Title : Anaerobic digestate centrate as a key resource to improve sustainability in biogas purification and upgrading to biomethane
Abstract:
This work aims to foster the deployment of biogas valorisation technologies by decreasing operating costs (OPEX) treating anaerobic digestate and biogas, as well as to decrease their pollutant emission. A disruptive circular approach that intensifies the biowaste-water-energy nexus focusing on resource recovery and valorisation in anaerobic digestion plants is proposed. The concept avoids the usage of chemicals, proposes a nature-based solution for centrate treatment valorising a solid waste from the water cycle, and decreases the impact of current biogas purification strategies based on ferric salts consumption supplied to the anaerobic digester and activated carbon filters for the removal of hydrogen sulphide (H2S) and methyl volatile siloxanes. Nitrification of centrate by innovative intensified constructed wetlands (CWs) using alum sludge from drinking water treatment plants as reactive substrate has been carried out. The CWs remove phosphorous, mainly adsorbed into the substrate and provides nitrite+nitrate as an electron acceptor source for anoxic biogas desulphurisation in suspended biomass bioreactors (SBBs). This combined integration of biotechnologies is totally disruptive in the scientific literature, being characterised by its very low OPEX. The SBBs favours elemental sulphur recovery from partial H2S oxidation (compared to fixed bed bioreactors), as well as the recovery of dissolved CH4 from the SBB liquid effluent using a membrane contactor. Non-treated centrate is also valorised as absorbent in a membrane contactor for biogas enrichment to biomethane quality, replacing chemical absorbents such as amines. The 3-stage-CW showed a nitrification capacity of 13-41 gN/m3/day outperforming conventional CW using gravel, in addition, the innovative CW obtained 100% phosphorous removal. The nitrified stream was successfully used in the SBBs, achieving >90% H2S removal efficiency at gas residence time down to 3 min. In the SBB, 70% of the dissolve methane was recovered by means of a commercial membrane contactor operating at sweep gas mode. A membrane contactor was also used for biogas upgrading to biomethane using the rich ammonium centrate as absorbent. A CH4 concentration >91.7% was obtained at an optimal biogas flow rate of ≤1.2 L/min and centrate flow rate ≥2 L/min (G/L maximum of 0.3). As G/L increased CH4 concentration decreased. These results prove the technical feasibility of integrating centrate and biogas treatment in these two innovative biological processes.
