Flight 93 National Memorial
In October 2012, the Office of Surface Mining Reclamation and Enforcement (OSMRE) made available more than $300,000 to the Flight 93 National Memorial. The grant will provide a way to help clean up acid mine drainage water from an underground coal mine near the crash site. To see how the money (which comes from civil penalties levied against mine operators that violated federal mining law) will benefit both the environment and visitors, please take a moment to view the OSMRE's Flight 93 video.
Earlier passive methods of cleaning the acid tainted water are effective to a great extent, but not completely. After a year of study, OSMRE determined that aerating the water from an existing pump well system would help separate the iron, increasing water quality. The study also indicated the site and the environment would benefit more with the construction of a wetland area, including a wide diversity of water-borne plants, similar to this previously built pond. Plants are very effective in binding the iron to themselves, removing the mineral from the water.
OSMRE held a technology transfer event on August 27, 2015, on the Flight 93 Memorial grounds, to discuss the passive technology capable of reducing total suspended metals to less than 2.0 mg/L (Enhanced Solids Fe Removal) and the passive technology capable of removing manganese to less than 1.0 mg/L at pH 7 (Passive Removal of Manganese at Circumneutral pH).
The Flight 93 site plan drawings, that were produced as part of the OSMRE grant, are available for download. The site plans were prepared by the United States Department of Agriculture.
Talking Points and Posters from the Treatment Tour
From the Flight 93 Treatment Tour, the talking points from the treatment tour were collected to capture the key information provided during the event. Available in PDF format, it can be viewed and downloaded as needed. Below is a listing of the five stops during the Flight 93 treatment system tour. The event posters that were designed and displayed during the event and tour can be viewed and downloaded by clicking on the corresponding stop or simply download all 6 event posters.
- Welcome/Registration Poster
- Stop 1: Venturi Site - Pumped Discharge
- Stop 2: Outlet Pond 3/Inlet to Pond 4
- Stop 3: Inlet of Engineered Wetland
- Stop 4: Wetland Outlet
- Stop 5: Manganese Removal Bed
View illustrations of what is occurring at the Flight 93 Memorial within the pond-wetland treatment system to remove iron from the mine water.
Common Treatment Methods for Metal Concentration
- Achieving total metal concentrations of less than 2.0 mg/L can be very challenging at mine drainage treatment sites. Recent implementation of total maximum daily load and other water quality standards have lowered effluent criteria for total metal concentrations to less than 2.0 mg/L in many watersheds. Historically, low total metal concentrations in the effluent have been achieved by using a high treatment pH or the use of flocculants. Both treatment strategies result in increased treatment costs and additional labor to ensure proper dose.
The Usage of Ponds with a Wetland
An extremely effective alternative to these strategies is to use a pond followed by a wetland to enhance solids removal. This treatment strategy can achieve total suspended metal concentrations of less than 2 mg/L.
The use of wetlands for mine drainage treatment has existed for several decades. The specific application of using wetlands to enhance solids removal to achieve less than 2 mg/L of solids concentrations is relatively new. Ponds and clarifiers use gravity as the primary settling mechanism. This mechanism is only effective if particles flocculate (growth of particles) to form non-buoyant suspended solids that can be pulled to the bottom of a pond by gravity. At times, small amounts of suspended particles fail to flocculate to form dense particles and remain in the water as “pin floc.” Pin Floc is often responsible for effluent violations when effluent criteria for total metals are less than 2 mg/L. Pin Floc contain charged surfaces (see diagram) that promotes electrical repulsion between particles and prevents particles from flocculating to form a non-buoyant floc. The overall surface charge on iron hydroxide floc is typically net positive at pHs below 8 (see graph). Humic organic matter (HOM) in wetlands is instrumental in helping to reduce electrical repulsion and allowing particles to settle by releasing humic carboxyl and phenolic groups into the water column. Like a commercial anionic organic polymer, these groups contain a negative charge that will neutralize the positive surface charge of the iron hydroxide particle, promoting flocculation and settling. Furthermore, a dense growth of wetland plants improves settling through the physical impaction of particles into plant material. The impaction causes flocculated particles to lose momentum and initiate settling, much like a baffle in a pond. Combined, the mechanisms can routinely achieve suspended metal concentrations of less than 2.0.
The pond-wetland treatment system on memorial grounds treats a 1,200 gallon-per-minute discharge to a total iron concentration of less than 1.0 mg/L.
Common Treatment Methods for Manganese
The most common method used to achieve manganese effluent standards is to use an alkali chemical to increase pH to 9.5 and precipitate Manganese Hydroxide. While effective, this method results in elevated chemical consumption and increased sludge production due to various nuisance reactions at occur at high pH.
The Usage of Limestone Beds
During the past 15 years, the use of limestone beds to passively remove manganese at a pH between 6 and 8 has been refined. The technology uses biotic reactions to precipitate manganese onto the surface of limestone. These beds have been proven to be easy to construct, highly effective, and low maintenance. Several sites have produced NPDES compliant water for over 15 years.
The manganese removal bed, installed on the Flight 93 memorial grounds, treats a 80 gallon-per-minute discharge to a total manganese concentration of less than 0.5 mg/L.