A Review of sewerage and drainage systems typologies with case study in Abidjan, Côte d'Ivoire: failures, policy and management techniques perspectives

Abstract

The failure of sewage and drainage systems in SubSaharan African cities is frequent and can be considered as a critical issue, both from an environmental standpoint and in terms of associated maintenance costs. This study analyzes the state of the sanitation systems, the elements behind the failures, the environmental concepts used to classify the problems, and the tools and methodological alternatives for ranking the various management solutions. This research illustrates the causes that contribute to the dysfunctions in the sewage systems of Abidjan as a typical example of sewerage systems management challenges in SubSaharan Africa’s large cities. Poor solid waste and wastewater management practices by residents, e.g., illegal dumping of solid waste into the sewers, unauthorized and defective connections to the network, structural dysfunctions related to the age of the network (cracked, denuded, or broken), urban agriculture in the vicinity of the channels, natural phenomena such as erosion, landslides in the undeveloped parts, and the high concentration of vegetation in the network, wholly contribute to the degradation of the network. A variety of decision support systems for the management of the assets of the urban sewage network were presented. The instruments have been categorized based on their capacity and functionality. The operating concept of each of these tools has been outlined, as well as their respective data needs. In addition, the study analyzes challenges related to the usage of existing decision support systems and provides an outlook on future research requirements in this area. This study offers a detailed analysis of the issues of sanitation management and could serve as a reference for other emerging nations in SubSaharan Africa.

Keywords:

• Decision support tools
• sewerage systems
• drainage systems
• ageing infrastructure
• management systems

1. Introduction

Sewer and drainage system management and maintenance are the most critical urban challenges worldwide, especially in subSaharan Africa (Yazdanfar & Sharma, 2015). The sewer and drainage systems must be carefully operated and preserved in a rational, and sustainable manner because it is a vital aspect of the urban water infrastructure (Mohammadreza Malek Mohammadi et al., 2019). The main objective of the technical management of such networks is to provide the population with a system that meets environmental standards. This requires precise knowledge of the network (Blindu, 2013) with regular network maintenance. Currently, many urban cities in Africa use deteriorating drainage infrastructure, exposing the communities to unforeseen failures that may disrupt both the sewer service and surface operations (TscheiknerGratl et al., 2020). Sewer leaks, sewer backups, network overflow, and aging infrastructure are all significant concerns, especially in places where groundwater is mostly used for potable water (Jagai et al., 2017).

Like many large African cities, the metropolis of Abidjan, the economic capital of Côte d’Ivoire, is not free from the problems of a sewerage system malfunction, (Kangah and Della 2015). Abidjan has the highest sewerage and drainage systems coverage rate in West Africa with 2,400 kilometers of pipes (C2D, 2017). The growth of urbanization (49.7%) associated with the proliferation of real estate companies has led to an increase in sewerage and drainage systems (Kangah and Della 2015). In the urban plan of Côte d’Ivoire, the real estate structures are forced to equip the locations where sewerage networks are built, so that all real estate operations are provided with sewage networks. In Abidjan, a large part of the land reserves is located in Cocody and Yopougon districts, resulting in large real estate concentration in these areas (Kablan, 2017). According to Ouattara et al. (2021), there were malfunctions in the network, resulting in the frequent obstruction of the sanitation facilities and the presence of leaks, breaks, and sewer backflows in these municipalities. During the rainy season, network backflows were observed (Kablan, 2017). This situation threatens the groundwater and potable water supply network. Faced with these challenges of groundwater and potable water contamination, it is urgent to find sustainable solutions to reduce the risks of groundwater contamination that threaten the wellbeing of people relying on this source of water.

The progressive aging of these networks, combined with poor human practices (dumping solids wastes in the networks, Illegal and defective connections on the sewers system), led to faults that complicate the tasks of managing these systems. These dysfunctions are mainly manifested through three characteristic symptoms (Blindu, 2013) including (1) an increase in the number of leaks and ruptures, (2) a decrease in carrying capacity, and (3) obstruction of the sewerage system. On the one hand, the ultimate solution to minimize faults is the rehabilitation of pipes (Bruaset, Rygg, and Sægrov 2018). However, reactive repair procedures and methods, on the other hand, are both costly and require capitalintensive infrastructural technologies (Goncharenko et al., 2018). For wealthy countries, a sewer pipe repair is a cost-effective option. Some countries, such as Australia and Germany, have successfully managed pollution in receiving waterways and reduced sewage leakage and other failures, according to Zhao et al. (2019). However, the expense of sewage rehabilitation is exorbitant in many developing countries, and the desired level of rehabilitation cannot be attained, resulting in poor customer service and harmful environmental effects (Tomczak and Zielińska 2017). Tušer & Oulehlová (2021) established the recurring questions that waste water system managers often ask about the maintenance and renewal of sewer systems as follows: “Do I need to renew the pipes? Which ones do I need to renew? What equipment and structures need to be repaired and when should the repairing be done?”

It is, therefore, vital to maintain sewer and drainage systems in expedient and economic conditions and efficiently regulate the rehabilitation process of the sanitation networks. Dongo et al. (2009) argued that the negligence imposed on the inspection and management of the structures of the sewerage systems in Côte d’Ivoire was worrisome. Malek Mohammadi et al. (2020) and Alnoaimi and Rahman (2019) added that the construction of infrastructure should not be the only concern. Aspects such as the maintenance, control of their operations, and establishment of an efficacious strategy to ensure their sustainable operation to meet sustainable development goals should equally be considered. Today, the fundamental concern of Public Sanitation Utilities (PSU) is to assess the objectives of the services provided to customers to control the network’s operation, maintenance, and investment costs under specified operational safety circumstances (Djukic et al., 2016). However, to attain the aforementioned goals, the question of which method to use to ensure effective sewer and drainage system management must be tackled. Bogotá et al. (2021) suggested that those responsible for the management and maintenance of sewer and drainage networks require a comprehensive methodology to assist in decision-making.

The current study aimed at discussing and proposing tools to assist the management of the collective sanitation network in Abidjan by particularly addressing four aspects of decision support: (a) a method for evaluating the renewal of degraded pipes, and (b) a method of diagnosing the malfunctions caused by aging and to ensure sustainable management of sanitation infrastructures in the city of Abidjan, (c) a method for diagnosing problems related to the aging and hydraulic capacity of the network, (d) Raising the population’s awareness of good practices to preserve the quality of sanitation networks and the environment while taking into account the potential diseases that could affect the population.

2. Material and methods

2.1. Research methodology

The methodology was based on an extensive literature review of research papers, case studies, study reports, scientific journals, and other relevant materials (Figure 1).

Figure 1. Categorization of the methodological framework.

We used a direct search of relevant peer-reviewed literature in google scholar and Scopus. To query scientific search engines and platforms, a Boolean search using keywords and phrases was performed. The initial search included factors of sewage system malfunction. The database was then extended to include research on “decision support tools for sewer and drainage systems management (SDSM)”. The keywords and phrases used were “sewerage and drainage systems management”, “factors of sewer and drainage malfunction”, “decision support tools for sewerage and drainage systems”, and “sewer leakage detection, and control methods”. All articles dealing with the management of collective sewerage systems, whether site-specific, sub-regional, national, or global, were included in the database. The database contained 120 papers that were thoroughly examined. Most of the articles included in the study were those published between 2015 and 2021. In addition, other articles before 2015 were included for a better understanding of the subject under consideration. A purposive stratified sampling approach was used for the research, as suggested by Nigussie et al. (2018) and Mkandawire and Mulwafu (2006). Professionals and practitioners (also referred to as key informants in this research) within the sanitation sector of the study areas were identified, to participate in semi-structured interviews. The selection criteria used for this group of participants was that they were required to be the head of the department within a sanitation institution in both study areas or possess a role within the institution that allowed them to make critical decisions concerning sanitation service delivery. To get their permission to participate, the key informants were contacted by phone and email. It was simple to obtain participant consent and schedule visits thanks to the networks SODECI and ONAD have within the sanitation sector in the study locations. A minimum of one person from each organization responsible for managing and providing sanitation services in each study area was to be found and contacted. The list of sanitation institutions where semi-structured interviews with professionals and practitioners were conducted is therefore provided in Table 1. Additionally, questionnaires that collected quantitative data were administered to staff employees that report to work in the department or unit as the key informants, which is also shown in Table 1. The number of questionnaires to be administered was determined by the sampling strategy (purposive stratified) utilized for this study. Every institution that was visited took part in the survey.

Table 1. List of sanitation institutions where semi-structured interviews and questionnaires with professionals and practitioners were conducted

