Abstract
The aim of the study was to evaluate the possibility of using the process of dark fermentation to convert kitchen waste into valuable volatile fatty acids in a semi-continuous process at different values of the organic loading rate (2.5 and 5.0 gVS/(L × d)) and hydraulic retention time (5 and 10 d) using anaerobic mixed microbial consortia. The experiments were performed in a bioreactor of working volume 8L with pH control. The maximum volatile fatty acids yield in a steady state (22.3 g/L) was achieved at the organic loading rate of 5.0 gVS/(L × d) and HRT of 10 days. The main products of dark fermentation were acetic and butyric acids, constituting, respectively, 35.2–47.7% and 24.1–30.0% of all identified volatile fatty acids. Additionally, at the beginning of the fermentation and in a steady-state condition, the microbial population analysis (16S rDNA) of the fermentation mixture with the most effective volatile fatty acids generation has been performed to monitor the DF microflora development. The dominant microorganisms at a phylum level in a steady state were Firmicutes (44.9%) and Bacteroidetes (30.1%), which indicate the main role of those phyla in the volatile fatty acids synthesis.
Highlights
The 21st century consumer-oriented lifestyle coupled with the constant growth of the human population has caused a vast increase in municipal solid waste (MSW) production globally
The biological process was considered to be at a steady-state condition when the volatile fatty acids (VFAs) were detected at a similar level, and the bioreactor was operated during 3 hydraulic retention time (HRT)
In the processes run at 10 days HRT, a steady state appeared after 12 days of fermentation, both at the organic loading rate (OLR) of 2.5 and 5.0 gVS/(L × d)
Summary
The 21st century consumer-oriented lifestyle coupled with the constant growth of the human population has caused a vast increase in municipal solid waste (MSW) production globally. According to the World Bank data, MSW generation worldwide is estimated to be about 1.3 billion tonnes per year, and it is expected to reach 2.2 billion tonnes/year by year 2025 (World Bank, 2012). MSW contains approximately 30% w/w (weight/weight) food waste (FW) [1]. The FW is a fraction of the bio-waste, which contains left-over meals and discarded foods from the stages of production, processing, retailing, and consumption. Incineration is energy demanding and unstable due to the high moisture content of the FW [4]. The equivalent of FW in a household environment is kitchen waste (KW) [5], which may include vegetables, fruits, as well as cooked and processed food, and is characterized by a high moisture content and low pH [6]. KW contains large amounts of available organic matter such as carbohydrates, proteins, and lipids, which form a valuable substrate for fermentation processes
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