Deploying a Water-Energy-Food (WEF) Nexus approach to design sustainable solid waste management in Onitsha, Nigeria

Municipal solid waste (MSW) management remains a critical environmental and social challenge for cities in Nigeria. In Onitsha, the situation has become increasingly unsustainable, contributing to environmental pollution, deterioration of city aesthetics, water contamination, unreliable energy supply, and food insecurity. This study explores the application of the Water-Energy-Food (WEF) Nexus framework as an integrated approach to addressing the solid waste management crisis in Onitsha, with a particular focus on resource recovery potential. A survey of 399 respondents was conducted to characterize the composition of solid waste generated in Onitsha and assess its impacts on residents. Principal Component Analysis (PCA) was applied to identify the dominant perceived effects of MSW on the resident population. To select the most suitable waste-to-energy (WtE) technology, the Analytical Hierarchy Process (AHP) combined with Multi-Criteria Decision Analysis (MCDA) was employed to evaluate five candidate technologies: incineration, anaerobic digestion, landfill gas, gasification, and pyrolysis. The dominant waste category identified was food waste, followed by plastics. PCA revealed that the major perceived effects of MSW on residents were susceptibility to waterborne diseases, air pollution, and flooding. The AHP analysis assigned the highest priority weight to anaerobic digestion (0.277), followed closely by landfill gas (0.276) and incineration (0.202), with pyrolysis ranked as the least preferred technology. These findings demonstrate that the WEF Nexus framework offers a viable, integrated pathway for addressing interconnected challenges of waste, energy, and environmental health in Onitsha. Anaerobic digestion emerges as the most contextually appropriate WtE technology, owing to the food-waste-dominant composition of the city's MSW stream. The study's outcomes carry practical implications for solid waste management policy and technology selection in Onitsha and other Nigerian cities facing comparable challenges, as well as for rapidly urbanizing cities in the Global South more broadly.

Urbanization attracts increasing populations to cities, causing lifestyle changes (Jiang et al., 2021). This causes increased demand for resources like food, energy and water, as well as for technology, leading to increasing solid waste generation and concomitant pollution impacts and deterioration of the environment. The problem of managing huge volumes of solid waste generated, lack of knowledge on the volume of solid waste generated, the inadequacy of the collection system, and inability to characterize municipal solid waste has become a vital environmental issue (Nwakoby et al., 2020) and as such, also a social and health issue bedeviling many cities in Nigeria. Nigeria generates some 32 million tons of solid waste per year, among the highest rates of solid waste production in Africa (Ayodele, 2022). Under current consumption behaviors, solid waste management practices, and the policies in place, the effect of population expansion on increased solid waste generation is likely to seriously augment implications for the sustainable development of Nigerian cities.

Governments have made efforts to tackle this problem, but few successes have been recorded, with most of the successes limited to waste collection efforts and disposal (Sharma et al., 2021). To achieve target 11.6 to “by 2030, reduce the adverse per capita environmental impact of cities including by paying special attention to air quality and municipal and other waste management” (Rodic and Wilson, 2017) of the UN Sustainable Development Goal (SDG) 11, the current solid waste management practices in the city of Onitsha requires rethinking to make the city and its human settlements inclusive, safe, resilient, and sustainable (SDG Compass, 2015). Cities tend to plan inside sectors, treating water, energy and food supply and demand, for example, usually separately. Albeit sustainable development is a complex issue, interlinkages between sectors are inherently apparent in our environment: waste pollutes water, etc. However, inside current administrative structures commonly in place in municipal governments, it is difficult for cities to take an integrated approach. Alongside the waste management issue, Onitsha is also facing an issue of energy access, as there are frequent power cuts, and renewable energy integration, as most energy is still being generated from fossil fuels. Food insecurity is also an issue in Onitsha.

As waste is becoming an increasingly popular source of energy, this study aims to identify dominant waste types by categories of settlements, and assess the Waste-to-Energy (WtE) potentials of these waste categories using different WtE technologies, in order to determine the preferred WtE option for Onitsha.

To facilitate the analysis of this issue, this study applies the Water-Energy-Food (WEF) Nexus approach (Hoff, 2011). The Nexus approach, which posits that the sectors water, energy and food and others such as solid waste, are inextricably linked, is an innovative research and policy framing approach that can support municipal governments to take more holistic and targeted sustainability decisions.

The Nexus approach highlights interlinkages between the water, energy and food sectors, such as that it takes much energy to supply freshwater and remove and treat wastewater, or that much water is needed to produce energy and food (ADB, 2013). The Nexus approach underlines responsible governance (GWSP, 2014) and that a new perception of water is needed, with the water–food link being of the highest social and political significance (ADB, 2013). The Nexus approach posits that applying a cross-sectoral perspective is a key opportunity to tackle sustainability challenges holistically. It can support identifying interconnections, trade-offs and synergy opportunities between different sectors that interact within urban systems to reduce natural resource use inefficiencies (Gondhalekar and Erlbeck, 2021) and hence GHG emissions. The Nexus approach can be deployed to optimize the use of water and energy in infrastructure systems whilst also optimizing food production. As such, the Nexus approach can support natural resource conservation as well as water, energy and food security, and thus climate change mitigation and adaptation.

The Nexus approach has gained much momentum in the past decade, particularly in the European Union, with support of the European Commission. Yet whilst the Nexus approach has enjoyed much attention from academia, So far, only very few cities globally have managed to implement an integrated Nexus approach (Gondhalekar et al., 2018), and its application in practice particularly to cities' sustainable development issues is lagging. This is despite of the Nexus approaches huge potentials to highlight how to leverage on urban waste streams for resource recovery (Gondhalekar and Ramsauer, 2017) and how to turn these wastes into economic benefits (Gondhalekar et al., 2021).

