Digestate recirculation rate optimization for the enhancement of hydrogen production: The case of disposable nappies and fruit/vegetable waste valorization in a mesophilic two-stage anaerobic digestion system

After the anaerobic digestion involvement in waste management, several attempts have been made for the biogas yields enhancement. The implementation of methanogenic effluent (digestate) recirculation has been proposed as an alternative for the pH adjustment in two-stage reactors, leading to decreased alkali solution requirements, which could subsequently reduce operational costs. In the current study, a mixture of 40–60% (w/w) fruits/vegetable waste-disposable nappies hydrolysate was valorized through the anaerobic digestion process. The digestate recirculation rate (RR) optimization took place in a continuous acidogenic reactor, followed by the two-stage system operation under various experimental conditions. Particularly, the RRs of 0-0.25-0.5-0.75-1 were tested, indicating up to 208% H2 production increase (RR0.25). The single- and two-stage configurations comparison followed, under the hydraulic retention time (HRT) of 25 d, while the two-stage operation was evaluated for pH adjustment (5.5) not only with alkali solution but also with the applied RR0.5 (reduced alkali solution requirements compared to RR0.25). During the two-stage system operation under RR, additional scenarios were investigated regarding the HRTmethanogenic reduction and the pH adjustment, while using exclusively digestate. Although the RR implementation seems promising, the digestate alkalinity content can significantly determine the system's efficiency.