Composting and Mechanical Biological Treatment for Reducing Greenhouse Gas Emissions in Bizerte, Tunisia

Abstract

This study seeks to develop effective solutions and strategies for waste management in the Tunisian municipality of Bizerte by addressing the urgent need for sustainable waste management practices in light of the global imperative to mitigate climate change. With a specific focus on reducing greenhouse gas emissions, this investigation aligns with Tunisia’s commitment to international agreements, including the Paris Agreement and the Global Methane Pledge. This study begins with an exploration of background information, followed by data collection and waste characterization to determine the current situation and to detect gaps that exist in terms of waste management in Bizerte municipality. Two scenarios are proposed as potential solutions for the municipality. The first examines a green waste composting facility with a 1000-ton yearly capacity, which has been operational since 2020. This study describes the compost production process and evaluates the quality of compost derived from green waste. This approach demonstrates its potential for delivering significant environmental and economic benefits. The second scenario delves into the implementation of a mechanical biological treatment (MBT) plant for sustainable handling of residual waste while contributing to a reduction in the municipality’s overall environmental footprint. Using the Sweet software, version 4.0.2, July 2022. this study quantifies the potential reduction in greenhouse gas emissions resulting from composting to be 3%, while the MBT achieves a reduction of 28%.

1. Introduction and Problem Statement

Waste generation rates are rising all around the world. In 2022, the world’s cities generated tons of solid waste, amounting to a footprint of 0.74 kg per person per day [1]. With rapid population growth and urbanization, annual waste generation is expected to increase by 70% from the 2016 levels to 3.40 billion tons in 2050 [1,2].

In developing countries, waste management presents two major challenges. Firstly, there is no clear strategy for waste management, and secondly, there is a lack of best practices to address the exponential increase in municipal waste. The prevailing approach to municipal solid waste disposal typically involves a two-step process: first, enhancing and structuring waste collection systems, and second, disposing of waste in unregulated landfills or controlled landfills. This is due to several factors. Specifically, municipalities are forced to manage ever-increasing amounts of waste [3,4] while dealing with limited regulations, resources, technical equipment, and financial means.

This approach, unfortunately, leads to surface and groundwater contamination, proliferation of diseases, and landscape degradation. Additionally, it contributes to a rise in greenhouse gas (GHG) emissions [5].

Tunisia is a developing country with an area of around 163,610 km² [6], which is divided into 24 governorates [7] and encompasses over 350 municipalities [8] including urban agglomerations, with a total population of 12.5 million [9]. The amount of municipal solid waste is about 2.6 million tons/year, with an estimated growth rate of 2.5% per annum. According to the National Waste Management Agency, the ANGED, this waste is also characterized by a high presence of biodegradable organic matter (63.2%). The country currently faces numerous challenges, including human health, environmental, and socio-political threats, prompting decision makers to enhance solid waste management (SWM) schemes with more integrated approaches [9].

Indeed, the major challenges facing Tunisia are the lack of operational enforcement of national regulations and strategies, sustainable financing, public awareness, trained staff, reliable data, basic know-how, and appropriate technologies. Thus, decision makers have access to limited tools when it comes to selecting adequate treatment/disposal techniques to increase the effectiveness of systems in terms of dealing with local conditions [10].

Confronting climate change and minimizing GHG emissions is a major challenge for the entire planet. Consequently, worldwide agreements have been launched to generate strategies to increase resilience against climate change from the perspective of sustainable development. Tunisia is one of the countries that, in 2016, committed to the low-carbon strategy of the Paris Agreement, and signed the “global methane pledge” declared during the COP 26 led by the United States and the European Union aimed at reducing methane emissions by 30% by 2030 compared to 2020 [11]. In this context, waste management stands out as a matter of global significance. Landfills, in particular, are responsible for nearly 5% of the total global GHG emissions and contribute to 12% of the world’s methane emissions, a potent GHG with an impact more than 20 times that of carbon dioxide [12]. Tunisia is responsible for 0.08% of global GHG emissions [13], and in 2022, the net GHG emissions were 48,883 kt of CO₂ equivalent [14]. The waste sector is responsible for 8% of emissions, largely dominated by solid waste making up 67% of the sector’s emissions [15].

The city of Bizerte is a coastal city located in the north of Tunisia on the southern shore of the Mediterranean Sea. This has made its location extremely strategic and has elevated it to the status of the chief town of the governorate of Bizerte and an important economic center [16]. The municipality of Bizerte covers an area of 45,882.00 hectares. The city is composed of five districts: Medina, Aïn Mariem, Zarzouna, Hached, and Louata. According to the municipality of Bizerte, the number of inhabitants increased from 114,371 in 2004 to 182,623 in 2022.

The municipality of Bizerte is interested in improving its strategy for waste management, with the aim of reducing landfilling, which currently accounts for 95% of all waste collected [17], as much as possible, increasing recycling rates, and promoting waste recovery. As a result, GHG emissions from the waste management sector will be reduced.

The amount of municipal waste generated in Bizerte in 2022 was 50,524 tons according to the municipality of Bizerte. In Bizerte, the absence of selective waste sorting complicates the application of recovery solutions, but in the long term, mechanical biological treatment (MBT) would appear to be an effective solution in line with the conditions pertaining to the Bizerte municipality. MBT facilities have become increasingly prevalent in the handling of municipal solid waste (MSW) as well as separately collected waste materials. The mechanical component of MBT plants is designed to efficiently recover materials such as plastics, glass, metals, and high-calorific alternative fuel, commonly known as Refuse-Derived Fuel (RDF). Meanwhile, the biological aspect of an MBT facility serves to stabilize the organic fraction of municipal solid waste (OFMSW) that has been mechanically separated from MSW through sieving [18,19,20].

