Characterization and Methanogenic Potential Evaluation of Faecal Sludge: Case of the Kossodo Biogas Plant in Ouagadougou

Abstract

The use of faecal sludge (FS) in anaerobic digestion (AD) requires a perfect knowledge of their composition. Considered as a very heterogeneous material, the high variability of FS can disturb biodigesters’ functionality and impact biogas production. Unique in West Africa, Kossodo’s biogas plant in Ouagadougou receives sludge from septic tanks and pit latrines. To evaluate the quality of sludge discharged in this treatment plant and its ability for AD, a characterization of 130 FS trucks from several onsite sanitation facilities was carried out. Physico-chemicals, including heavy metals and microbiological parameters, were analyzed using standard protocols. A biochemical methane potential test was employed to evaluate biogas yield. Results showed that raw sludge averaged 1.12% total solids (TS), 54.74% volatile solids (VS), 9253 mg/L chemical oxygen demand (COD), and 1645 mg/L biochemical oxygen demand (BOD). Settled faecal sludge exhibited higher levels of total coliforms, E. coli, helminth eggs, and heavy metals. Heavy metal levels met AD standards defined by VDI 4630, with decreasing toxicity order: Zn > Mn > Cu > Cr > Ni > Pb > As ≥ Hg. The carbon-to-nitrogen (C/N) ratio was 6.7 ± 4.3, indicating unsuitability for AD. Sludge settling increased C/N ratio by 46%, which was still below optimal AD conditions (20–30). Methane yield of raw and settled FS averaged 61 ± 0.2 and 156 ± 3.2 NL CH₄/kg VS removed, respectively. Co-substrate addition could enhance the methanogenic yield of these sludges. This study provides a valuable database on the characteristics of FS, supporting sustainable recovery options.

1. Introduction

Faecal sludge management (FSM) is a major challenge for many developing countries where onsite sanitation facilities are predominant. Globally, 2.7 billion people currently use onsite sanitation facilities, a number expected to reach 5 billion by 2030 [1]. In sub-Saharan Africa, more than 76% of the urban population relies on onsite sanitation facilities such as septic tanks (ST) and pit latrines (PL) [2]. About 50% of this population depends on traditional pit latrines [3]. On-site sanitation facilities produce large quantities of sludge that must be evacuated and treated safely. However, the safe management of FS should be accompanied by strategies proposing sustainable treatment solutions adapted to the socio-economic, technical and financial realities of populations. To address these concerns, several studies on the quantification and characterization of FS have been conducted to facilitate the design of FS treatment plants [4,5,6,7,8,9,10,11,12,13]. Treatment is a critical step in FSM to mitigate health and environmental risks and recover valuable resources from sludge [14]. Unfortunately, most FS treatment units fail after construction due to a lack of monitoring and ongoing funding for operations [15]. The failures related to this mismanagement affect natural resources and pose health, environmental and socio-economic risks [16]. Lack of funding for sanitation and increase of natural resource contamination related to improper waste disposal has led to advocacy for a transition to a circular economy (CE). A sanitation approach that aims to produce nutrients for agriculture, protein for animal feed or clean energy [17,18]. Faecal Sludge (FS) valorization is one of the viable solutions to solve inadequate sanitation problems and promote investment in sanitation systems [19].

The city of Ouagadougou, in Burkina Faso (West Africa), was chosen to treat FS by anaerobic digestion with biogas valorization. With funding from the Bill and Melinda Gates Foundation, the National Office of Water and Sanitation (ONEA) implemented the first biogas production plant from FS in 2016, located in Kossodo. The treatment plant will serve as a pilot to test the feasibility of biogas production from FS to duplicate the project on other sites. As the only biogas indistrual plant used FS in West Africa, this treatment plant has been designed to receive approximately 400 m³/day of raw sludge with 1.5% total solid content. A mixture of 80% of settled faecal sludge (SFS) and 20% co-substrates is expected as the input for digestion. The Kossodo biogas plant uses a Continuously Stirred Tank Reactor (CSTR) with a volume of 2500 m³ at 38 °C for approximately 20 days of retention hydraulic time (RHT). Although designed to produce 3000 m³ of biogas per day, it has been producing less than 1500 m³ per day since its inauguration.

The plant of Kossodo receives mostly sludge from city’s septic tanks and pit latrines. In Ouagadougou, about 77.6% of households use unimproved latrines, 14.3% use flush toilets, 6.7% use VIPs, 1.1% always defecate in the open air, and 0.3% are connected to the sewerage system [20]. Considered as pre-treatment equipment; sludges from septic tanks are supposed to be stabilized and not suitable for AD because of a long retention time. In addition, the characteristics of FS are variable and depend on human lifestyle, emptying methods, socio-cultural factors, and climatic and environmental conditions [21,22]. Quantification and characterization of sludge is an important indicator for plants design, but it is also an important indicator for plant monitoring, control and diagnostics.

The characterization of faecal sludge has two advantages. Firstly, it helps to identify the dangers to human health associated with the release of this sludge into the environment. Secondly, it allows plant operators to monitor their composition to improve the efficiency of process. This will help to reduce the impact of pollution on the environment and public health. It will also enable treatment plants to ensure regulatory compliance, optimize processes, and maximize resource recovery.

However, the recovery of faecal sludge into biogas poses enormous difficulties in practice. Literature reports that methanogenic potential of septage is low [23]. Studies have shown that sludge from public toilets (PT) had better methanogenic potential than pit latrines (PL) and domestic septic tanks (ST) [24,25]. However, these studies recommend the use of these sludges as an inoculum rather than substrate. They also recommended codigestion to improve the methanogenic potential of these sludges [26]. The evaluation of the methanogenic potential of SFS from several onsite sanitation facilities has been scarcely addressed in the literature. While most studies have focused on assessing the methanogenic potential of ST, PT and PL sludge, few studies like those of Afifah and Karne [27,28] have focused on FS treatment plants receiving mixed sludge, such as those of Kossodo, which receives sludge from a variety of sources and uses settled faecal sludge (SFS) for AD. Anaerobic digestion offers several advantages, such as reducing the quantity of FS by degrading organic matter, producing renewable energy through biogas, and reducing odor pollution. AD recycles the organic matter and nutrients contained in waste into a digestion by-product, i.e., digestate, which can be used in agriculture as a soil improver. As faecal sludge is a highly heterogeneous material, a lack of knowledge of its composition can be a limiting factor in the anaerobic digestion process. This high variability can impact some important parameters of AD such as pH, total solid content (TS), volatile solid (VS) content, which depends on sludge retention time in the pits, carbon-to-nitrogen ratio (C/N), which is an essential element for good progress of AD, the heavy metals in the sludge, which can inhibit biogas production, and total nitrogen content (TN), which depends on the degree of mineralization of FS and how users make use of it.

The main objective of this study is to contribute for the improvement of Kossodo’s biogas plant operation by raw and settled faecal sludge characterization and evaluation of its suitability for AD. Two parts will be presented in this article. A first part that deals with the characterization of raw and settled faecal sludge from the Kossodo plant, and a second part that will deal with the evaluation of the methanogenic potential of the sludge.

2. Materials and Methods

2.1. Faecal Sludge Characterization

2.1.1. Study Area

Ouagadougou, the capital of Burkina Faso, is located at 12°21′56.4″ N, and 1°32′2″ W. It has an area of 600 km², which shelters 12 districts and 55 sectors. The climate is characterized by a single wet season from May to October, with peak rainfall generally recorded in August, and a dry season from November to April. The mean annual rainfall was 770 mm/year during the period 1961–2015 [29]. Ouagadougou is characterized by a basement aquifer with a free water table with a depth of between two meters and ten meters [30]. Its population is estimated at 2,415,266 people with 572,169 households, of which about 77.6% use single pit latrines and 21% use VIP latrines and septic tanks [20]. Onsite sanitation systems are used by about 98% of the population. Faecal sludge production in Ouagadougou varies from 500 m³/day to 1000 m³/day, with an increase during the wet season [31,32]. Faecal sludge is collected by FS trucks.