Institutions	Mandates	Sanitation issue
Ministry of Sanitation	Responsible for rural and urban sanitation, Develop policies and guidelines for sanitation.	Entire service chain from containment to disposal/reuse
National Office of Sanitation and Drainage (ONAD)	Technical arm of ministry of urban health and sanitation in charge of planning and regulating infrastructures development of sanitation and drainage, Responsible to implement the national strategy for sanitation on behalf of the municipalities, Prepare project proposal to apply for finance, Develop city and town plans for sanitation, Rehabilitate and develop infrastructure for collective sanitation and drainage, To reinforce maintenance and operation of sewer sanitation and drainage infrastructure, Improve management of on-site sanitation, Awareness of the population, strengthening capacities of public and private actors, and develop coordination,	Entire service chain from containment to disposal/reuse
Water Distribution Company in Côte d’Ivoire (SODECI)	Responsible for operation and maintenance of the sewerage and drainage networks and facilities in the city of Abidjan, Management of storm water for Abidjan Management of subscribers, including billing and collection of sanitation fees from users, Execution of the work entrusted to SODECI (cleaning of underpasses, instruction of connection requests, cleaning of gutters of national interest, connections, and networks for collective use) on an exclusive basis.	Entire service chain for the maintenance of sewerage networks and the management of wastewater and rainwater to the treatment plants
The technical services of the District of Abidjan and Municipality vice Mayors office of Yopougon	Responsible for sanitation service delivery at the district level	Entire service chain of solid and liquid wastes management at the district level also preservation of public hygiene and improvement of the population’s living environment.
Directorate of Urban Waste and Drainage(DAUD)	Develop and ensure compliance with legislation and regulations on sanitation, drainage, roads and other networks; Ensuring the conformity of works related to primary sewerage and drainage networks with urban planning; Participating in the elaboration of the Sanitation Code; Assist local authorities in matters of sanitation, drainage, roads and other networks in liaison with the competent structures; Implement the institutional and regulatory framework for sanitation and drainage and ensure its monitoring; Organizing sanitation and drainage professionals; Ensure the institutional supervision of all sanitation, drainage, maintenance and upkeep operations and projects; Implement a programme for the construction of autonomous sanitation in peri-urban neighbourhoods.	Entire services chain for implementation of legislation and regulation of sanitation programs
Management Unit of the Guru Watershed Development and Integrated Management Programme (UGP)	Responsible for detailed engineering design of wastewater treatment, Responsible for detailed engineering studies of stormwater drainage and sedimentology, Environmental and Social Impact Assessment (ESIA)	Entire chain of wastewater and rainwater treatment and drainage engineering services
National Office of Technical Studies and Development (BNETD)	Responsible for carrying out studies of projects of public interest with a view to controlling costs, quality and deadlines. Responsible for advising and controlling projects of public interest with a view to controlling costs, quality and deadlines.	Entire service chain of construction of public sewerage infrastructure
sanitation department of the Ministry of Construction and Urban Planning (MCU)	Responsible for diagnosing the existing situation, i.e. recognition of the networks, inspections of the pumping stations and the Koumassi Digue pretreatment station, measurement campaign, hydraulic modeling and operating diagnosis; Establishment of basic data, i.e. data collection, household surveys, environmental situation reports, surveys of polluting industries, design of a GIS; Development of intervention strategies and variants, i.e. establishment of development scenarios, comparison of the envisaged solutions, technical studies, flood protection programme;	Entire service chain of construction, urbanization, land use and protection of sensitive areas, but also urban sanitation.
Université Felix Houphouët Boigny	No direct involvement in the sanitation sector No work done with informal institutions of sanitation Informal collaborations with sanitation sector	Dissemination of knowledge
Université Nangui Abrogoua	No direct involvement in the sanitation sector No work done with informal institutions of sanitation Informal collaborations with sanitation sector.	Dissemination of knowledge
Pit emptiers association	Get support from ONAD to perform their activities, Empty containment systems of household and deposit waste at designated deposit stations, Get financial support from ONAD, Sensitize households on their role to empty to prevent illegal disposal of wastewater to the environment.	Emptying, Transport and depositing at designated stations

2.2. Semi-structured interview and surveys

In addition to the literature review, different types of interviews were conducted to characterize the sewerage and drainage systems in the city of Abidjan. The diagnosis consisted of semi-structured interviews with establishments and institutions such as the Ministry of Sanitation; the Directorate of Urban Waste and Drainage (DAUD); the National Office of Sanitation and Drainage (ONAD); the Projet d’Amenagement de la Baie de Cocody (PABC), the Management Unit of the Guru Watershed Development and Integrated Management Programme (UGP) and the Priority Works Programme (C2D), Pit emptiers association, to understand the global functioning of the sewage systems, the problems linked to the functioning and the main role played by each actor. It also helped in obtaining the cartographic database on the different networks of Abidjan. The characterization of the systems resulted in a geographical survey which assisted in identifying the different factors which contribute to the dysfunction of the network. Also, a household survey was carried out to understand solid waste management along the system. The different questionnaires from the semi-structured interviews were divided into two themes. “General information about the respondent” was the first theme. The questions collected information on the respondent’s years of employment with the company, his or her degree of education and vocation, his or her position within the company, and the part the company plays in the sanitation sector. In addition to establishing a relationship with the respondents, these questions also gathered base information that was utilized to support the answers to the questions that followed. For instance, years of service and responsibilities held within an organization may affect one’s level of familiarity with the issues and sorts of challenges that urban sanitation systems confront, as well as the operation of collective sanitation in the city of Abidjan. In addition, the questions provided an understanding of the institutional structure and its prominent role in liquid waste management, its efficiency, and its weaknesses. The second theme was entitled “Institutional and mapping data needs”. The questions collected information on the capacity needs that are recognised within the institution in the specific study areas. The information collected from this theme allowed the location of the different collectors in the city of Abidjan to be determined and to obtain several cartographic data. This mapping data also allowed the tracing of the 21/22 collector, the Uniwax Drainage System and the M’Badon channel in the municipalities of Yopougon and Cocody. As for the household survey, the questionnaire focused on the level of education of the residents, how they manage their liquid and solid waste concerning the drains, their perception of the existence of the sewerage system, the illnesses they suffer as a result of the malfunctioning of the drains, and their stay in the study area to determine the reliability of the answers given. Since semi-structured interviews follow a flexible structure, where open-ended questions are asked, this left room for respondents to express their perceptions and meanings of the questions asked (Denscombe, 2014). This opened up the possibility of revealing underlying factors that were not considered in this research, giving a more comprehensive set of data collection (Creswell JW, 2018).

2.3. Ethical considerations

Ethical approval was sought from the Ethics Committee of the Committee for Human Research and Ethics (CHRE) of University of Energy and Natural Resources, Sunyani to conduct the study. Permission was sought from the Water Distribution Company in Côte d’Ivoire (SODECI) and the National Office of Sanitation and Drainage (ONAD) at the study sites. The purpose and importance of the study were communicated to the authorities and participants. Oral consent was sought from the participants before the start of the study.

Respondents were reassured that the information they provided was confidential. Participants were not financially incentivized or coerced to take part in the study, as it was explained to them that their participation was purely voluntary. Thus, the study was guided by the Belmont guideline (Belmont, 1979) regarding fairness in the selection of participants, consent to participate in studies, the confidentiality of information provided and respect for privacy.

3. Results and discussion

3.1. Sewer and drainage systems

According to Ansari et al. (2012), a sewerage system is a system that contains pipes of several lengths and is very important for the transport of rainwater, and wastewater, including domestic, residential, industrial, and commercial treatment services. Usually, the flow of wastewater in the sewerage systems is directly related to human activity—use for all kinds of activities (Ndongo et al., 2015). As for the drainage system, the flow of rainwater depends on rainfall.

In addition, it can also prevent flooding problems through rainwater drainage. The typical sewer system is based on gravity (Thong, 2018). This means that water flows under the force of gravity. There are two main groups of sewer systems in Côte d’Ivoire: the sewage system which is connected to a public pre-treatment plant and the drainage system which consists of transferring rainwater into a lagoon (Dongo et al., 2009). Figure 2 illustrates the wastewater flow pattern from the residential area to the treatment plant (SODECI, 2018).

Figure 2. Sewer flow diagram (Source: Water Distribution Company of Cote d’Ivoire (SODECI, 2018).

3.2. Types of sewer systems

Modern sewerage systems, according to Banik et al. (2017); are divided into two categories: domestic and industrial sewers, and rain sewers. For all types of wastewater and runoff, a combined system may supply only one network of pipes, mains, and discharge sewers (Sakson and Brzezińska 2018). The recommended system, on the other hand, has a sewer system for domestic and industrial waste, which is usually treated before release, and a separate system for rain water, which is diverted to temporary retention ponds or transported to a stream or river discharge point (Thong, 2018). Separate sewerage systems, combined sewerage systems, and drainage systems are the three types of sewerage systems in Abidjan (Dongo, 2006; SODECI, 2018).

3.2.1. Separate sewer systems

Two distinct pipelines are used in separate sewer systems. A first pipe transports rainwater from drains to local watercourses (Banik et al., 2017) with pollutants and trash in the precipitation water flowing untreated to local watercourses. The domestic wastewater is transported to the wastewater treatment plant via a second pipe, where it is cleaned at the “Pre-treatment and Discharge Station and Wastewater Deodorization Station.” In Yopougon and Cocody, the segregated network covers 522,587 m in Yopougon, or 70.87% of the network in this municipality, and 281.893 m in Cocody, or 67% of the municipality of Cocody (SODECI, 2018).

3.2.2. Combined sewer systems

Collective waste water and drainage systems have a single drain (Jia et al., 2018). Wastewater from homes and businesses flows into this pipe. In wet weather, rain water flows into the same drain and mixes with raw sewage. During dry weather conditions, all waste water is transported to the treatment plant, where the “pre-treatment, discharge and the waste water deodorization plant” clean the waste water. Often, when it rains heavily, there can be 10 times more rain water than raw sewage. Rain water can overflow the collective waste water and drainage systems. Treatment plants cannot handle large amounts of rain water, so waste water must bypass the treatment plants during rainfall and be discharged directly into receiving waters (BNETD, 2013). The combined system covers 554 m in Yopougon, i.e. 0.075% of the network in this municipality, and 177.65 m in Cocody, i.e. 1.97% of the municipality of Cocody (SODECI, 2018).

3.2.3. Rain drainage systems

Rain drains carry rain water and surface drainage (Figure 3), street washing water and other drainage water, but exclude sanitary waste water and industrial waste as well as effluent from septic tanks or other treatment processes (Koffi et al., 2012). The rain drainage network covers 254,116 m in Yopougon, or 34.46% of the total network in this municipality and 281,893 m in Cocody, or 1.97% in the Cocody municipality (SODECI, 2018).

Figure 3. Uniwax drainage system in Yopougon municipality.

3.2.4. History of sewerage and drainage systems management in Cote d’Ivoire

The beginnings of collective sanitation date back to the middle of the 19^th century under the impetus of the hygienist movement led by doctors and chemists. The latter had established correlations between stagnant wastewater and certain infectious diseases (De Bénédittis, 2004). The first steps consisted therefore the installation of ventilated underground piping systems to allow rapid drainage of water out of urban areas, without stagnation: these were the first unitary system that would constitute the bulk of sanitation until the 1950s (Davies et al., 2001). Indeed, over time, various factors such as the industrial revolution, the demographic explosion, social evolution, changes in mentalities, the development of urbanisation, and the progressive sealing of the soil have confronted the population with two new problems: the risk of flooding due to the saturation of the collectors and the increasing pollution flows due to the increase in the need for potable water (De. Bénédittis 2004). These quantitative and qualitative aspects of wastewater and rainwater management have shaped the current sanitation systems, with the development of new works: rain water overflows, treatment plants, separation networks, storage basins, infiltration basins (Ertl et al., 2002), etc.

