Recirculating-reducing and alkalinity producing system (ReRAPS) for the treatment of acidic coal pile runoff.

摘要

The electric utility industry has been subjected to increased environmental discharge restrictions and operational restraints regarding the treatment of stormwater runoff from coal storage operations. Pyrite oxidation processes within the coal combine with rain water to produce an acidic runoff, which can contain metal pollutants and is toxic to aquatic life. This research evaluated the treatment of coal pile runoff using an alternative constructed wetland design. This alternative design, which provided improved wetland performance, was based on the partial re-circulation of treated water into a detention basin located immediately upstream from a Reducing and Alkalinity Producing System. This modification created a semi-passive RAPS-based system which will be referred to as a Recirculating RAPS.; It was hypothesized that recirculation would moderate the pH in the detention pond resulting in the removal of metals such as Fe, Al, and Mn through co-precipitation chemical processes upstream from the RAPS component. This would therefore minimize the potential for Al hydroxide plugging in the RAPS substrate. It was further hypothesized that the entire ReRAPS wetland would remove sufficient amounts of contaminants from the CPR so that short term chronic toxicity tests would indicate that the whole water effluent was non-toxic to aquatic life.; After a three year stabilization period, the CPR had the following average influent concentrations: 12.8 mg/L of iron, 24.9 mg/L of aluminum, 2.9 mg/L of manganese, and 178.0 mg/L of acidity. The detention pond removed 82% of the total iron, 59% of the aluminum, and 35% of the acidity loading prior to the RAPS component. Manganese was not removed in the detention pond and in the RAPS component but was removed in the settling basin and drains. Follow-up toxicity testing found that the ReRAPS removed the toxicity of the CPR to levels which were only slightly toxic based on the seven day chronic toxicity test for larval Fathead minnow growth and Cladoceran reproduction. The residual toxicity may be due to the low levels of dissolved nickel and zinc. Ironically, the commonly used surrogate for trace metal removal, manganese, was found to have a possible protective effect against chronic toxicity.