2.1.2. Kossodo Treatment Plant

The biogas production plant is located in the northeast of Ouagadougou; more precisely, in the industrial zone of Kossodo, two kilometers from the national road n°3. Kossodo’s plant aims to help increase the population’s access to electricity by injecting the electricity produced from biogas into the network of the National Electricity Company of Burkina Faso (SONABEL). It also aims to strengthen agricultural productivity through the use of digestate for agricultural purposes.

2.1.3. Sampling Design

The objective of the sampling campaign was to determine the characteristics of faecal sludge discharged at the Kossodo treatment plant. This characterization considers the types of onsite sanitation facilities emptied and the impact of seasons on faecal sludge emptying in Ouagadougou. This investigation aims to assess the influence exerted by distinct categories of sludge from onsite sanitation facilities, and how it impacts the AD process. The study was conducted from February to May 2022 and June to September 2022, respectively, during the dry and wet season. For the purposes of the study, sludge sampling was performed in a targeted manner. Twenty (20) septage companies were involved in the study to have access to the desired types of sludge. The collection periods and sites were coordinated with the FS companies at a rate of four to six samples per week. A minimum of 30 samples per onsite sanitation facility was undertaken, adhering to the guidelines outlined in the faecal sludge management manual, which advocates for a minimum sampling range of 30 to 50 samples to ensure robust representation and validation of sludge characteristics [33]. Consequently, one hundred and thirty (130) samples were collected during the campaign: 40 samples from domestic’s septic tanks (ST); 30 samples from private pit latrines (PL); 30 samples from public toilets (PT); and 30 composite samples (CS). The distribution of on-site technologies and characterization of the type of faecal sludge conducted during this study is presented in

Table 1. Distribution of on-site technologies and characterization of the type of faecal sludge conducted during this study.

Designation		Onsite Sanitation Type	Description	Percentage
Septic tank (40)	Family cesspool connected to a septic tank and associated works	All water pits (34)	Modern toilet with manual connected to black and grey water	85%
		Septic tank only (6)	Modern toilet with manual flush siphon without connexion to black and grey water	15%
Pit Latrine (30)	Sludge from private latrines (traditional latrines, improved pit latrines)	Traditional latrine (30)	Excreta and anal cleansing materials (water or solids) are deposited in a pit	100%
Public Toilet (30)	Works with a large number of users. Collects sludge from restaurants, mosques, churches, and public services.	Septic tank only (4)	Modern toilet with manual flush siphon without connexion to black and grey water	13%
		Traditional latrine (9)	Excreta and anal cleansing materials (water or solids) are deposited in a pit	30%
		All water pits (17)	Modern toilet with manual connected to black and grey water come from restaurants, drinking establishment, and health centers	57%
Composite samples (30)	Mixture of sludge from two or three onsite sanitation systems	(latrine + septic tank + all-water tank, (30)	Mixtures of sludge from two or three on-site wastewater treatment plants	100%

.

Composite samples (30) were directly collected from FS trucks at the treatment plant. One hundred collection points for sludge from domestic and public services were reached during the emptying process, and those involved were interviewed. Sampling was performed when the truck returned to the dumping site.

2.1.4. Interview Guide

A survey was conducted among 100 households. The aim of this survey is to explain the behaviour of toilet users and the characteristics of the sludge analysed. The survey included private and public septic tanks and pit latrines. An interview guide was developed to collect information from households, which included the following elements: the profile of the client (household, public service, business, church, school, mosque, or other); the type of onsite facility emptied, the retention time of the containment system, the number of users of toilets, the year of the last emptying, the frequency of emptying, whether the latrine is waterproof, whether there was another connection, such as shower or laundry water, water added during emptying, household maintenance behavior, price of emptying, and season of most frequent pit emptying. This information was used to determine the average retention time of the sludge arriving at the plant. The KoboCollect tool was used for the survey. The onsite sanitation facilities involved in the collection and sampling are represented in Figure 1.

2.1.5. Sampling Method

The sludge sampling protocol developed by EAWAG-SANDEC was used for this study [34]. It consists of taking one (1) liter at the beginning of discharge (a few moments after opening the valve), two (2) liters in the middle of the discharge (when the tank is half empty), and one (1) liter at the end of the discharge, when the flow decreases. The three (3) collected phases were then thoroughly mixed and a sample was extracted (respectively, for each sampled truck). The samples were collected with a cane and inverted into sterile 500 mL borosilicate glass bottles for microbiological parameters: one (1) liter PET bottles for physico-chemical parameters, heavy metals, and decantation tests. The bottles were kept in a thermostatically controlled cooler until they were returned to the laboratory.

2.1.6. Analytical Parameters

Physico-Chemical Parameters

pH was measured using a pH meter WTW 3310 SET 2. Total suspended solids (TSS) was determined by differential weighing through filtration on a glass fiber filter (GFC) method, followed by oven drying at 105 °C for 2 h. Total solids (TS) was determined by oven drying at 105 °C after 24 h. Volatile solids (VS) was determined by oven combustion at 550 °C for 2 h, following the method described in the standard methods APHA [35]. Chemical oxygen demand (COD) was determined by digestion with potassium dichromate according to the standard method AFNOR T-90-101 [36]. Biochemical oxygen demand (BOD5) was determined using the respirometric method with an oxitop. Total carbon (TC) was estimated using the Walkley and Black method [37].

Total Kjeldahl nitrogen (TKN) was determited using Kjeldahl method [38]. Total phosphorus (TP) was determined using the Photometric Molybdovanadate Method with extraction. Major ions like Orthophosphate (P${\mathrm{O}}_4^{3-}$-P), Ammonium (${\mathrm{N}\mathrm{H}}_4^{+}$), Nitrite (${\mathrm{N}\mathrm{O}}_2^{-}$) and Nitrate (${\mathrm{NO}}_3^{-}$) were determined using colorimetric methods and read using a HACH DR 3900 spectrophotometer. Volatile Fatty Acid (VFA) And Total Alkalinity (TA) concentrations were determined through titration using the Biogas Titrator for FOS/TAC Analyses, Hach LANGE, employing the method outlined by NORDMANN [39].

Microbiological Parameters

Faecal coliforms and Escherichia coli (E. coli) were determined by inoculating the samples on ChromoCult Agar ES culture medium, followed by incubation at 44.5 °C for 18 to 24 h. Additionally, helminth eggs were characterized according to the method described by the World Health Organization (WHO) in 1996 [40].

Heavy Metals

Trace elements including zinc (Zn), copper (Cu), cadmium (Cd), lead (Pb), nickel (Ni), mercury (Hg), manganese (Mn), and chromium (Cr) were quantified using plasma atomic emission spectrometry. The aqua regia digestion method, modified according to EPA 200.7, was used for sample preparation. Aqua regia is a mixture of concentrated nitric acid and hydrochloric acid that is commonly used for digestion of samples to extract trace elements for analysis [41].

