Simultaneous supplementation of magnetite and polyurethane foam carrier can reach a Pareto-optimal point to alleviate ammonia inhibition during anaerobic digestion

Material addition is a potential method to alleviate ammonia inhibition in the anaerobic digestion process. In this study, seven materials were selected to examine their impact on methane production (MP). In the batch experiment, the reactors with “Magnetite + foam carrier” exhibited nearly a 56% increase in maximum MP rate. Different strategies of magnetite and foam carrier addition were further explored in semi-continuous experiment. Adding two materials together (simultaneous supplementation) resulted in almost 60% increase in MP, 28–41% higher than when supplemented separately. Simultaneous supplementation of the two materials was considered as the Pareto-optimal solution for MP maximization. The acetogenic and methanogenic activities analyses revealed that foam carrier addition accelerated propionate transformation (from propionate to acetate or valerate) while magnetite addition intensified the methanogenesis step. Direct interspecies electron transfer (DIET) was strengthened with over 50% enrichment in DIET-related bacteria. The Anaerobic Digestion Model No.1 was modified with the development of liquid metabolite and biofilm, result in a goodness-of-fit of R2 > 0.96.16sRNA gene sequencing demonstrated that the microbes in the biofilm had kinetic advantages over those in the liquid phase, coupled with higher km and lower Ks values, which contributed to the enhanced MP.