Vermicomposting of Sewage Sludge Produced at Camp SIC in Cité-Verte (Yaounde-Cameroon) as an Agricultural Amendment

The aim of this project is to reuse the sewage sludge produced at the Camp SIC Cité-Verte wastewater treatment plant (WWTP) as an agricultural amendment using the vermicomposting process. The methodological approach involved characterising this sludge, producing three types of vermicompost (E1, E2 and E3), characterising them and testing their effectiveness on maize. The WWTP produces a large quantity of sludge, 364.3 kg per day. The sludge is rich in organic matter, mineral elements (Total Carbon: 53.01%, Total Nitrogen: 8%, Total Phosphorus: 2500 mg/kg and Potassium: 2723 mg/kg) and pathogens (faecal streptococci = 128 CFU/100 ml and faecal coliforms= 166 CFU/100 ml). Analysis of the vermicompost obtained after sixty-five days showed a significant increase in the nutrients carbon (E3 :59%), potassium (E1: 2710 mg/kg, S2: 3125 mg/kg, E3: 2985 mg/kg), phosphorus (E1 : 2536 mg/kg, E2: 2836 mg/kg, E3: 3101 mg/kg); then a decrease compared to the initial substrate for carbon (E1: 44.02%, E2: 48%), nitrogen (E1: 2.06%, E2: 2.84%, E3: 3.05%) and a considerable decrease in pathogens in samples E1 and E2, unlike E3 where they are completely absent. The effectiveness of vermicompost showed germination rates of 65% and better results for maize growth and development. 

Introduction

Cameroon, like many developing countries, is experiencing rapid urbanisation and demographic growth. Cameroon’s population rose from 5.18 million in 1960 to 28.65 million in 2023, an increase of 453.4% in 63 years [1]. This demographic growth is leading to an increase in waste production, particularly sewage sludge generated by wastewater treatment plants [2]. The issue of managing the sewage sludge generated by these systems represents a major environmental challenge, as a considerable quantity is discharged into the environment in an uncontrolled manner. Such exposure of sewage sludge results in reduced fertility, porosity, permeability and lower agricultural production [3]. However, the demand for food is constantly increasing in line with this population growth, which means that agricultural production needs to be increased.

Faced with this situation, it is imperative to resort to fertilisation, which is the quickest way of providing plants with essential nutrients. Mineral fertilisation involves adding mineral elements to the soil to make it more suitable for agricultural use [4]. Intensive farming based on numerous inputs is showing its limits in environmental terms. The green revolution has increased yields but also accelerated many soil degradation processes [5]. Although the use of mineral fertilisers satisfies plant needs and promotes plant growth, it has a negative impact on the soil, the environment and human health [6], [7]. However, there are ways and means of getting out of this impasse, which revises the paradigms governing environmentally-friendly management. One of these is to recycle the sewage sludge by inoculating it with earthworms, which break down the organic matter. This is called vermicomposting.

Numerous studies show that vermicomposting is a highly effective method of converting solid organic waste into a valuable, useful and environmentally-friendly resource. This is an accelerated process involving the bio-oxidation and stabilisation of waste as a result of interactions between certain species of earthworms and micro-organisms [8], [9]. Moreover, these epigeal worms are made up of small species (1 cm–5 cm) and live on the surface (the first few centimetres) in piles of organic matter such as manure, forest litter, green waste, etc. [10]. They have a great capacity to rapidly degrade organic waste and activate it, making it available to plants without risk to the environment [11]. Vermicomposting appears to be a sustainable solution that contributes to a circular economy and is less costly. Not only does it reduce the volume of sludge and promote environmentally-friendly mineral fertilisation, it also produces a high-quality organic amendment rich in nutrients that are beneficial to the soil [12]. It helps combat pollution but also transforms waste into fertilising material [13].

The aim of this study is to recover the sewage sludge produced at the Cité Verte WWTP by vermicomposting as an agricultural amendment. The study will also help to reduce environmental pollution and produce organic fertiliser for plant growth.

Materials and Methods

Study Site and Sampling

The study was carried out at the Camp SIC de la Cité-Verte WWTP, located in the Abiergué catchment area, straddling the Yaoundé II district municipality and part of the Yaoundé VI district municipality, in the Department of Mfoundi, Central Cameroon region. The site has an equatorial climate with an average annual rainfall of 1600 mm and red ferralitic soils on acid crystalline rock [14]. Sampling was carried out in the field following the standard protocol [15]. Samples are taken using 250-mL double-capped polyethylene bottles. Before sampling, the effluent is shaken vigorously to homogenise the medium, and the device is rinsed with distilled water and introduced directly into the tank.