Faced with investments linked to the development of urbanisation and equipment, the control of aging urban infrastructures remained neglected until recently (Kracht, 2003). For most of the 20^th century, the maintenance and monitoring of sewerage systems were considered secondary to the development of other urban infrastructural networks, this being linked to the fact that their service is not directly perceived by the users, despite their important role in public health and environmental protection (Rieckermann & Gujer, 2002).

In Côte d’Ivoire, environmental issues have existed since independence, but they were taken into account and improved in 1981 with the creation of a ministry in charge of the environment, which demonstrated the concern for the preservation of the standard of the environment of Ivorians and the capital represented by nature (Colcanap & Dufour, (1982)). Before 1968, the problem of sanitation in Abidjan was hardly mentioned. At that time, the Ministries of Health and Public Works submitted a request to the United Nations within the framework of the United Nations Development Programme (UNDP; Dufour, 1982). Following serious epidemics, UNDP commissioned WHO to carry out studies on the subject, and in 1975 the initial Abidjan master plan of the sewerage system was developed. This scheme has been revised several times to take into account growing demography and financial difficulties (Table 2).

Table 2. Lists of revision time of the master plan from 1975 to 2018

	Type of Master plan	Objectives/key elements	Collection rate (%)
1975	Initial master plan	provided for the connection of all inhabited areas with a 75% connection rate	30
1981	First revision of master plan	provided for the construction of a wastewater treatment plant in Azito (Yopougon) and M’Pouto (Riviéra)	40
2018	Second revision of the master plan	Plan to connect all unconnected municipalities and repair the connected areas where the system was dysfunctional	60

The general objectives of this sanitation scheme were to connect all inhabited areas to a collective sewerage network and, as a first step, to reach a connection rate of 75% of the population in 1985, instead of 30% in 1975. The plan also provides for the protection of the lagoon and the surrounding environments (Colcanap & Dufour, (1982)). This network was composed of the main collector and secondary collectors in the different municipalities of the city. The function of the main collector is to convey all the waste water from the different districts directly into the Atlantic Ocean. This discharge is carried out 1.2 km from the coast after pre-treatment (screening, grit removal, and de-oiling) via the outlet into the Atlantic Ocean (Gnagne et al., 2015).

In 1980, it was estimated that 80% (1.3 million people) of the population in Abidjan were still not connected to sewers. Of these, very few had individual sanitation systems. There were optimistic forecasts that 40% (1.7 million people) of the population would still be without sewerage in 1990. Thus, despite the financial efforts made at the time, the population without adequate sanitation still was on a rise. (Dufour, 1982).

According to Code de l’eau (1998), in 1998 in Côte d’Ivoire, only seven out of 255 towns had sanitation and drainage master plans. Five of the existing master plans were being updated and four new master plans were being drawn up. The current rate of connection of users to the sewage system in Abidjan district was only 60% by 2018 (SODECI, 2018). Collective sewage of waste water is very little spread over the national territory. In addition to Abidjan, which has a significant collective sanitation infrastructure, the cities of Bouaké, Yamoussoukro, and San-Pédro have a promising collective sanitation infrastructure. The secondary towns do not have any. As a result, access to individual sanitation in urban areas represents nearly 80%. This situation generates the production of a large volume of sludge from the emptying of the on-site sanitation installations, which is dumped without any form of treatment (Code de l’eau, 1998). This exposes the surrounding environments to pollution risk and the population to public health problems.

3.3. Structures and facilities of Abidjan sewers (manholes, pipes etc.)

Many types of material can be used in the sewerage system. Those materials can be vitrified clay pipes, very dense polyethylene pipes, fortified cement, and reinforced concrete pipe (Pseau, 2019). Before selecting the material to be used (SODECI, 2018), some considerations need to be made, for example, cost of installation, the strength of the material, and the life span of the material. Different materials have different design features. In Cote d Ivoire, the most common material used in sewage systems is HDPE (high-density polyethylene) pipe with 300 mm diameter. Figure 4a, and Figure 4b shows the characteristics of manhole used in Cote d’Ivoire (SODECI, 2018). HDPE has a diameter ranging of 160 mm to 630 mm.

Figure 4. Sewerage System components used in Cote D’Ivoire.

It also has variable lengths of 6 m with many advantages. It is environmentally friendly with less energy consumption during construction and operation. Also, it does not allow toxic substances to leave the sewage to the ground. Other than that, HDPE is a material with a non-conductive polymer. It also does not rust and have a long life span. Due to its long life span, the maintenance cost is low (Majid & Elghorba, 2017).

In Cote d’Ivoire, the gravity sewer system is the most used (BNETD, 2013). This method is based on using gravitational force to discharge the waste water to treatment plan. This method is preferred because it has the lowest cost and is suitable for most landscapes. Gravity sewer system delivers the waste water by utilizing the potential energy resulting from the difference in elevation (Koffi et al., 2012). The purpose of the flow from upstream to downstream is to maintain flow velocity, avoid backflow and minimize the head loss in the sewer pipe (Li et al., 2019). The pipe must be located on a slope to allow sewer discharge by velocity between 0.8 m/s to 4 m/s. If the velocity is less than required then the solid waste will block the pipe, necessitating repairs. Other than that, the minimum size of sewer diameter is 200 mm and the depth is 1.2 m from the manhole (SODECI, 2018). For rain water drainage networks, the most used material in Abidjan is concrete bricks (Ouattara et al., 2021).

3.3.1. Characteristics of manhole

The manholes are needed on the surface of the ground to provide access for sewer maintenance or repair of the sewer (Thong, 2018). They also create minimum interference to the hydraulics of the sewer. The manholes must be strong and durable (Beg, Carvalho, and Leandro 2017). Usually, manholes are made from precast reinforced concrete and surrounded with concrete for protection and placed at the edge as well as outside of the building (M.N.A. Beg et al., (2017)). Manholes have their reference standard according to (BNETD, 2013). The manhole cover is made from cast iron with a minimum diameter of 600 mm and the foundation of the manhole should not be less than 150 mm thick. The maximum distance between manholes is 100 m in a sewer line (BNETD, 2013). Manholes are used for checking, maintaining, and repairing the sewer (Beg et al., 2016). Manholes also signify a change of pipe size or flow direction change in the sewer system. Besides that, manholes are located where there is a change in the gradient of the sewer pipe slope (Shinji et al., 2016). They will most likely appear intermittently along long sewerage pipelines. They will also appear at the sewer junction for easy servicing access (Ayasrah, 2020). Manholes are very important because it is the only way to access sewer for maintenance and to do an inspection (Hu et al., 2020). If the manhole cover is damaged, it should be replaced immediately as the gases it contains are hazardous to human health and safety. There are many ways to repair the manhole cover. One of the methods is to replace and reset a new cover. Other than that, renovating and renewing the fabric of the chamber can also solve the problem (Chen, Leandro, and Djordjević 2016). In conclusion, manholes need to be inspected constantly.

3.3.2. Overall operation of the sewerage systems in Abidjan

Abidjan is the most covered city in West Africa in terms of linear meters, with a coverage of more than 100 km of network (Dongo et al., 2010). The city sanitation is based on two types of systems: individual and collective.

Collective sanitation, which includes all the underground pipes for the transport of wastewater, is particularly dominated by the separate system (SODECI, 2018). However, two (02) municipalities of the city operate in a combined system, namely Treichville and Plateau. Indeed, concerning the transport of wastewater, the basic collector starts in Adobo with a diameter of 800 mm and ends in Port-Bouet with a diameter of 1800 mm before its discharge into the sea. The collector is oriented North-South, passing respectively through Abobo (800 mm), Williams ville (800 mm), Deux Plateaux (800 mm), Adjamé (800 mm), the east bank of the Plateau (1400 mm), the De Gaulle bridge (1400 mm), the eastern edge of Treichville (1800 mm), the Koumassi dyke (1800 mm), and finally the discharge into the sea on the Port-Bouet side of the town (1800 mm) (SODECI, 2018). It should be noted that the basic collector is made of concrete, particularly vibrated concrete, in order to support much higher loads. Although Yopougon municipality is the densest in terms of network, it is not connected to the base collector and discharges its effluents directly into the Ebrié Lagoon without any form of treatment. Contrary to the 1981 Master Plan, which provided for the construction of a wastewater treatment plant in Azito (Yopougon) and M’Pouto (Riviéra), neither of these plants has been installed and the quality of the lagoon’s water continues to deteriorate. Finally, the pre-treatment station initially planned for Port-Bouet by the two Master Plans was finally built at the root of the Koumassi dyke, which obviously led to the construction of additional works: the 3 km “discharge” section and the “loading chimney” with a diameter of 7 m and a height of 12.70 m (Dongo et al., 2006).

All the wastewater produced by the city of Abidjan is classified into three main categories: domestic wastewater consisting of household discharges (laundry water, dishwater) and black water, industrial wastewater, and wastewater from runoff (macro waste from urban and peri-urban activities; (Gnagne et al., 2015). The base collector, which is entirely made of tree mesh, is estimated to be about 25 km long, including about 1300 km of sea outfall. The topography of the terrain required the installation of discharge or lifting stations to enable all the effluent to be evacuated to the sea after prior pre-treatment (Figure Figure 5; SODECI, 2018). The first station upstream at Abobo, the S7 station and the last station 7 J1 at Port-Bouet. Throughout the city, there are 56 stations, 2 of which are out of service (SODECI, 2018). The De Gaulle station allows wastewater to be pumped from Abidjan North to Abidjan South. In Abidjan South, we have a total of 37 stations, 1 of which is out of order, and 19 in Abidjan North, 1 of which is also not working. To date, the equivalents population connected to the base collector is estimated at 594,000, i.e. 17% of the capacity of the structure. The areas that were supposed to be connected to the main sewer currently have an estimated wastewater production of 2.19 m³ /s and the current capacity of the treatment plant installed at the outlet of the main sewer remains 1.82 m³ /s. While the plant should be overused, it is still operating below its capacity. This indicates an inadequate connection policy to the main sewer.

