Key findings:

Municipal solid waste (MSW) generation is on the rise globally, posing serious ecological and health risks. Composting, a biological process, offers a sustainable solution by converting organic waste into biofertilizer. The process involves various phases driven by microorganisms. Compost has diverse applications, particularly in agriculture, promoting organic farming and environmental sustainability.

What is known and what is new?

This study emphasizes the escalating issue of MSW generation and the urgent need for effective management strategies. It highlights composting as a viable solution, detailing its microbiological phases and factors affecting compost quality. The shift towards organic farming and the increasing demand for organic products underscore the relevance of composting in modern waste management practices, emphasizing its potential benefits to the environment and society.

What is the implication, and what should change now?

The implication of this study is the urgent need for widespread adoption of composting practices to manage the escalating MSW generation. There is a critical need for enhanced waste management policies and infrastructure to support composting on a large scale. Individuals, communities, and governments should prioritize composting as a sustainable solution to reduce waste, protect the environment, and promote organic farming.

Municipal solid wastes (MSW) represent one of the global environmental challenges and a major source of environmental pollution [1]. The increase in population per capital income, with the development of modern societies and urbanization has led to an increase in the variety and amount of municipal solid wastes [2]. The latest World Bank report estimated an average municipal solid waste generation rate per capital at approximately 1.2kg per person per day and that by 2025, this may rise to 1.42kg per person per day, reaching 2.2 billion tonne of waste per year on a global scale [3,4].

The disposal and management of municipal solid wastes has been a major challenge especially for developing countries. It is well known that in Europe, the most prevalent form of waste management consists of land filling, 60.57% in 2012 and 57.70% in 2014 [5]. In recent years, attention has been shifted from land filling method (-10.31%) to other methods such as incineration (11.78%) and composting (22.43%) [5]. However, in Nigeria, waste disposal through open dumping by the roadside, water ways and burning are common practices. These practices, apart from being unpleasant to the sight, are harmful to human health and the environment.

Composting is the transformation of various biodegradable wastes into products that can be used safely and beneficially as bio-fertilizers and soil amendment [6]). Since compost can be applied as a fertilizer or as a material for production of plant nursery substrates, the production of compost and compost quality has been the subject of great interest to researchers [5]. Due to the presence of plant growth-promoting organisms and nutrient rich of compost, the use of compost in agriculture increases agricultural productivity and organic matter content of soil [7,8]. This to a larger extent helps to ensure food security. Apart from the use of compost as organic fertilizer, compost is applicable in bioremediation (Ventorino et al., 2019) [9], plant disease control (Pane et al., 2019) [10], weed control (Coelho et al., 2019) [11], pollution prevention (Uyizeye, 2019) [12], erosion control, landscaping and wetland restoration. The use of compost as organic fertilizer reduces the cost, health and environmental risk associated with chemical fertilizer.

Composting occurs through the activities of mixed microbial community notably bacteria, actinomycetes, molds and yeasts [13]. Of all these microorganisms, bacteria and fungi have been reported as being predominantly present during composting [14] Adequate environmental conditions of temperature, moisture and aeration must also be maintained for successful composting operation. For the final product to be useful, it must be rich in humus and contain in optimum amount macro elements such as nitrogen, phosphorus, potassium as well as some trace elements.

Although farmers imbibe the use of chemical fertilizers because of its strong and rapid effect on plant growth, the nutritional quality of these plants are less than the organically grown plants [15]. Moreover, the chemical content of these chemical fertilizers are toxic in nature and may accumulate in human body (Chandini et al., 2019) [16], leading to health implications. This could be the reason especially in developed countries; people are now choosing organic food as a substitute to chemically synthesized products. Thus, in line with the Sustainable Development Goal 12 (SDGs) of Responsible Consumption and Production to substantially reduce waste generation through prevention, reduction, recycling and reuse by 2030, composting is seen as a solution to properly manage waste to promote good health and wellbeing through sustainable practices [17]. This paper therefore reviews composting method and its relevance to nature and the environment as well as future prospects of composting.

