Influence of HRT on fermentative hydrogen production process by anaerobic activated sludge

Influences of hydraulic retention time (HRT) on fermentative hydrogen production process by anaerobic activated sludge were investigated. The bioreactor was fed with diluted molasses without adjusting the influent pH. The influent organic wastewater concentration was kept at 6000 mg COD/L with a temperature of 35°C. When the HRT decreased from 8 hours to 6 hours, the pH and ALK of the mixed anaerobic culture decreased from 4.3 and 250 mg/L to 4.2 and 180 mg/L respectively, ORP increased from -380--360 mV to-330--300 mV, and the bio-mass decreased from 8.7 gVSS/L to 7.2 gVSS/L, while hydrogen evolution rate increased from 0.14 L/(gVSS · d) to 0.22 L/(gVSS · d), and the fermentative end products such as volatile fatty acids and alcohol increased totally from 1106.1 mg/L to 1695.1 mg/L. Among the fermentative end products, as ethanol fermentation type purpose productions, the content of ethanol and acetic acid increased together from 90% to 85%. When HRT decreased further to 4h, pH and ALK of the mixed anaerobic culture decreased respectively to 3.7 and 75 mg/L quickly, while ORP increased to-210 mV. The dramatic change of ecological condition in the bioreactor caused mixed acid-fermentation type instead of ethanol fermentation type, and biomass decreased to 1.1 gVSS/L while hydrogen evolution rate decreased to zero. The results showed that the change of HRT had influence upon the bio-mass, biological activity and metabolic characteristics of biotic community of the anaerobic activated sludge system. HRT was the first influence factor that can be controlled.