In the short term, as the city of Bizerte is a green city with several public gardens and green spaces, the maintenance of trees and shrubs generates a large amount of green waste (GW). The idea of out of sight, out of mind can no longer be applied, especially when the amount of GW in Bizerte was estimated to be 2000 tons/year in 2018 according to the municipality of Bizerte. Therefore, the recovery of GW using a sustainable solution is a necessity, and given that GW is not mixed with other waste, composting would represent a judicious practice for recovering this waste and producing high-quality compost. Waste management in Bizerte is primarily based on landfilling all types of non-hazardous waste. There is currently no legislation prohibiting the landfilling of GW. The municipality of Bizerte is responsible for waste collection and transfer to the Bizerte Transfer Center operating under the authority of the ANGED, which handles waste landfilling. The municipality of Bizerte covers the landfilling expenses through the agency and bears the costs of collection and transfer. The primary expense involved in managing GW is associated with its collection and transportation to either landfills or treatment facilities.

Utilizing composting is an appropriate approach for recycling GW as the resulting compost serves as a valuable organic amendment and/or substrate that be reintegrated into the economic cycle effectively [21]. Composting organic waste involves the utilization of aerobic microorganisms to transform waste materials into compost, which serves as a valuable resource for soil enhancement [22,23]. A positive aspect lies in the fact that GW often displays low levels of micro-pollutants. This factor contributes favorably to the production of compost with suitable properties, aligning with the quality standards and utilization restrictions mandated for compost within organic farming systems [24]. In addition to the purpose of improving the quality of soil, reducing the negative impact of chemical fertilizers on the natural environment, and solving the problem of waste accumulation, composting is an approach well adapted to waste flow.

To achieve successful composting, influencing factors, such as temperature, moisture content, aeration rate, pH, C/N ratio, and composting materials, should be appropriately controlled. Among the influencing factors, the aeration rate is regarded as one of the key factors affecting the composting process and ensuring compost quality [25]. Our research consists of the setting up of a composting station established in the context of a project initiated in 2017 as part of a partnership agreement between the Tunisian municipality of Bizerte and the German municipality of Rostock.

Furthermore, to identify robust solutions adapted to the context of the municipality of Bizerte, based on credible data and an understanding of specific needs, this study is structured around precise objectives. The methodology commences with a comprehensive questionnaire designed for the municipality of Bizerte to determine the available data. An in-depth characterization of Bizerte’s waste is then needed, aiming at establishing a complete and clear database. This is followed by the first step to resolve the problem of waste management in Bizerte, which is the setting up of the composting station. Consequently, a description of the project is provided, highlighting the environmental and practical benefits of this initiative. This study also examines measures to control stability, maturity, and quality during compost production at the Bizerte composting station, ensuring compliance with standards. Finally, it assesses the overall impact of two waste management solutions—composting and utilizing an MBT plant—on reducing GHG emissions, demonstrating their contribution to environmental sustainability.

2. Background Information and Waste Characterization in Bizerte

To develop a comprehensive methodology and effective solutions for addressing waste management and mitigating greenhouse gas emissions in Bizerte, it is vital to conduct a thorough examination of the background information pertinent to the region’s waste management practices and associated environmental challenges. A key aspect of this understanding is the characterization of waste in Bizerte, which represents a critical step towards establishing a robust and comprehensive database in accordance with established standards.

2.1. Context of the Municipality of Bizerte

The municipality of Bizerte comprises five districts, including Medina, Aïn Mariem, Zarzouna, Hached, and Louata, as illustrated in Figure 1. It is located in the northern coastal region of Tunisia, bordered by the Mediterranean Sea. The municipality enjoys a strategic geographical position.

The ongoing population growth directly impacts the overall amount of waste generated, underscoring the importance of effective waste management to prevent environmental issues.

Table 1. Population percentage in Bizerte districts in 2014 [26].

District	Population
Medina	22.2%
Ain Mariem	40.5%
Hached	17.8%
Zarzouna	17.8%
Louata	1.7%

presents the percentage of population in every district in 2014.

As far as the waste management strategy of Bizerte is concerned, the focus is on the collection and transfer of mixed waste. It is important to mention that there are no source separation practices as Bizerte does not currently implement a sorting-at-source approach. The responsibility for waste collection and subsequent management lies with the municipality, specifically the Municipal Hygiene Service.

Based on the results of the questionnaire obtained from the municipality, waste management in the municipality of Bizerte is centered around the landfilling of waste at the public landfill of Beni Nafaa, which is located 12 km from the city center. Before being transported to the public landfill, the municipality directs waste to a transfer center. The Menzel Jmil transfer center is located 5 km from the Zarzouna district, while the El Massida transfer center is 3 km from the city center. For waste collection and transportation purposes, the municipality has around 31 gasoline-fueled trucks, while for waste handling, it has five items of equipment.

The amount of household waste generated by the municipality in 2021 was 50,524 tons per annum, in addition to 1500 tons per annum of commercial waste, 2500 tons per annum of construction waste, and 2000 tons per annum of green waste. Household waste is collected using garbage bags, containers, waste bins, and communal collection containers. Figure 2 presents a graphical representation of the quantity of waste collected in the year 2022.

