Manganese (Mn) is a difficult metal to remove in acid mine drainage because of high pH requirements for oxidation of Mn to form Mn oxide precipitates. However, Mn removal can be quickened in a gravel system providing a large surface area and high pH for Mn oxidation to occur with microbiological mediation. An experiment was designed with 12 gravel bed mesocosms that were 9.8 m3 × 6.7 m3 × 0.6 m3 to determine the best design criteria for optimizing Mn removal. Treatments consisted of two Mn loading rates (1.1 and 2.7 g/m2·d) and two gravel types (limestone and river gravel) and were replicated three times.
Water flowed through the experimental wetlands for 734 days. Manganese removal was more effective in limestone than in river gravel. Manganese removal was not affected by water temperature ranging from 5 to 30 °C in either rock material. Manganese removal rates ranged from 100 to 600 m/a in the limestone wetlands and 10 to 60 m/a in the river gravel wetlands. Greater pH in limestone (approximately 6.9) compared with river gravel (approximately 5.5) favored Mn oxide precipitation. Greater pH, coupled with oxidation–reduction potential (redox) values greater than 430 mV in the limestone, resulted in water chemistry near conditions predicting manganite to be the dominant Mn phase. Ideal pH and redox conditions for Mn removal are pH from 6.8 to 7.2 and redox greater than 500 mV.
The range of dissolved oxygen (DO) required to remove Mn in the various wetlands ranged from 3 to 5 mg/L, with approximately one‐half of the DO loss caused by Mn hydroxide formation and one‐half ascribed to biological consumption. The influent DO should be at least 0.35 mg/L for every 1 mg/L Mn removed. Removal rates for Mn ranged from 1 to 17 g/m2·d in limestone and from 1 to 2 g/m2·d in river gravel. Limestone is the material of choice for subsurface flow wetlands for Mn removal. Removal rates and required chemistry determined in this study can be used to design subsurface flow wetlands for optimum Mn removal.

Abstract. Constructed wetlands for treating acid drainage are preferred, low-cost, alternatives to conventional treatment. Drainage with high Fe (e.g., >50 mg/1) and zero alkalinity has not been amenable to treatment with wetlands alone, primarily due to Fe hydrolysis and resultant low pH which requires chemical treatment. Anoxic limestone drains (ALDs) increase the alkalinity of seepage that is then routed to a constructed wetlands. Increased alkalinity buffers the wetlands system from pH decreases, and enhances the effectiveness of wetlands treatment. The Tennessee Valley Authority has modified two low-pH constructed wetlands with ALDs. Results indicate retrofitted wetlands· function at efficiencies meeting effluent limitations without chemical treatment. Designs of 'ALDs are site-specific, but generically consist of an excavated seepage-interception trench backfilled with crushed limestone covered with plastic and clay soil. Dissolution rates of limestone in operational and simulated ALDs were experimentally measured in attempts to estimate design parameters and longevity of an ALO. Potential problems with ALDs include structural and hydraulic stability, plugging due to reaction products within the ALO, and inadequate design and installation.

Constructed wetlands are often a preferred alternative to conventional methods of treating acid drainage at mine sites, coal preparation facilities, and coal-fired power plants. The Tennessee Valley Authority (TVA) has designed and constructed wetlands in Alabama and Tennessee to treat acid discharges from these sources. Between June 1985 and August 1987, seven wetlands were constructed to treat acid drainage at an inactive coal preparation plant and adjacent mined area and four wetlands were constructed at TVA coal-fired power plants. Although sitespecific characteristics often restricted use of standardized methods, generic design, construction. and operation guidelines have been developed. Treatment efficiencies have ranged from 82%-99% removal for total iron and 9%-98% removal for total manganese. Preliminary design guidelines for required treatment area were: ph <

Abstract. A microcosm study was conducted to evaluate the use of anaerobic wetlands preceding aerobic wetlands for removal of Mn, Cu, Ni, Zn, and Pb in wastewater.Initial concentrations for Mn, Cu, Ni, Pb, and Zn were 20, 2.0, 1.5, 2.1, and 2.0 mg/L, respectively. Each experimental unit consisted of three cattlefeeding troughs (cells) set in series.The first cell was anaerobic and the last two were aerobic.Water was delivered to the wetland cells at 20 mL/min for a period of 380 days starting August 24, 1994.The anaerobic wetlands consisted of three treatments replicated two times.The aerobic wetlands consisted of two treatments replicated three times. One anaerobic treatment contained organic matter and limestone (SP).Another anaerobic treatment contained organic matter, limestone, and canarygrass (SP&CG).The third anaerobic treatment consisted of canarygrass planted in river gravel (RG).Water flowed into the top and was discharged from the bottom of each anaerobic wetland. The aerobic

Abstract. The Tennessee Valley Authority (TVA) operates 12 staged, aerobic, constructed wetlands to treat acid drainage at reclaimed coal mines, a coal preparation plant, and at coal-fired power plants.