Anaerobic co-digestion of glycerol and wastewater derived from biodiesel manufacturing

The anaerobic co-digestion of glycerol and wastewater derived from biodiesel manufacturing, in which COD was found to be 1054 and 428 g/L, respectively, was studied in batch laboratory-scale reactors at mesophilic temperature (35 °C). Glycerol was acidified with H3PO4 in order to recover the alkaline catalyst employed in the transesterification reaction (KOH) as agricultural fertiliser (potassium phosphates). Wastewater was subjected to an electrocoagulation process in order to reduce its oil content. After mixing, the anaerobic revalorisation of the wastewater was studied employing inoculum–substrate ratios ranging from 5.02 to 1.48 g VSS/g COD and organic loading rates of 0.27–0.36 g COD/g VSS d. Biodegradability was found to be around 100%, while the methane yield coefficient was 310 mL CH4/g COD removed (1 atm, 25 °C). At the equilibrium time, the proportionality between r ¯ ′ and the load was constant, showing 1.55 mL CH4/(g VSS g COD h). The results showed that anaerobic co-digestion reduces the clean water and nutrient requirement, with the consequent economical and environmental benefit.