This study explores the application of the WEF Nexus approach to the issue of solid waste management in Onitsha. As Onitsha faces high levels of water and environmental pollution, a gap in reliable energy provision, and food insecurity, the Nexus approach holds promise in tackling these issues simultaneously in an integrated approach by supporting the design of solutions to waste management as a key issue, and by taking a resource recovery focus.

Thus, the application of the Nexus approach is an innovative opportunity in x-raying solid waste management practices in Onitsha will help develop a sustainable framework for waste management by integrating the various levels of waste management in the city and setting the stage for a circular economy in solid waste management in the city. The Nexus approach examines the interests of the different sectors of the environment while respecting their interrelatedness and interdependencies for sustainable management of the environment and improving governance (Lipponen, 2021; Costanza and Kubiszewski, 2016). Within the Nexus approach, solid waste is further processed to produce a combination of valuable products and energy resources (Henze et al., 2001; Feng et al., 2020). With this approach, this study aims to shed light on the untapped wealth in the waste sector to forge a more viable solution to managing solid waste for Onitsha.

This study hypothesizes that a Nexus approach can support cities like Onitsha to develop innovative solutions that can tackle several issues with one approach rather than many parallel actions that are not connected. The Nexus approach has made headway in theory, but remains underrepresented in practice as decision makers require evidence to be able to choose alternative technology options. Hence, this study sets out to generate such much-needed evidence to turn the tables on the unsanitary and dangerous situation caused by inadequate solid waste management.

Background: solid waste management situation in Onitsha

One of the earliest studies on waste management in Onitsha focused on waste scavenging as a recycling option (Nzeadibe and Eziuzor, 2006). In the bid to find a solution to the waste collection problem in Onitsha, studies have explored route optimization for solid waste collection to minimize cost and distance traveled by collection vehicles (Ogwueleka, 2009a). Some of the challenges of waste management include inefficient collection methods, insufficient coverage of the collection system and improper disposal, lack of institutional arrangement, insufficient financial resources, insufficient information on quantity and composition of waste, and inappropriate technology: Thus, given the nature of the waste and prevailing socio-economic conditions in Nigeria, a workable waste management approach in Nigeria must be less capital intensive and labor intensive (Ogwueleka, 2009b). The dominant solid waste component identified from the waste stream is food waste with 40.5% followed by plastic polythene waste constituting 17.9% of the waste stream, and composite of rubber, leather, textile and wood making up 10.1%, and the rate of solid waste disposal in Onitsha is less than the rate at which wastes are being generated (Nwachukwu, 2010). The consequence is the proliferation of refuse heaps and poor sanitary conditions in Onitsha arising from inadequate equipment, poor funding scenarios, and inadequate planning. Emelumadu et al. (2016) examined the practice, patterns and challenges of solid waste management in Onitsha, Nigeria. The major challenge to waste management identified is the irregularity in the collection pattern by the waste management authorities. The authors opined that the state of solid waste management in Onitsha is still poor, as the major practices here include open dumping and burning of wastes, non-availability of ideal sanitary landfill or dump sites and lack of arrangement to collect solid waste from residents in a segregated form. Food waste is a dominant component in municipal solid waste in most cities of Nigeria.

The dominance of food waste in the waste stream of West African countries like Nigeria and its cities has been further asserted by Ogwueleka (2009a) and Nwoke et al. (2020), who further confirmed that the solid waste stream from six cities in Nigeria had organic wastes as the dominant waste category (see Table 1).

One of the most difficult operational problems in Nigerian cities, such as Onitsha, is solid waste collection arising from poor route optimization (Ogwueleka, 2009a). Route optimization is a process of minimizing the overall cost based on the distance traveled by vehicle by determining the most efficient route (Sarmah et al., 2019). The solid waste management situation in Onitsha has remained the traditional collection of solid wastes using trucks and dumping these wastes in landfills, followed by open incineration. Some have also adopted the method of burning waste, which adds to the problem of climate change and environmental pollution. These practices are unsustainable due to increased environmental pollution and the deterioration of city aesthetics. Despite the obvious interlinkages between the quantity of waste generated and the rate of urbanization, population growth, and economic prosperity in Onitsha, the solid waste management problems have persisted, mostly due to the inability of these scientific studies and relevant government agencies to address the importance of sustainability in waste management (Abubakar et al., 2022). Therefore, an innovative approach to solid waste management is needed.

The data needed for this study were collected from both primary and secondary data sources. The primary data was generated through a socio-economic questionnaire survey and an expert interview survey as described below. Secondary data were obtained in a desk-top review of published articles in academic journals related to the subject matter. QGIS software was used to map the city of Onitsha, depicting the location of dumpsites.

Socio-economic survey and principal component analysis (PCA)

The primary data needed for this study were generated through a socio-economic questionnaire survey, with distribution to the general public. The questionnaire was divided into four sections to generate demographic and socio-economic data on the solid waste management situation in Onitsha; perceived effects of solid waste in Onitsha; and the nature of solid waste management practiced in Onitsha.

The sampling of residents in Onitsha was carried out using the population data of Onitsha. By 1991 census, the population figure of Onitsha was 256,941 and using the National Population Commission's growth rate of 2.83 per cent, the 2006 population projected from the 1991 figure of 256,941 was 390,509 (UN-Habitat, 2012). Using the same growth rate of 2.83 and for this study, the 2023 population of Onitsha was projected to be 627,579. According to UN-Habitat (2012), this figure represents the night time population as the daytime population could be up to 1.5 million due to commuting workers and visitors. To determine the sample size, the Taro Yamane formula was used and is given as:

where n is the sample size; N is population size; e is error margin for the study is 0.05.