Settling Test

The Kossodo plant use thickened sludge for AD. The sludge was thickened by settling in a tank. To test the suitability of the sludge for settling, settling tests were conducted on all types of sludge on Imhoff cones. Subsequently, only the composite samples were used to assess the quality of the settled faecal sludge following Kossodo’s plant operation. The evaluation of settled faecal sludge was also performed to know their characteristics and its suitability for AD. Graduated Imhoff cones were used to determine sludge volume index (SVI). They were filled with sludge to be decanted for 30 to 60 min. The volume occupied by the settled sludge was then measured and expressed in mL/L. Sludge Volume Index (SVI) is the volume of settled sludge divided by the TSS concentration (in g/L). Samples were collected from each transport truck involved in onsite sanitation facilities to evaluate SVI. For a settled faecal sludge, a total of 30 samples were obtained through the decantation of composite samples for further characterization.

2.2. Methanogenic Potential Evaluation

2.2.1. Experimental Procedure

Evaluation of the Methanogenic Potential of Sludge

The assessment of the methanogenic potential of FS involved two types of samples: one obtained during the dry season, comprising a composite sludge sample (CS1) and a concentrated sludge sample (SFS1), and a second set of samples, consisting of a composite sample (CS2) and a settled faecal sludge sample (SFS2). The objective was to discern the respective impacts of each season on the quality of faecal sludge and its suitability for AD. No inoculum was added to seed the sludge, meaning that the sludge was tested without the addition of external microorganisms.

The specific methanogenic activity (SMA) of the composite sample was tested using microcrystalline cellulose (CAS: 9004 34-6) as the substrate, which is composed only of glucose as the monomer. The average SMA obtained was 357.3 L CH₄/kg VS (volatile solid), which is equivalent to 86.3% of the theoretical value of 414 L CH₄/kg VS, and was considered as good activity for anaerobic sludge according to references [42,43]. This indicates that faecal sludge has a high capability of degrading organic matter and producing methane gas without the need for additional inoculum or external microorganisms.

Batch Testing

The liquid displacement method was used for FS methanogenic potential determination. A series of batches was carried out in triplicates for each test. Experimental setup is composed of 2500 mL batch reactors, connected to 1 L graduated cylinders spilled in 20% saturated NaCl solution and acidified to pH = 2 to avoid dissolution of CO₂ in water. In total, 20% of reactor volume was allocated to headspace, leaving a 500 mL working space. The headspace was flushed with argon, which is an inert gas. Sealed, these reactors were maintained in a thermostatically controlled bath at a mesophilic temperature (35.5 ± 2 °C) and stirred manually every 24 h. A needle manometer was used to measure the different gas pressures. The biogas produced is trapped in the inverted test tube. The gas accumulated in the test tube was purged by a Biogas optima 7 gas analyzer to determine the CH₄ and CO₂ composition. The gas volumes obtained are corrected to standard temperature and pressure conditions: 0 °C and 1 atm. The results are expressed in normo liters (NL) of CH₄/Kg of VS removed. The volume of dry methane was calculated using the formula used by [44]:

$${\mathrm{V}}_0\mathrm{d}\mathrm{r}=\mathrm{V}\times (\left(\mathrm{P}-{\mathrm{P}}_{\mathrm{W}}\right)\times {\mathrm{T}}_0/({\mathrm{P}}_0\times \mathrm{T}\left)\right)$$

(1)

where V₀ dr was the volume of the dry gas in the normal state (NL); V was the volume of the gas as read off (mL); P was the pressure of the gas phase at the time of reading (hPa); Pw was the vapour pressure of the water as a function of the temperature of the ambient space (hPa); T₀ was normal temperature (=273 °K); p₀ was normal pressure (=1013 hPa); and T was the temperature of the fermentation gas or of the ambient space (°K).

2.3. Data Analysis

The XLSTAT Version 2016.02.28451, Rstudio 2023.06.1 and Tanagra software 1.4.41, were used to analyze the statistical data. Measures of minimum, maximum, and appropriate tendencies such as standard deviation (SD), mean, and median were used for descriptive statistical analysis. Spearman’s correlation matrix was used to evaluate the dependence of factors. A one-way analysis of variance (ANOVA) with α = 0.05 was performed to determine the significance of differences in the analysis data using XLSTAT Version 2016. The presentation of the results was adjusted to conform to the standard instrumentation scale and readings.

3. Results and Discussion

The onsite sanitation facilities concerned by this study are composed of 61% sludge from septic tank and 39% sludge from traditional pit latrines, as shown in Figure 2a. The spatial dispersion of collection points shows that the Kossodo treatment plant receives sludge from all districts of Ouagadougou, as shown in Figure 1.

The majority of septic tanks are all water tanks (51%), and ten percent (10%) of samples were sourced only from septic tanks. The survey revealed that 84% of respondents who use ST connect their shower and dishwater to these systems. This explains the high percentage of all water pits in the homes.

For traditional pit latrines users, about 93% of respondents connect their shower to the pit and only 7% use simple pits. Around 33% of the samples collected had a retention time of less than one year in the structures covered by the study.

The fastest emptying times were recorded among users of public toilets presented in Figure 2b. The users of these toilets also use traditional latrines rather than septic tanks.

The survey revealed that approximately 85% of users of public toilets and 15% of users of traditional pit latrines empty their pits at least once a year.

Forty per cent (40%) of the samples collected had a retention time ranging from 2 to 3 years and came mainly from all water tanks (20%), followed by pit latrine (15%) and 5% of septic tank only.

Twenty-five per cent (25%) of the samples collected had a retention time ranging from 4 to 5 years and come from all water pit (14%), septic tank (3%) and traditional pit latrine (8%). Only 2% of samples had a retention time of more than six years and come from traditional pit latrines. The survey results indicate that a significant portion (40%) of the analyzed sludge has an age range of 2 to 3 years. This suggests that the retention time of faecal sludge is influenced not only by the type of onsite sanitation facility, but also by factors such as the type of sludge and the number of users. For instance, sludge from public toilets tends to be emptied more frequently. Figure 2 provides a visual representation of the retention time of sludge categorized by onsite sanitation (a) and sludge types (b)

3.1. Raw Sludge Quality

Table 2 shows the results of analyses carried out on sludge 130 FS deposited at Kossodo treatment plant. The discussed physico-chemical parameters offer insights into the level of pollution in the faecal sludge. The minimum and maximum values obtained in this study were compared with the values found in other countries in the same onsite sanitation facilities used in the study. Temperature measurement plays a crucial role in faecal sludge treatment processes. It directly affects the speed of the biological and chemical reactions that take place in treatment plants. More specifically, it influences the activity of the micro-organisms responsible for the decomposition of organic matter [45,46]. Higher temperatures generally lead to higher reaction rates, which can improve treatment efficiency. In addition, temperature affects the solubility of gases, which has an impact on the release of potentially harmful compounds. Temperature control in faecal sludge treatment ensures optimal conditions for biological processes and helps maintain overall system performance. pH is an important factor in faecal sludge treatment processes because of its influence on the performance and efficiency of the process. It plays an important role in chemical and biological wastewater treatment systems [47]. A pH that is too basic inhibits the action of micro-organisms. If the pH is too acidic, it prevents certain bacteria from functioning and causes unpleasant smells. Phosphorus is a nutrient used by organisms for growth. Faecal sludge is relatively rich in phosphorus compounds [48]. It occurs in natural water and wastewater bound to oxygen to form phosphates. Phosphates come from a variety of sources, including agricultural fertilizers, detergents, domestic wastewater and faecal sludge. Monitoring phosphorus levels is essential to prevent environmental pollution and comply with regulations. By analyzing phosphorus levels, it is possible to determine the efficiency of treatment processes and ensure that the discharged water complies with environmental standards. Total nitrogen encompasses all forms of nitrogen in faecal sludge samples, such as organic nitrogen, ammonium, and nitrate/nitrite. It provides a comprehensive measure of the total nitrogen content of a given substance or environment. This parameter is essential in a variety of fields, particularly in environmental science, agriculture, and wastewater treatment, as it enables nutrient levels and potential pollution to be assessed. The discharge threshold values for these parameters are given in Table 2.

