Acidic metal-bearing aqueous discharges have been an environmental problem since mining first began. This problem is typically associated with abandoned coal mine operations; however, most mining activities have had some sort of water quality issue associated with them.
Up until approximately 20 years ago the only treatment option available to effectively handle these water quality issues were active treatment systems utilizing harsh chemicals and energy intensive machinery (i.e. pumps, agitators, aerators, presses, etc) which require constant attention and maintenance.
Within the last 20 years and primarily within the last 10 years, passive treatment systems utilizing environmentally friendly materials (usually limestone and compost) in a series of ponds, beds, and wetlands, requiring no electricity and little maintenance have been developed.
However, passive systems have historically (if 20 years can be called historical) not been dependable in meeting federal and state regulatory effluent limits to which mining companies are required to conform. In addition, these systems, with the exception of a few, have not been able to remove manganese and those that are removing manganese are not capable of reducing high concentrations to very low concentrations.