Co-treatment of fruit and vegetable waste in sludge digesters. An analysis of the relationship among bio-methane generation, process stability and digestate phytotoxicity

The co-digestion of a variable amount of fruit and vegetable waste in a waste mixed sludge digester was investigated using a pilot scale apparatus. The organic loading rate (OLR) was increased from 1.46 kgVS/m3 day to 2.8 kgVS/m3 day. The hydraulic retention time was reduced from 14 days to about 10 days. Specific bio-methane production increased from about 90 NL/kgVS to the maximum value of about 430 NL/kgVS when OLR was increased from 1.46 kgVS/m3day to 2.1 kgVS/m3day. A higher OLR caused an excessive reduction in the hydraulic retention time, enhancing microorganism wash out. Process stability evaluated by the total volatile fatty acids concentration (mg/L) to the alkalinity buffer capacity (eq. mg/L CaCO3) ratio (i.e. FOS/TAC) criterion was <0.1 indicating high stability for OLR < 2.46 kgVS/m3day. For higher OLR, FOS/TAC increased rapidly. Residual phytotoxicty of the digestate evaluated by the germination index (GI) (%) was quite constant for OLR < 2.46 kgVS/m3day, which is lower than the 60 % limit, indicating an acceptable toxicity level for crops. For OLR > 2.46 kgVS/m3day, GI decreased rapidly. This corresponding trend between FOS/TAC and GI was further investigated by the definition of the GI ratio (GIR) parameter. Comparison between GIR and FOS/TAC suggests that GI could be a suitable criterion for evaluating process stability.