Riboflavin modified carbon cloth enhances anaerobic digestion treating food waste in a pilot-scale system

Abstract

Previous laboratory-scale studies have consistently shown that carbon-based conductive materials can notably improve the anaerobic digestion of food waste, typically employing reactors with regular capacity of 1-20 L. Furthermore, incorporating riboflavin-loaded conductive materials can further address the imbalance between fermentation and methanogenesis in anaerobic systems. However, there have been few reports on pilot-scale investigation. In this study, a 10 m² of riboflavin modified carbon cloth was incorporated into a pilot-scale (2 m³) food waste anaerobic reactor to improve its treatment efficiency. The study found that the addition of riboflavin-loaded carbon cloth can increase the maximum organic loading rate (OLR) by 40% of the pilot-scale reactor, compared to the system using carbon cloth without riboflavin loading, while ensuring efficient operation of the reaction system, effectively alleviating system acidification, sustaining methanogen activity, and increasing daily methane production by 25%. Analysis of the microbial community structure revealed that riboflavin-loaded carbon cloth enriched the methanogenic archaea in the genera of Methanothrix and Methanobacterium, which are capable of extracellular direct interspecies electron transfer (DIET). And metabolic pathway analysis identified the methane production pathway, highly enriched on the reduction of acetic acid and CO₂ at riboflavin-loaded carbon cloth sample. The expression levels of genes related to methane production via DIET pathway were also significantly upregulated. These results can provide important guidance for the practical application of food waste anaerobic digestion engineering.

Keywords: carbon cloth; direct interspecies electron transfer; food waste; pilot-scale anaerobic digestion; riboflavin.

FIGURE 1

Metal accumulation simulation curves from the non modified period and the carbon cloth modified period.

FIGURE 2

(A) Schematic diagram and (B) photography of the reactor. (C) Water quality index and (D) gas composition from the carbon cloth modified period and the non modified period. Choose COD concentration and pH to reflect the removal capacity of the reactor, and choose CH₄ and CO₂ concentration to reflect the methane production capacity of the reactor.

FIGURE 3

(A) NH₄-N and (B) VFAs concentrations in the reactor from the carbon-cloth amended period and the non-amended period.

FIGURE 4

(A) Archaeal and (B) Bacterial community structures of the sludge samples from the reactor from the carbon-cloth amended period and the non-amended period determined by analysis of 16S rRNA gene fragments. Note: only sludge (OS, non-amended stage), Added carbon cloth sludge (ACCS, bulk sludge from carbon cloth amended stage), Added carbon cloth (ACC, sludge attached to the carbon cloth surface).

FIGURE 5

(A) The five key modules of KEGG metabolic pathways and (B) the proportion of four methane generation modules in samples from the control and the carbon-cloth amended periods for methane metabolism.

FIGURE 6

Prediction of key metabolic functional potential (percentage per million functional units) for (A) archaea and (B) bacteria based on KEGG database in the enhancing process with riboflavin-modified carbon cloth and its control.

FIGURE 7

(A) KEGG-annotated functional enzyme and (B) KEGG-annotated functional genes related to methanogenesis for archaea and bacteria, based on KEGG database in the enhancing process with riboflavin-modified carbon cloth and its control.

FIGURE 8

The variations in the abundance of key functional enzymes related to the pathway of reducing CO₂ and acetate to produce CH₄. The enzyme numbers are from Enzyme Commission numbers (EC numbers) formed a classification scheme for enzymes according to the reference information (see KEGG ENZYME).