Sewage Sludge Analysis Protocol

The sewage sludge was collected from the pre-treatment and primary treatment works, then analysed in the laboratory according to the standard protocol [16]. Samples were collected in 250-mL double-capped polyethylene bottles and stored in ice-lined coolers until they reached the laboratory. The physico-chemical analyses were carried out both in the field and in the laboratory, and included measurements of electrical conductivity, pH and temperature using a HANA multi-parameter HI 9829. Determination of total organic matter content by calcination, determination of total organic carbon using Giroux and Audesse’s [17] formula and total nitrogen using the Kjeldahl method in accordance with French standard [18]. The C/N ratio was determined from the total organic carbon and total nitrogen values obtained. The total phosphorus content of vermicompost was determined using the ‘molybdovanadate’ method [19].

Sludge Quantification Method

To quantify the sludge produced by the Camp Sic de la Cité Verte WWTP, we assessed the quantity of sludge removed in the pre-treatment and primary treatment works over thirty (30) days using a 10L container to measure the quantity of sludge skimmed off each day. The data collected was used to calculate the average quantity of sludge produced by the WWTP per day, per month and per year [20].

The average daily quantity of sludge produced by the WWTP (Qb in kg) is calculated using (1). The monthly quantity of sludge produced (Qmb in kg/month) is calculated using (2). The annual quantity of sludge produced (Qma in kg/year) is determined using (3).

(1) $$\begin{array}{r}Qb=\frac{V\times N}{T}\end{array}$$

where

V – bucket volume (in L)

N – number of buckets filled per day

T – number of skimming days

(2) $$\begin{array}{r}Qmb=Qb\times T\end{array}$$

(3) $$\begin{array}{r}Qma=Qmb\times C\end{array}$$

where

C – number of months in the year

Experimental Phase of Vermicomposting

This phase is carried out in four (4) stages: the collection of sludge and co-products, a pre-composting phase, vermicomposting and installation of the system, and finally the monitoring of parameters.

Household waste such as fruit peelings (plantains, pineapples, watermelons, papayas, oranges) are collected, placed in plastic bags and transported to the site. The sewage sludge is taken from two pre-treatment works using a shovel and a 10-litre bucket, and weighed using a balance. The Terminalia capata and Echinochoa pyramidalis leaf litter is recovered at the STEP, while the sawdust is obtained from a carpentry workshop. Co-products are cut up and ground to reduce particle size and increase the surface area in contact with the micro-organisms. After this preparation, all the co-products obtained were weighed using a balance and subjected to pre-composting for 24 days, with a total composition of 18 kg of sewage sludge, 2 kg of sawdust, 1.5 kg of household waste and 1.5 kg of leaf litter. In this pre-composting phase, three (03) samples were taken: sample (E1) comprising only sewage sludge, sample (E2) sewage sludge + sawdust, sample (E3) sewage sludge + litter + household waste. After the twenty-four (24) days of drying, the earthworms are injected into E2 and E3. These earthworms are collected under piles of discharged sewage sludge, identified in the WWTP.

Batch vermicomposting was set up and carried out in a wooden bin measuring 50 × 25 × 30 in cm. A grid has been placed at the bottom of the bin to prevent the earthworms from escaping. The wooden troughs are covered with a black plastic sheet to protect them from the elements and to maintain favourable conditions (humidity and temperature). The experiment lasted 40 days with a final composition: E1: only 6 kg of sewage sludge (i), E2: 6 kg of sewage sludge + 2 kg of sawdust + 200 earthworms (ii), E3: 6 kg of sewage sludge +1 kg of fresh leaves +1 kg of household waste +200 earthworms (iii).

