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Determination of the Composition of Waste and Estimation of Its Recoverable Energy Potential as an Essential Tool to Improve the Waste Management Plan: The Case Study of Nangui Abrogoua University in Côte d'Ivoire

Adjoumani Rodrigue KOUAKOU*, Abollé ABOLLE, Konan Edmond KOUASSI, Gaëtan Achiepo AKOTTO*

Laboratoire de Thermodynamique et de Physico-Chimie du Milieu, UFR Sciences Fondamentales et Appliquées, Université Nangui Abrogoua; 02 BP 801 Abidjan 02, Côte d’IvoireLaboratoire de Thermodynamique et de Physico-Chimie du Milieu, UFR Sciences Fondamentales et Appliquées, Université Nangui Abrogoua; 02 BP 801 Abidjan 02, Côte d’Ivoire

Abstract: Recoverable energy from waste is a major source of environmentally sustainable energy that is not currently unexplored in Côte d'Ivoire. This study evaluated the potential for energy recovery from waste, using waste produced at a university as a study model. It is the Nangui Abrogoua University. It was used as a study model because of its waste management system. The solid waste generated in this model has been characterized in its different components. The recoverable energy potential of the waste was then evaluated. The calorific value was used to predict the equivalent energy availability of the waste in kWh and equivalent tonnes of oil. The results of the study show a non-negligible energy potential that could reach 5687.1 MJ per day, i.e., 1579.8 kWh or 0.4 tonnes of petroleum equivalent per day. These results suggest the necessity to develop a waste management system impregnated with a policy of energy recovery from waste to supplement energy needs and obtain other social benefits from the implementation of such a policy.

Keywords: recoverable energy, waste management, environment sustainable, education

Received February 21, 2021; Revised April 04, 2021; Accepted April 12, 2021

Citation: Adjoumani Rodrigue KOUAKOU, Abollé ABOLLE, Konan Edmond KOUASSI, Gaëtan Achiepo AKOTTO, Determination of the Composition of Waste and Estimation of Its Recoverable Energy Potential as an Essential Tool to Improve the Waste Management Plan: The Case Study of Nangui Abrogoua University in Côte d'Ivoire. International Journal of Environmental Bioremediation & Biodegradation, 9 (1). 1-7. January 2021. doi:10.12691/ijebb-9-1-1

1. Introduction

The significant quantity of waste produced and the difficulty of disposing of it remain major problems in the world [1]. These wastes pose a risk to health and the environment, especially in developing countries [2]. The global energy crisis is felt more by developing countries, such as Côte d'Ivoire, where energy insufficiency has been identified as the source of social and economic poverty [3]. In addition, universities are seen as models in solid waste management studies, because they have several different buildings and the energy consumption is very high [4]. For this reason, universities must give an added value to solid waste, which they generate such as energy production [5]. Thus, energy recovery from solid waste has been identified by studies as a sustainable source of energy to supplement community energy needs, reducing dependence on fossil fuel sources [4]. The production of energy from waste is a solution that not only reduces energy costs, but also brings related benefits, including the reduction of landfill space and environmentally negative emissions [6]. The energy valorisation of solid waste can also be a source of energy procurement.

The University Nangui Abrogoua is located in the city of Abidjan, the economic capital, between the communes of Abobo and Adjamé. It is the second public university in Abidjan, with an estimated surface area of 138 hectares. It has four departments, a preparatory school for health, a health centre and buildings. It has about 13,000 students and 200 teachers. As all human beings produce waste, Nangui Abrogoua University is a potential source of solid waste. Therefore, it is necessary to recover energy from this source. In order to valorise solid waste, it is necessary to start by knowing its composition, its physical and chemical characteristics and to estimate its energy potential. Considering the absence of a study on the composition, physical-chemical characteristics and the estimation of the energy potential of solid waste from the Nangui Abrogoua University, which is essential for the promotion of sustainable development, we found it necessary to carry out this study.

The objective of this study is to determine the composition of solid waste and its physical and chemical characteristics and to estimate its recoverable energy potential.

