12}{\text{S}}_1=22850\text{g}/\text{mol}$ (5)

$\text{Minof}{\text{C}}_{450}=12\times 450=5400\text{g}/\text{mol}$ (6)

$\begin{array}{l}\text{\%}\text{carbone}=\frac{5400}{22850}=24\text{\%}\\ \text{\%}{\text{CH}}_4=\frac{239}{450}=53\text{\%}\\ \text{\%}{\text{CO}}_2=\frac{211}{239+211}=\frac{211}{450}=47\text{\%}\end{array}$ (7)

Consider 10,000 people (assumption c) and that each produces approximately 250 g of fecal matter per day. [12]

$\Rightarrow 10000\times 250=2500\text{kg}/\text{day}$

On the other hand, from 100 to 400 g of fecal matter is contained 30 to 60 g of dry matter [13].

Therefore, in the 2500 kg/day of faeces obtained above we will have 750 kg of dry matter, when producing biogas 50% of the organic matter can be degraded to total solids (TS) or 60% to volatile solids (VS) [4].

Now let’s use Equation (4) to determine the amount of biogas that can be produced from the dry matter of human excrement for the following composition:

· Carbon (24%) of 750 kg of dry matter obtained above

$\Rightarrow 750\times 0.24=180\text{kgcarbone}$

Using the assumption d to determine the amount of carbon that will be converted to biogas, we will have: 180 × 0.6 = 108 kg.

From Equations (4) and (7), we have: 53% CH₄ in the biogas, so the weight of methane carbon (CH-C) will be: 108 × 0.53 = 57.24 kgcarbone.

The weight of the methane will be: $57.24\times \frac{16}{12}=76.32\text{kg}{\text{CH}}_4$

Or, $\begin{array}{l}57.24\text{kg}=57240\text{g}\text{de}{\text{CH}}_{\text{4}}\\ \Rightarrow \frac{57240}{16}\text{moles}\text{de}\text{gaz}=3577.5\text{moles}\text{de}{\text{CH}}_{\text{4}}\\ 1\text{moledegazaNTP}=22.4\text{l}\\ 3577.5\times 22.4\times \frac{16}{12}=106847.9\text{l}\text{de}{\text{CH}}_{\text{4}}\end{array}$

Hence, the estimate of methane produced by 2500 kg/day of feces is 106.848 m³ CH₄ respectively.

The calorific value of biogas is variable depending on the quantity of methane contained, i.e. 22 - 26 MJ/m³ (5.6 - 7.2 kWh/m³) [14].

In order to determine the calorific value of the gas contained in the fecal matter calculated above, we consider the standard calorific value of biogas which is 22 MJ/m³.

We will have: 106.848 × 22 = 2350.656 MJ/m³ soit 15044.1984 kWh/m³/jour.

Now, let’s use the results obtained above on the population of the city of Kinshasa.

5.2. Estimation of the Biogas Production of the Population of Kinshasa

Table 4 gives an estimate of the quantity of biogas that we can produce in Kinshasa under normal conditions, which means that we exploit the totality of the human excrements of the city.

5.3. Production by Public Toilet

Table 5 shows the population of the city of Kinshasa by district according to hypotheses a and f.

We will start with the most populated district and end with the least. We will then have:

- Tshangu;

- Mount Amba;

- Lukunga and

- Funa.

To do this, all the hypotheses were taken into account for the Tshangu district alone, because it is the most populated. For the rest of the districts, only the last hypothesis (i.e., 3% access) was taken into account.

1) Tshangu

Biogas production from public toilets in this district under the high-need access assumptions is shown in Tables 6-9.

2) Mount Amba

The biogas production from public toilets in this district is shown in Table 10.

3) Lukunga

Biogas production from public toilets in this district is shown in Table 11.

4) Funa

Biogas production from public toilets in this district is shown in Table 12.

5.4. Interpretation of Results

With 1,240,220 parcels and a population of 15,628,085 in 2022, this results in an average of 13 people per parcel and under assumption b, an average of two families per parcel. And according to assumption a, each of these plots will have 10 people over the age of 10 and would produce 1.06 m³/day. This could power nearly 10 compact fluorescent lamps of 10 W or less for 5 hours. This would help the high and or students in their studies.

If we analyze the demographic growth of the city of Kinshasa as presented in Table 5, this causes a great problem and is the source of several diseases. If we look only at the year 2023, the population will be approximately 16 million. Energetically, it constitutes a great potential. Focusing on the analysis of the excrements produced by the population of age superior to 10 years, for 2023, we obtained:

Table 4. Overall biogas production in Kinshasa.

Table 5. Population of Kinshasa by district.

Table 6. Energy production using assumption 1 (i.e., 30% access).

Table 7. Energy production using assumption (i.e., 20% access).

Table 8. Energy production using assumption 3 (i.e., 10% access).

Table 9. Energy production using assumption 4 (i.e., 3% access).

Table 10. Energy production using case 4 (i.e. 3% access).

Table 11. Energy production using assumption 4 (i.e., 3% access).

Table 12. Energy production using assumption 4 (i.e., 3% access).

138355.7283 m³/day of CH₄ (Table 4) and that corresponds to 885476.66 kWh/day or 885.476 MWh/day. Now, according to the information contained in the introduction (Table 2), with this quantity of gas we can power 138,355 lamps of 60 to 100 W for six hours or nearly 70,000 lamps of 60 to 100 W for 12 hours.

