The operation performance of fermentative hydrogen production in anaerobic contact reactor (ACR) at different influent COD concentrations was investigated. The ACR could be kept at steady-state with ethanol-type fermentation

as influent chemical oxygen demand (COD) increased from 7000 mg·L-1 to 11000 mg·L-1 with constant HRT of 6 h. Specific hydrogen production of the ACR system increased from 2.43 m3·(m3·d)-1 to 3.51 m3·(m3·d)-1 as influent COD increased from 7000 mg·L-1 to 11000 mg·L-1

while specific hydrogen production of activated sludge peaked at 10.71 mol H2·(kg VSS·d)-1 at influent COD of 9000 mg·L-1. The results of polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) profiles showed that hydrogen-producing ethanol fermentative bacteria were dominant in the ACR. As influent COD concentration increased

Ethanoligenens harbinense YUAN-3 became more abundant

and propionate fermentative bacteria

i.e. Propionicimonas sp. F6 also started to be enriched.