Monitoring the maturation parameters (temperature, humidity, smell, colour) of the vermicompost is important to ensure that the compost is of high quality and can be used to feed plants safely. The temperature must be maintained between 15°C and 25°C [2], [21]. Moisture was assessed using the weighing method [22]. It includes:

Weight loss is calculated according to (4):

(4) $$\begin{array}{r}\text{P}W=\text{M}1-\text{M}2\end{array}$$

where

PW – weight loss

M1 – mass of fresh vermicompost

M2 – mass of vermicompost after drying

After obtaining the weight loss, we calculate the percentage of moisture according to (5):

(5) $$\begin{array}{r}\mathrm{\%}humidity=\frac{PW}{M1}\times 100\end{array}$$

Analysis of Vermicompost

After forty (40) days, the samples are air-dried and sieved using a 10 mm mesh sieve and 100 g of each sample is taken and applied to the germination tests.

Germination Test: Phytotoxicity

To carry out this test, the method used by Khan [23] was applied for five (05) days. The following were used:

• Maize seed to be germinated using different soil profiles.

• Pure moistened sand, used as a reference with a grain size of 2 mm.

• Ten (10) plastic planting bags were used, each containing five (05) maize seeds and divided into four (04) groups as follows (Table I):

Table I. Phytotoxicity Test Development

1st Group	2nd Group	3rd Group	4th Group
100% sand	100% E1 compost	25% sand + 75% E1	75% sand + 25% de E1
	100% E2 compost	25% sand +75% de E2	75% sand + 25% de E2
	100% E3 compost	25% sand + 75% de E3	75% sand + 25% de E3

These different piles are placed in an oven at 20°C and in the dark for 48 hours, then in the light for eight days. The boxes are sprinkled with water if necessary to maintain humidity. After 10 days, the number of germinated seeds per box is counted. The germination rate is then calculated in relation to the initial number of seeds sown. This percentage is expressed according to (6) [24] :

(6) $$\begin{array}{r}\mathrm{\%}germination=\frac{Numberofseedsgerminated}{Numberofseedsplanted}\times 100\end{array}$$

Determining the Effectiveness and Yield of Vermicompost

Choice of Plant Material (maize)

Maize is the most widely grown and consumed cereal in Cameroon. Its marketing is an important source of income for populations throughout the territory [25].

Type of Soil

Soil plays a fundamental role in terrestrial ecosystems [26]. The soil at Cité-Verte is ferralitic red on acid crystalline rock, which is poor in nutrients [27].

Field Test

The aim here is to monitor the effects of each vermicompost on the plant development of the maize. The maize is grown on a plot of land at the WWTP divided into several sections: section 1 consists of the soil used as a reference, section 2 consists of sewage sludge (E1), section 3 consists of vermicopost (E2) and section 4 consists of vermicompost (E3).

Parameter Monitoring

This stage marks the beginning of the process of recording the agromorphological parameters of the maize for a month, more precisely every Monday after spraying. It will also be necessary to determine:

• The intensity of the colour and the size of the stems;

• The diameter (D) of the stem obtained after measuring the perimeter (P) of the stem and determined by the formula:

(7) $$\begin{array}{r}D=\frac{2P}{\pi }\end{array}$$

The constant pi (π) equals 3.14.

Results and Discussion

Results

Quantification of WWTP Sludge

The sludge from the Camp SIC de la Cité-Verte WWTP was quantified over a period of thirty (30) days and the results are shown in Table II.

Table II. Quantification of Sludge Produced at the Camp SIC de la Cité-Verte WWTP

Period Type of structure	Daily production		Monthly production		Yearly production
	(kg)	(Tonnes)	(kg)	(Tonnes)	(kg)	(Tonnes)
Decanter	31,88	0,0318	956,5	0,956	11636,2	11,636
Sand trap	222,86	0,222	6686	6,686	81343,9	81,343
Manhole	12,91	0,012	387,5	0,387	4712,15	4,712
De-oiling unit	96,33	0,096	2889,9	2,899	35160,45	35,160
TOTAL	364,3	0,0318	10919,5	10,928	132852,7	132,852

Average daily production was 364.3 kg, giving a monthly production of 10919.5 kg. The average annual production of sewage sludge will be 13,2852.7 kg.

Physicochemical and Microbiological Analysis of Sludge

Table III shows the physicochemical and microbiological characteristics of the sewage sludge obtained before the vermicomposting process was implemented.