Categories of Municipal Solid Wastes

Municipal solid waste (MSW) is a pool of various wastes by towns and cities from different types of household activities. All solid wastes generated within a municipality’s territory, regardless of its physical and chemical nature and source of generation is classified as municipal solid waste [18]. Residential houses, hotels, markets, educational institutions among others are sources of municipal solid waste. These wastes may include but not limited to: food wastes, yard wastes, paper, cardboard, textile rags, leather, lignocellulose materials, waste oils, glass, metals and hazardous wastes.

On the basis of biodegradability, solid wastes can be categorized into biodegradable, moderately degradable and non-biodegradable [13]. Oxygenic and anoxygenic microbes are involved in decomposition of biodegradable wastes as well as slowly degradable wastes. Biodegradable wastes may include among others; yard wastes, food wastes and some agricultural wastes such as cow dungs, poultry droppings, while slowly degradable wastes may include among others, wood and cardboards [19]. Non-biodegradable are recalcitrant and persist for long periods. Such wastes include among others, wastes from mining operations, polyethene bags, leathers, plastics [20]. Thus in order to enhance compost operations and achieve the desired compost quality, waste separation must be carried out to remove non-biodegradable components.

Composting begins with waste collection, separation, processing and transformation. During the process of composting, the organic waste is transferred into a pile and allowed for microbial transformation under favourable environmental conditions of oxygen, temperature and moisture. CO₂ is released as a by-product [21]. Different composting methods exist with each having their merits and demerits. Thus, the composting method to be adopted depends on the one that best suits the goal of the researcher and the type of material to be composted. In vessel composting, the composting materials are placed in a vessel or container which uses forced aeration and mechanical turning to enhance the composting process. This method is labour and capital intensive [22]. The organic waste is placed on a long narrow windrow which should be turned on a regular basis for aeration in windrow composting. Although this method is costly, it is a very quick way of composting [22]. In vermicomposting, the earthworm is used for the composting process. They feed on a variety of organic wastes and their excreta called “casting” is rich in all kinds of nutrients required for soil fertility and plant growth [19]. Static composting is an old method whereby the organic waste is put into piles and allowed for passive aeration. This method is time consuming and results in slow degradation of organic wastes (Gonawala and Jardosh, 2018). Sheet composting involves spreading the organic waste as mulch onto the soil and tilled intermittently with a hoe, garden fork or spade for the waste to decay. This method is economical and a simple way of composting [15]. Indian Bangalore composting is recommended for the composting of night soil and refuse. This method was developed in Bangalore, India [23].

In a study involving the rate of degradation in composting, it was reported that organic waste containing aliphatic or polysaccharide compounds degrades faster than that containing aromatic compounds [24]. It is noteworthy that humic acid is an important parameter for determining mature compost. The water soluble carbon comprising sugars, phenolic substances, amino acids, peptides, hemicellulose and other easily biodegradable materials are the most common biological parameter which suggests compost maturity [25]. Galitskaya et al. (2017) [14] reported that there are chemical and biological changes during composting as a result of microbial succession, which is dependent on temperature changes. Hafeez et al. (2018) [26] reported that fungi and bacteria are predominantly present during composting.

