Unveiling the biotic and abiotic mechanisms of solids concentration affecting hydrolysis and acidogenesis during sludge thermophilic anaerobic fermentation

Abstract

Abstract The biotic factors (bacterial abundance, enzyme activity) and abiotic factors (sludge disintegration, organics solubilization) in sludge thermophilic fermentation at various solids concentrations were investigated to clarify the effects on hydrolysis and volatile fatty acids (VFAs) production. The results show that with the solids concentration increasing from volatile suspended solids (VSS) 5–40 g/L, the abiotic effects of particles disintegration played a leading role in hydrolysis, resulting in the decrease of hydrolysis rate constant (kH) (from 0.1181 to 0.0623) and solids degradation ratio (from 55.19% to 36.63%), while the biotic effects dominated above VSS20, reinforcing bacterial abundance and hydrolase activity. The maximal soluble chemical oxygen demand (SCOD) production yield positively correlated with VSS, and the value for VSS40 (436.52 mg/g VSSfed) increased by 50% over VSS5 (298.21 mg/g VSSfed). Additionally, the solubilization and conversion of carbohydrates were respectively enhanced by 2.6-folds and 13.4-folds at higher VSS, while for proteins they almost decreased by 50%. The VFAs production was comprehensively affected by the carbon substrate, ammonium and acid-forming enzyme, with the highest accumulation yield (232.17 mg CODVFA/g VSSfed) for VSS30; meanwhile, a higher percentage of acetate and n-butyrate but lower of propionate were achieved with VSS increasing.