Table III. Results of Physicochemical and Microbiological Analyses of Sewage Sludge

Parameters	Ws	Effluent discharge standard	AFNOR [32]
pH	8.2	6.5–8.5	/
T°C	25	> 30	/
CT (%)	53.01	30–80 [28]	/
PT (mg/kg)	2500	10 mg/l	>0.25
NT (%)	8	6 [29]	/
K+ (mg/kg)	2723	4–23 g/kg [29]	/
FS (CFU/100 ml)	128	88–100 (germs/ml) [30]	/
FC (CFU/100 ml)	3127	1000 UFC/ml [31]	/

Table III shows that there are high levels of nutrients, particularly total carbon (53.01%), total nitrogen (8%), phosphorus (2500 mg/kg) and potassium (2723 mg/kg), which are nutrients required for plant growth. In addition, these results show a multitude of live microorganisms, including streptococci (128 CFU/mL) and faecal coliforms (3127 CFU/mL).

Maturation Parameters During the Vermicomposting Process

Fig. 1 shows the results of monitoring maturation parameters during the vermicomposting process at the WWTP.

Fig. 1. Monitoring of maturation parameters during the vermicomposting process: a) Change in temperature during pre-composting; b) Change in temperature during the vermicomposting process; c) Humidity of the various samples; d) Change in colour during the vermicomposting process.

During the Pre-composting Phase

Fig. 1a shows the evolution of temperature during pre-composting. This pre-composting phase lasted 24 days, with an average temperature of 58.1°C, which was fairly high between day 1 and day 4. However, we observed a maximum temperature of 55.7°C, slightly lower than the average temperature between day 6 and day 1, followed by a decrease in temperature from day 14 to day 24, marking a phase of cooling of the compost and the start of the vermicomposting process.

During the Vermicomposting Process

Fig. 1b shows how temperatures change during the vermicomposting process. During vermicomposting, temperatures varied in samples E1 (27°C to 23.8°C), E2 (60°C to 23.4°C) and E3 (30°C to 24°C). A significant increase in temperature was observed at the start of the vermicomposting process in samples E2 and E3, followed by a gradual decrease between days 10 and 40, stabilising at around 24°C.

Humidity

Fig. 1c shows the moisture content of samples E1, E2 and E3 for four (04) weeks. Moisture levels varied in samples E1 (63% to 17.8%), E2 (60% to 62.2%) and E3 (57.14% to 63.1%). These results show that there is a decrease in E1, followed by a gradual increase in E2 and E3 over the four weeks.

Odour and Colour Change

From day 1 to day 22, the samples gave off an unpleasant sulphurous or ammoniacal odour. After 30 days, the odour of the various samples (E1, E2, E3) was no longer persistent.

Physicochemical and Microbiological Analysis of Vermicompost

Table IV shows the results of analysis of the physicochemical parameters of the vermicompost obtained after 40 days.

Table IV. Results of Physicochemical and Microbiological Analyses of Vermicompost

Samples parametrers	E1	E2	E3	FAO standards	AFNOR [32]
pH	8.11	8.22	8.40	≤8	/
Temperature	22	23	22.5	/	/
CT (%)	44.02	48	59	/	/
PT (mg/kg)	2536	2836	3101	/	>0.25
NT (%)	2.06	2.85	3.05	0.4–0.5	/
C/N	21.36	16.84	19.34	15–20	<20
K (mg/kg)	2710	3125	2985	4–23 g/kg	/
FS (CFU/100 ml)	38	Ab	Ab	/	/
FC (CFU/100 ml)	358	98	Ab	/	/

According to Table IV, the pH values obtained are 8.11, 8.22 and 8.40 for E1, E2 and E3 respectively. Nutrient analysis results for vermicompost samples E1, E2 and E3 show carbon contents of 44.02% for E1, 48% for E2 and 59% for E3, indicating a decrease in E1 and E2 compared to E3. As for the nitrogen content obtained in samples E1, E2 and E3, it varies between 2.06% for E1, 2.84% for E2 and 3.05% for E3. Potassium in sample E1 is 2710 mg/kg, in E2 it is 3125 mg/kg and in E3 it is 2985 mg/kg. Phosphorus levels were 2536 mg/kg (E1), 2836 mg/kg (E2) and 3101 mg/kg (E3), with E2 and E3 showing higher concentrations and differing significantly from the initial substrate. Finally, the C/N ratio of the samples varied, standing at 21.3% for E1, 16.8% for E2 and 19.3% for E3.

The results of the microbiological analysis of the vermicompost indicate that there was a considerable reduction in streptococci in sample E1, unlike samples E2 and E3, which showed a total absence of these pathogens. As for faecal coliforms, they completely disappeared in sample E3, although they were found at 358 CFU in sample E1 and 98 CFU in sample E2.