The calculated sample size is 399. Thus, a total of 399 questionnaires were administered. The data obtained were analyzed using charts and tables. Simple percentages were used to analyse respondents' responses to the questions posed to them. Principal Component Analysis (PCA) was used to isolate the key effects of municipal solid waste on the respondents based on the overall responses generated per district in Onitsha. These effects include the perception of risk pertaining to water-borne diseases, as water and environmental pollution from lack of effective solid waste management have been shown to impact water quality very seriously, as studies from neighboring towns like Awka, Anambra State have demonstrated (Onwuka and Ajator, 2018).

Multi-criteria decision analysis (MCDA) and analytical hierarchy process (AHP) to evaluate waste-to-energy (WtE) technology options

The efficient management of solid waste requires critical decisions to be made, such as the selection of optimal technologies to be employed. To determine an appropriate and suitable WtE technology for Onitsha, a multicriteria decision-making (MCDM) process was applied, under which decision-making takes place considering multiple criteria.

The application of Multi Criteria Decision Analysis (MCDA), a tool for environmental system analysis, is a powerful decision-support tool for identifying sustainable approaches in waste management, particularly in identifying and selecting suitable waste-to-energy (WtE) technology options based on all available criteria (Qazi et al., 2018; Kurbatova and Abu-Qdais, 2020; Ukpanyang et al., 2022) and for suitable siting of landfills (Ajibade et al., 2019). MCDA is a decision-support tool developed for complex multi-criteria problems that include qualitative and/or quantitative aspects of the problem in the decision-making process (Garfı̀ et al., 2009). This was achieved using the Analytical Hierarchy Process (AHP) developed by Saaty (Kurbatova and Abu-Qdais, 2020).

To be able to select the most appropriate WtE option in Onitsha, key experts in waste management in Anambra State were consulted, as well as reviewing key literature for the purpose of identifying criteria and sub-criteria for deciding on suitable WtE alternatives for Onitsha. As a result, three criteria, nine sub-criteria and five WtE alternatives were identified for the study.

The pairwise comparison of the nine sub-criteria was conducted under each of their main criteria, and this resulted in three pairwise comparison matrices with respect to each of the main criteria based on expert opinion. Guidelines were provided in the questionnaire to enable the experts to carry out the comparison, as well as matrices to conduct the pairwise comparison. Saaty's nine-point scale, originally (Kurbatova and Abu-Qdais, 2020), was adopted to enable the experts to scale their comparison in order of importance. The values of the pairwise comparison obtained from the experts for the goal, criteria and sub-criteria and WtE alternatives based on the criteria and sub-criteria were aggregated, and decision matrices were derived for calculating priority weights.

Nine experts with backgrounds in environmental-related issues were consulted and asked to make pairwise comparisons using the main criteria and sub-criteria provided. This small sample size reflects the level of availability of experts knowledgeable about the issue in Onitsha. According to Hsu and Lin (2013), this sample size is also sufficiently representative for the purposes of this study, as the Delphi technique does not require large samples (Hsu and Lin, 2013). The experts had adequate local knowledge of the state of municipal solid waste management. The categories of experts that participated in filling the questionnaire and the details of the experts consulted are shown in the Appendix.

Onitsha is a city in southern Nigeria, about 150 km from the Gulf of Guinea. Onitsha is located between longitudes 60 451E and 60 501 E, and latitudes 60 61 N and 60 101 N. The Onitsha urban area is made up of two local government authorities (LGAs) in Anambra State – Onitsha North and South LGAs. The western border of the city is bounded by the River Niger and Delta State, to the south by Ogbaru LGA, to the east by Idemili North LGA, while Oyi LGA is in the northeast border, Anambra to the East and West LGAs to the north (Figure 1). Onitsha has an approximate areal extent of 52 km 2 (Obeta et al., 2019).

As shown in Figure 1, Onitsha currently has three dumpsites: two (dumpsites A and B) at the eastern and one (dumpsite C) at the southern periphery of the city. The siting of dumpsites at the urban periphery implies long travel distances from the city center, where the majority of waste is being generated, e.g., at markets or central business district areas, to the dumpsites, by trucks or other means of collection of solid waste. This is expected to imply higher GHG emissions if dumpsites were located closer to the city center. In most cities, however, dumpsites are located at the urban periphery, due to lack of space in urban centers, but also because dumpsites are considered undesirable in inhabited spaces due to smell and other externalities, including public health risks from rats and water pollution, etc. As in Onitsha, dumpsites are also often situated on major roads to support accessibility by trucks. Dumpsite A is located directly on the Nkisi River, which is a tributary to the Niger River, implying that river water pollution is also expected to be an issue.

The results below are divided into two sections: results of the socio-economic survey characterizing the current solid waste management situation in Onitsha using the Principal Component Analysis (PCA), and results of the Multi-Criteria Decision Analysis (MCDA) using the Analytical Hierarchy Process (AHP) to evaluate suitable waste-to-energy technology options.

Results of principal component analysis (PCA) of socio-economic survey results on characteristics of the respondents

A total of 399 questionnaires were administered. It was observed that the respondents from the 8 districts of Onitsha were 58.9% males and 41.1% females. Out of 100% of the number of respondents, 15% of respondents were between 18 and 30 years of age; 20% of the respondents were between the ages of 31 and 40 years; 37% had ages ranging from 41 to 60, while 28% of the respondents were in the age group of 61 and above. On the other hand, given that only 28% of the respondents were above 61 years, the result also indicated that the majority of the respondents are within the productive age group, ranging from 18 to 60 years. The age distribution of the respondents showed that over 70% are above 30 years of age and understand the content of the questionnaire. In addition, 52% of the respondents have lived in Onitsha for more than 10 years, while 44% have lived between 7 and 9 years in Onitsha. Thus, the respondents are believed to have adequate knowledge of the environmental conditions in Onitsha.