2.2. Waste Characterization in the Bizerte Municipality

Undertaking sorting analysis and waste characterization is of significant importance with regard to effective waste management strategies. This process provides critical insights into the composition of municipal waste, thereby helping authorities tailor their approaches to address the handling of specific waste types. By understanding the proportions of organic, recyclable, non-recyclable, and hazardous materials, municipalities such as Bizerte can make informed decisions about resource allocation, recycling initiatives, and landfill diversion. Ultimately, such an analysis empowers communities to adopt targeted measures, minimize environmental impact, and pave the way for a more environmentally conscious and resource-efficient future.

The waste sorting analysis in Bizerte municipality adheres to established standards for accurate results. These standards include the SAXO method of Germany [27,28] and the MODECOM model, which has been adapted to Tunisian conditions. The MODECOM model is referenced in the standard NF XP X 30-413 [29] for bin sampling and the standard NF XP X 30-408 [30] for waste sampling. Both the SAXO method of Germany and the MODECOM model adapted to Tunisian conditions emphasize sorting waste into specific categories and subcategories. The flow chart in Figure 3 presents the process of the sorting analysis according to the German standards, the MODECOM model adapted to Tunisian conditions, and the procedure applied during the characterization of Bizerte municipal waste.

To determine the waste characterization of Bizerte’s municipal waste, especially as the population distribution is uneven, it is necessary to determine the different categories or sub-categories of waste in each district according to the following formula:

$$\mathbf{\%}\raisebox{1ex}{$\mathit{c}\mathit{a}\mathit{t}\mathit{e}\mathit{g}\mathit{o}\mathit{r}\mathit{y}$}\!\left/ \!\raisebox{-1ex}{$\mathit{F}\mathit{r}\mathit{a}\mathit{c}\mathit{t}\mathit{i}\mathit{o}\mathit{n}$}\right.=\sum {(\mathbf{\%}\mathit{M}\mathit{V}}_{\mathit{c}\mathit{a}\mathit{t}\mathit{e}\mathit{g}\mathit{o}\mathit{r}\mathit{y}}\times {\mathbf{\%}\mathit{p}\mathit{o}\mathit{p}\mathit{u}\mathit{l}\mathit{a}\mathit{t}\mathit{i}\mathit{o}\mathit{n}}_{\mathit{d}\mathit{i}\mathit{s}\mathit{t}\mathit{r}\mathit{i}\mathit{c}\mathit{t}})$$

As there is no specific guidance for the fraction below 20 mm in the Tunisian standard, the German standards were used, which indicates that this fraction contains 25% organic material. However, in this context, it is assumed that the organic fraction constitutes 30% of the materials for fractions under 20 mm. Given the uncertainty in these assumptions, it may be advisable to conduct further analyses at a later time to obtain more accurate data. Percentages are calculated using the following formula:

$$\mathbf{\%}\raisebox{1ex}{$\mathit{c}\mathit{a}\mathit{t}\mathit{e}\mathit{g}\mathit{o}\mathit{r}\mathit{y}$}\!\left/ \!\raisebox{-1ex}{$\mathit{F}\mathit{r}\mathit{a}\mathit{c}\mathit{t}\mathit{i}\mathit{o}\mathit{n}$}\right.=\sum (\mathbf{\%}\raisebox{1ex}{$\mathit{c}\mathit{a}\mathit{t}\mathit{e}\mathit{g}\mathit{o}\mathit{r}\mathit{y}$}\!\left/ \!\raisebox{-1ex}{$\mathit{F}\mathit{r}\mathit{a}\mathit{c}\mathit{t}\mathit{i}\mathit{o}\mathit{n}$}\right.\times \mathbf{\%}\mathit{F}\mathit{r}\mathit{a}\mathit{c}\mathit{t}\mathit{i}\mathit{o}\mathit{n})$$

The waste characterization of Bizerte municipal waste, obtained as a result of this research, is summarized in Figure 4, which presents a pie chart illustrating the waste composition.

The waste characterization analysis conducted in the municipality of Bizerte was carried out by systematically sampling all five districts of the municipality. This sampling approach considered various geographical, socio-economic, population density, rural, and urban conditions, aiming to achieve a representative sampling with regard to all parts of the municipality. The waste characterization analysis reveals a significant organic fraction, comprising 54% of the total waste, which is also a common finding in the majority of Tunisian municipalities. During the sorting process, it is noteworthy that citizens tend to initially separate kitchen waste, but it eventually gets mixed with other wastes. In cases where source sorting is absent, it would be advisable to commence the separate sorting of the organic fraction, starting with a small city as a pilot endeavor. Such a project could yield valuable insights and identify benefits. This approach aims to recover the organic fraction by composting, for example, and minimize the volume of waste sent to landfill.

The plastic fraction constitutes 12% of the total waste, with 61% of this plastic fraction being in the form of plastic bags and packaging. The quantity of plastic bags is particularly high. Although a ban on plastic bags had been mandated by law in Tunisia since 2020, under Government Decree No. 2020-32 dated 16 January 2020, which specifies the types of plastic bags whose production, importation, distribution, and possession are prohibited in the domestic market, it was only enforced in March 2023 when the Ministry of Environment prohibited single-use plastic bags in bakeries [32].

