Use of hydrophobic membranes to supply hydrogen to sulphate reducing bioreactors

This paper reports on the application of hydrophobic membranes to supply the gaseous substrates hydrogen/carbon dioxide (H2/CO2) to a sulphate reducing bioreactor. For this, two flat 0.016 m2 sheets of flouroplast microporous (0.45 microm) membranes were inserted in a 3.6 dm3 bioreactor for the supply of H2/CO2 gas as small gas bubbles. The bioreactor was operated at 30 degrees C and pH 7.0 and was also equipped with an external ultra filtration module for biomass retention. At a sulphate loading rate (SLR) of 1.32 g SO4(2-) dm(-3) day(-1) and a hydraulic retention time (HRT) of 61 h, a sulphate reduction rate (SRR) of 0.90 g SO4(2-) dm(-3) day(-1) was achieved. When the influent sulphate concentration was reduced from 3.36 to 0.75 g SO4(2-) dm(-3) by lowering the HRT to 10.3 h (SLR of 1.75 g SO4(2-) dm(-3) day(-1) the SRR dropped to 0.22 g SO4(2-) dm(-3) day(-1). The lower sulphate reduction efficiency was most probably caused by a too short biomass-substrate contact time or by irreversible sulphide inhibition. Mass transfer limitation of H2 and improper mixing of the reactor liquid were shown not to contribute to the low sulphate reduction efficiency.