An anoxic limestone drain
(ALD) is a buried bed of limestone constructed to intercept subsurface
mine water flows and prevent contact with atmospheric oxygen. Keeping
oxygen out of the water prevents oxidation of metals and armoring of
the limestone. The process of limestone dissolution generates alkalinity.
The sole purpose of an ALD is to provide alkalinity thereby changing
net acid water into net alkaline water. Retaining carbon dioxide in
the drain can improve limestone dissolution and alkalinity generation.
An ALD can be considered a pretreatment step to increase alkalinity
and raise pH before the water enters a constructed aerobic wetland.
In the aerobic wetland, metals can be oxidized and precipitated. ALDs
are limited to the amount of alkalinity they can generate based on solubility
equilibrium reactions. Also, the effectiveness and longevity of an ALD
can be substantially reduced if the AMD has high concentrations of ferric
iron, dissolved oxygen or aluminum. Vertical flow reactors (VFR) were
conceived as a way to overcome the alkalinity producing limitations
of ALD’s and the large area requirements for compost wetlands.
The VFR consists of a treatment cell with an underdrained limestone
base topped with a layer of organic substrate and standing water. The
water flows vertically through the compost and limestone and is collected
and discharged through a system of pipes. The VFR increases alkalinity
by limestone dissolution and bacterial sulfate reduction. Highly acidic
waters can be treated by running the AMD through a series of VFRs. A
settling pond and an aerobic wetland where metals are oxidized and precipitated
typically follow a VFR plan.
VFRs are sized based on retention times required to produce the necessary
alkalinity. Retention times of 12 to 15 hours are typically used for
sizing VFRs
and the amount of limestone necessary is calculated as shown above for
ALDs.
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