In response, on 2 June 2023, Tunisian factory workers employed in factories producing plastic bags organized a protest led by the Professional Association of Plastic Manufacturers against the Ministry of Environment’s decision to ban plastic bags [33]. To achieve a compromise between all the parties involved, workers are allowed to continue producing plastic bags and taxes are imposed on plastic bag production. The revenue generated in this way can then be utilized to support the recycling efforts in Tunisia. Another type of plastic waste is PET bottles, making up 19% of plastic waste, but the recycling sector remains poorly organized, depending heavily on the informal sector. To support recycling, municipalities can follow simple methods such as providing bins for the collection of plastic bottles and, in collaboration with the informal sector, can significantly reduce the amount of waste sent to landfill and minimize transport costs.

Paper and cardboard account for 9% of municipal waste in Bizerte. The demand for paper has increased considerably around the world in recent decades to around 420 million tons a year [34]. To avoid forests being cut down for the manufacture of paper, it is important to increase the recycling of this resource. In Tunisia, there are few entities active in the field of paper recycling. The creation of collection points for paper and cardboard is a simple and feasible solution, and setting up paper bins in primary schools is an adequate solution, especially as these schools are close to all citizens and, in this way, children’s awareness of recycling will also be raised.

3. Proposed Solutions for Bizerte Waste Management

Addressing the dual challenge of effective waste management in the municipality of Bizerte requires a comprehensive strategy for both sorted and residual solid wastes. In the current context of the municipality, which is characterized by an absence of sorting at the source, composting emerges as a practical solution for managing green waste from gardens and public spaces. When successfully implemented in a local project, green waste composting provides an eco-friendly method to convert organic waste into nutrient-rich compost, contributing significantly to sustainable waste reduction. As for unsorted residual waste, mechanical biological treatment (MBT) presents an alternative solution. Unlike composting, MBT is specifically designed to manage mixed and residual wastes, employing a combination of mechanical sorting and biological treatment processes. This approach aims to recover valuable materials and minimize the environmental impact associated with landfill disposal.

3.1. Bizerte Green Waste Composting

Composting green waste serves as Bizerte’s immediate strategy to minimize landfill contributions and, consequently, mitigate greenhouse gas emissions. This exploration delves into Bizerte’s green waste composting initiative, emphasizing compost materials, methods, and properties. The analysis aligns with scientific scrutiny and established standards and is complemented by a comprehensive financial assessment.

3.1.1. Materials and Methods

The municipality of Bizerte is responsible for the collection of GW from gardens and public spaces. The agents of the municipality collect this waste and crush it on-site. They then transport the crushed waste to the composting plant of the municipality of Bizerte, where it will be mixed and placed in windrows. To fix the humidity of these windrows, water will be added, and then temperature and humidity follow-up will be performed. During the composting process, the windrows are turned to ensure that aerobic fermentation takes place. After 24 weeks, the mature compost is gathered and screened. Finally, a sample will be taken and sent to the laboratory to be tested to ensure that it adheres to the standards. Figure 5 summarizes the working steps.

Description of the Composting Plant

The composting station is in a nursery of the municipality of Bizerte, which is located on the Bizerte Street Othman el Allouch, as depicted in Figure 6.

The composting method used is based on the principles of windrow technology. This takes place in an open site over a 6-month period. The composting plant is subdivided into parts, with each one being destined to realize an independent task, as illustrated in Figure 7. These include the reception area, the fermentation zone, the sieving area, and the storage zone.

Composting Materials and Pile Design

The research work was conducted on GW collected from public gardens in the municipality of Bizerte. This work was dedicated to achieving a high quality of compost and monitoring the necessary parameters, while adjusting the quantity of waste deposited in windrows based on the available waste, as well as the proportions of GW in the initial mixtures, in order to study the evolution of organic matter (OM) during the composting process. The GW exclusively included dead leaves, dry palms, tree branches, and grass clippings, which were chopped into approximately 5 to 10 mm pieces. The raking process was carried out on-site. The waste was then transferred to the station to be piled up. This article discusses the analyses of three of the windrows created at the composting station.

As the starting materials used were dry, the first watering was carried out using tap water as soon as they were placed in the windrows to keep the moisture level between 40% and 60%.

Sampling and Compost Analysis

The sampling was performed according to the French standard–[36] after the beginning of the composting process, and the compost was monitored for 6 months. Chemical analysis was conducted on two samples: sample t_i (initial mixtures from piles 1, 2, and 3) and sample t_f (final composts C1, C2, and C3). The representative samples were collected by dividing the pile into five equal sections. The samples were deep frozen (−20 °C) and stored until analysis. The temperature, pH, and moisture content were monitored daily during the composting period.

• Physical–chemical characterization

Table 2. Parameters of sampling and references.

Parameter	Method	Reference
Total Kjeldahl nitrogen (TKN)	Kjeldahl method using Kjeldahl Gerhardt Vapodest	NF ISO 11261 [37]
pH		NF EN 12176 March 1998 [38]
Temperature	Thermocouple with probe
Total organic carbon (TOC)	Colorimetric method	ISO 14235 [39]
C/N ratio	Expressed as ratio of (TOC/TKN) %
Mineral composition (Na2O, K2O, CaO, MgO, and P2O5) and heavy metals (Cu, Zn, Fe, and Mn)	Inductively coupled plasma atomic emission spectroscopy (ICP-AES) PERKIN ELMER 3300 RL	NF EN ISO 11885 [40]

summarizes the frequency of sampling, as well as the parameters that were analyzed, along with their corresponding standard methods.

