Digestate Post-treatment and Upcycling : Unconventional Moisturizing Agent for Food Waste In-Vessel Composting

Nowadays, Tunisia faces challenging environmental and energy issues which relate mainly to the implementation of an appropriate solid waste management system capable of dealing with the high production of biowaste on the one hand, and the increased need for water and energy resources on the other. Therefore, the current study is intended to develop a closed cycle technical concept treating mainly food waste (FW) through combined biological processes. In this approach, FW anaerobic digestion (AD) was destined to provide a valuable input material for FW in vessel-composting by exploiting the produced digestates. To this end, the gathered AD-effluents of three systems (D1, D2, D3) were entirely analyzed to select, as a further step, the most suitable one to be subject of the suggested post-treatment. Hence, several physiochemical parameters were examined as key performance indicators. It mainly consisted of moisture content, pH, C:N ratio and heavy metals contents. The current findings depicted that the generated digestates were characterized by a high moisture content which inspired its utilization as an unconventional moisturizing agent (MA) aiming to reduce the fresh water consumption during the composting process. Thus, two experimental setups were carried out to evaluate the effect of the selected digestate on FW-in-vessel composting process performance, as a non-standard MA inserted to A2, comparing to the unamended one (A1) which was moistened by fresh water. In fact, basing on the above-mentioned criteria, the relatively high C:N ratio (of around 15), as well as the relatively significant rate of the required macro- and micro-nutrients promoted the exploitation of biochar-rich digestate (D3) as not only an efficient unconventional MA, but also as a composting process booster. Furthermore, the results revealed that the addition of D3 improved significantly the composting process performance in terms of steering parameters including the attained temperature, MC and pH. When it comes to the maturity and stability assessment, the decreased profiles of C:N ratio, the nitrification index (NI), as well as the respiration activity (AT4) ascertained the fulfilment of the required conditions for both A1 and A2 to produce stable and mature end-products. However, regarding the compost quality, the examined concentrations of heavy metals met the requirement set by German standards and attested that both A1 and A2 generated highly-qualified products, rated as class B and A, respectively. Furthermore, it is noteworthy to mention that even the AD-effluents which were classified as an “inappropriate substrate” to be inserted to the composters, met the required criteria to be applied directly to land as a highly-qualified liquid biofertilizer. This latter ascertained, additionally, the efficiency and feasibility of the suggested closed cycle for a sustainable FW management. Graphic Abstract: [Figure not available: see fulltext.]