Figure 5. Overall operation of the base collector in Abidjan (SODECI, 2018).

3.4. Policy framework

As in most countries in Sub-Saharan Africa (Nigeria, Morocco, Cameroon, Senegal, etc.), several laws have been adopted for the integrated management of the environment and sanitation in Côte d’Ivoire. These different laws have allowed the decentralization of environmental and sanitation management in the city of Abidjan through the creation of ministries and institutions. The various laws and decrees regulating the environment and sanitation in Côte d’Ivoire are summarized in Table 3.

Table 3. Laws and decrees regulating the environment and sanitation in Côte d’Ivoire (MACOM, 2008)

Policy framework	Content
Environmental policy	-The Ministry of the Environment, Water and Forests (MEEF) is responsible for defining national guidelines and strategies for environmental management. -The National Environment Agency (ANDE) was created on 9 July 1997 by the Ivorian Ministry of the Environment. It is responsible for drafting the terms of reference in collaboration with the project owner in return for a charge of USD 747.547 (5,000,000 CFA francs).
Environmental Code	-The law on the environmental code was created on 3 October 1996. So, the environment code is completed by five (5) important decrees: i)determining the rules and procedures applicable to studies relating to the environmental impact of development projects; ii) the creation and organization of a public administrative establishment called the National Environment Agency (ANDE); iii) the protection of the marine and lagoon environment against pollution; iv) the creation and organization of the National Environment Fund, abbreviated as FNDE; v) Installations classified for environmental protection.
Sanitation policy	-The sanitation policy is under the responsibility of the Ministry of Construction, Urbanization and Housing (MCUH) - The Sanitation Directorate develops and implements national sanitation policies and strategies with the overall objective of contributing to sustainable development.
Institutional and regulatory framework of sanitation	-The law n°98-755 of 23 December 1998 on the “Code de l’eau” of Côte d’Ivoire, define sanitation as the collection, evacuation, and discharge or destruction according to sanitary requirements, with or without prior treatment of rainwater, wastewater or solid waste (Coulibaly, 2018; Ouattara et al., 2021)
Institutional framework	- National Office of Sanitation and Drainage (ONAD) - Ministry of Construction, Housing, Sanitation and Urban Planning (MCLAU) - Ministry of Economy and Finance (MINEF) - National Sanitation and Drainage Fund (FNAD)
Regulatory framework	- The decree on the creation and organization of the state-owned company called “National Office of Sanitation and Drainage (ONAD) - the decree on the creation of the national sanitation and drainage fund - the decree on the sanitation fee applicable to users of the public service in Abidjan - the decree on the approval of the leasing contract for the maintenance and operation of the networks, sanitation and drainage in Abidjan.

3.5. Sewer management models

According to (SODECI, 2018), two different techniques are commonly used in Côte d’Ivoire as well as in most developing countries such as Nigeria, Cameroon, South Africa. These techniques include closed-circuit television (CCTV) and Manual surveys. Closed-circuit television (CCTV), for infiltrations, this is an extremely helpful micro surveillance tool. Its use for exfiltration is limited because it relies solely on visual records (Cheng and Wang 2018). Although it is simple to detect faults visually that could lead to leakage, it is almost impossible to detect damaged joints. Monitoring the parts of the sewer below the flow is likewise nearly impossible unless the effluent is quite clear On a partially overloaded sewerage system, camera surveillance was also employed in conjunction with a sonar unit to provide a complete picture of the sewer, both above and below the wastewater flows (Cheng & Wang, 2018). As for the manual surveys, It’s merely a question of gradually opening the manholes during periods of low flow (such as at night) and noting any unusually clear flow (temporary plugs may be necessary). Although this is the most cost-effective approach to tracking seepage, it is only used in only few seepage studies (Hassan et al., 2019).

The technologies currently available on the market, according to Haurum and Moeslund (1996), are mostly focused on closed-circuit television (CCTV) systems. These are remotely controlled vehicles with television cameras that move through wastewater lines. However, some new alternative technologies have been created and are now widely used around the world (Ahn & Cho, 2018) including flow measurement techniques and applied mathematics. Thus we propose some existing technologies and also new technologies for a better management of pipe failures in Côte d’Ivoire and in sub-Saharan Africa:

3.5.1. Existing technologies

Numerical Modelling strategies: These models include everything from dry weather flow to current random models (Moczulski et al., 2016). Such methodologies only provide a very broad view for sewerage leakage operations, and should thus only be utilized for a general reading of the system. The most significant disadvantages are cost and time, as most systems rely on existing models that require many flow and sewer studies before they can be used. They do, however, give the simplest beginning point for locating defective locations. The latest computational programs, supported by random strategies, show great promise. Accuracy of the order of 100% has been achieved with very restricted information (For instance, volumes at key catchment places (Moczulski et al., 2016)).

Flow monitors (or meters): These are only useful for numerical modeling because their precision isn’t high enough to detect slight differences between manholes (Sun et al., 2021). Information on water use, and thus on sewage discharges, is currently mainly derived from applied mathematics and is therefore not sufficiently accurate for small studies. In conjunction with system modelling, a highly detailed overall image of a catchment area may be obtained at a reasonable cost. However, many strategically placed monitors allow the rapid inclusion or exclusion of network components in a study. The development of a large number of correct counters could improve the efficiency of this method.

Dye dilution: This is a technique that involves injecting a dye into a system at a specific dilution and measuring the dilution rate downstream (Pecly and Fernandes 2017). This method can be used as a monitoring method when inflow is suspected, but it can also be used to detect outflow if a flow meter is included at the sampling site.

Infrared thermography: As with the detection of sewer pipes, this can be carried out using an aircraft to fly over the region or by using specially equipped vans. While this method has shown good results, it is very sensitive to environmental conditions (especially rain) and is exorbitantly high in cost for general application (Bach & Kodikara, 2017).

Dye and smoke test: This method is commonly utilised in Australia and the United States, where sewer records are reliable. Because all connections must be known before the test testing, it is less common in some developing countries’ sewers (Tatiparthi et al., 2021).

Air pressure testing: They’re a common check for new pipes (Ulutaş et al., 2021). They were rarely used on older sewers until recently since it was nearly impossible to seal the laterals and other air leakage points. The plugs are now easier to install into the sewers thanks to new robotic procedures. This has now made pneumatic testing of historic sewers viable and cost-effective. However, this method only tests the main sewer, not the laterals or manholes.

Water tests: These are the only test methods that can guarantee 100% correct responses for both infiltration and exfiltration (Rojek & Studzinski, 2019). Regrettably, they are both time-consuming and inconvenient. It is therefore critical to choose the lengths that will be analyzed using different approaches. Table A1 lists the summary with advantages and disadvantages of the different technologies (See appendix 1).

3.5.2. New technologies

Current sonde: This is a very new method that was designed in Germany. When the sonde crosses a leaking point, it emits an electric current perpendicular to itself (and to the pipe), the intensity of which increases. The computer representation is difficult to grasp, and the screen does not distinguish between a leak and a link, which is one of the primary drawbacks. This challenge may be solved by progress in a German R&D project on this method (Rayhana et al., 2020).

Acoustic systems: They concentrate on the detection of frequencies and other mechanical mechanisms generated by mechanical wave propagation. They are good for detecting cracks and determining the state of connections and seats. Hammer blows create vibrations in the low-frequency band (100–10,000 Hz; Langeveld & Clemens, 2016). This technology has not yet shown to be suitable for detecting leaks in sewers due to its low resolution and accuracy.

Microwave signals: They are utilised to keep track of the state of sewer lines. On the one hand, ordinary microwave sensors are too huge to inspect tiny pipelines, and on the other hand, they are too expensive for many common activities. In Germany, a smaller and less expensive microwave backscatter sensor has been developed. It can identify irregularities behind the outer surface of pipes at a medium-range (Thiyagarajan, 2018). A microwave sensor that scans the wall was later developed. It is transported on a truck and may spin around the pipe’s axis. As a result, the entire pipe surface may be scanned with the maximum precision and resolution possible.

Ground-penetrating radar: Ground-penetrating radar was rarely used until recently because different soil types and pipe depths necessitated different radar frequencies, so the correct frequency had to be chosen. A brand new system, originally developed to locate plastic landmines, uses multi-frequency radar and can therefore operate in a diversity of soil conditions. At the moment, however, this system has been tested very little on sewage pipes and its effectiveness is difficult to measure (Alvarez et al., 2018).

Sonar: It is capable of producing a suitably detailed picture of the pipe wall’s profile and the surrounding environment. However, the results are quite subject to interpretation, necessitating the use of a highly skilled operator. It can, however, identify seepage in overloaded pipes since it can reveal underwater flow patterns (Moradi et al., 2019).

Probes for neutrons and gamma rays: They have been used to detect cracks in geophysical boreholes and to analyze soil parameters including density and moisture content in groundwater monitoring wells. The applicability of this technology to the identification of active leaking in sewers (large fluctuations in soil moisture) and holes after sewer pipes have been demonstrated in tests with radioactive sensors in sewers (different densities). Different subsurface physical conditions affect radioactive sensors, and their sensitivity and resolution have been limited in the past (Kurmer et al., 2019). In Germany, extensive investigations with radioactive sensors are planned in both laboratory tests and on test beds in various settings.

Ultrasonic sensors for air: For the detection and measurement of cracks and wall thicknesses, they have also been effectively modified and integrated into the German KARO multi-sensor system (Drenoyanis et al., 2019). The KARO system’s airborne ultrasonic sensors have a vast examination area, but their resolution and accuracy are poor. The depth and length of the tiniest fissures are difficult to discern.

