Abstract

Waste management in general and household waste in particular go through various treatment routes with the aim of recovery. Composting is a natural process of decomposition of organic matter by micro-organisms under well-defined optimum conditions to produce a stable compost which can be used as an organic “fertilizer”. Sorting, windrow formation and monitoring of physicochemical parameters are essential for the production of good compost. The raw household waste from the city of Soa contains 45.2% biodegradable organic matter for a yield of 27% of compost C/N is 28, pH is 6, humidity between 50 and 60%. When creating the windrow, its required average size must be 2m long, 1.5m high and 1.5m wide. The humidity to be maintained during composting must be between 40 and 65%. During the windrow composition phase, the temperature of the thermophilic phase is 60°C, and 30°C at the end. The compost obtained contains 34.40g organic matter, 2.12 g of potassium and 44.00g of phosphorus per 100.00g of compost. As parameters for verifying the safety we obtained 0.0045mg of lead per 100g of compost and trace of Cadmium. The presence of these compounds in the organic matter of the initial substrate and the finished compost was confirmed by the presence in the Fourier transform infrared (FTIR) spectrum of absorbance bands at the wavelength between 3200 and 3600 cm^-1 characteristics of OH phenols and also to the significant absorption of aliphatic compounds (O-H and N-H) at 3400 cm^-1. These results are in agreement with the standards

1. Introduction

The growth of the population in urban areas in Yaounde-Cameroon in general and that of Soa in particular has led to a significant increase in waste production of 0.62 kg/day/inhabitant ¹. The abundance and management of this waste is becoming an ecological and environmental preoccupation. Waste management and treatment methods are at the heart of this problem. The different waste treatment techniques aim for different objectives ranging from total to partial recycling, sometimes within the very process which gave rise to them or even to facilitate their material or energy recovery; “eco-compatible” return to the environment after detoxification or stabilization-solidification; and finally, the decomposition, more or less completely, into “harmless” chemical species ^{2, 3}. This last biological treatment technique is the most practical; its general principle is to exploit certain microbial activities by stimulating them in a controlled manner in order either to reduce the potential nuisances of waste (odors, health risks, polluting nature in the broad sense of the term), or to recover them in energy form or in matter. As a result, biological processes are in practice generally used for the treatment of essentially organic waste having a biodegradable nature, namely in particular waste associated with the exploitation or consumption of biomass ^{4, 5}.

The biodegradable fraction of this household waste varies between 50 and 60% ¹, hence the possibility of its recovery into compost using the composting technique. Composting is a controlled process of degradation of organic constituents of plant and animal origin, by a succession of communities of microorganisms (bacteria, fungi and protozoa) evolving in aerobic conditions and leading to the development of humified and stabilized organic matter. called compost ⁶. During the composting process Aerobic microorganisms break down organic matter and produce compost, carbon dioxide (CO2), ammonia, water, heat and many other compounds ⁷. Composting requires a lot of careful monitoring and ingenuity during its process, hence our theme on how to optimize this process on household waste from the city of Soa.

This work involves seeking the optimum values of organic matter and essential parameters for composting household waste from the city of Soa in order to obtain quality compost that can be used in agriculture as a soil amendment “organic fertilizer”. For this purpose, we must determine the biodegradable composition of this raw waste; determine the substrate formula to use; determine the values of the optimum parameters of the process; to analyze the compost obtained in the laboratory. Thus, will have contributed to the management and recovery of household waste, the reduction of waste destined for landfill, the fight against unsanitary conditions caused by waste, the reduction of the effects of chemical fertilizers and finally to reducing global warming ⁸.

2. Materials and Methods

For this work, we carried out two composting tests in order to find the optimum values of the different operational parameters of the process:

- For the first test: we started with 2500 kg of household waste which we had characterized, then carried out the first test of composting the 1322.5 kg of fermentable organic matter obtained. The one windrow composting trial allowed us to obtain 355.1 kg of finished compost.

- For the second test: we started with 22,315.8 kg of household waste which we had sorted to obtain 11,715.5 kg of compostable material, then carried out the composting tests in ten (10) different windrows from these 11,715, 5kg. The ten composting tests allowed us to obtain 3178.2 kg of compost.

Figure 1 below gives us an overview of the protocol that was followed to achieve the objectives sought in this work.

These composting operations are described as follows:

Sorting: This is the process to separate the components of heterogeneous raw waste. This operation allows us to obtain the compostable fraction (fermentable material). To do this, all the components are separated from each other in different enclosures.

Shredding: The aim is to reduce particle sizes. It is particularly useful for the coarsest fractions of organic matter, because when the material has a size grade, it is more difficult to biodegrade. For shredding efficiency, we use sharp machetes for the operation on a support or in bulk. The operation is repeated depending on the desired particle sizes.

