One of the most common global approaches to municipal solid waste management is landfilling (Awasthi et al., 2015). Landfilling is however challenged with secondary pollutants such as leachate, odorous compounds, and greenhouse gases mainly from organic fractions (Zeng et al., 2010). A sustainable approach that aims to close the loop by reducing the quantity of municipal solid waste and more importantly, the organic fractions diposed of at the landfill is desired. The concept of circular economy is one of such sustainable approaches (Ivanova et. al., 2019). Circular economy is a regenerative system in which resource input and waste, emission, and energy leakage are minimised by slowing, closing, and narrowing material and energy loops by providing a direction for material resource recycling and reuse (Geissdoerfer et. al., 2017). Composting/co-composting is a circular economy approach that could be applied to the organic fractions of municipal solid waste to reduce the burden of landfilling and provide a useful resource.
Composting and co-composting accelerate the degradation of organic matter by microorganisms. During the composting and co-composting process and under controlled conditions such as temperature and aeration the organic matter undergoes a thermophilic phase sanitising it and eliminating pathogenic microorganisms (Lung et al., 2001). Composting and co-composting are green methods, economical, sustainable, and relatively have a minimal environmental impact (Liu et al., 2011) compared to other methods of MOW management.
It is estimated that 44% of MOWs are generated globally, but only 5.5% arecomposted (World Bank, 2019). Developing economies like Ghana, for example, have about 60% of their waste being organic (Caucci, 2020). Although feedstock for composting is readily available, one feedstock does not yield an excellent compost product as compared to co-composting, which combines two or more feedstocks (Cooperband, (2000); Tognetti et al., (2007); Hubbe et al., (2010); Gupta, (2013); Kadir, Jamaludin and Azhari, (2016) and Giagnoni et al., (2020)). Relatively low levels of organic matter have been reported in some organic composts due to either low values or the complete absence of a carbon-based bulking agent (Garcı´a et al., 1991; Pascual et al., 1997, Tognetti et al., 2005)). Thus, an optimized combination of MOW feedstocks is critical to yielding high-quality compost products for use in the agrarian sector (He et al., 1995; Chefetz et al., 1996). Essentially, optimization balances the C:N ratios increases compost quality and nutritive value and makes the composting process more efficient. Consequently, to successfully co-compost MOW, feedstocks' physical and chemical characteristics must be investigated (Chai et al., 2013) to develop co-composting formulae and optimum mixing ratio models to yield high-end products (Gupta, 2013). These characteristics include pH, moisture content, C:N ratio, organic carbon and organic matter content, nutrients, and bulk density.
The quality of co-composted organic waste must be established empirically. Some known quality criteria are stability and maturity indices, pathogen reduction levels, organic matter content, heavy metals, trace and other available nutrients. Generally, compost quality depends on stability, soluble salts, pH particle size, the absence of weed seeds, heavy metals, phytotoxic compounds, undesirable objects, and product uniformity. Compost with particle sizes less than ½ inch in diameter, a soluble salt level less than 25 mmhos/cm, a pH between 6.0 and 7.8, a low respiration rate, no weed seeds are said to be of high quality (Rynk, 1992). Compost maturity and stability usually relate to the degree of completion of the composting process. Compost maturity refers to the degree of humification of the organic waste, and compost stability is the level of microbial activity. Both indicate the degree of organic matter decomposition and the phytotoxicity caused by inadequate composting process (Iwegbue, C. Egun and Emuh, 2006). Immature composts contain phytotoxins due to organic acids, which may suppress the germination of seeds, constrain root growth and limit crop yields (Butler et al., 2001). Generally, a lot of organic materials contain sufficient nutrient quantities for composting. Too much or too little nitrogen or carbon affects the whole composting procedure. Carbon helps microorganisms with energy and development, while nitrogen is responsible for protein provision and reproduction (Cochran et al., 1996). The balance of carbon and nitrogen influences the decomposition rate in the composting material (Sweeten and Auvermann, 1988). Compost has a relative stable source of nutrients, and these nutrients are made available to plants in a slow-release manner. On a pound -to-pound basis, huge volumes of nutrients are usually not available in compost compared to commercial fertilizers. Nevertheless, compost applied in greater quantities may have a substantial influence on nutrient availability (Council, 2008).
Although several studies on composting and co-composting has been done, a combination of organic matter, chicken manure and faecal sludge feedstocks is yet to be explored. This research therefore aims to explore co-composting these feedstocks which are readily avaible in Ghana and determine the appropriate mixing ratio for a high yield compost. The mixing ratio is based on organic municipal solid waste as the main compost feedstock. The other organic suggested materials (chicken manure and faecal sludge) are the bulking agents. The co-composted waste was evaluated using (1) the germination index to assess the maturity and phytotoxicity of compost products; (2) a self-heating test to evaluate compost stability as a measure of microbial activity (Bradley, 2012); and (3) a respiration test to determine compost maturity in terms of microbial metabolic activities and population (Hemidat and Nelles, 2018).