Settling ponds are simply ponds that collect water so that it can be relatively calm for a period of time, thereby allowing suspended solids time to precipitate or "drop out" of the water. For AMD-polluted water, solids such as iron (in the form of yellowboy or iron oxide) settle to the bottom of the pond and collect there. The water leaving the pond will not have as much metal or solids in it. The solids collected on the bottom of a settling pond gradually build up and have to be cleaned out periodically.
For AMD high in dissolved iron, settling ponds have another function: exposing the water to oxygen. The dissolved iron may be in one of two chemical states: ferrous (+2) or ferric (+3) ions. Ferrous iron will not drop out of the water, whereas ferric iron quickly combines with water to form the rust-like compound yellowboy (iron oxide), which does precipitate. A settling pond's large surface area permits oxygen from the atmosphere to dissolve into the polluted water. The oxygen reacts with ferrous iron to form ferric ions, which quickly become yellowboy and settle.
If mine water is alkaline and only contains iron, settling ponds may be the only treatment necessary. If the water is acidic or more polluted, settling ponds are used in conjunction with other treatment elements, such as vertical flow reactors.
Settling ponds may have commercial use: studies are underway to try and recycle the solids that collect on the bottom. For example, iron oxide can be use as a pigment in stains. In these cases, the bottom of the pond should have an impermeable liner to prevent dirt and clay from mixing with the collected metals.
Operations & Maintenance Considerations
As previously mentioned, metals that accumulate on the bottom of a settling pond need to be removed. They reduce the volume of the pond, decreasing the time that water is retained, thereby giving metals less time to settle out of it. Also, flows of water may create preferential paths through metal sludge over time. This results in short-circuiting, where water flows quickly in a current through the pond, without dispersing throughout or stalling long enough for metals to precipitate. One solution is to build berms, baffles, or other obstructions in the pond to deflect water from any preferential paths.
Of course, like most passive treatment systems, basic maintenance including mowing grass around a system and removing leaves that may clog conduits is necessary. Also, animals may damage a system, especially muskrats, whose tunnels must be blocked with dirt and rocks.