Fostering nature-based solutions and circular approaches in biogas purification: validation of digestate centrate nitrified by intensified multi-stage constructed wetlands as electron acceptor in anoxic biodesulphurisation.

Anoxic biodesulphurisation using a biogenic nitrified source as an electron acceptor is a potential alternative to aerobic H2S biotreatment. This work reports the successful combination of nitrified centrate obtained from innovative constructed wetlands (CWs) with anoxic suspended biomass bioreactors (SBBs) treating real biogas. The quality of the liquid medium on the biodesulphurisation process was evaluated. The SBBs (of 48 L and 130 L, respectively) shifted seamlessly from using nitrified centrate from activated sludge to nitrified centrate from CWs, but its low orthophosphate content did limit H2S removal. To overcome this limitation, raw centrate was supplied. When nitrate + nitrite and orthophosphate concentrations were above 10 mgN L-1 and 10 mgP L-1, 92 % of the H2S was removed in SBB1 (H2S elimination capacity (EC) of 3.0 gH2S Nm-3h-1), and 95 % in SBB2 (H2S-EC of 13.1 gH2S Nm-3h-1) at a gas residence time of 4.0 min and 3.1 min respectively.