2. Materials and Methods

2.1. Waste Collection and Composition

The composition of the samples was analysed using the MODECOM method on the basis of a collection conducted on two different days in six sites (Table 1) between May and September 2020 [7]. All solid waste during sampling was separated into different waste components such as textiles, polyethylene bags, plastics, glassware, foam, other packaging materials, cardboard, metals and wooden materials, etc. The waste was then separated into the following components: fabric, polyethylene bags, plastics, glassware, foam, other packaging materials, cardboard, metals and wooden materials, etc. They were then weighted to obtain the fraction of the different components of the university's solid waste.

Table 1. Sampling site

2.2. Determination of the Physical-chemical Parameters of Solid Waste

2.2.1. Density

A 16-litre container is filled with waste without compaction and then weighed. The densities are calculated using the following formula:

[picture] | (1)

: Density in kg m⁻³

M: weight in kg

V: volume of container in m³

2.2.2. Humidity and Dry Mass Content

Humidity was determined as quickly as possible to limit evaporation losses. The measurement is made according to the NF M03-002 standard which recommends drying at 105°C in an oven for 24 hours until a constant mass is obtained [7]. A mass varying from 10 to 100 g of each fraction has been dried, the dry (DM) and wet (H) mass contents are calculated according to the equations

[picture] | (2)

[picture] | (3)

Where % H: percentage of humidity.

M₀: initial mass of the sample before drying.

M₁: final mass of the sample after drying.

2.2.3. Organic Matter

The organic matter is determined by calcination of the dry matter at 550°C. The calcination time is four hours in a kiln. The organic matter (OM) is obtained by the difference in weight between the mass of the dry waste (M1) and the mass of the waste calcined at 550°C. It is determined in accordance with standard NF EN 13039 [7].

[picture] | (4)

%MO: percentage of organic matter

M₁: mass obtained after drying

M₂: mass obtained after passing through the furnace

W = % total humidity.

2.2.4. Lower Heating Value (LHV)

The lower calorific value of solid waste is determined using the formula [8]:

[picture] | (5)

LHV in kcal/kg, with:

R = % of plastics (dry mass);

P = % of paper-cardboard (dry weight)

H = % of other wastes (in dry mass);

Ru = % of rubber and leather (in dry mass);

T = % of textiles (by dry mass);

F = % of food waste (in dry mass);

G = % of wood and leaves (in dry mass);

W = % of total humidity.

2.2.5. Estimation of Recoverable Energy Potential

The calorific values of waste, as suggested by Shepherd et al., and Smith et al., [9,10]; were used to estimate the recoverable energy potential of each characterised waste [8]. The calorific value for each type of waste used in the estimate is presented in Table 2. The recoverable energy potential E in MJ/day for each waste characterised was obtained from the data in Table 2 by the following equation [11].

[picture] | (6)

where: LHVi = Lower heating value of each characterised waste

Mi: mass of waste category i per day.

Table 2. Calorific values used to estimate the energy value of waste

The total equivalent energy in MJ/day (Et) was obtained for the n numbers of waste categories by the following equation:

[picture] | (7)

where: LHVi = Lower heating value of each characterised waste

Mi: mass of waste category i per day

The equivalent energy in kWh/day is determined by the following formula

[picture] | (8)

The energy per tonne of oil equivalent (toe) per day is determined by the following equation

[picture] | (9)

3. Results and Discussion

3.1. Waste Composition

Wastes were collected over two periods: a teaching period and a vacancy period. The results are summarized in Figure 1 and Figure 2 for the teaching and vacancy periods respectively. Figure 3 shows the overall composition of waste during the two periods. These wastes are mainly composed of food wastes; paper/cardboard waste; plastic waste; textile waste; rubber waste; glass waste; metal waste; and hazardous waste. The food wastes consist of banana, yam and orange peelings, palm seed, lemon, and food residue (attiéké, rice, fish, meat, banana and yam). Figure 1 shows that during the work period, the mass of food waste varies from 0 (site 6) to 143.5 kg/day (site 5). It represents 56% of the total mass of wastes (Figure 3a). The maximum value observed on site 6 during the teaching period could be explained by the large amount of waste produced by the main restaurant of the regional university centre (CROU). Figure 2 shows that during the vacancy period, the mass of food waste varies from 0 (site 6) to 101.5 kg/day (site 4). It corresponds to 52% of the total mass of wastes (Figure 3b). The maximum value observed on site 4 could be explained by the presence of restaurants around this site. The high mass values of food waste observed on the sites during the teaching periods could be explained by the revival of the activities of the restaurant of the CROU.