In terms of the educational qualification of the respondents, about 20% had a primary school education as their highest education. Also, 48% of the respondents attest that they had secondary education as their highest level of formal education, while 24% of the respondents identified as university graduates in various fields. It also reveals that 3% of the respondents identified as having postgraduate degrees, while 5% of the respondents have no form of formal education. The educational characteristics of most of the respondents had a good understanding of the subject of the questionnaire upon explanation. In terms of the respondents' occupation, 33% were traders, 24% stated that they are public/civil servants, while 11% indicated that they are involved in other activities such as artisans/craftsmen mostly involved in metal works and furniture, and some 9% are retired men and women from the public service or that are no longer involved in active service or business. More so, 15% of the respondents are students, while 8% are categorized as others, which includes the unemployed.

Solid waste characteristics and management

To provide insight into the solid waste management situation in Onitsha, the respondents were requested to highlight the category of solid waste that dominates their waste stream. The respondents were asked to select at least two dominant solid waste types as they apply to them daily. This accounts for why the total response is higher than the 399-sample size. The total number of responses for each solid waste type was calculated using the sample size to derive percentages.

The outcome of the questionnaire distributed to obtain information from the respondents on waste characteristics in Onitsha is shown in Table 2. 55% of the respondents perceive food waste as the dominant solid waste generated in Onitsha, followed by sachet water/polythene waste, which accounts for 50%, textile accounting for 26% and plastic waste occupying 23%. Another important waste type is metallic waste at 17% while other waste types, which cover glass waste, take up 14%. Also, the responses were sorted based on the eight districts in Onitsha in order to portray in clear terms how these waste types vary across the districts of Onitsha and are depicted in Figure 2.

Questions were posed to the respondents at the household level to identify how wastes are stored and disposed of in their respective households and neighborhoods. Residents noted that two approaches to storing solid waste were dominant among the majority of the residents of Onitsha. The use of waste polythene bags ranked highest and followed by the storage of wastes in open containers. The use of closed containers ranked third, and 12.6% of the respondents showed that they dispose of their waste directly to dumps. This, however, is a worrisome observation given that it's obviously a common practice in Onitsha for residents to dump waste along street streets, which are not designated dumps (Figure 3). As observed, the percentage of respondents agreeing that, at the household level, wastes are stored in closed containers is lower than those who use open containers, raises a question regarding the safety of household members. As shown in Figure 2, the dominant waste category in Onitsha is food waste. Thus, leaving these wastes in open containers or even partially closed containers within the household endangers the health of household members due to the possibility of odor emanating from these waste containers when decomposition sets in.

On their disposal methods, the disposal of waste straight to open dump sites ranked highest with 41.4% of the respondents (Table 3). This high level of dumping of solid waste is consistent with the findings of other studies e.g., Bill et al. (2015), who found that 61.2% of respondents supported open burning of waste, and only about 34% of solid waste generated was collected at the dumpsites within the metropolis by the waste management agency in Awka, Anambra State. The collection of wastes using compactor trucks, either by government waste management authority or private contractors, ranked second. There is still evidence of disposal of wastes along roads which are not designated sites for waste disposal. The indication by a few of the respondents that burning of waste, disposal in drainage channels and throwing of waste into bushes as options for waste disposal is worrisome. This showcases the possibility of pollution of natural water bodies like the Nkisi River and River Niger that flowed through the city of Onitsha. More so, the choice of disposing waste in open dumpsites ranks highest, raising a concern about the inability of the government to deploy appropriate measures or engage a substantial number of partners or workers to handle the rising waste generation in Onitsha. It also buttresses the fact that residents are majorly involved in evacuating their own wastes with the mindset of getting the waste out of their own vicinity and never caring about how or where the waste that is not properly disposed of ends up. This has continued to pose a problem as the aesthetics of the city are in horrible condition, arising from waste litter defacing parts of the city.

Waste separation and resource recovery potential

The study also sought to identify if waste is separated before disposal at source. The idea of waste separation has not been a popular practice in Onitsha, and the responses obtained varied across the districts. The closest to solid waste separation is the scavenging activities that take place at different dump sites and landfill. As shown in Figure 4, it is evident that more people do not engage in waste separation before disposal despite being acquainted with the knowledge of waste separation as a form of waste management practice. However, a substantial percentage of the people agreed that they indulge in some sort of waste separation, and this is commonly seen in the GRA and Omagba parts of Onitsha, where the literate population and relatively wealthy residents are more concentrated. Only 6% stated that they have no knowledge that waste can be separated at source.

Further question was posed to examine the knowledge of the respondents on the benefits of waste generated within their localities. From Figure 5, it is clear to see that only 34% indicated that solid wastes are a source of revenue to individuals, especially those who indulge in materials scavenging, such as metal scraps and plastic waste collectors. The respondents believed that scavengers earn money from the sale of metal scraps and plastic materials, and as such, waste has provided a means of livelihood. While 58% have no idea of how wastes can be beneficial, 6% indicated that they understand that waste can be used for composting, and 2% responded that wastes can be a source of raw materials for biogas generation. Over the years, in the quest for revenue generation, there has been an upsurge of metal scrap businesses in Onitsha, such as Ozotex metals located at Ochanja market, Onitsha, and the Scrap Dealers and Suppliers' Association Fegge, Onitsha. The recycling of plastic materials and water sachets is also in top gear at the study area.

