The Office of Special Reclamation is required to treat polluted mine drainage from mine sites that have had their mining permits revoked and bonds forfeited. Polluted mine drainage may be acidic with pH values less than the lower acceptable limit for water quality standards (<6 on a standard pH scale) and high metal content, primarily iron, aluminum, and manganese, or alkaline with a higher pH (>6) and high metal content, usually iron.
Acid mine drainage, generally referred to as AMD, requires alkaline addition to raise the pH. Alkalinity is generally in the form of lime products such as limestone or hydrated lime, or other products such as sodium hydroxide, calcium hydroxide, or anhydrous ammonia.
Alkaline mine drainage, on the other hand, does not require any pH adjustments. This type of mine drainage is generally treated passively which requires ample space for treatment. If ample space is not available, alkaline mine drainage may require chemical addition in the form of oxidizers, coagulants, or polymers which enhance the settling of metal particulates resulting in clearer water.
OSR uses two basic approaches to treating mine drainage:
Active Treatment- uses a chemical neutralizing agent, such as lime or sodium hydroxide, that is added directly to the AMD at the source.
Passive Treatment- a treatment system in which the AMD passes through a neutralizing media, such as limestone, or a structure that enhances metal precipitation such as a wetland or settling pond.
Mine sites with larger volumes of polluted mine drainage and limited space for treatment may require an active treatment system similar to Omega Miningpictured below in the slide show. This treatment system utilizes a clarifier similar to those seen at municipal water treatment plants. Clarifiers are large tanks used to provide an adequate volume for retention while also using a mechanism that condenses or "rakes" the sludge into the center of the tank where it can then be easily pumped to a sludge drying area.