The most critical parameters for assessing the suitability of a substrate for AD are pH, TS, VS, C/N, and inhibitory parameters such as ammonium (${NH}_4^{+}$), VFA, and heavy metals. These parameters will be the subject of discussion in our results.

3.1.1. pH

Mean values of (7.62 ± 0.41), (7.72 ± 0.22), (7.56 ± 0.25), and (7.94 ± 0.29) were, respectively, recorded for septic tanks, pit latrines, public toilets, and composite samples. These results indicate that there is a significant variation in pH between composite samples and the other types of sludge (p-value = 0.0001). No significant difference was observed among septic tank, pit latrines and public toilets. These results are of the same order compared to those obtained by Wanda [13] and Ahmed [4] on septic tanks (7.66 ± 0.70; 7.66 ± 0.16) and public toilets (7.57 ± 0.32; 7.58 ± 0.20) from Cameroon and Accra. For pit latrines and composite samples, our results are contradict those found for pit latrines from Cameroon (6.96 ± 0.23) [13]. The pH plays an important role in the AD process. pH influences the chemical equilibrium of ammonia (NH₃), hydrogen sulfide (H₂S) and volatile fatty acids (VFA), which could inhibit microorganisms’ activities. The ideal pH range for AD would be between 6.5 and 8 [49]. Factors that may influence the variability of pH in different facilities include the rate of biological processes occurring in the faecal sludge storage technologies, environmental conditions (temperature, humidity, oxygen availability), and probably the mixing of sludge from several facilities such as composite sludge. The pH variation depending on different onsite sanitation facilities is illustrated in Figure 3a.

Table 2. Physico-chemical characteristics of faecal sludge from Kossodo’s plant based on sanitation facility type, with the comparative literature value ranges for reference.

	Results of Laboratory-Based Analysis Grouped by Septic Tank, Pit Latrine and Public Toilet						Mean and Range Values Reported in Litteraure
		Septic Tank (ST) n = 40	Pit Latrine (PL) n = 30	Public Toilet (PT) n = 30	Composite Sample (CS) n = 30	All Samples (130)	Discharge Threshold (BF *)	Septic Tank	Latrine Pit	Public Toilet
Parameters	Unit	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD
Temperature	°C	31.3 ± 1.4	30.9 ± 1.2	31.1 ± 1.2	31.1 ± 0.8	31.1 ± 1.2	18–40
pH		7.62 ± 0.41	7.72 ± 0.22	7.56 ± 0.25	7.94 ± 0.29	7.70 ± 0.34	6.5–9	6.9–7.9 b,c,d,e,f,k,m,t	7.1–8.2 d,h,k	7.48–7.9 a,l
Moisture	(%)	99.32 ± 0.38	98.90 ± 0.58	98.27 ± 0.74	98.89 ± 0.51	98.88 ± 0.66
TS	(%)	0.68 ± 0.38	1.10 ± 0.58	1.72 ± 0.74	1.11 ± 0.51	1.12 ± 0.66	50	0.19–7.2 b,c,d,p,t	0.64–19 b,c,d,e,h,i,j,k,p	1.59–5.25 b,o,s
TS	g/L	6.83 ± 3.78	10.95 ± 5.8	17.47 ± 7.82	11.13 ± 5.05	11.23 ± 6.81		1.92–72 b,c,d,k,p,t	6.4–190 b,c,d,e,h,i,j,k,p	15.9–52.5 b,o,s
VS	(% MS)	49.42 ± 7.79	51.62 ± 10.20	67.66 ± 7.43	52.03 ± 7.20	54.74 ± 10.83		45–76 b,c,d,e,k,r,t	43.2–77.2 b,c,d,h,i,k,p,t	56.9–84.92 a,g
VS	g/L	3.36 ± 2.11	5.82 ± 3.65	11.80 ± 5.5	5.77 ± 2.65	6.43 ± 4.74
TSS	g/L	3.87 ± 2.38	7.26 ± 4.15	12.77 ± 5.3	8.15 ± 4.23	7.69 ± 5.15
TC	(% MS)	29.6 ± 5.0	30.8 ± 5.7	39.24 ± 6.6	30.5 ± 4.4	32 ± 6.0		10.9 c	10.9 c
SVI	(mL/g)	23 ± 13	30 ± 15	55 ± 39	36 ± 20	35 ± 26
TP	mg/L	85 ± 46	125 ± 74	207 ± 73	148 ± 67	137 ± 78		89.5–150 g,l,n,q,r	450–521 l,n	110.8–450 g,l,r
${\mathrm{PO}}_4^{3-}$-P	mg/L	13.9 ± 10.8	15.7 ± 9.5	44.8 ± 15.4	19.43 ± 6.9	22.7 ± 16.5	0.8
TN	mg/L	431 ± 254	494 ± 220	1308 ± 847	532 ± 240	677 ± 572		190–1200 l,q,r,s		1397–1550 g,s
TKN	mg/L	405 ± 250	470 ± 216	1300 ± 845	421 ± 154	659 ± 574
${\mathrm{NH}}_4^{+}$-N	mg/L	255 ± 170	296 ± 168	592 ± 371	289 ± 136	350 ± 261		120–1200 r,s,t	140–3200 d,k	566–5000 a,r,t
${\mathrm{NO}}_3^{-}$-N	mg/L	23.5 ± 12.4	22.38 ± 7.4	8.3 ± 3.7	22 ± 11	15.9± 11.6	11.4
${\mathrm{NO}}_2^{-}$-N	mg/L	1.6 ± 1.1	1.5 ± 1.0	0.5 ± 0.3	2.1 ± 1.5	1.4 ± 1.2	0.9
C/N (ratio)	-	5.7 ± 3.9	6.9 ± 2.3	6.4 ± 3.6	8.1 ± 6.2	6.7 ± 4.3		1.2–3.7 u		3.2 u
COD	g/L	5.21 ± 3.3	9.06 ± 5.04	14.96 ± 6.4	9.12 ± 4.62	9.25 ± 6	150	1.8–43 b,d,f,k,p,t	6.4–129 b,d,h,i,k,p	15.5–49 b,s
BOD5	g/L	0.82 ± 0.39	1.5 ± 0.69	3.1 ± 0.85	1.44 ± 0.73	1.64 ± 1.08	40	0.84–3.9 g,t		4.3–7.6 b,g,s
COD/BOD5		6.2 ± 1.6	6.1 ± 1.7	4.7 ± 1.2	6.5 ± 2.1	5.89 ± 1.80		5–10 s	2.4–5.8 b,l	2.- 5.89 b,g,t
TS/COD		1.41 ± 0.33	1.25 ± 0.18	1.18 ± 0.22	1.27 ± 0.21	1.29 ± 0.26

* BF = Burkina Faso; n = number of sample; SD = standard deviation. ^a [50], ^b [13], ^c [51], ^d [9] ^e [10] ^f [52], ^g [4], ^h [53], ⁱ [54], ^j [12], ^k [55], ^l [56], ^m [57], ⁿ [58], ^o [59], ^p [5], ^q [60], ^r [61], ^s [62], ^t [63].

Table 2. Physico-chemical characteristics of faecal sludge from Kossodo’s plant based on sanitation facility type, with the comparative literature value ranges for reference.

