Abstract
Two semi-passive treatment systems for iron (Fe) and arsenic (As) removal in AMD were installed and monitored in-situ formore than a year. These technologies were designed to treat the As-enriched AMD (≈ 1 g/L Fe(II) and 100 mg/L As(III)) ofthe ancient Carnoulès mine. The treatment was based on biological Fe and As oxidation by indigenous bacteria, and subse-quent immobilization of As by ferric hydroxysulfates. Forced aeration and wood/pozzolana or plastic support were used forbiofilm attachment. The system performance ranged from 86 to 98% for Fe oxidation, 30 to 60% for Fe removal, and 50 to80% for As removal at a hydraulic retention time of 9 h. No significant difference were measured between the two biofilmsupports. The wood/pozzolana support had a shorter delay for performance recovery after interruptions. Iron oxidation rateswere similar to those obtained in the Carnoulès AMD stream and laboratory bioreactor, while As oxidation seemed to beenhanced. The sludge accumulated between 39 and 91 mg/g of As, mainly in the As(V) oxidation state; jarosite and amor-phous ferric hydroxysulfate phases were the main Fe and As scavengers. Challenging environmental conditions during thelong monitoring period confirm the robustness of the treatment units. The data will be useful in designing future full-scaletreatment systems adapted to As-rich AMD.