Effect of Hydraulic Retention Time and Organic-Loading Rate on Two-Staged, Semi-Continuous Mesophilic Anaerobic Digestion of Food Waste during Start-Up

Abstract

The start-up of two-stage, semi-continuous mesophilic anaerobic digestion (TSAD) of food waste is stabilized by altering the hydraulic retention time (HRT) and organic-loading rate (OLR). The volumetric biogas yield and composition are studied at OLR (0.25–0.50 gVS/L/d) and HRT (10, 20, 40 days) initiating at OLR 0.25 g VS/L/d and HRT of 20 and 40 days for the respective reactors. Methane (CH4) from the first stage of the two-staged reactor decreased from 18.20% to 0.06%, fostering hydrogen production in 44 days when the HRT was reduced from 20 to 10 days and OLR increased from 0.25 gVS/L/d to 0.50 gVS/L/d. During the alarming volatile fatty acids (VFA)/alkalinity ratio of 0.76, feeding to the second-stage reactor was halted until pH was restored to 7.00. The restoration of methanogens was evident by an increase in methane from 39.15% to 67.48%. A stable TSAD system produced 22.32 ± 4.16 NmL/gVS and 161.02 ± 17.72 NmL/gVS of yield in respective reactors. Thus, TSAD paves the path for multiple biofuels, i.e., H2 and CH4.