Effects of solid waste management on residents

On the potential effects of solid waste on inhabitants of Onitsha, the perception of the respondents on the subject matter across the districts of Onitsha is shown in Figure 6. The respondents perceived the major impact of indiscriminate disposal of municipal solid waste (MSW) within their locality to be blockage of drains, followed by degradation of natural beauty, flooding, air pollution, water pollution, and finally diseases.

The issue of blocked drains has been persistent in Onitsha, especially in areas of the city with low-income dwellers, such as Okpoko, Woliwo and where indigenous people of Onitsha reside, especially Otu Onitsha (Figure 7). This was followed by the destruction of aesthetics and the flood occurrence within Onitsha. This outcome is expected given that blocked drains, when not cleared, can lead to flash floods, which most often spread solid waste materials evacuated across the city, defacing it after the rains. There is also the problem of air pollution. This is common for most localities where wastes, especially biodegradable wastes, are dumped indiscriminately or where dumpsites are operated close to residential areas, a common practice in Onitsha. Respondents complained that most often, around morning and evening hours, unknown individuals usually set the waste dump on fire and this disturbs the air quality around their homes. In addition to this, Onitsha has previously been labeled one of the world's most polluted cities, given the high level of small particulate matter concentration (PM10) by Parke (2016). While it is clearly depicted that blocked drains were dominant in Woliwo, Okpoko, Odoakpu, Otu Onicha and Fegge, flooding was the dominant effect of solid waste in Awada, and degradation of natural beauty ranked highest in Omagba and GRA.

Further investigation was conducted to isolate the key effect of municipal solid waste on residents in Onitsha. This was achieved using principal component analysis (PCA). The PCA analysis collapsed the 6 identified effects of municipal solid waste into two significant and orthogonal components (Table 4). The two components explained 76% of the variance in the data with Eigen values greater than 1. The variables with the highest loadings on each of the components were picked. Based on the extracted variables, it is shown that two of the perceived effects of MSW on residents is loaded highly on component 1, and this implies susceptibility to water-borne diseases in the study area due to solid waste. In component 2, two related variables loaded highly, implying the effects due to air pollution and flooding in Onitsha (Figure 8).

Based on the extracted variables, it is shown that two of the perceived effects of MSW on residents are highly loaded on component 1, and this implies susceptibility to water-borne diseases in the study area due to solid waste. In component 2, two related variables loaded highly, implying the effects due to air pollution and flooding in Onitsha.

Sustainability of solid waste management in Onitsha

Respondents were asked to identify the number of waste collection points designated for them across the eight districts of Onitsha. It became obvious that the number of available waste collection points is grossly inadequate. 42% responded that the waste collection points or waste receptacles provided within their locality vary from 3–4 waste receptacles or collection points, 29% showed that these designated waste receptacles vary from 5 to 6 points across the districts. Only 17% acknowledge that the collection receptacles provided for them vary from 1–2 points, and lastly, 12% indicated that no provision was made for waste collection in their locations.

This situation has made solid waste disposal and management inadequate and unsustainable. More so, despite the inadequacy in the number of available waste receptacles or collection points spread across the city, the inhabitants demanded complete removal of all the receptacles from their localities due inability of the Waste Management Authority (ASWAMA) to haul these receptacles when filled with waste. Many of the respondents noted that the designated waste collection points where these receptacles are kept have become a breeding ground for mosquitoes and a persistent source of odor. There is also a clear case of inefficiency on the part of ASWAMA, as it has become evident that the garbage piles up faster than it is removed from the collection points. The unsustainable nature of the municipal solid waste situation in Onitsha is a result of an ineffective solid waste management system being operated in the city, which has been characterized by inadequate equipment, poor staffing, inadequate funding, poor planning, and over-utilization of the existing solid waste management facilities despite increasing population.

Multi-criteria decision analysis (MCDA) and analytical hierarchy process (AHP) to identify Nexus approach-based suitable waste-to-energy (WtE) technology option

The selection of a more sustainable approach to managing waste in Onitsha is important for protecting water and land resources and the public from the effects of solid waste materials. It is evident that a substantial percentage of municipal solid waste generated in Onitsha is disposed of indiscriminately along roadsides and dumped into rivers and drainage systems leading to rivers. These impact surface and groundwater quality, both for human use and aquatic life. Therefore, the Nexus approach to solid waste management in Onitsha focuses on identifying alternative uses of the waste material to bridge the sustainability gap in managing solid waste in the city.

To this end, this study explored the potential of a waste-to-energy (WtE) process in Onitsha, which is currently neither being practiced nor utilized anywhere in the city. Having obtained information on the characteristics of wastes generated across the city, the feasibility of applying various types of WtE options in maximizing efficient use of waste resources, while reducing the current environmental impacts, was explored.

The efficient management of solid waste requires critical decisions to be made, such as the selection of optimal technologies to be employed, considering multiple criteria. Thus, multicriteria decision analysis (MCDA) was employed in determining a suitable WtE technology option to address solid waste management in Onitsha based on expert opinions. The WtE technology options were evaluated using the analytical hierarchical process (AHP) developed by Saaty (Kurbatova and Abu-Qdais, 2020).

Key experts in waste management in Anambra State were consulted, and literature reviews were conducted for the purpose of identifying criteria and sub-criteria for deciding on suitable WtE technology options for Onitsha. As a result, three criteria and nine sub-criteria and five WtE alternatives were identified for the study (Table 5).