Microbiology of Composting

The microbiology of composting or the composting process is divided into three phases; the 1^st mesophilic phase, the thermophilic phase and the 2^nd mesophilic phase known as the cooling or maturation phase as shown below (Fig. 1). Different microorganisms predominate at different phases of composting. The initial composition of organic matter, moisture content, quantity and composition of the microbial community, determines the duration of each stage [27]. In the first mesophilic stage, mesophilic bacteria, fungi and actinomycetes are actively present. They grow at temperature of 15 to 45^oC with an optimum growth range of 30 to 39^oC, utilizing the carbon-rich substrates at the initial stage of composting [28]. The microbial activities during the decomposition process will result in an increase in temperature and a decrease in pH, which could stimulate the proliferation of fungi that breakdown the readily digestible carbon sources into organic acids, leading to a drop in pH of the compost [29]. Strong organic matter component of the composting materials is broken down by fungi such as molds and yeast; this enables bacteria to synergistically continue the decomposition process. Thus many compounds such as sugars, amino acids and other simple organic matter are rapidly degraded in this phase. Moreover, the transformation of organic matter results in rapid increase in temperature and the composting will undergo the second phase known as the thermophilic phase [30]. The thermophilic phase is the stage where most of the degradation takes place. At this stage, fats, cellulose, hemicellulose and some lignin are degraded by thermophilic bacteria and fungi. Mesophiles are succeeded by thermophiles such as actinomycetes, which grow at optimum temperature between 40 to 80^oC [31]. As the carbon source gets exhausted towards the end of the thermophilic stage, the temperature gradually decreases as the composting enters the cooling or maturation phase [32]. At the second mesophilic or cooling phase, the amount of substrate for growth becomes limiting, resulting to a decline in microbial activity. As a result of the depletion of nutrients and low temperature characteristics of this final maturation stage, the activity of thermophilic microorganisms ceases and mesophilic microorganisms recolonize the organic matter and degrade the remaining organic materials. At this stage, the compost is mature and the microflora present plays a crucial role in compost maturity and the inhibition of plant diseases because of the metabolism of phytotoxic compounds [17]. Maaroufi et al. (2019) [33] reported that few fungi species can completely degrade lignin which actinomycetes and bacteria are not capable of degrading.

Fig.1. Different stages of composting in relation with temperature changes

Factors affecting the quality of Compost

For optimum application of the compost as a fertilizer for plant growth, the quality of the final product is of paramount importance, and is measured on the basis of the biodegradability of organic matter [34]. The pH, carbon/nitrogen (C/N) ratio, organic matter content, humification ratio as well as cation exchange capacity (CEC) are key parameters used to determine the quality of a stable compost [34]. Mature compost has a pH between 7 and 9, while an acidic pH is an indicator of immature compost [35]. It was reported that alkaline pH is most desirable for composting, while an acidic pH destroys the microorganisms involved in composting, thus slowing down the composting process [36,13]. Mature compost has C/N ratio in the range of 15 to 20, but a C/N ratio in the range of 10 to 15 is also regarded as stable compost [37]. However, Gonawale and Jardosh (2018) reported that a C/N ratio of 30 is optimum for composting operation. Higher C/N ratio hinders the activities of microorganisms which will invariably slow the decomposition process [38]. Since nitrifying bacteria convert ammonia to nitrate (NO₃^-), they cause a reduction in ammonium (NH₄⁺) content and increase in NO₃^- during composting. Thus, monitoring the level of NO₃^- and NH₄⁺ from the beginning to the end of composting could be a useful parameter in determining mature compost [34]. A mature compost is also expected to have a humic/fulvic acid (HA/FA) ratio greater than 1 but less than 3 (Wichuk and McCartney, 2013) [39] . Cation exchange capacity (CEC) is expected to increase during composting. Thus CEC greater than 60 meq.100g^-1of organic matter is an indicator of mature compost [34]. Moreover, mature compost rather than having the smell of ammonia, has a pleasant odor. It also has a constant and low temperature and appears dark in color, a feature that distinguishes mature compost from immature or raw organic waste [40].

Other factors that may affect the quality of the compost may include but not limited to oxygen, moisture content, particle size as well as raw material texture. Oxygen is very important during composting. When oxygen is in short supply, there is a tendency that oxygen may be used up during microbial decomposition of composting materials. As a result, an anaerobic environment may be created, leading to the generation of foul odor as well as gases such as methane, CO₂, and ammonia [22]

Moisture is very important for microbial activities to take place. Moisture content in the range of 40 to 60% should be maintained during composting process [36]. Moisture content beyond that range may lead to poor oxygen diffusion which will in turn slow down the metabolic activities of microorganisms. When the moisture content is too low, it will not only affect the pathogens in the compost but will inhibit the beneficial microorganisms which act as a starter culture during composting [17]. Chennaou et al. (2018) [41] reported a decrease in moisture content as the composting process progressed.

The particle size of the materials in the range of 1 to 2 inches in diameter is usually best for composting operations [13]. This is because it brings about a higher surface area which helps to enhance microbial activities thereby speeding up the composting process. The rate at which aerobic microorganisms decomposes organic matter increases as the particle size decreases. Nevertheless, very small particles may decrease O₂ diffusion inside the pile, thus slowing down the composting period [42].