	Results of Laboratory-Based Analysis Grouped by Septic Tank, Pit Latrine and Public Toilet						Mean and Range Values Reported in Litteraure
		Septic Tank (ST) n = 40	Pit Latrine (PL) n = 30	Public Toilet (PT) n = 30	Composite Sample (CS) n = 30	All Samples (130)	Discharge Threshold (BF *)	Septic Tank	Latrine Pit	Public Toilet
Parameters	Unit	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD	Mean ± SD
Temperature	°C	31.3 ± 1.4	30.9 ± 1.2	31.1 ± 1.2	31.1 ± 0.8	31.1 ± 1.2	18–40
pH		7.62 ± 0.41	7.72 ± 0.22	7.56 ± 0.25	7.94 ± 0.29	7.70 ± 0.34	6.5–9	6.9–7.9 b,c,d,e,f,k,m,t	7.1–8.2 d,h,k	7.48–7.9 a,l
Moisture	(%)	99.32 ± 0.38	98.90 ± 0.58	98.27 ± 0.74	98.89 ± 0.51	98.88 ± 0.66
TS	(%)	0.68 ± 0.38	1.10 ± 0.58	1.72 ± 0.74	1.11 ± 0.51	1.12 ± 0.66	50	0.19–7.2 b,c,d,p,t	0.64–19 b,c,d,e,h,i,j,k,p	1.59–5.25 b,o,s
TS	g/L	6.83 ± 3.78	10.95 ± 5.8	17.47 ± 7.82	11.13 ± 5.05	11.23 ± 6.81		1.92–72 b,c,d,k,p,t	6.4–190 b,c,d,e,h,i,j,k,p	15.9–52.5 b,o,s
VS	(% MS)	49.42 ± 7.79	51.62 ± 10.20	67.66 ± 7.43	52.03 ± 7.20	54.74 ± 10.83		45–76 b,c,d,e,k,r,t	43.2–77.2 b,c,d,h,i,k,p,t	56.9–84.92 a,g
VS	g/L	3.36 ± 2.11	5.82 ± 3.65	11.80 ± 5.5	5.77 ± 2.65	6.43 ± 4.74
TSS	g/L	3.87 ± 2.38	7.26 ± 4.15	12.77 ± 5.3	8.15 ± 4.23	7.69 ± 5.15
TC	(% MS)	29.6 ± 5.0	30.8 ± 5.7	39.24 ± 6.6	30.5 ± 4.4	32 ± 6.0		10.9 c	10.9 c
SVI	(mL/g)	23 ± 13	30 ± 15	55 ± 39	36 ± 20	35 ± 26
TP	mg/L	85 ± 46	125 ± 74	207 ± 73	148 ± 67	137 ± 78		89.5–150 g,l,n,q,r	450–521 l,n	110.8–450 g,l,r
${\mathrm{PO}}_4^{3-}$-P	mg/L	13.9 ± 10.8	15.7 ± 9.5	44.8 ± 15.4	19.43 ± 6.9	22.7 ± 16.5	0.8
TN	mg/L	431 ± 254	494 ± 220	1308 ± 847	532 ± 240	677 ± 572		190–1200 l,q,r,s		1397–1550 g,s
TKN	mg/L	405 ± 250	470 ± 216	1300 ± 845	421 ± 154	659 ± 574
${\mathrm{NH}}_4^{+}$-N	mg/L	255 ± 170	296 ± 168	592 ± 371	289 ± 136	350 ± 261		120–1200 r,s,t	140–3200 d,k	566–5000 a,r,t
${\mathrm{NO}}_3^{-}$-N	mg/L	23.5 ± 12.4	22.38 ± 7.4	8.3 ± 3.7	22 ± 11	15.9± 11.6	11.4
${\mathrm{NO}}_2^{-}$-N	mg/L	1.6 ± 1.1	1.5 ± 1.0	0.5 ± 0.3	2.1 ± 1.5	1.4 ± 1.2	0.9
C/N (ratio)	-	5.7 ± 3.9	6.9 ± 2.3	6.4 ± 3.6	8.1 ± 6.2	6.7 ± 4.3		1.2–3.7 u		3.2 u
COD	g/L	5.21 ± 3.3	9.06 ± 5.04	14.96 ± 6.4	9.12 ± 4.62	9.25 ± 6	150	1.8–43 b,d,f,k,p,t	6.4–129 b,d,h,i,k,p	15.5–49 b,s
BOD5	g/L	0.82 ± 0.39	1.5 ± 0.69	3.1 ± 0.85	1.44 ± 0.73	1.64 ± 1.08	40	0.84–3.9 g,t		4.3–7.6 b,g,s
COD/BOD5		6.2 ± 1.6	6.1 ± 1.7	4.7 ± 1.2	6.5 ± 2.1	5.89 ± 1.80		5–10 s	2.4–5.8 b,l	2.- 5.89 b,g,t
TS/COD		1.41 ± 0.33	1.25 ± 0.18	1.18 ± 0.22	1.27 ± 0.21	1.29 ± 0.26

* BF = Burkina Faso; n = number of sample; SD = standard deviation. ^a [50], ^b [13], ^c [51], ^d [9] ^e [10] ^f [52], ^g [4], ^h [53], ⁱ [54], ^j [12], ^k [55], ^l [56], ^m [57], ⁿ [58], ^o [59], ^p [5], ^q [60], ^r [61], ^s [62], ^t [63].

3.1.2. Total Solid Content (TS)

Total solid (TS) represents all organic and mineral materials in suspension and dissolved salts contained in a substrate. TS content values recorded in this study are presented in Figure 3b. The highest TS content was found in public toilets (17.47 g/L ± 7.82), followed by composite samples (11.13 g/L ± 5.05), and pit latrines (10.95 g/L ± 5.8). Septic tanks exhibited lower TS values (6.83 ± 3.78 g/L). A significant difference was observed between public toilets and other categories of sludge, p-value < 0.05. Typically, TS content for public toilets and pit latrines exceeds 3.5% (35 g/L) [63,64]. However, studies conducted in other sub-Saharan African countries have reported a lower TS content. The results found in this study are similar to those found in Cameroon, with an average TS of 15.9 ± 2.31 g/L [13].

The average TS content (1.12 ± 0.66%) of all samples indicates that these faecal sludges are liquid. Comparable TS contents (TS = 1.18%) were previously reported in 2013 for FS collected in Ouagadougou [5]. In contrast, lower TS contents were recorded for FS in Senegal (TS = 0.92%) and the same treatment plant in Ouagadougou (TS = 0.86 ± 0.58%) in 2016 [57,65]. These findings highlight a distinct contrast influenced by various factors, including community lifestyle, religious practices, emptying practices, and inherent sludge characteristics, as reported by [21,22]. Survey data revealed that a significant proportion of individuals with latrines (93%) reported the practice of connecting their showers to the latrine system.

The results also indicate distinct behaviors regarding public toilets, with approximately 56% of the surveyed individuals reporting the use of water for their toilets. The settled sludge in the pits presents challenges in the emptying process, as the majority (96%) of surveyed vacuum truck operators stated that they add water to the toilets before emptying. This action is often requested from households before the arrival of the vacuum truck, due to the limitations in the truck’s capacity to efficiently collect the accumulated and compacted solids at the bottom of the systems. The prevalence of flush toilets in septic tanks contributes to their high moisture content, owing to the water-intensive nature of these installations.

About 39% of samples also come from all water pits. This explains the low TS content of the sludge analyzed. For AD of liquid matter, TS must be in the range of 2–5% min and 15–20% max [66]. For a large-scale AD system, several types of reactors are used. When the total solid content is low (TS < 3–5%), the typical reactors, like up-flow anaerobic sludge bed (UASB), expanded granular sludge bed (ESGB), expanded bed (EB), fluidized bed (FB), internal circulation (IC) and anaerobic fixed bed (AFB), are suitable, and biomass concentration inside is maintained in the range of 3.5–4% [67]. When using CSTR reactors, like the one used in Kossodo, a TS content between 1% and 10% is allowed [68]. However, for optimal operation, this content should be between 3 and 10% [69]. The sludge received at the treatment plant does not satisfy these conditions; it is too liquid to be digested due to its low TS content.

