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

Abstract

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 9kgCOD/m3 /d. Free ammonia concentrations in the digesters were varied between 37 and 966mgNH3 -N/L, while maintaining all other operational conditions constant. A decrease in volatile solids reduction from 54±5% (at <554mgNH3 -N/L) to 35±6% at the maximum free ammonia concentration of 966mgNH3 -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±222mgNH3 -N/L for methanogens was estimated based on the digester steady-state methane rates dynamics. This study showed that current THP AD digesters (typically 110-260mgNH3 -N/L) operate under 12%-18% ammonia inhibition for methanogenesis. Operation under SRT of 15days, 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 554mgNH3 -N/L, decreased volatile solids reduction and biogas yield were observed. A volatile solids reduction of 35±6% at maximum free ammonia concentration of 966mgNH3 -N/L was still achieved. A Monod inhibition constant for methanogens of 847±222mgNH3 -N/L was estimated. It was estimated that current THP AD systems (110-260mgNH3 -N/L) operate under 12%-18% NH3 inhibition for methanogenesis.