Anaerobic digestion of food waste with aerobic post-treatment: Effect of fruit and vegetable content

A mesophilic anaerobic digester, followed by a psychrophilic aerobic post-treatment, was used to treat food waste (FW) with different proportions of fruit and vegetable waste (FVW). Two types of FW were used: low fruit and vegetable mix (LFV, with 56.5% of FVW) and high fruit and vegetable mix (HFV, with 78.3% of FVW). The anaerobic digester fed with LFV failed at an organic loading rate of 1.6 g VS.L^-1.d^-1 (volatile fatty acid (VFA) = 6000 mg.L^-1) due to high ammonia (reaching 3000 mg.L^-1). It was shown that, in an unstable anaerobic environment, ammonia is highly correlated (r² = 0.77) with VFA and is negatively correlated with volatile solids, total solids, and chemical oxygen demand (COD) removal rates (r² = 0.88, r² = 0.71, and r² = 0.91, respectively). In contrast, the anaerobic digester fed with HFV exhibited a stable performance (VFA = 1243 mg.L^-1), with limited ammonia accumulation (940 mg.L^-1). Methane generation was affected by the FVW content and reached 531 ml CH₄.g VS^-1 (CH₄ = 52%) with LFV feed and 478 ml CH₄.g VS^-1 (CH₄ = 57.4%) with HFV. The overall TS, VS and COD removal rates (all ranging between 94% and 97%), were closely similar for LFV and HFV. Accordingly, the aerobic post-treatment seems to compensate for the reduced performance of the disturbed anaerobic system fed with LFV.