Result of Phytotoxicity Test

Phytotoxicity is a key index for assessing the effects of toxic substances on plant growth [33]. It is therefore necessary to determine the phytotoxicity of the different groups obtained in order to assess the degree of maturity. Fig. 2 shows the results of the phytotoxicity test on different samples.

Fig. 2. Graphical representation of the results of the phytotoxicity test.

Group 1, made up of sand as a reference sample, had a germination rate of 80%, higher than groups 2, 3 and 4 made up of organic fertiliser. In group 2 (100% of E1, E2 and E3), the germination index was higher in E2 and E3 than in E1. However, in group 3 (25% of E1, E2 and E3) the germination rate increased. However, E2 and E3 in groups 3 and 4 showed a significant reduction in toxicity.

Results of the Vermicompost Efficiency Test in the Field

The efficiency of maize growth was carried out on a plot at the STEP of the Camp SIC in Cité-Verte, Yaounde. The plot was divided into four (04) sections to test the effectiveness of different treatments on maize (Table V–VIII).

Table V. Results of the Vermicompost Efficacy Test in the Field: Number of Leaves

	Portion 1		Portion 2 (E1)		Portion 3 (E2)		Portion 4 (E3)
	L1	L2	L1	L2	L1	L2	L1	L2
Plant 1	4	7	4	7	8	9	9	11
Plant 2	4	7	3	6	8	9	8	10
Plant 3	3	6	3	7	7	9	9	10
Plant 4	3	4	3	7	7	8	10	9

Table VI. Results of the Vermicompost Efficacy Test in the Field: Stem Size

	Portion 1		Portion 2 (E1)		Portion 3 (E2)		Portion 4 (E3)
	L1	L2	L1	L2	L1	L2	L1	L2
Plant 1	4	6.3	16.5	18.8	15	49.1	59.5	78.4
Plant 2	4.2	5.9	16.1	19.1	35.2	47	22	68.2
Plant 3	4.5	7	6	16.5	8	42.2	12	66.9
Plant 4	4.8	7	11.6	18.3	33.4	37	7	59.6

Table VII. Results of the Vermicompost Efficacy Test in the Field: Leaf Colour

	Portion 1		Portion 2 (E1)		Portion 3 (E2)		Portion 4 (E3)
	L1	L2	L1	L2	L1	L2	L1	L2
Plant 1	YG	VC	LG	LG	DG	DG	DG	DG
Plant 2	YG	VC	LG	LG	DG	DG	DG	DG
Plant 3	YG	YG	LG	LG	DG	DG	DG	DG
Plant 4	YG	YG	LG	LG	DG	DG	DG	DG

Table VIII. Results of the Vermicompost Efficacy Test in the Field: Diameter of Stems

	Portion 1		Portion 2 (E1)		Portion 3 (E2)		Portion 4 (E3)
	L1	L2	L1	L2	L1	L2	L1	L2
Plant 1	0.5	2.01	0.96	0.7	1.59	2.54	6	4.14
Plant 2	0.57	2.2	0.89	0.75	1.43	3.18	3.8	4.26
Plant 3	0.45	1.89	1.27	0.81	1.4	2.86	5.7	3.8
Plant 4	0.51	2.15	1.21	0.83	1.33	4.07	5.9	4.07

Bio-indicators such as plant height, stem diameter and number of leaves were used to test the effectiveness of vermicompost.

Number of Leaves

Table V shows the number of leaves on the different maize plants. This shows that the plant with the highest number of leaves is found in section 4, with a maximum of 11 leaves, followed by section 3, which varies from 7 to 9.

Stem Size

Table VI shows the different heights of maize stalks. The results show that portion 4 was the largest at 78.4 cm, followed by portion 3 at 49.1 cm and portion 2 at 19.1 cm. As for portion 1 which serves as a control sample, the heights are largely preserved below portions 2, 3 and 4.

Leaf Colour

Table VII.c shows the variations in colour observed on maize leaves. Section 1 is yellow-green, section 2 is light green, while sections 3 and 4 are dark green.

Diameter of Stems

Table VIII shows the different diameters obtained over the course of the maize’s development. The largest diameter was observed on plants in section 4, at 4.26 cm, followed by section 3 at 4.07 cm and section 2 with a maximum diameter of 1.27 cm.