To inform the AHP model, five WtE technology options were identified, which are described as follows Figure 9:

Incineration: Involves thermochemical conversion of solid waste by subjecting the waste to combustion at high temperature. Heat is produced from the process and can be used to generate electricity. Emissions from the incineration plants can be harmful, and a major residue is ash;

Anaerobic digestion: This involves the use of biodigesters and microorganisms to convert the organic fraction of municipal solid wastes into biogas;

Landfill gas: Sanitary landfill with a gas plant erected on the landfill to recover gas generated as a result of anaerobic degradation of the organic component to produce heat;

Gasification: A thermochemical process (partial oxidation) that converts waste with carbon content into syngas and other valued products at a high-temperature range between 750 to 1,000 °C, with the aid of controlled air and steam. The syngas can be used to produce electricity, and

Pyrolysis: Pyrolysis is a thermal process that takes place without air at a temperature around 400–1,000 °C, where the municipal solid waste is degraded to produce gas (syngas), liquid (pyrolysis oil), or solid fuel (char; National Society for Clean Air and Environmental Protection, 2022). This process requires professional labor, huge capital, operation and maintenance costs (Ouda et al., 2016; Qazi et al., 2018).

Nine experts with backgrounds in environment-related issues were consulted and asked to make pairwise comparisons using the main criteria and sub-criteria provided. The experts had adequate local knowledge of the state of municipal solid waste management. Guidelines were provided in the questionnaire to enable the experts to carry out the comparison, as well as matrices to conduct the pairwise comparison based on Saaty's scale of 9 points (Kurbatova and Abu-Qdais, 2020). The categories of experts that participated in filling the questionnaire are shown in Table 6, and the details of the experts consulted are shown in the Appendix.

The values of the pairwise comparison obtained from the experts for the goal, criteria and sub-criteria and WtE alternatives based on the criteria and sub-criteria were aggregated, and decision matrices were derived for calculating priority weights (Table 7).

Expert assessment of the suitability of Waste-to-Energy (WtE) technology options

The application of a Nexus approach in this study sought to identify a suitable WtE technology option for Onitsha. This was achieved through an analytical hierarchical processes (AHP) technique based on multicriteria decision analysis. The result of the pairwise comparison done by the experts on the main criteria and their associated priority vectors for the main criteria yielded a consistency ratio of 0.08, as shown in Table 8. Consistency ratio plays a key role in determining whether the pairwise comparison is consistent and acceptable. Studies like Agbejule et al. (2021) have reported that a consistency ratio of less than 0.1 or 10% is acceptable. The pairwise comparison of the main criteria revealed that the environment criteria had a priority weight of 0.707, weighed highest, followed by technical consideration and socioeconomic criteria, based on expert judgement. Thus, the experts considered the environment more important in considerations relating to the choice of WtE technology alternative for Onitsha as expressly shown in Figure 10. The pairwise comparison also showed that the socio-economic criterion is the least important, following experts' judgement. Thus, this may be the result of the fact that solid waste generated in Onitsha is still being managed traditionally, with the majority of waste authorities' focus on waste collection and disposal to landfills. Such a traditional method of waste management has been fraught with environmental and public health impacts that impact the environment as well as residents of Onitsha (Kurbatova and Abu-Qdais, 2020).

The pairwise comparison of the nine sub-criteria was conducted under each of their main criteria, and this resulted in three pairwise comparison matrices with respect to each of the main criteria based on expert opinion. From Table 8, it is obvious that under the main criteria Environment and Health, pollution and production of hazardous residue (weight = 0.444) and GHG emission (weight = 0.444) were found to be the most important, sharing the same priority weights, while public and occupational health impact received the least priority (Table 9). This outcome is consistent with that of Qazi et al. (2018), where GHG emissions and Pollution/production of hazardous residue criteria. The high priority assigned can be explained by the fact that during the field exercise, it was obvious that poor management of landfills and open dumps in parts of Onitsha is associated with the burning of waste heaps, which affects the air quality conditions in the city. This has become common practice, especially around evening hours when people are supposed to be at home taking their rest.

For the sub-criteria under the technical criteria, technical know-how received the highest priority of 0.458, followed by complexity of the technology selected with a weight of 0.416, while the least priority is received by potential of energy production. This outcome shows that, based on the experts' ranking, the availability of technical know-how should be the main issue when considering WtE options for Onitsha, followed by consideration for the complex nature of the technology. This implies that the capability of energy generation from the selected WtE technology is not as important as the other two sub-criteria. This result could be a result of lack of trust in the power sector, government's inability to provide adequate power, the poor state of infrastructure in the energy sector and inadequate grid system.

Looking at the sub-criteria under the socioeconomic criterion, capital cost ranked highest with a priority weight of 0.619, followed by the operation and maintenance cost with a weight of 0.284. Job creation has the least priority with a weight of 0.096. Similar outcome on the importance of capital cost sub-criterion in evaluating socio-economic aspect of selecting and implementing WtE technologies has been reported in studies of Agbejule et al. (2021), Kurbatova and Abu-Qdais (2020) and Rahman et al. (2017). In another study, Qazi et al. (2018), operation and maintenance cost sub-criterion was reported as the key component of socio-economic criterion in considering WtE technologies.

Furthermore, pairwise comparison was conducted for the WtE options based on the nine sub-criteria (Figure 11). The response of the experts on the pairwise comparison between WtE option with respect to the criteria created nine matrices. The findings revealed that for the pollution and production of hazardous residue (PPH) sub-criteria, landfill gas as the technology with the least environmental pollution, followed closely by anaerobic digestion and incineration. Gasification and pyrolysis are the least preferred impacts. A similar finding—where experts identified landfilling and anaerobic digestion as the best alternatives in terms of pollution potential—is reported in Kurbatova and Abu-Qdais (2020). Likewise, Agbejule et al. (2021) found that anaerobic digestion had the lowest pollution potential and was therefore the most preferred option. Under the GHG emissions sub-criteria, the experts viewed landfill gas as the most preferred WtE option for Onitsha, with the least tendency to emit greenhouse gases. This was followed by anaerobic digestion and incineration, while gasification and pyrolysis had the least priority weight and, as such, the least preferred options. Considering the role of public and occupational health sub-criteria in the pairwise comparison for WtE technology alternatives, landfill gas emerged as the technology with the least potential to impact public health, followed by anaerobic digestion and incineration. The least preferred were gasification and pyrolysis.

