A passive treatment system for removing metals, especially manganese, from mine drainage and other aqueous solutions. The Hybrid Flow Pond (HFP) is an open, pond-like structure utilizing a horizontal or vertical operational flow path through a bed of treatment media together with a vertical flow path for flushing accumulated solids from the bed. The bed preferably contains basic steel slag, limestone, marble, blast furnace slag, other types of slag, organic material, or mixtures thereof. In another aspect of the Hybrid Flow Pond, a tiered piping system divides an interior of the treatment bed into cells. Each cell has its own header and adjustable riser whose output level can be raised or lowered to change the head experienced by each cell.