Effect of hydraulic retention time on the methanogenic step of a two-stage anaerobic digestion system from sewage sludge and wine vinasse: Microbial and kinetic evaluation

Abstract

The aim of this study was to investigate the effect of hydraulic retention time on methane production in the methanogenic step of a two-stage thermophilic-mesophilic anaerobic digestion system. The methanogenic step was carried out in a continuously stirred tank reactor under mesophilic conditions. The methanogenic digester was fed with a mixture consisting of 50% sewage sludge and 50% wine vinasse previously acidified in a thermophilic acidogenic step. Eight hydraulic retention times (20, 16, 10, 6, 5, 4, 3 and 2 days) were tested to determine the effect of each operational variable. Maximum methane production (1.8 L/LR·d) was reached at a hydraulic retention time of 2 days, whereas maximum methane yield (159.4 mL CH4/g CODremoved) and Archaea activity (11.6·10-9 L CH4/cells) were achieved at a hydraulic retention time of 4 days. From this time onward, an accumulation of volatile fatty acids took place, reaching values of over 1000 mg/L. Although the Stover-Kincannon model is more suitable to predict the chemical demand oxygen concentration in the effluent, both the Grau Second-order and Stover-Kincannon models adequately describe substrate removal in the methanogenic step of an anaerobic digestion system with a high correlation coefficient (R2 > 0.996).