3.1.3. Volatile Solid (VS)

Volatile solids (VS) represent the biodegradable fraction of total solids (TS) undergoing conversion into methane and carbon dioxide. It provides an approximate measure of organic material and is expressed as a percentage of TS. Figure 3c illustrates the dispersion of organic matter content across various faecal sludge type. Public toilets exhibit the highest VS content, averaging (67.66 ± 7.43%), followed by composite samples (52.03 ± 7.20%), latrines (51.62 ± 10.20%), and septic tanks (49.42 ± 7.79%). The notably elevated VS content in public toilets results from frequent emptying, rendering the sludge fresher and richer in organic matter. Survey findings indicated that 85% of respondents emptied public toilets frequently, with some doing so monthly and others up to four times a year.

Comparative analysis of VS content reveals intriguing variations. A study conducted on public toilets in Cameroon reported VS contents of (66.19 ± 8.6%), aligning with the same order of this study [13]. In contrast, investigations into public toilets in Ghana unveiled higher VS contents, reaching (84.92 ± 2.65%) [4].

The low VS content recorded in the septic tank means that these sludges are partially mineralized. Sludge solid retention time was between 3 and 4 years old for septic tanks only and 2 to 3 years for the all water septic tank. A septic tank is a pretreatment structure for sludge. It is designed to sustain a long period of 4 to 5 years. Unfortunately, the majority of the samples concerned by this study were collected from all water septic tanks. These onsite sanitation facilities have very short filling frequencies, which reduces the storage time of the sludge in these pits. That is why they are partially digested. These results are close to those found for FS in Uganda with VS (50.9 ± 18.9%) [53]. However, other studies have found VS of 66.46± 9.2% and 58.7% for septic tanks in Cameroon and Senegal [13,57]. Studies carried out in Ouagadougou have shown that the volatile solid content of sludge can vary according to the season. In Ouagadougou, an average of 53% volatile solids (VS) was observed during the dry season, while it increased to 61% during the wet season. However, no statistically significant difference was detected between the two seasons [5]. This shows that VS content depends on the sludge emptying frequency or retention time in the pits. The survey of collection points revealed that about 52% of respondents emptied more of their pits during the wet season because of ground water capillary rise into sanitation systems. This led us to investigate the influence of the different seasons on sludge quality received in Kossodo’s unit presented in

Table 3. Faecal sludge all samples characteristics during dry and wet conditions.

		Dry Season (65)		Wet Season (65)
Parameters	Unit	Mean	SD	Mean	SD
Temperature	°C	31.12	1.14	31.09	1.23
pH		7.7	0.34	7.71	0.33
Moisture	(%)	98.7	0.7	99.01	0.60
TS	(%)	1.24	0.7	0.98	0.60
TS	g/L	12.3	7.1	10.14	6.40
VS	(% TS)	53.3	10.6	56.1	10.9
VS	g/L	7	4.9	6	4.6
TC	(% TS)	31.1	6.1	32.9	6.14
TN	mg/L	577	455	776	658
C/N		7.78	4.35	5.65	3.94
COD	g/L	10.10	6.15	8.39	5.73
BOD5	g/L	1.73	1.03	1.55	1.12
COD/BOD5		6.04	1.84	5.75	1.75

.

3.1.4. Biodegradability

The biodegradability of organic matter depends directly on the biochemical composition of the substrate to be treated. The COD and BOD₅ values are shown Figure 3d,e. An indicator of the degree of biodegradability of faecal sludge is the COD/BOD₅ ratio. It provides the basis for determining whether they can be treated biologically. If this ratio is less than 3, the sample can be said to be readily biodegradable, otherwise it is not [70]. The analytical results obtained show a great variability of COD and BOD₅ between the different types of FS presented in Table 2. The average ratio is about 6.2± 1.6 for ST, 6.1 ± 1.7 for LT, 4.7 ± 1.2 for PT and 6.5 ± 2.1 for CS. These reports indicate that faecal sludge is not easily biodegradable. Low biodegradability, characterized by a high COD/BOD₅ ratio, indicates that the sludge has been stored for long periods or that it contains a significant amount of inorganic pollutant [63]. Our results conform with those found in Ouagadougou with an average of 6.1 ± 4.2 on composite samples and those found in Ghana on PT [4,5]. On the other hand, our results were not the same as those found in Cameroon (2.31 ± 0.26) on PT [13].

3.1.5. The C/N Ratio

The recorded mean carbon-to-nitrogen (C/N) ratio for different types of sludge are as follows: 5.7 ± 3.9 for ST sludge, 6.9 ± 2.3 for PL sludge, 6.4 ± 3.6 for PT sludge, and 8.1 ± 6.2 for CS. The overall average C/N ratio across all samples is 6.7 ± 4.3. A notable discrepancy in the C/N ratio was observed between CS sludge and ST sludge, with a p-value of 0.0001, signifying a statistically significant difference between these two types of sludge. Conversely, no significant differences were found among the C/N ratios of CS, PL, and PT sludge.

The carbon-to-nitrogen (C/N) ratio plays a crucial role in the AD process by ensuring a balanced microbial reaction medium. Typically, a C/N ratio ranging from 10 to 30 is considered optimal for stable AD [71], while some studies suggest a narrower range of 25 to 32 for optimal operation [72].

The existing literature indicates a carbon-to-nitrogen (C/N) ratio ranging from 5 to 16 for faecal material [73], while other studies reported values ranging from 0.8 to 26 for faecal sludge [54]. In the same study, an average value of 3 ± 1.2 was found for public toilets in Kampala (Uganda), and values of 1.1 ± 0.7 and 2 ± 4.1 were recorded for households in Kampala (Uganda) and Naivasha (Kenya), respectively. Notably, these C/N ratios are lower than those observed in this study.

Figure 3f illustrates the C/N ratio of faecal sludge based on its origin within the scope of this study.

3.2. Influence of Season on Faecal Sludge Characteristics

Sample characteristics according to the dry and wet season are presented in Table 3. The average TS content is 12.3 ± 7.1 g/L for the dry season and 10.1± 6.40 g/L for the wet season. TS content during the dry season is higher than TS content of the wet season (p-value = 0.04). A study conducted on FS in Ouagadougou [5] found lower TS (10.6 ± 8.2 g/L) contents in the dry season and higher contents in the wet season (12.9 ± 10.9 g/L), but no significant difference was observed for FS between the dry season and wet season. This result indicates the high variability of faecal sludge characteristics, which are influenced by emptying methods, sociocultural factors, and the design of on-site sanitation facilities.

The volatile solid (VS) content in the dry season (53% of TS) is lower than that in the wet season (56% of TS). A higher percentage of volatile solids during the wet season shows that these sludges are fresher and therefore less degraded than dry season sludges. However, the concentration of volatile solid (VS) remains lower in the wet season than dry season due to the dilution of the pits, which leads to a dilution of the total solid (TS) content. Most of the private pit latrines in this study are not waterproof and can be infiltrated by rainwater, as reported by 68% of the respondents in the surveys. Emptying is performed much more frequently during the wet season because of water infiltration, so FS seems to be fresher. However, no significant difference was observed in VS content between dry season and wet season sludge (p-value = 0.212). Therefore, the high TS content observed during the dry season has an impact on the total quantity of VS. This indicates that more settled faecal sludge tends to contain higher organic matter, which is consistent with the strong correlation observed between settled faecal sludge matter contents and chemical oxygen demand (COD); a parameter used to measure the organic content of wastewater and sludge is presented in Figure 4.

