Sustaining thermo-alkaline anaerobic digestion at high organic loading rates by ammonia recovery in a high-rate...

Thermophilic anaerobic digestion at high organic loading rates (OLRs) may be inhibited by ammonia. This study coupled thermophilic co-digestion of food waste and sludge with vacuum stripping of ammonia in a recirculation line. OLR increased stepwise from 0.7 to 7.0 g VS/L_reactor/d over 223 d of semi-continuous operation at 55 °C. Starting at OLR 4.0 g/L/d, 25% of working volume in three digesters was replaced once a day with vacuum-stripped digestate having pH 9.80 ± 0.09. Digestate replacement immediately decreased ammonia concentration but resulted in robust alkaline conditions at OLR 5.0-7.0 g/L/d. Methane production in the controls without vacuum stripping stabilized around 2.40 L/L/d for 46 d at OLR 5.0 g/L/d, then began to collapse when total ammonia nitrogen exceeded 1983-2273 mg/L, followed by sharp accumulation of volatile fatty acids to 95.9-308 mmol/L and pH drop to 5.51-5.77. The treatment digesters maintained total ammonia nitrogen below 1018-1085 mg/L while methane production rate increased from 2.83 L/L/d at OLR 5.0 g/L/d to 3.34 L/L/d at OLR 6.5 g/L/d. The treatment digesters became unstable upon daily shocks of pH 8.52-8.64 at OLR 7.0 g/L/d. Digestate microbiome was dominated by fermentative bacteria in genus Defluviitoga and ammonia-tolerant hydrogenotrophic Methanothermobacter. These anaerobic thermophiles were viable upon 5-h vacuum stripping of digestate boiling at pH 9.22-9.80, 65 °C and 25-27 kPa. Methanothermobacter had a strong syntrophic association with Defluviitoga. The interwoven effect of OLR, pH and concentrations of acetic acid, propionic acid and ammonia drove microbial community restructuring and process failure.