Passive treatment of acid rock drainage (ARD): state of the practice

Aerobic wetlands, anoxic limestone drains, open limestone channels, successive alkalinity producing systems, and sulfate-reducing bacteria (SRB) anaerobic bioreactors have been used to treat acid rock drainage (ARD). Each has been successful, under limited circumstances, with the degree of success dependent on the chemistry of water to be treated and on-site characteristics. For metal-mine ARD, solid-substrate SRB bioreactor has been the “passive” method of choice. However, it is likely to be most successful when used with other treatment methods to minimize problems associated with any one particular component. Although system designers reference 30-year substrate life spans from a geochemical perspective, no solid-substrate SRB bioreactor designed to treat heavy metal-laden ARD has operated continuously for more than 3 ½ years without some substrate maintenance. Until the physical aspects of SRB substrate design and long-term effects of biodegradation are better understood, operating cost estimates should reflect maintenance of the SRB substrate on a 4- to 5-year schedule. Liquid-reactant SRB bioreactors may potentially be useful in overcoming some inherent problems with the traditional solid-substrate SRB bioreactor applications, but at higher operating cost.