Urban solid waste management is one of the most pressing and serious environmental problems facing urban governments in developing countries. This challenge will become even more severe in the future given the trends of rapid urbanization and the growth in the urban population (Arafat et al. 2015; Ferronato & Torrett, 2019).
Improper collection and disposal of waste poses a serious health risk to the population causing a clear environmental degradation in most cities of the developing world (Meylan et al., 2018). With increasing public pressure and environmental legislation, waste experts are being called in to develop more sustainable methods of dealing with municipal waste (Abbasi & Gajalakshmi, 2015; Abu Hajar et al., 2020). One of the steps in improving the current situation of solid waste is to enhance resource recovery activities. Recycling of inorganic materials from municipal solid waste is often well developed by the activities of the informal sector (Aparcana, 2017). However, the reuse of organic waste materials, which often contributes more than 50% of the total amount of waste, is still limited but has an interesting recovery (Ardolino et al., 2020). Combined approaches to reduce reliance on landfills as a method of disposal and biological treatment is increasingly becoming a standard requirement for the vast majority of biodegradable waste (Bhatia et al., 2018).
Among all management options for organic waste, composting is the most approved method (Ardhaoui et al., 2019). It is an effective strategy to divert Solid Waste (SW) from landfills and improve the heating value of feedstock in case of energy recovery (Carabassa et al., 2020). Previous studies confirmed that composting reduces the volume of organic materials by more than 30% (Awasthi et al., 2020) and converts waste into a hygienic and valuable product ( Chaher et al., 2020b; ChenYu et al., 2018).
Availability and variety of raw input materials, less of prerequisites, ease of technology, simplicity of concept, the environment and socio-economic benefits create a great opportunity in Tunisia to produce compost from organic waste (Aydi, 2015; Mahjoub et al., 2020). However, the opportunity to use the different types of organic waste as compost requires scientific studies that endorse it to guide users concerning the aspects behind the better management of the composting operation. In conjunction with the quantitative and life cycle-based evaluations, a comprehensive technical-scientific view of bio-waste composting should also include increasing the currently limited knowledge of the process performance in terms of monitoring and controlling the crucial factors affecting the efficiency of the composting units (Asadu et al., 2019; Chaher et al., 2020a). In this context, Moisture content (MC) is a critical factor in the composting process. The optimal MC for effective composting depends on the specific physicochemical properties and biological features of the materials to be composted (Kim et al., 2016; Xu et al., 2020). However, the optimum MC required for biological activity during composting is between 50% and 60% ; Chaher et al., 2020b; Hemidat et al., 2018).
Several studies confirmed that moisture content has a remarkable effect on the composting process (Al-Bataina et al., 2016; Barthod et al., 2018; Du et al., 2018; Tibu et al., 2019). It influences the oxygen uptake rate, free air space, microbial activity and the temperature of the process. During composting, the MC is vital for the distribution of soluble nutrients needed for the microbial metabolic activity (Fan et al.,2019). According to Xu et al. (2020) loss of moisture during the composting process can be counted as a strong indication of the decomposition rate. Very low MC could cause early dehydration during composting and that may hinder the biological process and slow down microbial activity under the low moisture range (Franke-Whittle et al., 2014).
It is well known that compost production is a very water-consuming process, as ensuring the required level of moisture requires large quantities of water. Many studies have claimed that every ton of ready-made compost needs one cubic meter of water, and this is a significant amount that should be taken into account when planning such projects, especially in countries that suffer from water scarcity (Bacenetti, 2020; ChenYu et al., 2018; Tibu et al., 2019;.Tunisia is one of those countries; it is considered one of the countries in the world with the scarcest water resources (Abdulrahman, 2018; Ardhaoui et al., 2019). Tunisia is a water stressed country with per capita renewable water availability of 486 m3—well below the average of 1200 m³/capita for the Middle East and North Africa Region (MENA) regions (I et al., 2013). Indeed, the rapidly increasing population began to use more water than the country could provide (Mahjoub et al., 2020).
Therefore, there is an urgent need to seek an alternative to conventional water resources to be used in aerobic composting to ensure the required level of moisture content for an efficient composting process. To reduce the use of conventional water resources during the composting treatment, the research work aims to exploit an unconventional one; digestate produced from food waste (FW) anaerobic digesters to feed FW aerobic digesters. This option might be of considerable value by providing high-acclimated microbial diversity as well as micro- and macro-nutrients to enhance the process performance and the end product quality. A further objective is to examine its effect on FW in-vessel composting treatment as a moisturizing agent (MA).