Comparison Between Single and Two-Stage Anaerobic Digestion of Vegetable Waste: Kinetics of Methanogenesis and Carbon Flow

Abstract

This study aims to compare the performance and kinetics between the single-stage anaerobic digestion (SAD) and the two-stage anaerobic digestion (TAD) of vegetable waste (VW). The SAD was performed using continuously stirred tank reactors. Meanwhile, the TAD experiment was set up using a combined system involving a continuously stirred tank for hydrolysis/acidogenesis and an upflow reactor for methanogenesis. The hydrolytic reactor operated as a batch process with a retention time (RT) of 9 days, while the methane reactor was a continuous process operation with RT of 20 days. Both TAD and SAD were controlled at a temperature of 36 °C. The SAD experiments lasted for 143 days, and were characterised by the kinetic rate constant k = 0.02 day−1 which was much lower than that for the TAD (k = 0.66 − 2.16 day−1). The SAD seemed to be inhibited by high concentration of free ammonia and low inoculum to substrate ratio; herein, only 17.8–22.3% of the initial carbon could be converted into biogas (equivalent to 91–110 Nml/g-VSadded) with low methane content (44.1–48.7%). Meanwhile, TAD converted 41.67% initial carbon to biogas (equivalent to 299.0–374.6 Nml/g-VSadded) with high methane content (71.68–81.0%). Moreover, methanogenesis in the TAD was highly stable which enabled the digestion process to return to normal state within a few days, even though the concentrations of the influent increased to double (6.5–24.5 g-COD/l). As per these results, the TAD was much more stable, faster, and stronger than the SAD.