Discussion

Physicochemical analysis of the sludge shows that the percentage content of total carbon is 53.01. This value is in line with that of ADEME [28], which estimates that the concentration of organic matter in sludge can vary from 30 to 80%. As for total nitrogen, the level is high compared with the recommendations of the FAO [29]. The C/N ratio of 6.62% is below the values appropriate for the vermicomposting process, falling short of the range (25%-30%) for the composting material required as suggested by Ndegwa and Thompson [34]. As a result, the N, P, K content obtained in our work is in variable quantities to be used directly as an agricultural amendment and soil health as explained by Cai et al. [35]. Faced with this, it has been recommended to add filler-rich high-carbon fillers [36] to optimise the nutrient content in sewage sludge. Albrecht [37] explains that optimising the bulking agents in sewage sludge has a major impact on the degradation of organic matter by earthworms in order to produce the high-quality compost needed for plant growth, while also minimising the environmental impact. In addition, these results show a high proportion of live microorganisms such as streptococci (128 CFU/mL) and faecal coliforms (3127 CFU/mL). Streptococci and coliforms are indicator bacteria for the presence of faecal contaminants in wastewater, and are therefore a cause for concern in terms of environmental impact and public health.

Fig. 1a shows an average temperature of 58.1°C during vermicomposting, which was fairly high between day 1 and day 4. The sharp rise in temperature is due to microbial activity in the sewage sludge, which degrades organic matter. Manantsoa et al. [38] explain that the microorganisms present in the sludge during pre-composting are responsible for heat production, which contributes to a rise in temperature. This rise in temperature during the pre-composting process marks the thermophilic phase and destroys pathogenic germs, i.e. hygienises the compost. During the vermicomposting process, the temperature in the control sample (E3) varied from 30°C to 24°C. For the other samples, E1 varied from 27°C to 23.8°C and E2 from 26.1°C to 23.4°C. A significant increase in temperature was observed at the start of the vermicomposting process in samples E2 and E3. This was more than 2°C above ambient temperature. The different temperature variations indicate that biochemical reactions are taking place during the composting process. Compaoré and Nanéma [39] reported that the increase in temperature during the first week of composting could be the result of high microbial activity induced by the presence of organic matter. This metabolic activity releases heat, which contributes to a rise in temperature. They also reported that the sharp drop in temperatures observed could be explained by a slowdown in the activity of microorganisms due to the depletion of easily biodegradable organic matter. Also our results are in line with those obtained by Liegui [40] who found a significant temperature increase at the beginning of the household waste vermicomposting process.

With regard to the nutrients in vermicompost, namely carbon, nitrogen, potassium and phosphorus, the results of analyses carried out on the three samples (E1, E2 and E3) show that total carbon decreased in samples E1 and E2 compared with sample E3. However, the carbon content in E2 and E3 appeared really high compared with the initial substrate. The reduction in carbon may be due to the fact that during the vermicomposting process, micro-organisms and earthworms use carbon as a source of energy for the mineralisation and decomposition of organic matter. Singh et al. [41] attributed the decrease in organic carbon at the end of vermicomposting to the consumption of carbon by the worms and its conversion into CO₂ by the earthworms’ respiratory activity. Similarly, the increase in the carbon content of E3 may be due to the high carbon content of household waste. Ramnarain et al. [42] observed similar results in vermicomposting different organic wastes for 120 days using Eisenia fetida. In the final vermicompost, sample E3 was significantly different from samples E1 and E2. This difference may be due to the initial nitrogen content of the different substrates. Tripathi and Bhardwaj [43] reported that during the vermicomposting process, the addition of nitrogen content is due to the release of nitrogenous excretory substances, mucus, enzymes and the death of certain earthworm species. Our results are close to those obtained by Amouei et al. [44] in vermicomposting different organic wastes. In addition, the potassium value is quite high, particularly in samples E2 and E3. However, the significant increase in potassium concentration in samples E2 and E3 may be due to the initial raw material of the sample. These results are similar to those of Gorakh et al. [45] who found a slight increase in potassium in vermicompost from any combination of wastes than the initial mixture. This is due to the addition of nutrients during pre-composting compared with the initial substrate. Suthar [46] also explains that waste treated by earthworms contains a high concentration of potassium due to the increased microbial activity during the vermicomposting process. Finally, the phosphorus concentration in samples E2 and E3 appeared higher than in samples E1 and was largely different from the initial substrate. This increase is caused by the different compositions of the three samples, and is also due to the formation of organic acid during the decomposition of organic waste, which solubilises phosphorus, making it readily available [47]. Considering the C/N ratio, we find that the vermicompost obtained in sample E1 (21.36%) is of good quality, as it complies with the value recommended by standard [32]. As for the E2 and E3 samples (16.84%, 19.34%), they are relatively low, but these samples are also of good quality, as they fall within the standard range recommended by the FAO [29].