Formation of windrows: The compostable fraction (biodegradable organic material) of the sorted waste is placed in a windrow to allow fermentation (composting). In order to have an exact and controlled size, we used the plows that we hoist to the edge of the swath to allow it to evolve in a regular volume. At the bottom of the windrow is placed a waterproof plastic which allows excess watering water to be collected in the form of leachate. Rectangular windrows are formed under a shed. When windrowing, the humidity is controlled and, if necessary, water is added by sprinkling.

Watering: Watering makes it possible to moisten the windrow, hence increasing the humidity level to stimulate the biological activity of microorganisms. It is carried out manually using watering cans and above all by collecting excess water in an enclosure through a waterproof tarpaulin spread at the bottom of the windrow and then reintroducing it into the windrow.

Decomposition: This is an operation carried out naturally by micro-organisms. During this phase, organic matter is converted into various compounds such as carbon dioxide, water, finished compost as well as other compounds, under the action of microorganisms naturally present in the waste. This activity is characterized by a sharp rise in temperature, loss of humidity and high oxygen consumption. Consequently, this phase requires the control of humidity, temperature and oxygen levels to be able to correct the water and oxygen supplies through the air. Here aeration is done by turning or blowing air using a perforated pipe which crosses the windrow.

Turnaround: Windrows are turned using the fork. To do this, the entire windrow is moved from one point to another and then the composting waste is placed in a layer one on top of the other. After depositing a specific quantity, a quantity of water poured relative to the quantity of waste deposited.

Maturation: During this phase, the organic matter having already undergone decomposition, always continues to undergo a slow transformation leading to its definitive chemical and biological stabilization. Carried out naturally in the windrow, the maturation of the compost requires neither turning nor watering during its period but rather protection against pests.

Screening: The screening of the material generally takes place at the end of the maturation of the compost. Its purpose is to remove coarse elements which could not be eliminated by sorting and which have not been decomposed. Screening rejects mainly consist of inert elements (rubble, stones, pebbles, and pieces of broken glass), pieces of coal, non-biodegraded wood particles and hard and soft plastics. The mesh sizes of the sieves used are approximately 20 mm.

Sampling is carried out at several locations in the windrow to form representative samples. They are homogenized before being sent to the laboratory or analyzed on site depending on the type analysis to be carried out.

The temperature is taken using a probe thermometer. The operation is carried out by pushing the probe of the lit thermometer into the windrow to a depth of approximately 50 cm, then waiting approximately five minutes before reading the temperature value.

The pH is measured in a beaker into which 10 g of sifted compost and 100 ml of distilled water are introduced in order to take the pH after ten minutes of homogenization using a pH meter or using pH paper to get the idea of the pH interval.

The humidity is estimated through the oven: the compost sifted using a sieve with a pore diameter of 20 mm, then dried in an oven at 105°C for at least 48 hours. The mass is weighed before (M1) and after passing through the oven (M2). The humidity rate (H%) is calculated by the following formula:

(1)

In order to assess the quality of water to be watered on a windrow, a quantity of substrate is taken, then weighed (m1) and packaged, then put back into the windrow to a depth of approximately 50 cm. After a few hours or days everything is removed from the windrow then reweighed (m2) again. The rate of water lost (m%) is evaluated by the following formula:

(2)

Then the quantity of water to add to the windrow (Ma) is calculated from the initial mass of the windrow (M0) by the following formula:

(3)

Organic matter is determined from organic carbon by the Walkley and Black method described by Nelson and Sommers. The principle is based on the oxidation of carbon by potassium dichromate (K₂Cr₂O₇) in a strongly acidic medium (H₂SO₄). Then we calculate the organic carbon by the following formula:

(4)

P is the test portion of the sample (approximately 1g)

The ash rate is calculated after calcination of the sample. It is determined on the dry matter: in fact, 5g of sample of the substrate or compost is placed in the oven at 105°C until the weight of the dry matter has stabilized. After obtaining the dry material, all the crucibles and their contents are brought to a muffle furnace at 550°C for approximately 8 hours. These crucibles are then weighed after being cooled in a desiccator. The calculation of the ash rate is then done as follows:

(5)

With: P0 = weight of the empty crucible; P1 = weight of the crucible + sample dried in an oven at 105°C and P2 = weight of the crucible + calcined residue

NTK nitrogen, which is the sum of ammonia nitrogen and organic nitrogen, was determined using the Kjeldahl method:

(6)

Where: TB = volume of sodium hydroxide used for the determination of the blank, TE = volume of sodium hydroxide used for the determination of the sample, P = test portion.

