Effect of hydraulic retention time on suppression of methanogens during a continuous biohydrogen production process using molasses wastewater

This study was undertaken to investigate the reduction of the hydraulic retention time (HRT) to decrease methane generation and recover hydrogen production during the long-term operation of biohydrogen production in a continuous stirred tank reactor (CSTR) using molasses wastewater. Reduction of HRT can be a simple and economic method to immediately control unfavorable methane generated during continuous operation of a hydrogen production system. The steady-state performance of the CSTR showed a hydrogen content of 41.3 ± 3.30% and a hydrogen production rate (HPR) of 63.7 ± 10.01 mmol-H₂L⁻¹d⁻¹ under an organic loading rate (OLR) of 29.7 g CODL⁻¹ at an HRT of 24 h. Increase in the methane level above 40% during long-term operation caused decrease in the hydrogen content and HPR to 5.9 ± 1.6% and 2.1 ± 1.1 mmoL-H₂L⁻¹d⁻¹, respectively. When methane increased to a high level over 40%, the CSTR at the HRT of 24 h was operated at the HRT of 12 h. Reduction of the HRT from 24 to 12 h led to decrease in the methane content of 12.1 ± 4.44% and recovery of the HPR value to 48.9 ± 15.37 mmol-H₂L⁻¹d⁻¹ over a duration of 13–22 d. When methane is generated in a continuously operated reactor, reduction of the HRT can be an easy way to suppress methanogens and recover hydrogen production without any additives or extra treatments.