Figure 1. Composition of solid waste during the teaching period (G: garden waste; F: food waste; P: paper/cardboard waste; R: plastic waste; T: textile waste; Caou: rubber waste; V: glass waste; M: metal waste; Da: hazardous waste; Re: other waste)

Figure 2. Composition of solid waste during the vacancy period (G: garden waste; F: food waste; P: paper/cardboard waste; R: plastic waste; T: textile waste; Caou: rubber waste; V: glass waste; M: metal waste; Da: hazardous waste; Re: other waste)

Figure 3. Total composition of solid waste (G: garden waste; F: food waste; P: paper/cardboard waste; R: plastic waste; T: textile waste; Caou: rubber waste; V: glass waste; M: metal waste; Da: hazardous waste; Re: other waste)

Concerning paper/cardboard waste, the mass of waste varies from 0 (site 6) to 52 kg/day (site 3) during the teaching period (Figure 1). It contributes to 18% of the total mass of wastes (Figure 3a). The highest average paper/cardboard waste mass is observed at site 3 during all periods. This high mass of paper/cardboard waste at Site 3 could be explained by the presence of the classrooms around this site. During the vacancy period (Figure 2), the mass of paper/cardboard waste varies from 0 (site 6) to 36 kg/day (site 1). It corresponds to 25% of the total mass of wastes produced during the holiday period (Figure3a).

Figure 4. Humidity and organic matter according to waste composition (G: garden waste; F: Food waste; P: Paper/cardboard waste; R: Plastic waste; T: Textile waste; H: Other waste (2% rubber waste, 1% glass waste, 1% waste and the remaining 11% waste); M1: Mixed sample of all waste; M2: Mixed sample of all waste except garden waste)

Waste plastics occupy the third most important fraction in the percentage distribution of wastes in all periods. Analysis of the results shows that the mass of plastic waste varies from 0 kg (site 6) to 23.1 kg/day (site 3) during the holiday period (Figure 2). It corresponds to 12% of the total wastes mass (Figure 3a). During the teaching period (Figure 1), the mass of plastic waste varies from 0 kg/day (site 6) to 63.4 kg/day (site 3). This represents 15% of the total mass of waste during the teaching period (Figure 3a). Plastic waste consisted mainly of plastic packaging. The presence of plastic waste on site 3 could be due to the commercial activities around Amphitheatres A and C.

Concerning the textile waste, it is mainly found at (site 2) with masses of 9.75 kg/day and 4.2 kg/day respectively during the vacancy and teaching period. These masses represent 4% of the total mass of wastes (Figure 4). The presence of these wastes in this site is due to the proximity of the professors' houses, which are the residences for some students.

The fraction of other waste is 7% during vacancy periods (3% rest of waste, 2% rubber, 1% metal, 0.99% glass and 0.1% hazardous waste) and 8% during working periods (5% rest of waste, 1% rubber waste, 1% metal waste, 0.99% glass and 0.1% hazardous waste). During the teaching period and the vacancy period, hazardous waste is generated mainly at sites 1 and 2. Hazardous waste includes metal and glass sharps waste, solid waste of unused medicine, and laboratory waste. The mass of hazardous waste observed on sites 1 and 2 comes from laboratories and the health unit respectively. The metal waste during both periods comes from site 1, site 3 and site 5. The metal waste was largely composed of tin cans and iron. The metal waste observed on site 5, possibly came from the restaurant of the CROU. Figure 3 also shows that the proportion of glass waste is roughly equal over the two periods. Glass waste comes from site 1 and site 3 and consists mainly of beverage bottles and unusable glassware.

3.2. Physico-chemical Parameters

3.2.1. Density

The mass varies from a few decades of kg/m3 to several kg/m3. It is 625 kg/m3 for food waste; 487 kg/ m3 for the textile, 137.5 kg/m3 for the paper/board; 375 kg/m3 for the sheet waste and 112.5 kg/m3 for the plastic waste. The overall value is 406 kg/m3. This high value is due to the high content of putrescibles. In fact, the density of putrescibles is between 250 and 500 kg/ m3. The overall density of this waste corresponds well to that of developing countries [12].