For the technical criterion and its first sub-criterion, technical know-how, the outcome showed that, considering the availability of the skilled personnel for WtE technology, landfill gas tops the list of choices among the experts, followed by both incineration and anaerobic digestion sharing the same priority weights. The least preferred choices were gasification and pyrolysis. The choice of landfill gas as a preferred option, considering technical know-how, is similar to the findings of Farooq et al. (2021), Agbejule et al. (2021), and Kurbatova and Abu-Qdais (2020). The consideration of the complex nature (CNT) of WtE technology, incineration received the highest priority weight, followed by anaerobic digestion and landfill gas. This shows that incineration is the most preferred choice as it is considered to require low-skilled labor by Farooq et al. (2021), especially for a city in a developing country like Nigeria. This is followed by anaerobic digestion and landfill gas. However, it is clear that the least priority technologies under CNT sub-criteria, gasification and pyrolysis, require highly skilled labor given the complex nature of their processes. Considering the potential for energy generation and following experts' judgement, pyrolysis ranked highest, followed by landfill gas and gasification technologies. They also believed that anaerobic digestion and incineration possess the lowest electricity generation potential, given their low ranking.

On capital cost, anaerobic digestion was identified as the most preferred, with the lowest capital cost, followed by landfill gas. The least preferred in relation to capital cost is incineration. Moreover, anaerobic digestion was ranked highest as the technology with the lowest operational and maintenance costs, based on expert judgment, followed by incineration and landfill gas. Agbejule et al. (2021) note that experts' selection of incineration as the second most preferred waste-to-energy (WtE) option under the cost sub-criteria is linked to the fact that its operating costs largely reflect labor expenses and the lack of a waste-sorting requirement, which may explain its appeal.

With respect to job creation, respondents also prioritized anaerobic digestion as the technology with the greatest employment potential, followed by incineration. Gasification and pyrolysis were ranked lowest in terms of job creation opportunities.

The concluding part of the AHP requires that the overall priority regarding the WtE technology alternatives obtained from expert choice be determined. This is determined by combining the priority weight of all the criteria, sub-criteria and the alternatives. The overall priorities are shown in Figure 7. It can be observed that anaerobic digestion and landfill gas are the best expert choice of waste to energy with global weights of 0.277 and 0.276, respectively. Incineration of solid waste, with a global weight of 0.32, ranked as the second preferred technology (Figure 12). The gasification and pyrolysis processes were ranked as the least preferred options for solid waste management in Onitsha. The findings of the analytical hierarchical process on selecting a more suitable waste-to-energy technology for Onitsha are similar to the findings of Kurbatova and Abu-Qdais (2020) in Moscow, where landfill biogas ranked highest, followed by anaerobic digestion as the second preferred option, and Qazi et al. (2018) in Oman, with anaerobic digestion ranking highest.

This study has explored the application of the Water-Energy-Food Nexus approach to the issue of solid waste management in Onitsha in order to evaluate feasible WtE pathways. The findings reveal three central insights: (1) the waste stream is dominated by organic and plastic waste fractions; (2) residents experience acute environmental and public health impacts linked to inadequate waste infrastructure and poor disposal practices; and (3) anaerobic digestion and landfill gas recovery represent the most context-appropriate WtE technologies. These findings contribute to a growing body of literature on sustainable waste governance in rapidly urbanizing African cities and offer direction for planning integrated, resource-efficient waste systems.

First, the study reaffirms that food waste and polythene sachets account for the largest proportion of municipal solid waste in Onitsha. This confirms earlier work by (Ogwueleka 2009a), who identified putrescible waste as constituting over half of the waste stream in Onitsha and comparable Nigerian cities. Similar trends have been observed in Lagos (Kofoworola, 2007), Makurdi (Aguoru and Alu, 2015), Enugu (Onyishi Chinedu et al., 2023), and Kumasi in Ghana (Addo-Fordwuor and Seah, 2024; Asibey et al., 2019), indicating a consistent pattern in West African cities where informal food markets, limited refrigeration, and high consumption of sachet water shape waste composition. These proportions contrast sharply with high-income cities such as Tokyo, Berlin, or Amsterdam, where paper, plastics, and packaging form the largest fractions due to different consumption modes and stricter product regulation (Campbell-Johnston et al., 2019).

The persistence of organic dominance despite increasing urbanization suggests structural continuity in consumption patterns, but also underscores the technological opportunities for biological conversion processes. Organic-rich waste streams are particularly well-suited for anaerobic digestion (AD), composition, and landfill gas recovery—options that perform well under variable feedstock quality. This contrasts with thermochemical WtE processes like pyrolysis or gasification, which require relatively homogeneous, low-moisture feedstock rarely found in low-income urban settings.

Second, the study reveals widespread unsanitary waste practices, including open dumping, waste burning, and disposal into drains, which reflect deep governance and infrastructural challenges. This aligns with long-standing research documenting chronic inefficiencies in Nigerian solid waste management systems (Adeniyi et al., 2022). The inadequate distribution of waste collection points mirrors findings from Abuja (Imam et al., 2008), Benin City (Ogu and Iyamu, 2013), and Ibadan (Kayode and Omole, 2023), where households often rely on improvised containers and informal dumping due to insufficient municipal support. The recurrence of these patterns highlights a structural governance issue common across Sub-Saharan African cities, where waste policy reforms are rarely matched with operational capacity, budget allocations, or institutional accountability.

