An integrated evaluation strategy for anaerobic digestion monitoring based on acid-base balance and thermodynamics of volatile fatty acid degradation

Process monitoring is a top priority for anaerobic digestion (AD), yet it poses a significant challenge due to its inherent inaccuracies and case-specific nature in diagnosing reactor status. Herein, a long-term instability simulation experiment using lignocellulosic biomass was conducted with varying organic loading rate (OLR) and hydraulic retention time (HRT) levels. The increase in OLR (0.75 to 20 kg VS m−3·d−1) and decrease in HRT (20 to 4 d) led to substantial total volatile fatty acids (TVFA) accumulation (>14 g/L) with a concomitant decrease in methane yield and organics removal. Full processing capacity with considerable conversion efficiency at four steps (hydrolysis: 60.43 ± 2.82 %, acidogenesis: 56.13 ± 3.27 %, acetogenesis: 56.03 ± 3.31 %, methanogenesis: 55.93 ± 3.32 %) and optimal methane yield (3.13 ± 0.17 L/(L·d)) was achieved at an OLR of 18 Kg VS·m−3·d−1 and HRT of 4 days. An integrated evaluation strategy for AD process stability is proposed based on the acid-base balance and thermodynamics of VFA degradation. Propionate, iso-valerate, TVFA/bicarbonate alkalinity (BA), and BA/total alkalinity (TA) were identified as potential auxiliary indicators for diagnosing AD imbalances, while ΔG for propionate and valerate degradation served as the early warning signs. To enhance the efficacy and universality of early warnings, relative variations for acid-base indicators based on steady-state (∼15 %) and the absolute threshold value for ΔGPropionate (-40 KJ/mol) and ΔGValerate (-55 KJ/mol) were adopted. This approach integrates considerations for microbial adaption, thereby enhancing an assessment of whether a given signal reflects irreversible acidification or is merely a temporary fluctuation induced by reversible acidification. The proposed strategy enables a reliable and accurate early warning, holding great promise for practical applications.