The inhibitory impact of ammonia on thermally hydrolyzed sludge fed anaerobic digestion

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/m(3)/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.

Automated summary

This study found that ammonia limits methanogenesis, but high free ammonia concentrations did not impact the final dewaterability. Operating under a 15-day SRT can compensate for lower methanogens rates, enabling the system to operate under higher ammonia concentrations without performance impacts.