Treatment of AMD
In 1968, Pennsylvania instituted strict effluent
discharge limitations on mine operations. Many mining companies used
chemical treatment methods to meet these new effluent limits. In these
chemical treatment systems, the acidity is buffered by the addition
of alkaline chemicals such as calcium
carbonate, sodium hydroxide, sodium bicarbonate or anhydrous ammonia.
These chemicals raise the pH to acceptable
levels and decrease the solubility of dissolved metals. Precipitates
form and settle out of solution. But these chemicals are expensive and
the treatment system requires additional costs associated with operation
and maintenance as well as the disposal of metal-laden sludges.
Passive Treatment of AMD
As early as 1978, many variations of AMD passive treatment systems were
studied by numerous organizations in laboratories. During the last 15
years, passive treatment systems have been implemented on full-scale
sites throughout the United States with promising results. The concept
behind passive treatment is to allow the naturally occurring chemical
and biological reactions that aid in AMD treatment to occur in the controlled
environment of the treatment system, and not in the receiving water
body.
Passive treatment conceptually offers many advantages over conventional
active treatment systems. The use of chemical addition and energy consuming
treatment processes are virtually eliminated with passive treatment
systems. Also, the operation and maintenance requirements of passive
systems are considerably less than active treatment systems.
The first passive technology involved the use of natural Sphagnum wetlands
to improve the water quality of AMD without causing other detrimental
impacts on the ecosystem. Although this concept had its limitations,
it spawned research and development into other passive treatment technologies
that did not follow the natural wetland paradigm.
Designing a passive treatment system for AMD requires the understanding
of mine water chemistry, and available treatment techniques. Sampling
the AMD affected water and analyzing its chemistry is extremely important
in the selection of appropriate treatment technologies.
References
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