Assimilable phosphorus is an important fertilizing element for plants. Knowing its composition in the compost is important. The principle of its dosage is based on the fact that phosphate ions form, in the presence of the phosphomolybdic reagent (or nitro-vanado-molybdic reagent), a yellow-colored phospho-vanado-molybdate complex measurable by atomic absorption spectrophotometry at 430 nm.

The concentrations of heavy metals in our work were carried out by the method of atomic absorption spectrometry (AAS) using an air/acetylene mixture (2500°C) or a nitrous oxide/acetylene mixture (3100°C). for refractory elements.

IRFT spectroscopy analysis was carried out on the dry sample of compost with potassium bromide (KBr), using a Perkin Elmer 1600 FTIR spectrophotometer at a frequency ranging from 400 cm^-1 to 4000 cm^-1 and with a speed of 16 nms^-1. A few drops of the mixture are placed on the tip of a spatula between plates made of KBr.

The formation of the windrows consisted of the successive deposits of compostable materials so as to obtain a well-organized pile whose dimensions are known. In order to study the influence of the size (length, width and height) of the windrow on the process and quality of the finished compost, five windrows of different sizes were made under the shed:

• A windrow 1 of 2.5m in length, 2m in width and 2m in height, corresponding to 10m³ and 2480 kg of starting compostable material (initial substrate);

• A windrow 2 of 2m in length, 1.5m in width and 1.5m in height, corresponding to 4.5m³) and 1324kg of compostable starting material;

• A windrow 3 of 1.5m long, 1m wide and 1m high, corresponding to 1.5m³ and 525kg of compostable starting material;

• A windrow 4 of 1m long, 0.5m wide and 0.5m high, corresponding to 0.125m³ and 78.5kg of initial compostable material;

• A windrow 5 of 0.5m in length, 0.25m in width and 0.25m in height, corresponding to 0.031m³ and 22.5kg of initial compostable material.

3. Results and Discussion

Waste characterization is a process which consists of sorting this waste into categories (fermentable or putrescible waste, paper and cardboard, textiles, plastics, glass, metals, unclassified combustibles, unclassified incombustibles, hazardous or special waste, composite materials and materials fine) or in subcategories; by following the MODECOM method (Method for Characterization of Household Waste) which is a standardized French method.

To carry out this characterization, we started from a quantity of 2500 kg of household waste collected in the different strategic points of the city of Sao in order to have a significant representativeness of each component. We obtained the following results (Figure 2):

From these results obtained, it appears that household waste in the city of Soa is made up mainly of biodegradable organic materials which represents on average 45.2% of the total quantity of raw waste (41.5% in dry season and 49% in rainy season). On the other hand, the raw waste of the city of Soa is poor in hazardous waste which represents only 0.8% of raw waste. These results are close to those achieved by ¹ which was 58.9% for organic matter and 0.2% for hazardous waste.

This significant composition of organic matter in raw waste testifies to the intense consumption of the population of Soa in food products and also knowing that this composition also depends on the season ¹. This high percentage of biodegradable organic waste during the rainy season could be explained by the abundant presence of food products during this period of fresh food as well as green waste which abounds in the trash cans. Putrescible organic matter consists mainly of food waste (67.5%) and green waste (32.5%).

Concerning compostable waste which consists of waste from the putrescible waste and paper-cardboard categories, which gives a percentage of compostable waste of approximately 52.9% of raw household waste. Of this compostable waste, paper and cardboard represent 14.7%, compared to 57.3% for food waste and 28% of green waste (Figure 3).

Compared to hazardous waste, their low presence (0.8%) could be explained by the fact that the sources of household waste, which are households, do not generate enough of it. This small quantity which is found in raw waste is generally the product of medical activities as well as various handling of household or hospital chemicals. This hazardous waste can also come from equipment or household appliances ⁹. Most of this hazardous waste from the raw waste of the city of Soa consists of batteries, sharp objects (razor blades, knives, syringes, needles, etc.) and other various chemical products (bottles containing or not pesticides).

Given this high presence of organic matter in the raw waste of the city of Soa and the low presence of hazardous waste, we can, a posteriori, judge that the household waste of the city of Soa can be valorized by the composting technique. By implementing a very effective sorting technique to eliminate these unwanted elements.

The fermentable fraction (biodegradable organic matter) of the sorted waste is piled up to form a windrow. Substrate parameters are shown in Table 1.

