Syntrophic acetate oxidation during the two-phase anaerobic digestion of waste activated sludge: Microbial population, Gibbs free energy and kinetic modelling

- A two-staged (meso/thermophilic) configuration for WAS digestion is proposed.
- Acetoclastic archaea were absent in the thermophilic stage at TAN = 1.4 g L−1.
- Syntrophic Acetate Oxidation (SAO) was modeled with a modified ADM1 version.
- The model was used to detect the feeding strategy capable to stabilize the process.
- The energetic loss due to SAO corresponded to 20% of the acetoclastic pathway.

A system using a two-phase anaerobic configuration (mesophilic/thermophilic) was tested by feeding waste activated sludge (WAS). The first acidogenic stage presented a hydraulic retention time (HRT) of 3 days, while the second methanogenic stage had an HRT of 10 days. Both raw and ultrasonically pretreated WAS samples were utilized for the experiment. Previous Fluorescence in Situ Hybridization (FISH) observations, revealed that in the thermophilic phase, the acetoclastic methanogenesis was likely replaced by a nonacetoclastic pathway, namely, syntrophic acetate oxidation (SAO). A modified version of Anaerobic Digestion Model n°1 (ADM1), accounting for the SAO pathway, was implemented and calibrated. The proposed model addressed the relationship between the hydrogen concentration and Gibbs free energy and showed the thermodynamic feasibility of the SAO pathway, while simultaneously highlighting the role played by hydrogenotrophic methanogens in maintaining a sufficiently low hydrogen partial pressure so that the SAO was energetically feasible. The estimated energy loss was estimated to be approximately 20% due to the switch of the microbial pathway from acetoclastic methanogenesis to SAO.