3.3. Statistical Relationships of Measured Parameters

The Spearman correlation matrix was used to assess the dependency between the variables analyzed. A high correlation is observed between TS and COD (r = 0.94), TS and TC (r = 0.95), and TS content VS (r = 0.95). The relationships identified between TS and measured parameters can be useful for the design, operation, and selection of treatment processes, such as settling tanks, drying beds, and/or digesters [14,22].

The specific correlation between COD and TS is depicted in Figure 5. A high correlation was observed across all samples, yielding an R² = 0.89. It is evident that samples with the lowest TS exhibit the lowest COD levels. These characteristics are more commonly observed in the majority of sludge originating from septic tanks, with a TS/COD ratio of 1.41, indicating a lower presence of organic material. This suggests that this sludge is partially mineralized. The samples exhibiting the highest total solids also demonstrate the highest levels of chemical oxygen demand. These characteristics are predominantly observed in samples collected from public toilets. The survey revealed that 33% of the samples were less than one year old, indicating a comparatively fresher composition compared to other categories of sludge. The TS/COD ratio of 1.18 illustrates that a substantial majority of the total solid content comprises organic material. This study found a similar slope than previous studies (COD = 0.83 × TS). COD to TS (R² = 0.89) shows strong correlations, similar to studies conducted on faecal sludge in Ouagadougou, Dar Es Salaam and Kampala, which show a strong correlation between TS sludge and COD with R² of 0.84, 0.90 and 0.93, respectively [12,74].

3.4. Characteristics of Settled Faecal Sludge

The concentration of faecal sludge could be a solution for improving the total solid content and thus increasing the volatile solid content.

Table 4. Characteristics of settled faecal sludge and composite sample.

		Composite Sample (n = 30)	Settled Faecal Sludge (n = 30)
Parameters	Unit	Mean ± SD	Mean ± SD	Range	Start of Inhibition	Reference
pH		7.94 ± 0.3	7.52 ±0.4	6.5–8.0		[48]
TS	(g/L)	11± 5	80.9 ± 20	(2–5)–(15–20)		[66]
VS	(g/L)	5.8 ± 2.6	39.5 ± 10.5	≥60		[75]
TAC	(mg CaCO3/L)	1603 ± 408	2973 ± 748	1500–5000		[76]
VFA	(mg/L CH3COOH)	72 ± 31	254 ± 32	---
${\mathrm{NH}}_4^{+}$-N	(mg/L)	289 ± 136	976 ± 161	≤1500	1500–3000	[77,78]
TC	(%TS)	30.5 ± 4.4	35.3±6.5
TN	(%TS)	4.9 ± 3.87	3.1 ± 1
C/N		8.1 ± 6.2	11.7 ± 2.7	20–30		[79]
FC	CFU/100 mL	1.5 × 106 ± 1.3 × 106	7.5 × 106 ± 6 × 106
E-coli	CFU/100 mL	8.1 × 105 ± 7 × 105	2.1 × 106 ± 1.7 × 106
Helminthes	œufs/L	1.2 × 104 ± 1.5 × 102	1.7 × 104 ± 3.8 × 102
As	mg/L	0.016 ± 0.013	0.03 ± 0.01	-
Cr	mg/L	0.043 ± 0.041	0.76 ± 0.83	0.005–50	28–300	[78]
Cu	mg/L	0.45 ± 0.24	2.06 ± 2.20	-	5–300
Hg	mg/L	˂*DL	0.03 ± 0.01	-
Mn	mg/L	1.21 ± 1.32	5.76 ± 6.99	0.005–50	1500
Ni	mg/L	0.02 ± 0.005	0.47 ± 0.1	0.005–0.5	10–300
Pb	mg/L	0.20 ± 0.18	0.45 ± 0.1	0.02–200	8–340
Zn	mg/L	1.71 ± 1.65	9.67 ± 12.1	-	3–400

*DL = Detection limit; DL = Detection limit; hydrogen potential (pH);Total solid (TS); Volatile solid (VS); Total Alkalinity (TAC); Ammonium-nitrogen (${\mathrm{NH}}_4^{+}$-N); Total Carbon (TC); Total nitrogen (TN); Carbone to Nitrogen ratio (C/N); Fecal coliform (FC); Escherichia coli (E-coli); Volatile fatty acids (VFA); arsenic (As); chromium (Cr); copper (Cu); mercury (Hg); manganese (Mn); nickel (Ni); lead (Pb); zinc (Zn).

compares the results of the settled faecal sludge with those of the composite sludge used in the settling tests.

3.4.1. Total Solid and Volatile Content

The settled composite samples resulted in an average TS content ranging from 11 ± 5 g/L to 80.9 ± 20 g/L. The average VS content of the sludge increased from 5.8 ± 2.6 g/L to 39.5 ± 10.5 g/L. Settled faecal sludge has the advantage of increasing the organic matter content available for digestion and has a high total solid content, which can be suitable for certain types of anaerobic digesters. This higher concentration of organic matter and total solids in the sludge can potentially improve the efficiency of the AD process, leading to higher biogas production and better waste treatment. However, it is important to carefully monitor and manage the increased concentration of bacteria, parasites, and heavy metals in the settled faecal sludge to avoid potential adverse effects on the AD process and the environment. Proper measures such as pasteurization and monitoring of heavy metal concentrations should be implemented to ensure safe and effective AD of the sludge. Figure 6 shows the dispersion of total solids (TS) and volatile solids (VS) content of the settled faecal sludge and the composite samples.

3.4.2. Carbon/Nitrogen/Phosphorus Ratio

The C/N ratio varies from 7 to 19 for the settled faecal sludge, whereas it averages at 8.1 for the composite samples. Compared to the composite samples, the settled faecal sludge increases the C/N ratio, with an average of 11.7, indicating an increase of 46%. The graphical representation of the relationship between total solids (TS) and carbon-to-nitrogen (C/N) ratio in Figure 7 shows that the most settled faecal sludge have the highest C/N ratio. This could be justified by the high presence of nitrogen, certainly in dissolved form, in the liquid fraction of the sludge. Nutrients, including carbon, nitrogen, phosphorus, and sulfur, play a crucial role in the performance of microorganisms during AD. The optimal nutrient requirement for AD is generally in the range of a C:N:P:S ratio of 500–1000:15–20:5:3. The C/N ratio of the composite sludge in the study has an average of 8.1 ± 6.2, which is relatively low for optimal AD conditions. For optimal performance, the C/N ratio should ideally be between 16:1 to 25:1 [78].

3.4.3. Content of Microorganisms and Heavy Metals

The concentration of sludge during AD offers advantages, but it also results in an increased concentration of bacteria and parasites, leading to a higher average total coliform count of 7.5 × 10⁶ CFU/100 mL for settled faecal sludge, compared to the initial average of 1.5 × 10⁶ CFU/100 mL. The average count of E. coli also increases from 8.1 × 10⁵/CFU 100 mL to 2.1 × 10⁶ CFU/100 mL with sludge concentration. Pasteurization of sludge before or after AD could be a potential solution, as recommended for such inputs.

The concentration of trace metal elements also increases with sludge concentration. Significant levels of heavy metals can be found in sludge from waste discharge, and when their concentration exceeds the inhibition threshold, they can be toxic to the bacterial population during AD. Based on the results obtained in the study, the toxicity classification of heavy metals, in descending order, is as follows: Zn > Mn > Cu > Cr > Ni > Pb > As ≥ Hg. The highest concentration was recorded for zinc (9.67 ± 12.12 mg/L), while the lowest was for mercury (Hg) (0.03 ± 0.01 mg/L). However, not all heavy metals are harmful to anaerobic digesters, and their toxicity can be ranked in increasing order: Cu > Ni > Pb > Cr > Zn > Fe [80].