Optical triangulation method: Another method for obtaining 3D optical measurements of a sewer pipe is to utilize a laser. A new device was created and incorporated in Germany (KARO) and Australia (PIRAT). It enables systematic pipe geometry measurement (diameter and deviations from the circular shape) while the robot is moving. The range of the 3D sensor utilized in these systems is relatively extensive because it is directed straight in the direction of the robot’s movement; nonetheless, the resulting resolution and accuracy remain low. Small cracks (less than 1 mm) are not noticeable (Sirazitdinova et al., 2018).

Hydrochemical sensors: Preliminary tests using movable probes within pipes in Germany have shown that this technology can be used to accurately find groundwater seepage. Electrochemical sensors must be tuned to the content of the wastewater because they are only very sensitive to a few chemical components and have a comparable poor precision. While it has been demonstrated that groundwater seepage can be detected, wastewater exfiltration is almost always hard to detect. In both laboratory and field studies, numerous studies have been carried out to improve the accuracy and reliability of these sensors (Selvam et al., 2018).Table A2 shows the summary of advantages and disadvantages of the different technologies (See appendix 1).

4. Status of sewer systems in Abidjan, Côte d’Ivoire

The current configuration of the sanitation system does not conform to either of the two Sanitation Master Plans (1971 Master Plan and 1981 Master Plan), although there are some similarities. Thus, to improve the situation, the Ministry of Urban Sanitation and Sanitation (MSUA) obtained funds from World Bank under the Second Debt Relief and Development Contract (C2D) to finance a priority programme of sanitation and drainage works in Abidjan District. The programme consists of carrying out priority works to strengthen sanitation and drainage infrastructure in the city (MSUA, 2016). These works do not concern slum and Peri-urban areas as they are not included in the urban development plans. These slum and Peri-urban areas seem to be ignored by the administration; this may explain the presence of wastewater in the streets, on wasteland, and in front yards, which are sources of disease (Dongo, 2006).

4.1. issues affecting sewerage networks in Abidjan and most Sub-Saharan African countries

The factors that degrade safe water networks also act on the degradation of sewerage systems (Totaro & Piccinni, 2020). However, certain factors or the response of a structure to a degradation factor are specific to the type of network and its use. For example, safe water networks are pressurised and the materials used are different (cast iron, steel, PVC), whereas sewerage networks operate mostly on the free surface except during important rainy events, and the materials used are generally concrete, cement, cast iron, PVC or sandstone. Sewerage and safe water systems do not react in the same way to internal and external stresses. Kakoudakis (2019) identifies several factors that can contribute to the degradation of sewerage systems, which are presented in Table 4. From the point of view of practitioners Kakoudakis (2019), the predominant parameters influencing the degradation of sewerage systems are: Land use, the age of the collectors, investment history, the thickness of the cover, the nature of the surrounding soil, the diameter of the collectors, the construction standard, and the connections.

Table 4. Factors influencing the structural and operational degradation in Sub-Sharan African sewerage systems (Kakoudakis, 2019; Ouattara et al., 2021)

Construction features	External local features	Others factors	Anthropogenic factors
Construction Method	Land use	Presence of rats	Dumping of solid waste in or around the network
Know-how	Surface load	Effluents characteristics	Uncontrolled connections of wastewater networks to the system
Pipe diameter	Surface type	Unsuitable maintenance	Urban agriculture in or around the system
Thickness of the cover	Soil movement	Investment history	Commercial activities on both sides of the network
Pipe layering method	Maintenance by public services	Age	Behaviour of local residents
Pipe materials	Groundwater regime	Height of sediment
Length of a driving unit	Infiltration/exfiltration	Hydraulic overload
Levelling	Properties of the encasing soil	Hydraulic capacity
Branch connections	Root penetration
Type of joints	Leaks and breaks in the network
Quality of the backfill

The most dominant factors responsible for structural and operational failures of sewers in most Sub-Saharan African countries that use collective sewerage are related to the ageing of the collectors and to socio-environmental factors. These factors mainly include poor solid waste and wastewater management practices by residents, i.e. illegal dumping of solid waste in the sewers, unauthorised and defective connections to the network, structural malfunctions related to the age of the network (cracked, exposed, clogged, broken), urban agriculture in the vicinity of the channels, natural phenomena such as erosion, landslides in the undeveloped parts, and the high concentration of vegetation in the network, whose roots have a significant impact on the drains. They highlight the degree of exposure of urban sewerage systems in large sub-Saharan African cities.

For example, along the Uniwax drainage system (9.79 km) which is one of the longest sewer located in the city of Abidjan, precisely in Yopougon municipality, the geographical survey identified a total of 169 uncontrolled dumping of solid waste, 110 unauthorised and defective wastewater network connection, 41 unauthorised urban agriculture and vegetation on both sides of the system, 189 problems related to structural and operational failures of the channel and high concentration of vegetation in the undeveloped parts (Figure A1, A2, A3, A4, A5, A6 and A7 in appendix 2). These different factors are partly responsible for the failure of the Uniwax drainage system. This possibility has been verified as more than 78.6% of the different sections of the channel are clogged.

A study conducted by Ouattara et al. (2021) on the diagnosis of the sewers identified several factors responsible for the malfunctioning of the 21/22 collector in Yopougon Nouveau quartier. Like the Uniwax drainage system, these factors mainly concerned the deposit of household waste in the manholes, obsolete and defective connections in the sewer, as well as poor management practices of the inhabitants with regard to the network. Out of the 5.65 km length of the sewerage network, a total of 272 illegal and defective connections and 234 waste deposits were identified. Combined with the age of the collector, these factors are responsible for the structural and operational dysfunctions of the collector. Figure 6 shows the spatial distribution of the different major degradation factors of two collectors (Collector 21/22 and Uniwax Drainage System) in the city of Abidjan. These dysfunctions are generally materialised by backflow at the level of the manholes, obstructions and leaks (breaks, longitudinal and circumferential leaks and holes). It should be emphasized that these factors are not specific to the city of Abidjan, but are identical in most of the large cities in Sub-Saharan Africa. Several studies conducted in different urban areas have highlighted this situation. The city of Ziguinchor in southern Senegal has obstructions in its drainage systems due to solid waste, which has a negative impact on urban drainage (Bouly et al., 2019). In a similar way, the cities of Isiolo in Kenya, Lomé in Togo and Maroua in Cameroon face significant drainage challenges according to studies conducted by Karanja (2014), Ndongo et al. (2015), and Gbafa et al. (2017). The same is observed in Enugu metropolis in Nigeria (Okoye et al., 2018). In addition, the high concentration of solid wastes and uncontrolled and faulty connections constitute a blockage to the flow of wastewater. Thus, these wastewaters are discharged into the houses through backups impacting on the health and well-being of the residents as highlighted by (Tuo, Coulibaly, and Ake-Awomon 2019). Indeed, the rapid population growth of municipalities in large urban areas in sub-Saharan Africa has rendered previously designed urban plans and other master plans non-operational. At the same time, it has accelerated the uncontrolled development of different neighbourhoods in the cities. This increase in needs of all kinds, out of all proportion to local availability, has led to a breakdown in the capacity of existing infrastructure, particularly in terms of rainwater and domestic drainage and other networks. Faced with the difficulties encountered by the municipal authorities to satisfy the demands expressed, the populations have settled without right or title, most often in areas unsuitable for habitation, thus creating a proliferation of precarious and unhealthy neighbourhoods. This situation is at the origin of the increase in waste of all kinds, which is one of the major causes of the insalubrity of the neighbourhoods, the main receptors of which are sewerage systems (Adouni et al., 2022). Bangoura (2018), in a study on the management of solid household waste in the city of Conakry in Guinea, showed that 76.6% of household waste was generated by precarious neighbourhoods whose destination was the urban sewage systems thus favouring the obstruction and blockage of collectors. In the specific area of solid waste, management remains weak in the municipality of Yopougon in Abidjan, despite the efforts of municipal technical services. Collection is irregular and unsystematic, and the material means of collection are insufficient and most often unsuitable and inappropriate. In the recent past, the rate of household waste collection has varied from 32% (2001) to 98% (1998) over the period from 1994 to 2007 (Aké, 2008). The same observation was made by NGambi (2016) in five African capitals (Dakar, Bujumbura, Yaoundé, Cotonou and Nairobi). His study showed that the rate of waste collection in all the cities taken into account does not exceed 50% and the average is 36.7%. This waste therefore ends up in the drains, which causes the drains to malfunction.

Figure 6. Spatial distribution of the different major degradation factors along Collector 21/22 and Uniwax Drainage System in Abidjan (Ouattara et al., 2021).

With regard to wastewater management, several uncontrolled and defective connections were observed on the sewers in Abidjan. A study conducted by Kablan et al. (2017) revealed that 33% of the malfunctions observed in the sewerage networks in Cocody municipality are due to defective and unregulated connections. Similar results were obtained by Dadjo (2018) who observed that on average 53% of the pipes studied in the city of Cotonou in Benin are experiencing progressive dysfunctions due to bad practices of faulty connections, 15% of which are deliberately made by residents. Table 5 presents the frequencies of the different malfunction factors in some major sub-Saharan African cities

Table 5. Frequency of failures in urban sewerage systems in some Sub-Saharan African cities (Bangoura, 2018; Dadjo, (2018); Ouattara et al., 2021)

Failures factorsSub-Saharan African cities	Obstructions or blockages due to solid wastes (%)	Failures related to unauthorised and defective connections (%)	Structural failures related to the age (Breaks, leaks, holes) (%)	Natural phenomena (landslide or erosion (%))
Abidjan	65	33	23	2.6
Dakar	61.33	13.6	28	2
Cotonou	61.3	15	26	1.8
Isolio	51.2	19	20	1.5
Maroua	75	32	31	2.1
Lomé	70.15	36.5	29	1.6
Edo Ekiti	61	21	18
Conakry	76.6	29	31
Enugu	66	17	17
Yaoundé	71	25	30
Accra	31.30	11	17
Johannesburg	28.65	8	9
Bujumbura	36.7	19	9
Nairobi	32.6	11.6	9

4.1.1. Backflow

Backflow is identified as a situation in which the free flow capacity in a downstream section of a collection system has been fully reached (Figure 7) and acts as a disruption to further flow (Irwin et al., 2018). The phenomenon of backflow in wastewater and rain water systems is mostly due to blockages in the network (Ouattara et al., 2021).

