Characteristics of kitchen waste and the formation of Floating Brown Particles (FBP) in the anaerobic digestion process

Five types of kitchen waste (KW) from China - including hop pot (HP), fast food (FF), Hebei cuisine (HC), university canteen (UC), and other mixed KW (Other) - were investigated as feedstock for potential biogas and methane production. The biodegradability of KW was measured using batch anaerobic digestion (AD) tests and feedstock at an inoculum ratio (F/I) of 0.5. Gompertz and Cone models were used to determine the kinetic parameters of KW degradation, biogas, and methane production. Results showed that HP had the highest lag phase time of 5.46 days. Methane production varies with different sources of KW. HP had the highest methane yield of 363.9 mL/g-VS_added as compared to a sample of FF (334.8 mL/g-VS_added), other (278.5 mL/g-VS_added), UC (239.2 mL/g-VS_added), and HC (236.0 mL/g-VS_added). The biodegradability of KW ranged from 39.5% to 50.4%. During the AD process a certain amount of floating brown particles (FBP) were formed, which may be the main inhibiting factor of methane production. Analysis of ¹³C NMR and FTIR revealed that the main component of FBP was calcium stearate. The formation mechanism of calcium stearate may contribute to the relatively high lipid content (18.6% to 30.9%) of the KW sample, which subsequently resulted in over-accumulation of long-chain fatty acids (LCFAs) and reaction with Ca²⁺. Using lipid-rich substrates as feedstock may be an efficient approach to adding Ca²⁺ artificially for reducing the inhibition of LCFAs.