Changes in flow rates (), pH (●), concentrations of soluble zinc (◆) and soluble iron (▾), and numbers of planktonic‐phase bacteria (◊) in bioreactor II. The composition of the feed liquor was based on that of an acidic (pH 2.5) stream draining the abandoned Cwm Rheidol mine in mid‐Wales, and contained 3 mM ferrous iron and 3 mM zinc and varying concentrations of aluminium, as major soluble metals.