Figure 7. Backflow from a manhole in Cocody Angré chateaux.

The system therefore begins to accumulate residual water behind the bottleneck in the system, creating a hydraulic level line (HLL) upstream of the bottleneck, which has a smaller gradient than the pipe segment. Figure Figure 8 is an illustration of a scatter plot obtained at the flowmeter location in a 33 inches conduit in which the flow enters backflow states when it reaches a depth of about 15 inches. Beginning at a depth of approximately 15 inches, the flow rate stays fairly constant at 9 mgd (note that the data follows the Iso-Q™ line of 9 mgd) as the pipe continues to fill until full levels are reached, then jumps to a HGL depth of approximately 125 inches (Paul et al., 2005a). The operating capacity of this conduit is 43% (9 mgd/ 21 mgd) of its designed capacity because of the downstream bottleneck. This condition could be characterised as a 57% backflow (Pichler et al., 2019).

Figure 8. Scattergraph produced by Downstream Bottleneck (Paul et al., 2005a).

4.1.2. Obstructions

Obstruction manifests itself as a blockage of the pipe by preventing the normal flow of water through the pipe (Ouattara et al., 2021). In general (Gutierrez-Mondragon et al., 1999), the main causes of sewer line blockages are:

1. Accumulated deposits of grease, solid waste, or unwanted material in the main or lateral sewer line.

2. Partial or total obstruction of the sewer line resulting from tree root intrusion (Figure Figure 9).

   Figure 9. Blockage of rain sewer in M’Pouto (Abidjan, Cocody-Riviera) (Ouattara et al., 2021).

   

3. Collapse of the sewerage network as a result of obsolete and defective sewer pipes.

4. Debris penetrating the sewerage system as a consequence of unauthorized pipe connections.

4.1.3. Leakage

The level and degree of leakage in sewerage systems varies from country to country, but also from region to region within a country (Ali & Choi, 2019; Peche & Graf, 2021). A sewer leak can be defined as the escape or release of wastewater from a sewer pipe or pipeline due to a crack, holes or other defects in the system (Elmasry et al., 2002). One of the most telling signs of a sewage leak is an unpleasant smell. This smell is usually found in the basement, but can also be present outside (Cao et al., 2019). These odours are usually caused by moulds, which grow in the presence of moisture from leaking pipes. The leakage in an underground wastewater pipe can be caused by the presence of wastewater on the surface or by moisture on the ground. Leaks in open drainage systems are observed visually due to defects in the network (Ouattara et al., 2021).

4.1.4. Causes of sewerage pipe leaks and failures

Sewerage systems Leaks and failures are caused by a range of factors (Saghi and Ansari, 2015). However, understanding the source of these leaks is critical to better prevent them in the future and repair them appropriately (Mawasha, 2018). Sewer pipes are susceptible to corrosion, which causes perforations in their walls. The main cause of leakage and failures in sewerage systems is external corrosion (Foorginezhad et al., 2021); “Corrosion is the main cause of most metal pipe failures. There is generally a tendency for metals to want to return to their ore stage” (Moskvicheva et al., 2016). Wastewater pipes installed in wet and saturated soils and environments are more susceptible to external corrosion than those installed in drier soils; the very low redox potential and low strength of wet soils create severe corrosion problems (Jiang et al., 2015). As corrosion is a completely natural process, electrical stray currents are likely to contribute significantly to corrosion processes in metal pipes (Grengg et al., 2020). Indeed, corrosion not only leads to the development of holes in pipes but also weakens the strength of drains to the point of causing other problems that can create serious environmental consequences (Oualit, Jauberthie, and Abadlia 2019) (Oualit and Abadlia 2019).

4.1.5. Factors affecting leakages

Although the corrosion process is the most important determinant of pipe leakage, many more factors act with or independently of corrosion to cause leakage or failure in the pipes (Saghi and Ansari, 2015). These various factors include the structural properties of the pipes, the type, and behaviour of soil, the pressure in the pipe system, the construction methods, etc. Table 6 lists many of the important factors that contribute to leakage.

Table 6. Factors influencing leakage/breaks in pipe systems (Saghi and Ansari, 2015)

Physical pipe characteristics	Age of the pipe Corrosion (metal pipes) Diameter Pipe material
Soil Behavior	Erosion and soil loss Freezing soil around the pipe Frost penetration Landslides Settlement (due to loading on the soil) 6. Subsidence (pumping water in the area)
Operation & Maintenance	High-pressure zones Improper tapping of service connections Main flushing New construction in the vicinity of a pipe altering the loads that a pipe must bear. Water hammer
Installation of a new main	Manufacturer defects in the pipe Pipe damaged in shipment Pipe laid improperly Poor backfill and bedding 5. Poor joining methods
External Conditions	Contact with other structures Construction equipment driving over mains Excavation and external damage Heavy traffic loads causing vibrations 5. Highly corrosive soil conditions 6. Stray electric current
Water Conditions	Aggressiveness Pressure (excessive pressures in the network) Temperature

4.1.6. Typology of leakage

The work carried out by Zeng et al. (2020) differentiated the main types of leakage in a network:

1. Diffuse leaks, which are technically impossible to eliminate. They are often located at joints between pipes and at connections.

2. Leaks due to breakage. By breakage we mean the phenomenon that leads to the repair of the pipe as soon as it is detected.

Anbari et al. (2017) also added that the possibility of pipe failure or deterioration with increasing age of the pipe, as corrosion is likely to deteriorate it. As corrosion is the most frequent source of leakage in pipelines, both directly as well as indirectly, an essential consideration is that different materials are more resilient to corrosion than other materials. Table 7 lists many types of water and sewer main leakage and their associated causes.

Table 7. Types of leaks (Anbari et al., 2017)

Type of leak	Cause	Repair Mechanism
Full break	Corrosion (Graphitization)	Replace pipe or section of pipe
Joint	Displaced gaskets Expansive soils Improperly tightened joints Overloading. Subsidence	Seal the joint
Service Connection	Excessive pressure	Replace connection
Circumferential	Excessive loading Poor bedding	Replace pipe section and restore bedding
Longitudinal split	Pipe fatigue Water hammer	Sleeve
Puncture/Hole	Corrosion (Pitting) Rocks or clay mounds in the bedding	Patch

4.1.7. Consequences of sanitation networks failures and malfunction

Sewer failures and malfunctions can have many impacts on society and the environment (Ouattara, 2021). Figure Figure 10 illustrates some of the socio-economic and environmental consequences of sewer and drainage system failures.

Figure 10. Social and environmental impacts of pipe failures (Ouattara et al., 2021).

4.2. Description of decision-support tools (DST) for sewerage systems management

Currently, there are several available management assistance tools for sewerage systems (El-Housni et al 2018). These tools are diverse in their scope as well as in their purpose. The key points of the different tools are detailed in the following figure (Figure Figure 11).

Figure 11. The generic infrastructure asset management system with the corresponding sewer asset management tools applicable at different stages (Ana & Bauwens, 2007, September; Lemer, 1999).

(a) Group 1: Performance analysis tools

The maintenance decision support and control tools under this category refer to those that deal primarily with performance modelling.

Hasegawa et al. model (Japan): This model is an approach that aims to estimate the degree of need for rehabilitation of existing pipes while focusing on four different aspects: 1) reduction and gradual decrease of flow capacity, 2) possible probability of degradation and collapse of roads, 3) sewer overflow and flooding influenced by inflow and infiltration, and 4) increase of treatment cost due to inflow and infiltration (Hasegawa et al., 1999). The scale of urgency for repair is determined at the conduit level. Using the results of CCTV inspection, the reduction in flow capacity is calculated. Based on the flow reduction capacity value and the type of pipe (i.e. sanitary or combined), the pipes are ranked into three categories: rank 1—a little flow capacity left, rank 2—left flow capacity is half the original one, and rank 3—left flow capacity is more than half of the original. The possibility of road collapse is evaluated using a road collapse probability index, which was constructed based on observed pipe defects such as breakage, defective lateral, disconnected joints, etc. An index is assigned to a given sewer, based on observed defects from CCTV inspection. The sewers are classified into three categories, based on the index values: rank 1—the high possibility of road collapse, rank 2—the high possibility of road collapse if no measures are taken, and rank 3—the low possibility of road collapse. The influence of sewer surroundings is factored in the determination of the possibility of sewer collapse. Using pipe profile (age and material) and circumstances surrounding (layer height, groundwater level, road category, traffic, terrain typologies, and the presence of other underground structures). Pipes are assigned a rank; the possibility of road collapse is then considered to increase in the proportion of this ranking. The influence of inflow/infiltration on sewer overflow and flooding is determined using a simulation model. The possibility of overflow/flooding is evaluated based on the relation of the wet weather peak flow (Q) and the design (Q_d) or maximum pipe flow (Q_max) capacities: rank 1- Q ≥ Q_max, rank 2- Q_d ≤ Q < Q_max, and rank 3—Q < Q_d. Then, the associated increase in treatment costs for excessive inflow/infiltration is calculated. To come up with the final repair priority, the ranking of each sewer based on the four viewpoints are combined; the pipe with the highest combined rank is prioritized first.

Bengassem and Bennis model (Canada): This model is an approach to determine and evaluate the structural and operational conditions of sewerage and drainage systems, using a fuzzy reasoning system, to facilitate the design of a repair plan (Bengassem & Bennis, (2000)). The technique includes hardware verification and hydraulic modelling to judge the performance of the sewer system components. Fuzzy theory is therefore applied at the level of the pipe sections for an integration of all evaluation factors, which allows an estimation of the quality of the sewer system. Three parameters are taken into account in the assessment of structural performance: 1) intrinsic parameters (i.e. pipe failures), 2) extrinsic parameters (i.e. pipe features and environmental characteristics that affect the deterioration of the pipe, e.g., geotechnical conditions and hydrogeological parameters), and (3) site sensitivity (i.e. site constitution, soil density, etc.). Each pipe is then rated from 0 to 100 to show its condition to these three different aspects. The hydraulic performance of the system focuses on a system’s carrying capability and is estimated through the use of the Bennis method (Bennis et al., 1999), which defines the overload resistance factor of a conduit. To define the performance level of the sewer system, 3 fuzzy schemes are constructed: a structural fuzzy system (FSS), a flow fuzzy scheme (FHS), and a general fuzzy system (FGS). FSS and FHS jointly determine the performance of the structures (based on intrinsic, extrinsic, and site sensitivity) and the hydraulic performance (based on site sensitivity, SRF, and SEF values). FGS then aggregates all the other parameters to calculate the Global Operational Factor (GOF) for each pipe section in the sewer system.

