Abstract
Powdered activated carbon (AC) is commonly used as an effective additive to enhance anaerobic digestion (AD), but little is known about how the metabolic pathways resulting from adding AC change the microbial association network and enhance food waste treatment. In this work, the use of AC in an anaerobic digestion system for food waste was explored. Using bioinformatics analysis, taxonomic trees and the KEGG pathway analysis, changes in microbial network and biometabolic pathways were tracked. The overall effect of these changes were used to explain and validate improved digestion performance. The results showed that AC accelerated the decomposition of edible oil in food waste, enhancing the conversion of food waste to methane with the optimized dosage of 12 g AC per reactor. Specifically, when AC was added, the proponoate metabolic pathway that converts propanoic acid to acetic acid became more prominent, as measured by 16S rRNA in the microbial community. The other two metabolic pathways, Lipid Metabolism and Methane Metabolism, were also enhanced. Bioinformatics analysis revealed that AC promoted the proliferation of syntrophic microorganisms such as Methanosaeta and Geobacter, forming a highly intensive syntrophic microbial network.
Highlights
The increasing food waste (FW) generation around the world has increased the demand for reduction and effective utilization of FW in recent years[1]
In the initial 22 days of organic loading rates (OLRs) up to 3 g volatile suspended sludge (VS)·L−1·d−1, the average Cumulative methane yield (CMY) and specific methane production (SMP) in the five anaerobic digestion (AD) reactors were increased to 15.6 L and 0.32 L.g−1 VS, respectively, and all reactors operated stably with an average effluent pH of 7.4
When the OLR was increased to 6 g VS·L−1·d−1, the CMY in AD reactors with the addition of activated carbon (AC) greater than or equal to 12 g was maintained between 68.5 L and 71.2 L and SMP was kept between 0.36 L.g−1 VS and 0.37 L.g−1 VS with an average effluent pH of 7.3
Summary
The increasing food waste (FW) generation around the world has increased the demand for reduction and effective utilization of FW in recent years[1]. Further studies showed that direct interspecies electron transfer (DIET) has been considered as an alternative to IHT for syntrophic metabolism[9] This process can be explained as the mechanism of direct electron transfer from the bacteria to the electron-accepting methanogens via biological electrical connections, e.g. electrically conductive pili[10] and outer surface c-type cytochromes[11]. Deep insight into the effects of AC on the interactive relationship in a broad profile of microbial networks including hydrolyzing and fermenting bacteria, syntrophic bacteria, and methanogens was conducted by the metagenomic shotgun sequencing using high-throughput sequencing technology, which typically generates millions to billions of reads for the metagenomic DNA extracted from sludge samples of AD reactors. To the best of our knowledge, this is the first study to conduct multi-analysis regarding the taxonomic resolution of microbial communities and metabolic pathway of anaerobic microbes as well as association analysis of microbial network in AD process assisted by the addition of AC
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