The reduction or even total disappearance of pathogens in samples E1, E2 and E3 can be explained by the variations in temperature during vermicomposting compared with the initial substrate. According to the [48], the elimination temperature for pathogenic germs is around 70°C. In our study, the temperature reached 58.1°C and helped to eliminate these pathogens.

Phytotoxicity is considered an important index for assessing the effects of toxic substances on plant growth [33]. Fig. 2 shows the percentage germination rate in the different samples. The good rate obtained by group 1 is due to the fact that the sand contained in this group is a well-draining substrate which allows better aeration of the roots and enables the maize to absorb the oxygen necessary for germination. In addition, sand offers better moisture retention, which favours germination [49]. In the other cases, E1 has a low germination index with a higher phytotoxicity potential than the others. However, E2 and E3 in groups 3 and 4 showed a significant reduction. This is due to the action of earthworms during vermicomposting, which can eliminate contaminants in the sludge and promote plant growth [38].

For the test of the effectiveness of vermicompost on foliage, we found that portion 1, which is the reference sample, contains the plant with the lowest number of leaves (03). As for portion 2, the number of leaves varies from 3 to 7. All these observations and the behaviour of the plants are due, on the one hand, to the fact that the vermicompost applied to portions 4 and 3 is rich in nutrients (nitrogen, phosphorus, potassium, etc.) and, on the other hand, to the fact that the earthworms, by mineralising the organic matter present in the soil, transform it into nutrients that are easily assimilated by the maize. The effectiveness of vermicompost can also be seen in the colour of the leaves. Section 1 differs from the other sections because of the low proportion of nutrients in the control soil. The soil in the study area is known to be poor in nutrients due to its iron oxide content, which makes essential nutrients less available [50]. The size of the maize stalks will also explain the nutritional value of the vermicompost with portion 1, the control sample, whose heights remained well below those of portions 2, 3 and 4. Edwards and Burrows [51] demonstrate in their work that the vermicompost obtained by vermicomposting sewage sludge is a source of nutrients for plants and stimulates their growth.

Conclusion

With a view to contributing to good environmental and sanitation practice, the overall aim of our work was to treat and reuse the sludge produced at the Camp SIC de la Cité-Verte WWTP as an agricultural amendment using vermicomposting. The diagnosis revealed that the Camp SIC de la Cité-Verte WWTP produces huge quantities of sludge that is rich in organic matter and nutrients important for plant growth, but could not be used directly because of the high concentration of pathogens and other polluting substances.

The characteristics of the final vermicompost showed an increase in nutrients, particularly carbon (E3 :59%), phosphorus (E1 :2536 mg/kg, E2 :2836 mg/kg, E3 : 3101 mg/kg), potassium (E1 :2710 mg/kg, E2 :3125 mg/kg, E3 : 2985 mg/kg) due to the combined action of earthworms and micro-organisms resulting in a significant input of nutrients, followed by a decrease compared to the initial substrate in carbon (E1:44.02%, E2:48%) and nitrogen (E1:2.06%, E2:2.84%, E3:3.05%). In addition, there has been a considerable reduction in pathogens, which can have a considerable impact on the environment and user health. The C/N ratios (E1: 21.3%, E2: 16.8%, E3: 19.3%) are less than 25%, in line with FAO and AFNOR standards. The phytotoxicity results, which enabled us to test the effectiveness of our vermicompost, showed that samples E2 and E3 in groups 2, 3 and 4 presented no danger to maize germination. Finally, the results of the field test showed the contribution of the vermicompost from the three samples to the maize plants. The portion receiving E3 vermicompost has higher nutritional values, followed by the portion receiving E2 vermicompost. As a result, waste recovery is a sustainable solution that requires particular attention.