Temperature is the primary indicator of the composting process because it reflects the release of heat by microorganisms during their decomposition activities. The values of this temperature are recorded daily during the first month, every 2 days for the second month then every 5 days for the rest of the time until the end of composting. The values obtained allowed us to draw the curve in Figure 4. The temperature is taken in the middle, approximately 50cm from the top of the windrow.

The trend of all the curves shows that the maximum temperature increases with the size of the windrow because we observe that the curve of the Windrow1 has a maximum of 72.5 °C higher than that of the Windrow 2 of 68.9 °C, and that of windrow 3 (58.0 °C), then of Windrow 4 (34.2 °C) and finally of Windrow 5 (31.6 °C). This order can be explained by the fact that the larger the size of the windrow, the less the temperature is exchanged with the outside because when there is exchange with the external air, the temperature decreases ¹⁰ due to heat transfer phenomena, at this time, the activity of the microorganisms is more intense inside the larger windrow due to good heat conservation.

Between the 1st and 2nd day the temperature jumps are sudden (we leave from approximately 28°C to 68°C). We can understand by this effect that microorganisms reach the maximum level of their activities in just a few hours (24 hours).

The shape of the curves shows that the thermophilic phases between 60 and 75°C ¹¹ are reached in less than two (2) days. This phase lasted on average 9 days for windrow 1; 7 days for windrow 2; 6 days for windrow 3; there was no thermophilic phase for windrow 4 and 5. The number of days of the thermophilic phase is important in the hygienist process (destruction of pathogenic microorganisms) of the compost and also facilitates the decomposition of organic matter but for it to be effective, it must last at least 5 days ¹². It is during this phase that we note the exhaustion by microorganisms of the substrate whose activities result in the heat released around the physiological functioning of their organs, hence the increase in temperature.

We can also observe on the curves that the thermophilic phase is between the 2nd and the 70th day of composting with a decrease in the activities of the microorganisms, followed by the mesophilic phase between the 70th and the 108th and finally by the maturation phase between the 108th and 128th day. These results allow us to note that we obtained the young compost in 70 days and a mature compost at approximately 128 days which is not far from that carried out on household waste of the same type by ⁶ of 100 days in average. ¹⁰ showed that the temperatures of the analyzed composts have a maximum of 54 to 69 °C at 8 days, then decrease up to 180 days to reach 35 to 28 °C during the composting of sewage sludge and of green waste.

Along the same lines, ¹³ also showed that the temperature in a windrow compost can increase during the first days up to 70°C, then gradually decreases to reach a constant temperature close to ambient temperature.

We notice throughout the composting that the curve drops before rising suddenly, this can be explained by the fact that each time the windrow is turned over, we obtain a new small thermophilic phase. These small thermophilic phases follow one another but have lower values than the previous ones. The increase in temperature with each turning could come from the fact that by turning, we bring back the undecomposed materials from the outside of the windrow towards the inside, which again triggers the decomposition which is more intense inside. of the windrow ¹⁴. As the composting process continues, and several turnings have been carried out, the fermentable materials become exhausted, hence the reduction in temperature jumps.

It is determined each month using the Walkley and Black method ¹⁵. The results obtained are grouped in Table 2 and schematized in Figure 5.

We observe through Figure 5 that the quantity of organic matter decreases with time, it appears from the percentages of OM obtained that:

• In windrow 1, the starting substrate contains 57.5% OM, and the finished compost has 32.3%, a difference of 25.2%.

• In windrow 2, there is 52.8% OM in the starting substrate and 34.4% in the finished compost; a difference.of 18.4%.

• In windrow 3 we have 48.1% OM in the starting substrate and 36.5% in the finished compost, a difference of 11.6%.

• In windrow 4, we have 54.9% OM in the starting substrate and 50.2% in the final compost, a difference of 4.7%.

• In windrow 5, we have 53.3% OM in the starting substrate and 51.7% in the finished compost, a difference of 1.6%.

This difference in organic matter between the initial substrate and the finished compost resulting in its decomposition by microorganisms, this reduction, during the composting of organic matter, has often been reported and assimilated to the mineralization of organic matter. by microorganisms ¹⁶. This reduction in OM contents shows that carbon disappears in greater proportions than the total dry matter. Carbon loss can also be in the form of volatile fatty acids ⁶.

It appears that the larger the size of the windrow, the more intense the decomposition. This observation is in agreement with that of the temperature experienced previously. For good composting, this difference is between 10 and 40% ¹⁰; however, in windrows 4 and 5, this difference is negligible (4.7 and 1.6%), which confirms that decomposition is not effective. We can conclude that the optimum windrows in terms of organic matter are the windrows 1, 2 and 3.