• Microbiological analysis

This analysis consisted of the enumeration of fecal coliforms, total coliforms, Escherichia Coli, salmonella, and helminth eggs. For Escherichia Coli, the samples were incubated at 44 °C for 24 h in peptone water. The positive tubes formed a pink ring in the presence of a few drops of Kovacs reagent. For fecal streptococci, inoculation was in Rothe’s medium and incubation was at 37 °C for 24 h. The positive tubes showed cloudiness and a white ring at the bottom of the test tube. Confirmation was performed using Litsky’s medium with incubation at 37 °C for 48 h. The tubes that presented turbidity and a purplish ring were considered positive. In the case of salmonella, detection was performed in 4 steps according to the standard NF V 08-052 (1997).

3.1.2. Results and Proprieties of the Produced Compost at the Bizerte Composting Plant

Physical and Chemical Characteristics of the Raw Materials

The physico-chemical characteristics of the raw materials are important factors to consider to guarantee a successful start to the composting process. The characteristics of the organic matter used are summarized in

Table 3. Characterization of raw materials and initial blends.

Parameters	C1	C2	C3
TS (%)	75.1	6.25	42.84
pH	6	5.2	5
Temperature (°C)	20.9	25	19
Organic carbon (%C·TS)	75.1	14	42.84
Total Kjeldahl nitrogen (NTK) (%N·TS)	2.5	0.56	1.02
C/N ratio	30.04	25	42
Calcium (%Ca·TS)	8.32	6.09	5.075
Potassium (%K·TS)	0.565	0.165	0.232
Phosphorus (P) (%P·TS)	0.211	0.312	0.132
Sodium (Na) (%Na·TS)	0.125	0.201	0.95
Iron (mg·kg−1·TS)	2.56 × 103	1.24 × 103	1.56 × 103
Manganese (mg·kg−1·TS)	96	120	150.3

.

C/N Ratio

Measuring the carbon-to-nitrogen (C/N) ratio at the beginning of the composting process is of paramount importance. This ratio serves as a crucial indicator of a compost’s potential for effective organic matter decomposition [41]. Insufficient nitrogen sources decelerate the decomposition process, whereas mixtures containing a low carbon content release excess nitrogen as ammonia during turnover [42]. By assessing the C/N ratio, we can determine the balance between carbon-rich materials, such as dried leaves and straw, and nitrogen-rich components, such as green plant waste and manure. Achieving an optimal C/N ratio is vital for promoting the activity of composting microorganisms, as it ensures a favorable environment for their growth and efficient organic matter breakdown. By carefully measuring and maintaining the C/N ratio during the initial stages of composting, we can enhance the overall composting process, accelerate decomposition, minimize odors, and produce nutrient-rich compost that benefits plant growth and soil health. The C:N ratio at the beginning of the bioprocess is generally between 20 and 30 [43,44,45].

Temperature Evolution

Temperature is the main parameter that indicates good progress during the composting process. Ambient temperature is needed to understand the evolution of a windrow’s temperature. Daily measurement of temperature taken over 6 months made it possible to draw the curve shown in Figure 8. The study of the evolution of temperature shows that C1 reached the peak temperature of 69 °C after 4 months, which shows that the mesophilic phase is slow. In the case of C2, the maximum temperature of 70 °C was obtained after about 3 months and 22 days, while C3 reached 71 °C after 2 months. Consequently, it was observed that windrow C3 presented the shortest mesophilic phase and reached the maximum thermophilic phase compared to C1 and C2 [46,47]. Therefore, as the percentage of grass increases, the thermophilic phase is reached more quickly and lasts for a shorter period (4 months). This means that thermophilic microorganisms are more important in C3 than in C1 and C2, and the condition for the proliferation of such organisms is more favorable.

The maintenance of a temperature above 55 °C for 4 consecutive months meets the compost evaluation requirements in terms of guaranteeing the elimination of pathogens [48]. The increase in temperature is followed by a decrease. This can be explained by a decrease in the activity of thermophilic microorganisms [44]. The active phase of composting is followed by a period of maturation during which the temperature of the compost gradually decreases. At this stage, another group of microorganisms appears, which are responsible for the important step of decomposition of materials composed of vegetable cell membranes such as cellulose. The maturation period removes the risks associated with the use of immature compost [49]. Finally, the temperature decreases to the ambient temperature.

Monitoring of End-Product Quality Indicators

• Heavy metal content vs. Tunisian compost standards

The presence of heavy metals (HMs) is crucial for promoting plant growth; however, excessive concentrations can have detrimental effects on plant metabolism. Extensive research has demonstrated the importance of evaluating the nutritional value of a compost throughout the composting process, taking into account additive amendments and the degradation of organic matter. This assessment is vital to ensure that the compost maintains an optimal balance of nutrients, facilitate healthy plant development, and mitigate the potential inhibitory effects of high heavy metal levels. By monitoring and optimizing the nutritional content of compost, we can enhance its efficacy as a plant growth promoter, while minimizing the risks associated with elevated heavy metal concentrations [50,51].

Heavy metals can contaminate the human and animal food chain [42]. According to Gobat et al. (2010), the quality and use of the final compost is directly influenced by the degree of contamination of the starting substrates by toxic substances [52]. Trace elements accumulate in the final compost during the mineralization of organic matter and are not degraded [53]. Their solubility depends on many environmental factors, such as pH and the presence of ions, chelants, etc. [52]. The content of trace elements is one of the parameters that has to be controlled to ensure the quality of compost [54].

Based on our results shown in

Table 4. Heavy metal contents vs. Tunisian compost standards.

