## 1 Introduction  
Energy accessibility and waste management are some of the most significant challenges developing countries face, including South Africa [1]. Energy demands exceed the existing energy supply due to the continual increase in population globally. Regularly used energy resources, such as oil, coal, and natural gas, are diminishing and they emit greenhouse gases that contribute to climate change [2]. Accordingly, the research focus in many countries has shifted to finding and implementing efficient and green alternatives, such as renewable resources, as solutions to these conventional energy sources, which are detrimental and waning [3]. Examples of renewable energy resources are those that can be replenished naturally, such as solar photovoltaic and wind power. However, the energy demands of a constantly growing population using coal-powered stations cannot be supplemented with only solar and wind energy as these are predominantly weather-dependent [4]. Waste generation and management have been dealt with in many ways, one of which is using it as a resource for producing valuable products, such as biogas, which contributes to the circular economy [5]. Further, this is in line with the UN's Sustainable Development Goals 5, 7, 8, 9, and 13. Biogas can be produced from many substrates, including organic solid waste, wastewater/effluent, etc., as substrates/feedstock in biodgestion. Feedback is any substrate that can be converted to biogas/methane through anaerobic bacteria. These range from solid wastes to readily degradable wastewater and sludge. This waste must contain a substantial amount of organic matter, which is then converted into biogas [6]. Conventionally, anaerobic digestion is a practical way to treat animal and agricultural waste, macroalgae, and sewage sludge from aerobic wastewater treatment plants [7, 8, 9]. However, a change happened after 1970, as soon as environmental consciousness grew in connection with the demand for renewable energy reforms and new waste-management strategies [10]. Industrial and municipal waste has also been identified as eligible for anaerobic digestion, as shown in Figure 1. This resource is one of the sustainable and viable routes to help many developing countries manage massive amounts of the waste left unattended in landfills and discharged into water streams and oceans [11]. This waste negatively impacts the environment and, consequently, life in such an environment, i.e., human and animal lives [12, 13]. Clean, renewable energy in the form of biogas can be achieved through anaerobic digestion (AD) of waste matter [15]. AD comprises a series of biochemical reactions that result in the production of biogas, a mixture of methane, carbon dioxide, and negligible traces of other elements. Different waste matter, including the organic part of municipal solid waste, industrial waste, wastewater from manufacturing processing, and agricultural waste produced from livestock and crop production, is used in the AD process. Notably, the biogas formation process produces some by-products, such as slurry (digestate), which provides an added benefit, since the spent waste (slurry) can be used as organic compost by farmers due to its nutrient composition [16]. However, it is crucial to check the safety of this digestate for its use as a fertilizer since there can be potential incidences of heavy metals and pathogenic bacteria [17]. In addition to the environmental benefits of waste management, there are also socio-economic rewards. It is envisaged that countries would raise their annual turnover for different sectors, generate thousands of jobs, and save billions of dollars a year after fully implementing waste-management solutions [18]. Since research has shown that the digestion of a single substrate produces less biogas than a co-digestion of two or more substrates, municipal solid waste and sugarcane processing wastewater are deemed to be good co-substrates for biogas production. Fur  
Figure 1: Sources of suitable substrates for anaerobic digestion [14].