Kinetic and thermodynamic effects of temperature on methanogenic degradation of acetate, propionate, butyrate and valerate

This study elucidates volatile fatty acids (VFAs) accumulation reasons during organic solid wastes degradation via kinetics and thermodynamics assessments of degradation mechanisms of representative VFA (acetate, propionate, butyrate and valerate) under mesophilic (35 degrees C) and thermophilic (55 degrees C) conditions. Acetate exhibited the highest maximum microbial specific growth rates of 0.18 d(-1) and 0.33 d(-1) at mesophilic and thermophilic conditions, respectively, during the degradation process, followed by butyrate at 0.08 d(-1) and 0.08 d(-1). However, propionate showed a stronger inhibition to methanogenesis at thermophilic conditions, especially at the substrate concentration exceeding 5000 mg COD/L. The largest specific maximum methanogenic activity (SMA) decreased from 0.029 to 0.022 g CH4-COD/g-VSS/d. Furthermore, an inability of microorganisms to adapt to thermophilic propionate degradation conditions (at maximum lag time of 12 days) indicated substantial substrate inhibition. As an intermediate product of valerate degradation, the propionate degradation rate was considerably lower than that of valerate (0.036 g COD/d vs. 0.104 g COD/d). Hence, propionate caused a substantial rate limitation during the valerate degradation process under thermophilic conditions. In addition, under thermophilic conditions, valerate converted to methane via a long three-step syntrophic oxidation process. A large amount of hydrogen production accompanied this process. The inhibitory effect of propionate on methanogens and high hydrogen concentration (approximately 700 ppm) required time to subside. Thus, a 4-day plateau associated with a positive Gibbs free energy (1.45-2.11 kJ/mol) occurred during valerate degradation.