Baik model (US): The Baik model is an estimation of state-level transition probabilities in Markov chain-based degradation models for sewer networks, with the use of an ordinary proportional model (Baik et al., 2006). The aim is the prediction of the future state of sewers so that managers will be able to prepare an inspection, renovation, and renewal interventions efficiently and cost-effectively. In one of the several cases, the model needs data from inspections of the existing system to assess the potential for change. For structural assessment, an internal inspection is required and for hydraulic assessments, models are used. Infiltration and inflow are also studied. The assessment of the quality of the piping is based on the maintenance and structural points (assessed on the analysis of 108 criteria, e.g., deformation, presence of roots) of the inspection.

(b) Group 2: Performance and decision analyses tools

From data collection to performance modeling and decision analysis, the tools in this category cover the first three stages of a complete asset management system.

KureCAD (Finland): KureCAD is a Geographic Information System (GIS)-based software solution for sewerage rehabilitation that includes the following features: 1) asset data storage, 2) sewer rehabilitation prioritization, and 3) documentation of rehabilitation plans (Stone et al., 2002).The three primary sets of data in KureCAD are structural condition (strength and morphology), operational condition (ability to discharge water), and leakage rates (evaluation of pipe leaking). The user can assign a rating from 1 (good condition without repair) to 4 (extremely bad condition requiring) urgent intervention based on data from internal inspections or repair reports. The state of the pipe with relation to the three basic data kinds is shown here. KureCAD then translates the condition index into a GIS presentation by combining the scores. Following this, a pipe status assessment can be carried out, which includes prioritizing sewer rehabilitation, determining the best repair method, and calculating the cost. KureCAD generate the planning and design documentation needed to begin rehabilitation efforts, such as accurate site maps, precise construction specifications, and contract conditions.

PRISM (Canada): PRISM (Proactive Rehabilitative Infrastructure Sewer Management) is a computerised financing cost model for prioritising the scheduling of sewer upgrades, based on several budgetary constraints (Ariaratnam & MacLeod, 2002). Through linear programming, the PRISM model strives to reduce capital expenditure across a planning horizon while maintaining full annual budgets and prioritizing pipe repair to the highest priority pipe categories. Pipe types are identified according to their age, thickness, material, type of landfill, and an average depth of cover. An estimate of the probability of deficiency, which is the possibility that a collector is in a structurally defective state, calculated from historical data using a log-linear statistical model, is made for each pipe class. A pipe significance factor is determined based on the frequency of failures, the kind of pipe (mixed, sanitary, storm), and the pipe size to create two or more classes of pipes with equal failure risks. The pipe classification with the highest factor is the first to be considered in rehabilitation planning.

APOGEE (France): APOGEE is a decision-making tool that was created to improve sewer network annual planning and renovation (Rommel et al., 1989). It is made up of three basic elements:

A database—this contains relevant information gathered through inspections about the symptoms of degradation at different sections of the network;

An expert system—this makes a diagnosis on the state of the sewer network, based on the entries in the database. Its knowledge base concerns the modeling of mechanisms affecting sewer network failure based on five categories: hydrology and hydrogeology, excess loading on the network, abrasion and aggressivity of the effluent, pressurized flow in the collector, and history of the construction modes. Its inference engine is implemented in Prolog; it is capable of mixed chaining and can carry out reasoning based on a first-order logic, i.e., predicate calculus;

A planning module—this schedules the interventions to repair the network and prescribes the manner of repair. It employs a multi-criteria approach in defining, evaluating, and selecting rehabilitation actions based on technical and environmental criteria.

AQUA-WertMin (Germany): AQUA-WertMin (Berger et al., 2016) is a computer-based program designed to support utility companies in their TV inspection, renewal, and new construction strategies for sewerage systems. The implementation of the Herz distribution (Herz, 1996), which calculates the transition of pipes from one condition category to the next lower condition category over time, is its distinguishing characteristic. Based on audits, users enter pipe condition scores into the application (e.g., CCTV). The software then divides the pipes into six categories: from those in excellent condition or with no visible faults to those that are faulty or need immediate replacement. In a second step, the software calculates the possibility that a pipe segment will change its condition and move to the nearest lower class. In this way, the degradation of the pipes and the future requirements for renovation of the drainage systems are planned. The programme also includes mechanisms that help operators to assess rehabilitation costs based on estimated costs and repair times.

(c) Group 3: Total sewer asset management tools

This group of tools represents an entire sewage asset management system that encompasses all stages of the complete infrastructure asset management system.

Hydroplan (Belgium): Hydroplan is an integrated method to sewer systems quality management that includes structural, hydraulic, and environmental risk evaluations applied to the sewerage network’s strategic components. The procedure (Van Herzeele et al., 2006) starts with a list of the present state of affairs and historical knowledge, to set up associate degree initial quality information. Then, at the pipe level, strategic analysis is carried out. The pipes that will cause the most serious impacts in the event of collapse are assessed based on a variety of parameters, including monetary, social, and environmental consequences. Coefficient factors area unit applied to the scores to designate the worldwide strategic level of a given sewer. The chances of sewage failures are estimated while calculating the impact of failures. This is often done at the structural level (pipe condition) mistreatment aging models (e.g., Herz distribution) and scrutiny results, and the hydraulic and ecological levels employing a marking fluid mechanics model (e.g., Info Works). Score area unit are attributed to the various aspects of failure, per fastened criteria, such as the likelihood of collapse (or calculable residual lifetime), frequency of flooding, etc. The results of the strategic analysis, as well as the failure probabilities, are combined to come back with the global risk score. This stage provides a collection of important pipes in areas where proactive repair efforts are the most cost-effective. The ultimate outputs of the procedure are: 1) a short action list that might take the network to the next performance and: 2) a mid- and long-run operation and maintenance strategy that might maintain the high level of performance. Long-term investments are determined using a comprehensive life cycle model and Monte Carlo simulations, which incorporates all prices by monetizing risks and preventive investments. The sewerage data is continuously updated with newly obtained data, and the procedure is coiled through to ensure the quick detection of recent concerns and to allow for faster development of the most cost-effective remedies.

CARE-S (Europe): CARE-S (Computer-Aided Recovery of Sewerage Systems) is a DSS programme developed to assist local engineers in the construction and efficient control of sewerage systems (Saegrov, 2006). The CARE-S method is characterised by four steps:

Step 1. Initial planning—this involves setting the framework for sewer rehabilitation, determining the relevant performance indicators (PIs) and identifying and prioritising areas for rehabilitation. At this level, CARE-S provides a tool to generate relevant PIs for the rehabilitation decision;

Step 2. Diagnostic study—this consists of investigations into the structural, hydraulic, environmental and operational performance of the prioritized sewer sub-network (as specified). CARE-S includes a number of network-level and detailed individual models that ensure the assessment of the hydraulic (e.g., Info Works, FLUENT), environmental (e.g., CSO Assessment Tool), and structural (e.g., GompitZ, WATS 2.0) conditions of the network and their progression over time;

Step 3. Solution selection—the concept of a possible solution to a sewer problem identified in step 2 involves the development of integrated solutions. CARE-S uses multi-criteria decision-making approaches to prioritise pipes for rehabilitation (using the interactive elimination process; Baur and Herz, (2001b) and to select appropriate rehabilitation strategies (using the balancing and positioning strategy; see, Strassert (2000)). The prioritization of rehabilitation works is based on several physical, hydraulic, environmental, and socio-economic factors and other associated attributes. The decision on rehabilitation technology is supported by the CARE-S system of information on different rehabilitation methods, which contains an extensive database of repair and restoration technologies. In addition to this, it also provides a tool for developing long-term rehabilitation action, based on predictions of the future condition of the sewers, as planned in step 2;

Step 4. Implementation and monitoring—this stage includes carrying out the rehabilitation activities, implementing the operational scheme, modifying the hydraulic and environmental models, monitoring the key performance indicators (kPIs), continuing a condition monitoring process, verifying the success of the rehabilitation scheme and modifying the scheme if necessary. To accomplish these operations, the CARE-S rehabilitation manager provides general strategic guidance for their implementation.

4.3. Data requirements

Table 8 show a summary of the relevant data for each of the sewer asset management decision support tools described above.

Table 8. Sewer asset management decision-support “tools relevant data” (Ana& Bauwens 2007)

4.4. Selection of decision support systems (DSS) for Sewerage systems for developing countries

Watsan specialists and international development businesses can use a variety of decision-making resources to replace, install, and maintain DSS systems for water and sanitation services. Mannina et al. (2019) examined 120 existing Watsan technology selection guide tools. Assessment tools, process guidance, technical briefs, technical references, and policies papers were defined as the five types of guidance tools. Mannina et al. (2019) chose those tools that are expected to offer the most comprehensive decision support tools to managers of collective sanitation and water services based on an evaluation of these 120 decision support tools. These different decision-making tools are, in fact, the closest to an ideal decision-making aid for water supply and sanitation. Table 9 summarizes these findings. Palaniappan recognized four primary features of a suitable decision support system from these four supporting resources: first, the sector, the locality, the subjects, and the users.