3.2.2. Humidity and Organic Matter

Food waste represents the wettest fractions with a percentage of humidity of 73% (Figure 4). Organic matter is highest in food waste (Figure 4). The optimal humidity and organic matter contents for biological waste treatment for microbial activity are respectively above 50% and 40% [13]. Consequently, food waste will be favourable to all biological treatment and disadvantageous to all thermal treatment.

3.2.3. Lower Heating Value of waste

The Lower heating value (LHV) of the waste was determined from equation 7. The LHV is therefore calculated based on the content of the following categories: paper/cardboard, textiles, food waste, plastics, wood and foil waste, and other waste. The result gives an LHV of 3573 kcal/kg in the two periods. Thermal treatment of waste with energy recovery is therefore possible. Indeed, waste combustion with energy recovery is conditioned by a LHV of more than 1200 kcal/kg (Janajreh et al., 2013).

3.3. Estimation of Recoverable Energy Potential

The estimated value of the recoverable energy potential per day for each characterised waste is presented in Table 3. The calculated value of the total equivalent energy (Et) using equation 9 is 5687.1 MJ per day. Similarly, using Equation 10, the total energy produced (Eq) is 1579.8 kWh per day. Using Equation 11, the tonne of oil equivalent (toe) is 0.4 tonnes per day.

Table 3. Estimated recoverable energy potential of the UNA

Although biodegradable food waste has the lowest LHV value (14.7 MJ/kg), it has the highest recoverable energy potential (1889.3 MJ per day) of all solid wastes (Table 3). This represents 44.7% of the recoverable energy of all characterised waste. A significant portion of the waste is also plastics (33.2%). This includes non-biodegradable waste such as polyethylene bags, plastic bottles, plastic food packaging and polystyrene food packaging.

The recoverable energy potential of the characterised waste is 5687.1 MJ per day. This can produce energy equivalent to 1579.8 kWh per day. With this recoverable energy potential, about 1000 institutional personnel could cook with a 1000 W electric cooker for one hour and thirty minutes per day [11]. However, about 20-50% of this energy may be recoverable, depending on the quantity of recoverable energy [10]. Thus, the 50% recoverable energy (equivalent to 789.5 kWh per day) will be sufficient for another 40 minutes of cooking per day for the same number of staff, while the 20% recoverable energy (equivalent to 316 kWh) will power a 1/8 horsepower (0.09321 kW) refrigeration unit for about 5 hours of the day for each of the 1000 personnel.

The recovery of energy from solid waste generated in the academic community of study and similar communities such as institutions of higher education, densely populated cities and even rural communities could lead to the provision of an energy bonus. With waste energy recovery technology, this energy bonus could be added to the energy needs of the communities. In addition, this approach to recoverable energy could also be a way to reduce the non-biodegradable waste of plastics which, although not environmentally sustainable, constitutes the majority of waste that is naturally and generally disposed of as waste in a community [11]. This is particularly important as the alternative of recycling these thermoplastic wastes could be difficult as they could contain different types of polymers.

4. Conclusion

In this study, waste characterisation was carried out using the waste produced at Nangui Abrogoua University as a model community. This characterisation was used to assess the recoverable energy potential of the waste. The following conclusions can be drawn.

• The determination of the composition of the waste identified biodegradable food waste as the most important fraction (52-56% of the total mass of solid waste).

• The Lower heating value of all solid waste (LHV = 3573 kcal/kg) revealed that thermal treatment of waste with energy recovery is possible. However, humidity and organic matter showed that biodegradable food waste is favourable for biological treatment. Despite their relatively low LHV, these food wastes have the highest recoverable energy potential (2540 MJ/day). As for plastic waste, the estimated recoverable energy potential is 1889.3 MJ/day.

• These results show a large recoverable energy potential of the characterised waste. This potential is 5687.1 MJ per day. This is equivalent to 1579.8 kWh per day or 0.4 tonnes of oil equivalent per day.

These results reveal the need to integrate the policy of energy recovery from solid waste into the waste and energy management policy. This could improve the current energy supply from untapped waste resources in the country. In addition, it is recommended that energy conversion facilities be built to maximise energy recovery from waste generated in cities. The energy recovered from these systems can then be usefully annexed to supplement the country's energy needs. This approach to the energy recovery system, if well developed, has environmental benefits in terms of job creation and reducing the amount of waste that could have been landfilled. Thus, the transformation of waste into energy is a great advantage for the achievement of a sustainable economy and environment.

References

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