Residents' lived experiences, as revealed through PCA, deepen this understanding. The strong association between waste mismanagement and waterborne diseases is consistent with empirical evidence linking unmanaged waste to cholera, typhoid, and diarrhoeal outbreaks in urban Nigeria (Raji and Ibrahim, 2011). Likewise, flooding related to waste-clogged drains has been documented in Tanzania (Kitole et al., 2024) and Ghana (Addo-Fordwuor and Seah, 2024; Afrane et al., 2022), highlighting a regional pattern in which ineffective solid waste management contributes directly to climate vulnerability. Onitsha's situation appears particularly acute given its riverine topography, high commercial density, and limited drainage infrastructure, which exacerbates flood risk and exposure to polluted run-off.

The PCA component related to air pollution also aligns with Onitsha's past designation as one of the world's polluted cities, with extremely high levels of particulate matter (PM10), attributed in part to open waste burning (Valentine Ifenna et al., 2025). This confirms that solid waste management is not a peripheral urban service but a core environmental health determinant with strong cross-sectoral linkages, precisely the rationale for applying a Nexus approach in this study.

Third, the use of the AHP to evaluate WtE technologies adds methodological and practical value. The expert prioritization of environmental and health criteria over technical and economic issues contrasts with studies from high-income contexts where energy efficiency, cost, and circularity often dominate decision-making (AlNouss et al., 2024; Tian et al., 2021). In contrast, the prioritization observed in Onitsha reflects the immediate pressure of pollution, disease, and environmental degradation that shape local perceptions of risk and feasibility. The ranking of WtE is consistent with similar multi-criteria evaluations, where AD and landfill gas recovery emerged as preferred options due to their flexibility, lower emissions, and compatibility with organic-dominated waste streams (Ferdoush et al., 2024). The relatively low ranking of pyrolysis and gasification corresponds with findings by Phounglamcheik et al. (2020), who highlight the unsuitability of these technologies for high-moisture, heterogeneous waste typically found in low-income regions. The lack of awareness on the process of pyrolysis and on the opportunity of converting waste to wealth has also been underlined as a reason for low uptake, which leads to the inefficient handling of waste that cannot leverage energy recovery options in neighboring locations (Onwuka and Ajator, 2018).

The preference for AD therefore aligns with both the environmental priorities of experts and the technical realities of Onitsha's waste profile.

For policy and governance, the study reinforces that solid waste management reforms must go beyond technology procurement to address systemic issues in waste collection, regulation, and institutional coordination. Without improving the distribution of receptacles, enforcing anti-dumping technology will face implementation challenges. Fragmented governance, a common challenge across Nigerian cities (Valentine Ifenna et al., 2025), remains a barrier that must be addressed through clearer mandates, improved funding structure, and performance accountability (Asibey et al., 2019). For urban sustainability planning, the findings highlight the value of embedding WtE options within broader Nexus thinking. AD and landfill gas recovery can contribute simultaneously to energy access, sanitation improvement, and agricultural productivity through digestate application—aligning with SDGs 6, 7, 11, and 13. The biological co-products of AD have been shown to enhance soil fertility and reduce dependence on chemical fertilizers (Nkoa, 2014), offering opportunities for local circularity, especially in peri-urban farming systems around Onitsha. Lastly, for research, the study underscores the importance of context-specific, data-driven decision tools for WtE planning. The successful application of PCA and AHP demonstrates their capability to synthesize diverse community and expert perspectives—an approach that can be replicated across other Nigerian and West African cities where empirical data remain limited.

This study explored the application of the Nexus approach to the issue of solid waste management in Onitsha. As Onitsha faces high levels of water and environmental pollution, a gap in reliable energy provision, and food insecurity, the Nexus approach holds promise in tackling these issues simultaneously in an integrated approach by supporting the design of solutions to waste management as a key issue, and by taking a resource recovery focus.

This study highlighted the unsustainable practices in the management of solid waste generated in Onitsha. The implication of these outcomes points to the need for a more sustainable approach toward managing waste in a commercial city like Onitsha. This study applied a Nexus approach to support identification of suitable WtE technology options for waste to energy resource recovery in order to support water, energy and food security, and environmental health and sustainable social and economic development more generally, in an integrated manner in Onitsha.

According to the key findings, the main priorities for consideration of any choice of WtE technology are environmental and health considerations, availability of technical know-how, and the pollution level of such WtE option. The experts were fed up with the poor handling of open dumps in Onitsha, which is connected to the incineration of waste heaps that affect the air quality conditions in the city. In addition, the capital cost of the WtE technology for Onitsha was considered of high priority in selecting WtE options, and this implies that the choice of a less capital-intensive technology will be suitable for Onitsha, given the economic realities on the ground, being a city in a developing economy. Therefore, also operation and maintenance costs are a key priority for determining the most preferred WtE option.

Based on the various criteria and sub-criteria, the selected WtE technologies were ranked, and the result showed that anaerobic digestion was the most preferred, followed by landfill gas and lastly incineration. The choice of anaerobic digestion by the experts is clearly a suitable option as the waste stream in Onitsha is dominated by biodegradables. In addition, the digestate produced from an anaerobic digester can be used directly in farms as manure and saves costs for farmers from buying fertilizers. However, in relation to the production of residue, an anaerobic digester is a more suitable technology for Onitsha than landfill gas. This study, therefore, recommends that the state waste management agency (ASWAMA) embark on developing an implementation strategy in order to leverage on the application of the Nexus approach to support water, energy and food security, and more specifically to harness energy from waste for the benefit of Onitsha and its residents. The resource recovery sector for waste to energy projects, but also for plastic recycling and metal scraps should be properly organized and regulated by the Anambra State government through its relevant agencies to improve the revenue generation of those involved.