Methane production and microbial adaptation in high-load vacuum-enhanced anaerobic digestion: Addressing ammonia and propionate toxicity

• IntensiCarb technology processed 6 times the OLR of conventional AD. • Methane yield decreased linearly with increasing propionate concentrations over 2.20 g/L. • Side-stream vacuum increased ammonia toxicity resilience by a factor of 3. • Population of propionate degraders in IC was different from conventional AD. • Methanosaeta dominated conventional AD, while Methanosarcinaceae dominated IC reactors. Ex-situ vacuum-enhanced anaerobic digestion (IntensiCarb-AD) of primary and thickened waste activated sludge (PS/TWAS) was tested at an organic loading rate (OLR) of 11.0–11.3 kg COD/m 3 -d. IC-AD reactors with similar solids retention times (SRT = 20 d) and OLR, were operated at hydraulic retention times (HRT) of 5 days (IF defined as SRT/HRT of 4) and 3.33 days (IF6). IF4 had a higher steady-state methane yield of 0.214 ± 0.009 L-CH 4 /gCOD fed compared to 0.148 ± 0.029 L-CH 4 /gCOD fed in IF6. COD removals of 53.5 %±2.25 % were obtained with IF4 but fluctuated with IF6 (37.0 %±7.25 %). Both IF4 and IF6 processed 6 times the OLR of a conventional AD with comparable volatile solids reduction efficiency. The relative instability in IF6 was linked to a greater reduction in microbial activity due to longer vacuum application and higher propionate concentrations (>2.20 g/L), which affected the performance of hydrogenotrophic methanogens. Methanosarcinaceae and Methanobacteriaceae populations were 10.8 % and 35.2 % more abundant in IF4 than in IF6, respectively, corroborating the significantly higher methane yields observed in IF4 compared with IF6. Ammonia recovery efficiencies of 52.0 % and 49.1 % were observed in IF4 and IF6 respectively. Batch ammonia toxicity tests indicated a higher inhibitory constant (K i ) for IC-AD (5.60–5.70 gN/L) compared to conventional AD (1.90 gN/L). Acetate degradation was effective even with ammonia and propionate concentrations as high as 2,000 mg/L and 4,000 mg/L, respectively.