Arsenic rich AMDs (Acid Mine Drainages) represent a major source of pollution foraquatic ecosystems. Microbially driven iron (Fe) and arsenic (As) oxidation and precipitation represent a promising strategy to treat this pollution. A better understanding of thebiogeochemical mechanisms involved is required prior any further exploitation of this microbial potential. A field-scale pilot was implemented at the Carnoul`es mine (France) for thetreatment of AMD. It is an ergonomic and passive aerobic system: five treatment units of1.5 m2 are combined vertically in series and fed with the AMD water by gravitation flow.Biogenic precipitates (corresponding to Fe- and As-rich biofilms) covered the bottom of theunits. Inlet water and biogenic precipitates were collected over a 7 months period. Wedetermined the bacterial community structure in the precipitates by fingerprint (ARISA),metabarcoding (16S rRNA gene) and qPCR targeting arsenite oxidase gene aioA. Chemicaland mineralogical analyses were conducted on the precipitates and on the feed water. Thebacterial communities in the precipitates developed from the indigenous communities of theAMD used to feed the pilot. Our results showed an evolution of these communities overtime associated with an increase of the potential genetic for As oxidation. The proportionof As(V) in the precipitates and arsenic removal efficiency fluctuated, with maximum levelsof 99% and 97 % respectively. This work provided information about microbial dynamicsand pollution removal efficiency in a treatment pilot under field conditions. It will serve forfuture design of a bioremediation system to treat As-rich AMD.