Raw materials texture also influences the composting process. Hard/tough textured or high lignin degradable organic materials usually take longer time to compost than soft textured organic materials [13]. Moreover, the presence of physical barriers like thorny leaves may cause the composting process to take longer time than expected.

Relevance of Compost

Compost is extremely important to nature and the environment. The use of compost as organic fertilizer in agriculture is a welcome development because of the present-day campaign against the use of synthetic fertilizer [13]. Compost is a product of microbial decomposition of organic waste; as such it is relatively cheap compared to synthetic fertilizer and pesticides. In addition, the use of compost in agriculture is cost effective because it will reduce or eliminate the dependence on expensive chemical fertilizer [43]. Application of compost in agriculture can aid in improving soil fertility as well as promoting plant growth, due to the presence of plant-growth-promoting bacteria. It was reported that the organic matter of the compost contains biocontrol microorganisms that suppress phytopathogens, thereby protecting plants from diseases (Somerville et al., 2020) [44], thus ensuring food security and improved agricultural yield. Lin et al. (2014) [45] reported that Bacillus sp. present in compost proved very effective in controlling plant wilt and damping-off disease. Surface application of compost as organic soil amendment was reported to be very effective in improving water holding capacity of the soil, maintain soil structure and aggregate stability [22]. Therefore, compost is very effective in controlling soil erosion due to the presence of humus that binds the soil and as well act as soil glue, holding the soil constituents together [46]. Compost is also useful in bioremediation of soil polluted with heavy metals through adsorption, precipitation, complexation and redox reactions [47,48]. Compost is also rich in microorganisms that are capable of degrading hydrocarbons, solvents, wood preserving chemicals, pesticides, petroleum products, explosives, among others, thereby reducing the toxicity of the chemical pollutants [49]. Composting is an eco-friendly way of transforming solid wastes that could have been dumped into water bodies, road sides or could have been burned, into various beneficial purposes [50,22].

Future Prospects of Composting

The opportunities for the future of compost market are abound and the most common are in agriculture, landscaping, home gardening, horticulture and even construction industries [17]. Waliczek et al., (2020) [51] reported an estimate of $ 9.2 billion with a Compound Annual Growth Rate (CAGR) of 6.8% beginning 2019 is expected to be attained in 2024 in the global compost market. In line with the SDGs, most developed countries are targeting to minimize the adverse effect of waste generation through maximizing waste management practices that will be of benefit to humanity and to the environment [52].

There is an increasing demand for organic products probably due to growing awareness of the effects of chemical fertilizers and pesticides to humans and the environment. This has resulted in a shift in preference of compost as organic fertilizer, which is expected to be capable of providing optimum growth in plants for the agricultural sector. However, the ability of compost materials to recycle nutrients back into the soil and minimize organic waste makes it a multifunctional soil improver. Moreover, composting process is in accordance with the concept of circular economy of transforming waste and resources for better use in order to minimize wastage [17]. Value is being added into the output through circular economy for as long as possible.

It is noteworthy that many countries all over the world have set up rules guiding the production of compost. For instance, the United Kingdom, Canada and Australia have all enacted regulations guiding the compost production and use in order to ensure the quality of the compost material [52]. MSW compost is undoubtedly a promising material with a significant amount of benefits to the environment, economy and society. With the advent of modern technologies, knowledge of microbial diversity will assist in optimizing and developing high quality compost material in accordance with global requirements.

MSW composting is no doubt an up-and-coming technology with enormous benefits to the environment, economy and the society. It is an economically and eco-friendly way of turning organic waste into several beneficial uses. As the world is moving towards organic foods in preference to chemically produced products due to increasing awareness of the effects of chemical fertilizer and pesticides on human health and the environment, organic fertilizer compost is seen as a gate way to achieving this desire. However, more awareness still needs to be created on the benefits of compost to humanity for its full acceptance and implementation by farmers.

The authors declare that they have no conflict of interest.

No funding sources 

The study was approved by the Institutional Ethics Committee of University of Kassala

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