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

Nour El Houda Chaher,Abdallah Nassour,Michael Nelles,Moktar Hamdi

doi:10.1007/s12649-021-01565-0

Abstract

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

Highlights

The inappropriate collection and disposal of organic wastes creates serious concerns in terms of environmental degradation, health risks and socioeconomic problems which conducted to an unavoidable instability, in developing countries [1]
As the experimental work was undertaken in Germany as part of the framework of «Renew_Value project» destined to be applicable in Tunisia, the selection of food waste (FW) was rigorous in terms of being as similar as possible in terms of its composition to FW generated in the study area
Several parameters were considered as efficient indicators, to check, on one hand, whether the collected anaerobic digestion (AD)-effluents could influence the effectiveness of the compost applied to agricultural land, and on the other hand, to assess the feasibility of digestate post-treatment via composting

Summary

Introduction

The inappropriate collection and disposal of organic wastes creates serious concerns in terms of environmental degradation, health risks and socioeconomic problems which conducted to an unavoidable instability, in developing countries [1]. Increasing public pressure, as well as the consequent environmental legislation, has driven stakeholders, decision makers and experts to introduce stronger waste management strategies [2]. These strategies are intended to deal with the environmental and socioeconomic conditions of the affected countries, and to ensure the efficient recovery of significant volumes of biowaste [3], [4]. Biological treatments have come to be considered to be one of the most suitable options for handling organic residues. Some scientific works have revealed that anaerobic or aerobic co-digestion might, to a certain extent, overcome the outlined issues in term of biological treatment performance, and simultaneously enhance the effectiveness of worldwide organic waste management [9], [10]

Methods

Results

Conclusion