Ammonia inhibition mitigation in anaerobic digestion of high solids content of thermally hydrolysed secondary sewage sludge by the NH₄⁺ affinitive clinoptilolite and a strong acid type ion-exchange resin S957 was investigated. Continuous NH₄⁺-N removal was achieved through ion-exchanging at both temperatures with average removals of 50 and 70% for the clinoptilolite and resin dosed reactors, respectively. Approximate 0.2–0.5 unit of pH reduction was also observed in the dosed reactors. The synergy of NH₄⁺-N removal and pH reduction exponentially decreased free NH₃ concentration, from 600 to 90 mg/L at 43 °C, which mitigated ammonia inhibition and improved methane yields by approximately 54%. Microbial community profiling suggested that facilitated by ammonia removal, the improvement in methane production was mainly achieved through the doubling in bacterial density and a 6-fold increase in population of the Methanosarcinaceae family, which in turn improved the degradation of residual volatile fatty acids, proteins and carbohydrates.