Continuous H₂ and CH₄ production from high-solid food waste in the two-stage thermophilic fermentation process with the recirculation of digester sludge

A thermophilic two-stage fermentation process using 10% total solids (TS) food waste was tested at varying organic loading rates (OLRs). The system was configured to produce H₂ and CH₄ in conjugation with the chemical oxygen demand (COD), nitrogen removal, and adjustment of the pH by returning sludge as an alkali buffer from the sludge storage tank for denitrification. The pH in the H₂ fermentation reactor was maintained in the range of 5.4-5.7 using sludge recirculation (Q_r/Q_i ratio 1). The average H₂ (11.1 l-H₂ l^-1-fed d^-1) and CH₄ (47.4 l-CH₄ l^-1-fed d^-1) production rates were achieved at OLRs of 39 (H₂ fermentation reactor) and 4.16 gCOD l^-1 d^-1 (CH₄ fermentation reactor), respectively. These results suggest that long-term stability of the continuous two-stage process can be successfully achieved by recirculation of high-alkalinity sludge of 6.7-7.5 g l^-1 as CaCO₃, without any added external chemical buffer.