Abstract: Ten passive treatment systems, located in Pennsylvania and Maryland, have been intensively monitored for up to ten years. Influent and effluent water quality data from ten anoxic limestone drains (ALDs) and six reducing and alkalinity-producing systems (RAPS) have been analyzed to determine long-term performance for each of these specific unit operations. ALDs and RAPS are used principally to generate alkalinity, ALDs are buried beds of limestone that add alkalinity through dissolution of calcite. RAPS add alkalinity through both limestone dissolution and bacterial sulfate reduction. ALDs that received mine water containing less than 1 mg/L of both ferric iron and aluminum have continued to produce consistent concentrations of alkalinity since their construction. However, an ALD that received 20 mg/L of aluminum experienced a rapid reduction in permeability and failed within five months. Maximum levels of alkalinity (between 150 and 300 m&) appear to be reached after I5 hours of retention. All but one RAPS in this study have been constructed and put into operation only within the past 2.5 to 5 years. One system has been in operation and monitored for more than nine years. AIkalinity due to sulfate reduction was highest during the first two summers of operation. Alkalinity due to a limestone dissolution has been consistent throughout the life of the system. For the six RAPS in this study, sulfate reduction contributed an average of 28% of the total alkalinity. Rate of total alkalinity generation range from 15.6 gd''rn-'to 62.4 gd-'mL2 and were dependent on influent water quality and contact time.

Abstract: Acid mine drainage (AMD) is one of the most serious environmental problems in the metal-mining industry. AMD is formed by the chemical and bacterial oxidation of sulphide minerals, and it is characterized by low pH values and high sulphate and metals content. The most common method to treat AMD is chemical neutralization. The chemical treatment requires high capital and operating costs and its use is problematic at the closed mines sites. Outokumpu has studied and used sulphate reducing bacteria (SRB) as an alternative method for the treatment of AMD. SRB existing in many natural anaerobic aqueous environments can reduce sulphate to sulphide which precipitates metals as extremely insoluble metal sulphides. Full scale experiments were begun in summer 1995 in the Ruostesuo open pit (depth 46 m) by adding liquid manure as a source of bacteria and press-juice as a growth substrate. The average Zn content of the whole column has decreased from 3,5 mg/l to 0,8 mg/l and below 25 m zinc is 0 mg/l. Similar results have been reached with nickel in the Kotalahti old nickel mine, where bacteria were brought in 1996. We have found that the same bacterial mechanism acts in peat-limestone filters, which Outokumpu has built at several mine sites since 1993.