At present, the problem is how to collect this energy? Hence, the possibility of producing it by using public toilets in large squares in each district.

By doing the study for the Tshangu district for the four access hypotheses, we have:

- For Hypothesis 1: For 30% of the population to have access to public latrines by the year 2023, gives:

· 16187.62 m³/day (103600.7 kWh/day or 103.6 MWh/day) which can light nearly 16,000 lamps from 60 to 100 W for six hours or nearly 8000 lamps from 60 to 100 W for twelve hours.

- For Hypothesis 2: For 20% of the population to have access to public latrines by the year 2023, gives:

· 10791.74 m³/day (60967.17 kWh/day or 60.9 MWh/day) which can light nearly 10,000 lamps of 60 to 100 W for six hours or nearly 5000 lamps of 60 to 100 W for twelve hours.

- For Hypothesis 3: For 10% of the population to have access to public latrines by the year 2023, gives:

· 5395.87 m³/day (34533.17 kWh/day or 34.5 MWh/day) which can light nearly 5000 lamps from 60 to 100 W for six hours or nearly 2500 lamps from 60 to 100 W for twelve hours.

- For Hypothesis 4: For 3% of the population to have access to public latrines by the year 2023, gives:

· 1618.762 m³/day (10360.07 kWh/day or 10.36 MWh/day) which can light nearly 1600 lamps from 60 to 100 W for six hours or nearly 800 lamps from 60 to 100 W for twelve hours.

For the district of Mont-Amba, Lukunga and Funa, considering only hypothesis 4 (i.e. 3% of the population uses public latrines for great need) and for the year 2023 alone, we have the following situation.

- Mont-Amba, we produce: 1402.927 m³/day (8978.73 kWh/day or 8.97 MWh/day) which can light nearly 1400 lamps from 60 to 100 W for six hours or nearly 700 lamps from 60 to 100 W for twelve hours;

- In Lukunga, we produce: 946.35 m³/day (6056.66 kWh/day or 6.056 MWh/day) which can light nearly 900 lamps from 60 to 100 W for six hours or nearly 450 lamps from 60 to 100 W for twelve hours;

- Funa, we produce: 182.629 m³/day (1168.83 kWh/day or 1.17 MWh/day) which can light almost 180 lamps from 60 to 100 W for six hours or almost 90 lamps from 60 to 100 W for twelve hours.

6. Conclusions

To conclude, we say that the increase in the population of the city of Kinshasa in particular, and that of DR Congo, in general, can be translated into an excellent opportunity for development in the field of energy with environmental respect. From this fact, the rule of three Rs (Reduce, Reuse, and Recycle) offers an unparalleled opportunity.

Hence, the objective of this article by valorizing the human excrements of the city of Kinshasa by transforming it into energy. In order to better manage this waste, we have just demonstrated how much it will be beneficial for our dear city.

Indeed, 1 m³ of biogas can light a 60 - 100-Watt bulb for 6 hours or cook 3 dishes for a family of 6 people. For our case, this biogas by human excrements through a process of biological degradation called methanization and due to the biological fermentation of fermentable organic matter in an anaerobic environment, that is to say deprived of oxygen.

To do this, the analytical method, using the Buswell equation for the determination of the amount of biogas contained in human excrement.

Focusing on the analysis of the excrements produced by the population of age superior to 10 years, for 2023, we obtained: 138355.7283 m³/day of CH₄ (885476.66 kWh/day or 885.476 MWh/day), which, energy can light: 138,355 lamps of 60 to 100 W for six hours or nearly 70,000 lamps of 60 to 100 W for 12 hours.

However, the recovery of this energy directly is difficult, that is why we proposed to produce it in public latrines implanted in each big place of each district and to arrive to make the calculations certain assumptions. Considering the last one which offers the lowest access rate, that is 3% of the district population to these latrines, we have:

· In Tshangu, we produce: 1618.762 m³/day (10360.07 kWh/day or 10.36 MWh/day) which can light nearly 1600 lamps from 60 to 100 W for six hours or nearly 800 lamps from 60 to 100 W for twelve hours;

· Mont-Amba, we produce: 1402.927 m³/day (8978.73 kWh/day or 8.97 MWh/day) which can light nearly 1400 lamps from 60 to 100 W for six hours or nearly 700 lamps from 60 to 100 W for twelve hours;

· In Lukunga, we produce: 946.35 m³/day (6056.66 kWh/day or 6.056 MWh/day) which can light nearly 900 lamps from 60 to 100 W for six hours or nearly 450 lamps from 60 to 100 W for twelve hours;

· Funa, we produce: 182.629 m³/day (1168.83 kWh/day or 1.17 MWh/day) which can light almost 180 lamps from 60 to 100 W for six hours or almost 90 lamps from 60 to 100 W for twelve hours.

Moreover, this can be extended to large places such as markets, universities etc. and the government through its Ministry of Urbanism and Habitat, could build a law around this solution by directing future constructions to valorize the waste from septic tanks.

This solution could be implemented throughout the republic. And in rural areas, it would encourage the population to raise animals such as pigs and to use their excrements and this will significantly reduce deforestation.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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