The pH values are between 6 and 7 in the initial substrate and between 7 and 8 at the end of composting. The acidic pH of the substrates would justify the acidity of the fermentable materials of household waste in general ¹⁷. While the basic pH obtained at the end of composting justifies the decomposition of organic matter, the products formed are generally more alkaline ¹⁸.

The physicochemical elements that were analyzed on the compost constitute plant nutrients and elements involved in verifying the safety of the finished compost. These nutrients consist, in addition to organic matter, of potassium, assimilable phosphorus, cadmium and lead. these results are presented in Table 3.

Phosphorus is an element that ensures good root development and resistance to plant diseases. This value obtained of 44 g/100g of finished compost demonstrates the good mineralization of organic matter by microorganisms.

Potassium is a plant nutrient; it promotes flower development and fruit growth ¹⁹. Our compost contains 2.12 g of potassium per 100g of finished compost. So much so that its quantity is not obligatory in the finished compost but is very necessary to support the plant in the assimilation of element K.

Our compost contains trace of cadmium and 0.045 mg of lead per 100g. This quantity of lead is not exorbitant because it is below the threshold provided for by the standard. Lead is a heavy metal sometimes found in household waste because it is part of several components of household appliances. Their admissible value in compost defined by the French standard is between 3 and 41 mg/kg of compost. Our value obtained is acceptable because it is within the acceptable threshold, this would be justified by the fact that today households use less batteries and accumulators which are the main sources of cadmium in household waste.

The evolution of the absorbance of the Fourrier Transform Infra-Red spectrum of the substrate and the finished compost was carried out, the spectra obtained are presented in the following Figure 6 and Figure 7:

The finished compost and the initial substrate have slightly different IR spectra; however, they are partially superimposable with greater absorbances for the finished compost (Figure 6 and Figure 7). The absorption peak at 1650 cm^-1 (C=C aromatics, C=O amides, N-H amides, O=C quinones) is almost as well marked as the peak at 1430 cm^-1 (C-H fatty acids in particular). The significant presence of aliphatic compounds also appears at 2925 cm^-1 and 2850 cm^-1, and probably at 3400 cm^-1 (O-H) ^{4, 5, 8}. These composts are rich in polysaccharides (C-O at 1030 cm^-1). The intensity of the absorption band at 1030 cm^-1 compared to 1430 cm^-1 shows the degradation of polysaccharides ⁸.

The presence of aliphatic compounds is characterized by the significant band around 2900cm^-1 and at 3620 cm^-1 indicator of the O-H phenol functional group. The fairly clear distinction of the peaks at 2925 cm^-1 (asymmetric-CH₂) and 2850 cm^-1 (symmetric-CH₂) shows a greater proportion of symmetric CH₂. The spectrum characterized by a broad mass between 3550 cm^-1 and 3000 cm-1 reflects the diversity of compounds present in the waste; The band at 1050 cm^-1 is mainly due to the presence of cellulose and hemicellulose. The absorption band at 1510 cm-1 (aromatic C=C) characteristic of lignin. The peaks between 1100 and 1040 cm^-1 would be due to polysaccharides (C-O, C-C), in particular cellulose ^{4, 8}. A significant peak is observed at 1317 cm-1 which may correspond to the presence of lignin (C-O of phenols).

The peak around 950 cm^-1 which was clearly visible for the initial substrate, is less visible in the finished compost, which reflects the degradation of numerous aliphatic compounds, but also phenolic compounds and the degradation of polysaccharides. ²⁰ showed that when a compost reaches a certain level of stability, it becomes a homogeneous mixture whose characteristics are independent of the nature of the composted waste.

4. Conclusion

The general objective of our work was to determine the optimum values of the essential parameters of the composting of household waste from the city of Soa in order to obtain quality compost that can be used in agriculture as a soil amendment. Our study focused on determining the ratio of compostable waste in household waste in the city of Soa, defining the windrow formation protocol on the site, obtaining the optimum values of the substrate formula ( the organic matter content, the C/N ratio, the particle size, the humidity and the pH) intended for composting, and finally the laboratory verification of the operational physico-chemical parameters (the organic matter content, the temperature, humidity and pH) and the compost obtained (potassium, phosphorus, sodium) during the composting process without forgetting safety elements such as heavy metals (lead and cadmium). We can say that our objective has been achieved because we have obtained the optimum parameters which allow us to optimally compost the household waste of the city of Soa.

ACKNOWLEDGMENT

The authors thank laboratory of the Ministry of Agriculture and Rural Development (MINADER)-Cameroon for carried out in the analyzes.

Conflicts of Interest

The authors declare no conflict of interest or personal relationship that could have appeared to influence the work of this article

Data Availability

The experimental data used to support the findings is included within the article.

References