Table 9. List of the most comprehensive water supply and sanitation support resources (Mannina et al., 2019)

Authors	Decision support resources
Australian Agency for International Development	Safe water guide for the Australian aid program (2005)
Brikké, F. and Bredero M.	Linking technology choice with O&M in the context of community water supply and sanitation (WHO 2003)
Cotruvo, J. et al.	Providing safe drinking water in small systems: Technology, operations, and economics (NSF, WHO and PAHO 1999).
Department of Water Affairs and Forestry, South Africa	Introductory guide to appropriate solutions for water and sanitation (2004)
Deverill et al.	Designing water supply and sanitation projects to meet demand in rural and peri-urban communities (2002)
Huuhtanen, S. and Laukkanen	Guide to working in sanitation and hygiene for those working in developing countries (2006)
A. Lantage et al.	Household water treatment and safe storage options in developing countries: Review of current implementation practices (2007)
ROLAC—UNEP	Recommendations on basic sanitation and municipal wastewater for Latin America & the Caribbean (2003)
Skinner, B.—WELL	Small-scale water supply: a review of technologies (2003)
Smet, J. Van Wijk, C. IWSC	Small community water supplies (2002)
UNEP	International source book on environmentally sound technologies for wastewater and rain water management (2000)
UNICEF	Towards better programming: A sanitation handbook (1997)
Finney, B.A. and Gearheart, R.A—U. of Humboldt	Water and wastewater technologies appropriate for reuse model
WELL	Guidance document on water supply and sanitation programmes (1998)
World Bank	Manual on low-cost sanitation technologies for Ger Areas, Mongolia (2006)
WSP and WUP	Water and sanitation for all: A practitioners’ “companion” (2003)

5. Conclusion

The deterioration of urban sewerage systems is recognized as one of the most alarming problems in sub-Saharan Africa and particularly in Côte d’Ivoire. Multiple socioenvironmental variables are responsible for the growing malfunction of these metropolitan sewage and drainage systems. In Abidjan, the failures of the drains have been aggravated by the aging process of the collectors, the poor practices of residents such as the dumping of solid waste and uncontrolled and defective connections in the networks, poorly planned peri-urban agriculture in the vicinity of the collectors, and natural phenomena such as erosion, landslides, and the high concentration of vegetation in the network. The current study suggested that cities in Sub-Saharan Africa, in general, and Abidjan should implement cutting-edge methods that are efficient in terms of cost and network management strategies in order to alleviate the difficulties caused by malfunctioning sewer systems. These tools are classified into three main categories: (i) performance analysis tools that primarily deal with sewer performance modelling, (ii) performance and decision analysis tools that cover the first three stages of a full asset management system, and (iii) total sewer asset management tools that represent a comprehensive sewage asset management system that incorporates all phases of the entire infrastructure asset management system. In addition to the aforementioned strategies and/or procedures, it is imperative that the general public be actively involved in the maintenance of sanitary infrastructure. To ensuring the efficacy of sanitation systems and their capacity to be maintained over time, educational and training programs for households, as well as awareness campaigns, should be carried out.

Author contributions

ZAO and designed the research, ZAO performed the data analysis and wrote the main manuscript text; ATK, AK and KD supervised and revised article. All authors have reviewed the manuscript and approved it for submission.

Acknowledgements

The authors are grateful to the collaboration of the “Water Distribution Company in Côte d’Ivoire (SODECI), The National Office of Sanitation and Drainage (ONAD), Ministry of Sanitation and Municipality vice Mayors office of Yopougon.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data Availability Statement

The data presented in this study is available on request from the corresponding author. The data is not publicly available due to confidentiality.

Additional information

Funding

This study was supported by the Regional Center for Energy and Environmental Sustainability (RCEES) of the University of Energy and Natural Resources under the African Higher Education Centers of Excellence (ACE Impact) Project of World Bank.

APPENDIX 1

Table A1. Advantages and disadvantages of the existing technologies

Existing technologies	Advantages	Disadvantages
Numerical Modelling strategies	Can be safer and cheaper than the real world. Able to test a product or system works before building it. Can use it to find unexpected problems. Able to explore “what if … ” questions. Can speed things up or slow them down to see changes over long or short periods of time.	Mistakes may be made in the programming or rules of the simulation or model. The cost of a simulation model can be high. The cost of running several different simulations may be high. Time may be needed to make sense of the results. People’s reactions to the model or simulation might not be realistic or reliable.
Flow monitors (or meters)	Technology that provides a global and very detailed image.2 Raisonnable cost.	These instruments are only useful for numerical modeling, as their accuracy is not sufficient to detect slight differences between manholes. Information on water use, and thus on wastewater discharges, is currently mainly derived from applied mathematics and is therefore not sufficiently accurate for small studies
Closed-circuit television (CCTV)	Very simple technology to identify leaking defects visually Completely non-destructive Fast and precise Pinpoints accurate location (including depth) of problem Incorporates fully integrated software which enables real-time results and decision-making as well as recordable data for future use/referencing Easy to operate Environmentally friendly	The use of this technology for exfiltration is limited as it relies solely on visual records Cannot detect damaged seals Monitoring of parts of the sewer below the flow is also almost impossible
Infrared thermography	Technology that is very effective at detecting thermal contrasts on the surface of a road due to water leakage. Infrared technologies detect wavelength ranges greater than 0.7 μm, allowing the temperature distribution within the pipe and the surrounding environment to be detected to identify any temperature anomalies that may indicate a leak	cost is exorbitant for general application Infrared thermography uses wavelengths limited to the electromagnetic range between 0.4 and 0.7 μm. It is sensitive to environmental conditions (especially rain)
Dye and smoke test	Efficient and easy to locate leaks and other faulty pipe connections Convenient and effective technology to detect sewer pipe defects and sources of unwanted rainwater in sanitary sewer systems due to broken manholes and other defects Very reasonable costs	It is less common in the sewers of some developing countries because the networks are not well known or are completely blocked
Air pressure testing	No filling of the test section of the sewer with water, thus no problems with obtaining the water and no water damage with sudden pipe breaks. A constant test pressure prevails throughout the whole test section of the sewer, even at the high point of the test length, irrespective of the laying gradients. Faster installation of the test equipment and quick execution of the test. Thus, only a short period of down time of the sewer to be tested is required. The tests can be carried out at temperatures below 0° C (secure against frost). Better construction and light use of the wear-free test device. Acoustic leak localisation by determination of the source of the noise caused by escaping medium is possible. Economical.	It only tests the main sewer, not the laterals or manholes.
Water tests	It is the only test method that can guarantee 100% correct responses for infiltration and exfiltration It is precise It is non-destructive Reasonable cost	It is both time-consuming and not very practical. Low sensitivity, The tendency of leaks to get clogged by viscous food materials during testing, The sensitivity of the test is not high, due to operator dependence.
Manual surveys	Simple and practical technology Most cost-effective approach to monitoring infiltration,	It is not very practical for exfiltration

Table A2. Advantages and disadvantages of new technologies

New technologies	Advantages	Disadvantages
Current sonde	Low total cost of ownership Accuracy at low flow rates High-resolution measurement Good rate of change (flow range)	Computer representation is difficult to grasp The screen does not distinguish between a leak and a junction
Acoustic systems	High resolution to detect cracks and determine the condition of joints and gaskets.	Unsuitable technology for sewer leakage detection due to low resolution and accuracy.
Microwave signals	High-performance technology for scanning with the highest possible accuracy and resolution of images. Very practical in the identification of anomalies behind the external surface of pipes at medium range	Microwave sensors are too large to inspect tiny pipes. Technology too expensive for many common activities
Ground-penetrating radar	It detects metal and non-metal objects, as well as voids and underground irregularities. It makes it possible to measure the dimensions, depth and thickness of targets. Data is provided quickly and can cover a large site area.	The most significant performance limitation of GPR is in high-conductivity materials such as clay soils and soils that are salt contaminated. Performance is also limited by signal scattering in heterogeneous conditions (e.g., rocky soils). Technologie très peu testé sur les canalisations d’assainissement et son efficacité est difficile à mesurer
Sonar	Technology capable of producing a sufficiently detailed image of the pipe wall profile and the surrounding environment However, it can identify seepage in overloaded pipes, as it can reveal underwater flow patterns	The results are quite difficult to interpret and require a highly qualified operator.
Probes for neutrons and gamma rays	Very suitable technology to detect cracks in geophysical boreholes and to analyze soil parameters, including density and moisture content, in groundwater monitoring wells. Practical technology for identifying active leaks in sewers (large fluctuations in soil moisture) and holes after sewer pipes	Different subsurface physical conditions affect radioactive sensors, and their sensitivity and resolution are limited
Ultrasonic sensors for air	Highly effective in detecting and measuring cracks and pipe wall thickness,	The airborne ultrasonic sensors of the KARO system have a large area of examination, but their resolution and accuracy are poor. The depth and length of even the smallest cracks are difficult to discern.
Optical triangulation method	Technology that provides 3D optical measurements of a sewer pipe using a laser. It allows the geometry of the pipe (diameter and deviations from the circular shape) to be measured systematically while the robot is moving.	The resolution and accuracy of the images obtained remain low. Small cracks (less than 1 mm) are not discernible
Hydrochemical sensors	Highly reliable technology for accurate detection of groundwater infiltration.	Electrochemical sensors must be adapted to the wastewater content as they are very sensitive to only a few chemical components and have comparable poor accuracy. Difficult to detect seepage of wastewater with these sensors

APPENDIX 2

Figure A1. High concentration of solid waste in the network and its immediate surroundings in Uniwax drainage system in Abidjan (Ouattara et al., 2021)

Figure A2. Discharge of waste in the system in the first part of 21/22 collector In Yopougon municipality (Abidjan) (Ouattara et al., 2021)

Figure A3. Unauthorised and defective connections on sewer in Abidjan (Ouattara et al., 2021)

Figure A4. The phenomenon of road collapse due to the backflow from the solid wastes blockage and defective connection in Saint-François roundabout (Abidjan) (Ouattara et al., 2021)

Figure A5. Structural malfunctions of the channel related to the age of material in Abidjan (Ouattara et al., 2021)

Figure A6. Erosion and landslide phenomena along Uniwax drainage system (Ouattara et al., 2021)

Figure A7. Banana farm and High concentration of Chinese bamboo in Uniwax drainage system (Ouattara et al., 2021)