A process of biological reduction of sulfate was investigated using a
3.2-1 laboratory UASB reactor (35 degrees C) with conventional anaerob
ic sludge and ethanol (a relatively cheap reagent) as the sole electro
n donor and carbon source. High sulfate-conversion efficiencies (SCE)
of up to 80% were obtained at sulfate loading rates (SLR) of up to 6 g
SO42-/(1 day) (hydraulic retention times (HRT), 5-0.85 days; influent
sulfate concentrations, 0.84-5 g/l). Further increasing the SLR to 10
g SO42-/(1 day) caused the SCE to decrease to 60%. The SCE slightly d
ecreased with a decrease in HRT, was almost independent of the influen
t sulfate concentration, and linearly increased with the increase in m
edium DH, i.e., with the decrease in the concentration of undissociate
d H2S. Thus, to reach a high SCE at a high SLR, additional measures sh
ould be taken to eliminate undissociated H2S from the reactor medium.
Sulfate-reducing bacteria (SRB) won the competition with methanogens f
or the energy and carbon sources in this system. The predominance of S
RB was continuously enhanced with time, although complete cessation of
methane production was not observed even after 8 months of the experi
ment. It is speculated that the microbial sulfate reduction can be fol
lowed by a stripping of the H2S formed or chemical (biological) proces
ses of further conversion of H2S to sulfur and/or sulfuric acid.