Parameters	C1	C2	C3	NT 10.44
Zinc (Zn) (mg·kg−1·TS)	114	98.2	62	600
Copper (Cu) (mg·kg−1·TS)	30	96	53.26	300
Lead (mg·kg−1·TS)	52.7	32.7	32	180
Cadmium (mg·kg−1·TS)	0.388	0.198	1.02	3
Nickel (mg·kg−1·TS)	6.57	3.57	9.2	60
Arsenic (mg·kg−1·TS)	4.63	2.63	3.12	18
Chromium (mg·kg−1·TS)	17.5	20.3	12.9	120

, we can see that all concentrations of heavy metals are lower than the Tunisian standards. In addition, the results indicate the high quality of the three composts.

• Microbiological tests

During aerobic composting, temperature fluctuations are the primary drivers governing the metabolic reactions [41]. These fluctuations significantly impact the variety, quantity, and types of microorganisms involved in the biological process. In fact, elevated temperatures lead to the deactivation and elimination of most pathogens [42]. Microbiological tests are important for evaluating the sanitary quality of a compost. In situations where fertilizers carry pathogens such as E. coli and Salmonella, there is a potential for soil and plant root contamination [55].

The results are positive for total coliforms, fecal coliforms, Escherchia coli, and fecal streptococci. However, their concentrations remain below the limit values authorized by the NFU 44-051 standards, as shown in

Table 5. Microbiological test.

Parameter	C1	C2	C3	NFU 44-051
Total coliforms	2.3102	4.26102	9.121	<102 g/TS
Fecal coliforms	92	105	5.31	<102 g/TS
E. Coli UFC/g	36	45	1.021	<102 g/TS
Fecal spectrococci UFC/g	1.1102	5.1302	1.0610	<102 g/TS
Salmonella	Absent	Absent	Absent	Absent

.

• P and K contents of composted material

The determination of compost quality should be restricted to the abundance of desirable nutrients, specifically potassium (K) and phosphorus (P) [56]. From an agricultural perspective, fertilizers play a crucial role in supplying soil with vital nutrient elements necessary for the various growth stages of crops. Macro-nutrient elements in the form of nitrogen (N), potassium (K₂O), and phosphorus (P₂O₅) have a significant impact on plant development. Typically, microbial activity mineralizes a fraction of these nutrients throughout the composting period, contributing to the enriched nutrient content of the end products [57]. The end products C1, C2, and C3 were characterized by a K rate of 0.648, 0.213, and 0.103 K%TS, respectively, which are adequate according to the Tunisian standards NT10.44, as is the case with regard to the P rates of 0.184, 0.103, and 0.295 P% TS for C1, C2, and C3, respectively, as outlined in

Table 6. K and P rates compared to the Tunisian standards.

Parameter	C1	C2	C3	NT10.44
K%TS	0.648	0.103	0.213	<102 g/TS
P% TS	0.184	0.295	0.103	<102 g/TS

.

3.1.3. Financial Analysis

A notable contrast exists between developing and developed countries when it comes to waste management approaches. Developed nations prioritize investment in high-cost systems for waste management and treatment, while developing countries continue to rely on inefficient management and treatment practices [58].

In contrast to the prevailing waste management approaches used in developing countries, there have been notable efforts in Tunisia in recent years to address the issue more effectively. The investment in Bizerte in terms of the composting plant reflects a shift towards adopting sustainable waste management practices in the region.

Composting offers a more environmentally friendly and cost-effective solution for treating GW compared to traditional inefficient practices.

The cost analysis of establishing a composting plant in Tunisia offers a comprehensive evaluation of the financial aspects associated with the project. The initial investment costs encompass land acquisition, infrastructure construction, equipment procurement, and permit acquisition to ensure compliance with regulations. Additionally, the operational costs include personnel salaries, feedstock preprocessing, utilities, maintenance, and monitoring expenses. It is crucial to consider the costs associated with feedstock management, including collection and transportation from various sources, as well as any necessary preprocessing steps. It is important to conduct a thorough financial analysis, such as calculating the Return on Investment (ROI) and the Payback Period.

The ROI measures the profitability of an investment relative to its cost. It can be calculated using the formula:

$$\mathit{R}\mathit{O}\mathit{I}=\frac{(\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{j}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{ }\mathrm{R}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{u}\mathrm{e}-\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{ }\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{ }\mathrm{C}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{s})}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{ }\mathrm{i}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{ }\mathrm{a}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t}}$$

The Payback Period represents the time required to recover the initial investment. It can be computed as follows:

$$\mathrm{P}\mathrm{a}\mathrm{y}\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}\mathrm{ }\mathrm{P}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{o}\mathrm{d}=\frac{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{ }\mathrm{I}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{ }\mathrm{A}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t}}{(\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{j}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{ }\mathrm{R}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{u}\mathrm{e}-\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{ }\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{ }\mathrm{C}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{s})}$$

A sensitivity analysis allows the examination of potential variations in compost sales prices, feedstock availability, and operating costs in order to understand their influence on a project’s feasibility. By considering these cost factors and conducting a rigorous financial analysis, decision makers can gain valuable insights into the economic sustainability of establishing a composting plant in Tunisia. The costs of construction and acquiring technology for the treatment, as well as ancillary costs, were estimated based on the concept explained above. The costs of taxes and duties were not taken into account.

Table 7. Investment, operating, and utility costs.

