A process for the precipitation of trivalent arsenic sulphide in sulphate-reducing condition at low pH would be very attractive due to the high arsenic content (60%) in the final precipitate. A bacterial consortium able to reduce sulphate at pH 4.5 served to inoculate column bioreactors that were continuously fed with As(V) or As(III), glycerol and/or hydrogen, at pH values between 2 and 5. The diversity, functionality and evolution of the consortium colonizing the bioreactors were characterized by means of biomolecular tools, in relation with operating parameters (pH, As, sulphide, acetate). The highest As removal rate obtained during these experiments was close to 3 mg.l-1.h-1 using As(V) as the initial arsenic form, while precipitation rates were improved using As(III). When glycerol was replaced by hydrogen in a bioreactor containing a mature biofilm, sulphate-reducing activity increased roughly. Organisms related to Desulfosporosinus were the only sulphate-reducing bacterium (SRB) detected in the bioreactor. arrA genes, involved in As(V) dissimilatory reduction, were also detected and suggested that As(V) was reduced by a Desulfosporosinus-like organism. Molecular fingerprints evidenced an evolution of the bacterial population structure according to changes in operating conditions.