At certain concentrations, trace metal elements can increase the biogas potential in anaerobic digesters [81]. Low concentrations of Cu²⁺ (0–100 mg/L), Fe²⁺ (50–4000 mg/L), Ni²⁺ (0.8–50 mg/L), Cd²⁺ (0.1–0.3 mg/L) and Zn²⁺ (0–5 mg/kg) promote biogas production, while high concentrations can inhibit AD [82]. Some heavy metal ions, such as Cu, Pb, Cr, and Zn, can also inactivate enzymes, thereby inhibiting the growth of bacteria and disrupting the operation of the anaerobic digester [83]. However, the levels of trace metals obtained in this study are below the recommended limit for good AD.

3.5. Methanogenic Potential of Faecal Sludge

The biogas volumes obtained during the experimental period ranged from 24 to 39 days of digestion for both the composite sample (CS) and settled faecal sludge (SFS). The calculated dry methane volumes under standard pressure and temperature conditions averaged between 215 mL and 665 mL for CS1 and CS2, respectively. Meanwhile, the methane volumes obtained were 4177 mL and 7168 mL for SFS1 and SFS2, respectively. The characteristics of the faecal sludge and the inputs used for the digestion process leading to biogas production are detailed in

Table 5. Characteristics of sludge use for biogas production (mean values).

Season	Designation	pH	TS	VS	TA	VFA	NT	TP	COD	BOD5	COD/BOD5	C/N
	Unit		%	%	mg/L	mg/L	mg/L	mg/L	mg/L	mg/L
	CS1	8.1	1.68	48.1	1206	89	1322	232	7800	1200	6.5	4.2
Dry	SFS1	7.8	7.95	49.3	1814	386	2160	426	38,200	5000	7.6	11.2
	CS2	7.6	1.82	55.4	1020	92	864	182	12,400	3400	3.6	7.2
Wet	SFS2	7.4	7.52	54.2	2010	620	2082	787	42,400	9000	4.7	12.3

and

Table 6. Digestion input (mean values).

	FS Volume	OLR	Abatement Rate		Digestate		Methane Yield (NL CH4/Kg)
Unit	L	gVSL−1	% (VS)	% (TS)	TS (%)	VS (%TS)	of VS Rem	of TS Add
CS1	2	8.08	46.8	39.9	1.01	42.6	28	6.4
SFS1	2	39.2	52.3	42	4.61	40.5	102	26.3
CS2	2	10.08	54.2	47.8	0.95	48.6	61	18.3
SFS2	2	40.75	56.2	44	4.21	42.4	156	47.5

OLR = Organic load rate, rem = removed add = added.

.

To accommodate for both seasons, we observed that the methanogenic potential of settled faecal sludge yielded the most favorable outcomes. The lowest methanogenic potential was observed in the composite samples, characterized by its low methane content and diminished methanogenic potential.

The biogas yield and composition are depicted in Figure 8a,b. The findings indicate that raw faecal sludge contains a lower methane content compared to settled sludge. These sludges exhibit elevated nitrogen levels, resulting in a low carbon-to-nitrogen ratio. Specifically, the methane content of CS1 during the dry season is below 30%, while that of CS2 is 39%.

The methanogenic potential of the raw sludge, comprising a blend of septic tank and latrine components obtained in this study, is approximately 28 NL CH₄/KgVS in the dry season and 61 NL CH₄/KgVS in the wet season. A low methane content, approximately around 23%, along with a diminished methanogenic potential, was also noted in faecal sludge in Bobo Dioulasso, Burkina Faso, as reported by Christiane [84]. These values align closely with those reported in the literature, ranging between 45 and 50 NL CH₄/KgVS for septic tank sludge [85]. On the other hand, other works have found values ranging from 0.009 m³ to 0.028 m³/kg VS [27,86]. However, in India, anaerobic digestion of FS produced a considerable amount of biogas at a production rate ranging from 0.02 to 0.12 m³/(kg TS) under mesophilic conditions and 0.05 to 0.21 m³/Kg of TS under thermophilic conditions [28]. Biogas production from faecal sludge is a function of the residence time of the sludge in the pits. The raw sludge used in this experiment is a mixture of sludge from domestics septic tanks and private pit latrines, with an average retention time of 2 to 4 years. This explains their low degradability. The low methane content obtained for the raw sludge shows that it is already mineralized. Equally, this result is explained by the fact that the raw sludge is very liquid (TS < 2%) with a very low organic load (<20 g/L), and very low biomass input ratio, VFA/TAC = 0.03. The biogas production depends on the amount of non-digested material in the sludge to be treated. For a good AD, respecting the optimal conditions, the organic load in the reactor in batch mode must be between 20 g/L and 60 g/L max [42]. Composite sludge has also a C/N ratio < 10. The C/N ratio is important for the stability of the process. For anaerobic digestion, the C/N ratio is, roughly, between 10 and 30 [71]. A higher ratio leads to a poor degradation of carbon; conversely, a ratio that is too low leads to an important production of ammonia, which inhibits the bacteria.

The observed trend indicates that sludge concentration during the wet season yields optimal methanogenic potential and methane content outcomes. This phenomenon is attributed to the higher organic matter content in the wet season sludge compared to that concentrated during the dry season. Additionally, it is worth noting that these sludges exhibit a chemical oxygen demand (COD) to biological oxygen demand over 5 days (BOD₅) ratio that is lower than that of SFS1. Settled faecal sludge has a higher methane content (48%) and (54%), with better methanogenic potential 102 NL CH₄/Kg VS for SF1 and 156 NL CH₄/Kg VS for SFS2. These values demonstrate that SFS has an advantage for biogas production, as it exhibits an interesting methanogenic potential, which can be further enhanced through co-digestion, resulting in high production yields suitable for industrial-scale operations like that of Kossodo.

4. Conclusions

Knowledge of faecal sludge characterization is crucial for sustainable faecal sludge management. It can reduce the cost of parameter analysis, facilitate the design and sizing of new faecal sludge treatment plants, and, above all, guide decision-making on the choice of the appropriate type of recovery. This study provided information on the characteristics of the FS discharged at Kossodo’s biogas plant and strengthened the knowledge on their complex nature. Based on the results of this study, the main conclusions on FS characterization are as follows:

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Kossodo’s plant FS characteristics show a high heterogeneity according to the origin of the sludge.

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Public toilets’ sludge is most suitable for energy recovery because it has volatile solids ≥ 60% and TS content around 2%. This study highlighted the contrast between sludge characteristics in the pits and the emptying methods that have an impact on the quality of the sludge discharged in the plant.

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For energy recovery, sludge concentration is an alternative to increase the total solid and solid volatile content of sludge.

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Settled faecal sludge have a better C/N ratio, which is an advantage for large scale AD.

Assessment of the methanogenic potential of faecal sludge revealed a low methanogenic potential for raw sludge. Settled faecal sludge, on the other hand, showed more promising potential than raw sludge. Biogas production was highest in the wet season, particularly for settled faecal sludge, which was relatively richer in organic matter than dry season sludge. This underlines the importance of the length of time the sludge remains in the pits for its recovery by anaerobic digestion. However, co-digestion has the potential to enhance the methanogenic potential of the sludge by improving the C/N ratio of the settled faecal sludge, bringing it closer to the recommended optimal operating C/N ratio range of 20–30. The incorporation of a cosubstrate with higher carbon content could further elevate this ratio, facilitating efficient anaerobic digestion.

This indicates a favorable path toward the sustainability of faecal sludge recovery in the future.