This study evaluated the impact of ammonia on mesophilic anaerobic digestion (AD) with thermal hydrolysis pretreatment (THP) treating a mixture of primary sludge and waste activated sludge and operated under constant organic loading rate of 9 kg COD/m³ /d. Free ammonia concentrations in the digesters were varied between 37 and 966 mg NH₃ -N/L, while maintaining all other operational conditions constant. A decrease in volatile solids reduction from 54 ± 5% (at <554 mg NH₃ -N/L) to 35 ± 6% at the maximum free ammonia concentration of 966 mg NH₃ -N/L was observed at steady-state conditions. No impact of free ammonia on final dewaterability was detected. Free ammonia thus mostly limited methanogenesis. A free ammonia Monod inhibition constant of 847 ± 222 mg NH₃ -N/L for methanogens was estimated based on the digester steady-state methane rates dynamics. This study showed that current THP AD digesters (typically 110-260 mg NH₃ -N/L) operate under 12%-18% ammonia inhibition for methanogenesis. Operation under SRT of 15 days, about 2 times more than needed to retain methanogens, can compensate for lower methanogens rates and avoid performance impacts. The later shows a good potential to operate under higher free and total ammonia concentration without jeopardizing performance. PRACTITIONER POINTS: Only from a free ammonia concentration above 554 mg NH₃ -N/L, decreased volatile solids reduction and biogas yield were observed. A volatile solids reduction of 35 ± 6% at maximum free ammonia concentration of 966 mg NH₃ -N/L was still achieved. A Monod inhibition constant for methanogens of 847 ± 222 mg NH₃ -N/L was estimated. It was estimated that current THP AD systems (110-260 mg NH₃ -N/L) operate under 12%-18% NH₃ inhibition for methanogenesis.

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