Designation	Cost [TND]
Investment costs for civil engineering (net) Preparation and design Building and construction Outdoor installations	988,000 TND
Investment costs: material/equipment (net) Equipment:    Backhoe loader    Rotary drum screen    Crusher    Steel construction crusher    Other equipment    Conveyor belt Office equipment	507,000 TND
Operating and utility costs    Personnel/year    Utilities/year	80 TND/T 23,400 TND 13,439 TND

summarizes the costs.

According to the municipality of Bizerte, the total investment amount is 1,495,000 TND. Projects that have an environmental impact cannot be solely classified as businesses with material gains, but rather as ventures with social, ecological, and economic implications. The current price of a ton of compost in Tunisia is approximately 400 TND for 1 ton without packaging.

Table 8. Return on Investment and the Payback Period.

Parameter	Cost
Investment Costs	1,495,000
Projected Revenue	192,000 TND/Year
Total Production Costs	38,400 TND/Year
ROI	10%
Payback Period	9.7 years

provides an analysis of the ROI and the Payback Period associated with the project. The table details the key parameters, including investment costs, projected revenue, total production costs, ROI percentage, and the estimated Payback Period. These financial metrics offer valuable insights into the economic viability and sustainability of the undertaken initiative.

From the perspective of a circular economy, this project proves to be highly profitable in terms of managing GW. It not only reduces the amount of waste sent to landfills but also has agronomic benefits as the compost produced can enrich soil and enhance agricultural production. Moreover, by minimizing the emission of greenhouse gases resulting from the fermentation of landfilled waste, this project also contributes to combating climate change. Furthermore, composting indirectly impacts human health by promoting the consumption of organic products instead of those contaminated by pesticides and chemical fertilizers, which can lead to both short-term and long-term diseases.

3.2. Mechanical Biological Treatment of Residual Waste in Bizerte

The most common types of MBT processes are as follows: MBT with a significant landfill fraction, incorporating both aerobic and anaerobic processing to generate biogas, either in wet or dry conditions; MBT for the production of Solid Recovered Fuel (SRF/RDF); and MBT with wet separation to achieve high material recovery. For Bizerte, the most suitable MBT approach appears to be the one with a significant landfill fraction, combining aerobic biological treatment.

The MBT process yields a high-calorific fraction as an output, which is then utilized to produce RDF. This RDF serves as an eco-friendly alternative energy source. Moreover, the presence of a nearby cement plant, which harnesses this heat to elevate temperatures to the range of 1450 °C for the calcination of raw materials [59], resulting in the production of clinker, presents a synergistic opportunity to make efficient use of this RDF. The process also yields recyclable materials, and it effectively stabilizes the organic fraction, significantly reducing the quantity of waste destined for landfill disposal.

The diagram in Figure 9 presents the input and output processes of the MBT with the percentages inputted into the Sweet software simulation.

4. Greenhouse Gas Emissions

For the municipality of Bizerte, waste management is not only an opportunity to increase the recycling rates and to recover waste in such a way as to ensure sustainability and a circular economy, but it is also an opportunity to minimize greenhouse gas emissions from the waste sector when compared to the usual landfilling scenario. This section is dedicated to describing the creation of two scenarios, one in the short term, with its implementation started in 2020, which is the opening of the green waste composting station, while the other scenario is a long-term and reliable solution for waste management for the municipality, which is the implementation of the MBT plant. These scenarios were developed based on data collected from questionnaires within the municipality. The composition of the waste used was based on the result of the waste characterization mentioned at the beginning of the paper. These scenarios were simulated using the Solid Waste Emission Estimation Tool as part of the Sweet software, allowing us to compare greenhouse gas emissions from the municipality of Bizerte with the baseline scenario in the form of usual landfilling. Abt Associates and SCS Engineers, in collaboration with the Environmental Protection Agency (EPA) and the Climate and Clean Air Coalition Municipal Solid Waste Initiative, jointly created this tool. This tool was designed to facilitate the estimation of annual pollutant emissions, with a particular emphasis on methane and black carbon [60].

The baseline scenario only accounts for landfilling and recycling, with the quantity of recycled waste in Tunisia not exceeding 4% of the total household waste generated annually [61].

The public landfill in Beni Nefaa, Bizerte, was established in 2004. In 2012, the government initiated the extraction of biogas from this landfill. According to the ANGED, the amount of methane recovered is 5959 cubic meters of methane per annum.

The tool provides a detailed breakdown of emissions from various pollutants. However, the discussion primarily focuses on the total emissions of CO₂, black carbon, CH₄, and organic carbon due to their significant contribution to global warming. Black carbon appears to be a significant contributor to positive radiative forcing, a key factor in climate change, whose main impact comes from the direct absorption of light energy in the atmosphere. In addition, the global warming potential (GWP) of carbon monoxide is highlighted as being 1.9 times greater than that of carbon dioxide [62], as reported by the Intergovernmental Panel on Climate Change (IPCC) in 2007. Elemental carbon resulting from combustion has a remarkably higher GWP at 680 times that of CO₂ [63], as shown in the studies by Bond and Sun in 2005. This underlines the urgency of tackling the emissions of these specific components, each of which makes a distinct contribution to the complex dynamics of climate change, thus requiring targeted strategies to mitigate their impact on the environment. Figure 10 illustrates the total emissions from the waste management sector in the municipality of Bizerte, measured in terms of CO₂ equivalents. This is under the Business-As-Usual (BAU) scenario. The graph clearly indicates that emissions will continue to rise as waste production increases, especially in the absence of effective treatment facilities. It is important to add that recycling in Bizerte began in 2009, which explains the initial decrease in the curve. However, we then see a consistent increase in emissions as waste production grows and treatment facilities remain inadequate. By 2050, emissions are projected to reach 46,432 metric tons of CO₂ equivalent.

