Anaerobic co-digestion of food waste and dairy manure: Effects of food waste particle size and organic loading rate

• Reducing food waste particle size from 8 to 2.5 mm enhanced CH4 production.
• Reducing food waste particle size significantly improved digestate dewaterability.
• Energy used to grind food waste is minor compared to heating value of CH4 produced.
• Specific methane yield was highest (0.47–0.63 L/g VS) at 2 g VS/L/d of loading.
• Methane production was highest (1.40–1.53 L/L/d) at loading rate of 3 g VS/L/d.

This study was to comprehensively evaluate the effects of food waste particle size on co-digestion of food waste and dairy manure at organic loading rates increased stepwise from 0.67 to 3 g/L/d of volatile solids (VS). Three anaerobic digesters were fed semi-continuously with equal VS amounts of food waste and dairy manure. Food waste was ground to 2.5 mm (fine), 4 mm (medium), and 8 mm (coarse) for the three digesters, respectively. Methane production rate and specific methane yield were significantly higher in the digester with fine food waste. Digestate dewaterability was improved significantly by reducing food waste particle size. Specific methane yield was highest at the organic loading rate of 2 g VS/L/d, being 0.63, 0.56, and 0.47 L CH4/g VS with fine, medium, and coarse food waste, respectively. Methane production rate was highest (1.40–1.53 L CH4/L/d) at the organic loading rate of 3 g VS/L/d. The energy used to grind food waste was minor compared with the heating value of the methane produced.