Acid Rock Drainage - Mining and the Environment: From Ore to Metal - page 706

FIGURE 17.10
Anoxic Limestone Drain

Vegetated
Crown

Monitoring
Well 1

Monitoring
Well 2

Source:
Nairn et al. 1992

Clay Soil

Plastic Liner

High Quality
Limestone
(>90% CaCO 3 )

Monitoring
Well 3 Clay Soil

Original
Surface

Drain
Exit

Plastic Liner

Limestone

FIGURE 17.11
ALD - Wetland Passive Treatment
System

Well 1

Well 2

Source:
Nairn et al . 1992

Limestone
Pit

Well 3

Pipe

Constructed Wetland
Cell No.2

Shallow
Ditch

Limestone
Pit

Settling
Pond

Constructed Wetland
Cell No.1

Limestone
Pit

Metals such as aluminium, iron, and manganese are commonly removed in aerobic wetlands
and various co-precipitation reactions also result in the removal of other metals. Aerobic
wetlands are particularly well suited to provide a i nal 'polishing' stage of metal removal.
Many factors are involved including the layout of wetlands, the selection of suitable aquatic
plants and the residence time required for the processes to occur. A typical system involving
anoxic limestone drains and aerobic wetlands is depicted schematically in Figure 17.11 .

(3) Anaerobic treatment systems
These systems depend on metal removal and acid neutralization from sulphate reducing
bacteria. Accordingly, they need to create the conditions in which these organisms can
thrive. This requires the supply of an organic substrate which may use low cost agricul-
tural or silvicultural wastes, and requires relatively deep water to prevent oxygen from
reaching the substrate. This substrate through which the acidic water must percolate, pro-
duces the reducing environment as well as fuel for the bacteria. Bacteria reduce sulphate