According to Paul J et al. [64], composting can be utilized for municipal organic waste management to mitigate methane emissions resulting from the disposal of new organic waste in landfills. The first scenario with regard to green waste composting reveals a slight decrease in greenhouse gas emissions by 2050 compared to the BAU scenario, amounting to only a 3% reduction to 44,978 metric tons of CO₂ equivalent. However, according to the second scenario, the implementation of the MBT plant significantly reduces greenhouse gas emissions by an impressive 28%. This reduction is so substantial that, by 2050, it is anticipated that emissions will amount to only 33,610 metric tons of CO₂ equivalent. These results are depicted in the curve shown in Figure 10 for “Total emissions by scenario including CO₂, BC, CH₄, organic carbon”.

Additional results obtained from the software simulation analysis include a breakdown of baseline emissions by sector. According to these findings, as illustrated in Figure 11, the largest contributor to total emissions is landfills and landfill gas combustion, which is projected to exceed 38,301 metric tons of CO₂ equivalent by the year 2050. The concentration of landfill gas is higher because the municipality of Bizerte uses landfilling extensively. The percentage of organic waste is high, which increases the production of landfill gas. Methane, a greenhouse gas, is particularly concerning as it is approximately 25 times more effective at trapping heat in the atmosphere than carbon dioxide (CO₂) over a 100-year period, and about 84 times more effective over a 20-year period [65,66,67]. This makes methane a significant contributor to the greenhouse effect and to climate change, especially when emitted in large quantities [68]. Following closely is waste handling equipment, accounting for 5872 metric tons of CO₂ equivalent emissions. Lastly, waste collection and transport, assuming a consistent number of vehicles each year, is estimated to contribute 2259 metric tons of CO₂ equivalent emissions by 2050. The municipality of Bizerte uses waste handling equipment and operates gasoline-powered trucks instead of diesel ones, which results in higher greenhouse gas emissions. Diesel vehicles are less harmful as they emit 20% less CO₂ than gasoline vehicles [69].

5. Conclusions

In conclusion, this study addressed the issue of waste management and its implications for climate change in the municipality of Bizerte, Tunisia. To obtain credible data, the use of questionnaires for information collection and waste characterization according to the established standards was carried out. These measures were necessary to provide reliable and suitable scenarios for the conditions pertinent to Bizerte.

The municipality of Bizerte currently deals with a waste composition consisting of 54% organic waste, 15% plastic, and 9% paper/cardboard, as revealed by the waste characterization results. In light of these findings, two scenarios were proposed to address the waste management issue.

The first scenario involves the composting of green waste generated from gardens and green areas within the municipality. This is achieved through the operation of a composting station producing 480 tons per annum of high-quality compost, which meets the standards for essential nutrients such as potassium and phosphorus, both of which are crucial for plant growth. From a financial perspective, the investment in the Bizerte composting station reflects a commitment to adopting sustainable waste management practices. Although the initial investment costs are substantial, the economic benefits and contributions to a circular economy make composting a sound choice. Green waste composting serves as a successful model of sustainable waste management, combining environmental responsibility with economic viability.

The second scenario involves the implementation of an MBT plant, which increases recycling, a practice that does not exceed 4% in Tunisia. It provides Refuse-Derived Fuel (RDF) for the cement plant in Bizerte and stabilizes the organic fraction, thus reducing the quantity of waste directed to public landfills. Waste management in the municipality of Bizerte goes beyond recycling and sustainability; it also plays a pivotal role in mitigating greenhouse gas emissions. The composting plant reduces greenhouse gas emissions by 3%, while the MBT plant reduces them by 28%. This significant reduction contributes to Tunisia’s positive results, especially as it is a participant in the Global Methane Pledge. Addressing greenhouse gas emissions in the waste sector is a crucial step towards Bizerte’s sustainability and environmental well-being. While green waste composting represents a positive initiative, the MBT plant emerges as a game changer in terms of significantly reducing emissions and aligning waste management practices with climate change mitigation goals.

The composting station represents an initiative that would be highly beneficial if implemented across all municipalities. Its success in Bizerte highlights the potential for replication and the widespread adoption of sustainable waste management practices. Similarly, the MBT project holds significant promise if its reach could be extended beyond the municipal boundaries of Bizerte. Establishing MBT facilities for the entire Bizerte governorate, rather than just the municipality of Bizerte, would allow a higher input of waste and the resolution of waste management challenges at the regional level. This approach would lead to more efficient resource utilization and a comprehensive waste management solution that would benefit the entire governorate. Moreover, the applicability of both solutions extends beyond the borders of Bizerte and has relevance for all of Tunisia and the wider MENA region. The concepts demonstrated in this manuscript provide scalable models that can be adapted to varying municipalities and regions with similar waste management challenges. While the focus is currently on Bizerte, the potential replication of these initiatives at the national and regional scales could offer sustainable solutions to assist decision makers in finding reliable waste management solutions.

However, it is essential to acknowledge the limitations of this study. The MBT plant, as presented, is particularly suitable for cities generating residual waste volumes ranging between 50,000 and 500,000 tons per year. This implies that while it is a robust solution for numerous urban areas, its optimal effectiveness is within a specific waste generation range. Future research could explore adaptations or alternative approaches to address waste management challenges in regions with varying waste generation profiles.