System stability associated with different lipid contents during mesophilic anaerobic digestion of lipid-rich food waste

Abstract

Lipid content is an essential indicator which affects the operation performance and efficiency of an anaerobic digestion system. This study investigated the relationship between system stability and lipid degradation during mesophilic anaerobic digestion process of lipid-rich food waste. As the experiment designed, the hydraulic retention time was 30 days and influent total solid (TS) concentration of the substrates was kept at 10 %, in which lipid content gradually increased from 10 % to 60 % (on the TS basis). The anaerobic digestion process proceeded successfully, achieving a methane yield of 587 mL/g-VS (volatile solids) at lipid/TS ratio of 50 %. However, the system collapsed after the lipid/TS ratio exceeded 50 %. It was found that the accumulation of long-chain fatty acid was not the primary factor leading to system failure, as the total concentration remained below 500 mg/L even at lipid/TS ratio of 60 %. Instead, the C/N ratios resulting from different lipid contents significantly impacted system stability. As lipid/TS ratio increased to 60 %, the C/N ratio of substrate reached 45.47, causing system failure due to an efficiency imbalance between alkalinity production and VFAs accumulation. The mechanism was illustrated by carbon and nitrogen distributions. Additionally, the synergistic effect between lipids and food waste was also discussed, peaking at lipid/TS ratio of 40 %. These results highlighted that the C/N ratio had more prominent impacts on system stability than long-chain fatty acids accumulation during the designed lipid-rich food waste anaerobic digestion process.

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