SOME PASSIVE AND SEMI-PASSIVE TREATMENT METHODS

FOR ACID MINE DRAINAGE

Anoxic limestone drains (ALD's) and ponds (see diagram below)

• If alkalinity is needed, a ditch filled with limestone gravel, then covered with (sometimes) plastic and soil is used to add up to 300 mg/L alkalinity under the following conditions:

1) very low dissolved oxygen (DO)

2) no Fe3+ or Al present to clog drain by forming precipitates (at least according to conventional wisdom)

• ALD's are usually followed by oxidation ponds to allow Fe2+ to oxidize & iron hydroxides to precipitate
• Oxidation ponds may be followed by downflow wetlands, which do several things

1) Yellow boy precipitates

2) Organic matter (such as compost) allow sulfate reduction, which adds alkalinity

3) Alkalinity may also be added by including limestone in the wetland below the compost (a SAPS system)

• Ideally, by the time the water leaves such a system, the water will have pH near 7, net alkalinity, iron < 2 mg/L. Mn is not usually removed very well, but there are exceptions. The systems require significant area to build the ponds; they must be large enough to allow enough retention time for oxidation/precipitation and for the iron hydroxides to have time to physically settle out

SAPS (successive alkalinity-producing system) (see diagram below)

• When one ALD doesn't add enough alkalinity or when water has significant DO
• Pond has limestone underneath an organic compost layer; water flows down through compost, then through limestone (both add alkalinity; compost allows for reduction of Fe3+ to Fe2+ so iron hydroxides will not clog limestone)

Aluminator

• When water has significant Al (probably can't use ALD because drain may clog with Al)
• Start system with a SAPS unit that is several feet higher than next pond
• Water flows down through compost, then through limestone
• Valve allows pond to be drained every 6 months which helps to flush aluminum solids out of limestone
• Aluminator followed by successive ponds or more aluminators if elevation change sufficient

Limestone sand dosing

• A relatively new practice, about the cheapest option
• Add limestone sand (fine particles) directly to stream
• Best for situations where Fe & Al relatively low (few ppm) with low pH (2-5)
• Limestone must be replenished, probably at least yearly; iron and aluminum hydroxides still accumulate in stream, although in shorter reaches than before treatment

Limestone channels

• limestone gravel (± sand) is used to line a stream channel
• best where water velocity is relatively rapid and where Fe & Al relatively low (few ppm) with low pH (2-5)
• Limestone must be replenished, probably at least yearly; iron and aluminum hydroxides still accumulate in stream, although in shorter reaches than before treatment

Limestone diversion wells

• Relatively new, relatively cheap
• Create small dam, pipe portion of stream to vertical concrete culvert
• Add limestone gravel to culvert ( every two weeks)
• Best for situations where Fe & Al relatively low (few ppm) with low pH (2-5)
• Dams can break, leaves clog pipes, requires easy access and frequent limestone addition, iron hydroxides still accumulate in stream, must have 4-5 ft elevation change between dam and culvert

SOME IMPLICATIONS FOR LANDOWNERS

Sampling and treatment cannot proceed without landowner cooperation. In all cases, some maintenance is required, so permission to access discharges and treatment facilities must be granted by landowners. Some sites may require improved access so trucks can reach the site. Access will also be needed for chemistry and flow monitoring and possibly visits by tours or classes.

Anoxic limestone drains (ALD's)

• Virtually no impact except must have somewhere to put dirt removed (may be used to construct pond)
• After construction, ALD will not be visible
• likely will require rebuilding and replenishing limestone in 15 - 25 years

Settling Ponds (oxidation ponds, SAPS, Aluminator)

• ties up land (although it may be possible to relocate ponds at a later time)
• pond must be maintained (muskrats may dig holes in dams)
• dams may fail, although this is unlikely with good design (ponds do not usually collect much surface drainage; most water comes from ground water)
• iron hydroxides must eventually be dredged out and disposed of ( 25 years); these should have low concentrations of trace metals - the sediments are mainly iron and aluminum hydroxides
• sediments have potential as an iron oxide resource (see below)

Limestone dosing (and channels)

• limestone sand washes downstream during floods
• iron hydroxides still precipitate in stream, although in a shorter reach than before treatment
• requires yearly treatment, must have truck access to stream

Limestone diversion wells

• Small dam required on stream; dams may burst during floods
• A small portion of the stream is diverted underground through PVC pipe
• Concrete culvert structures must have truck access for limestone addition every two weeks
• alternatively could install hoppers to store more limestone - still requires someone to add limestone often
• iron hydroxides still accumulate in stream, although in a shorter reach than before treatment

Public Relations

• Some landowners see cooperation with a citizens' group as good public relations.
• With landowner permission, the SCRA would like to publicize the fact that landowners are helping implement treatment by providing land, limestone, equipment, or time.

WHERE DOES THE MONEY COME FROM? WHERE DOES IT GO?

• The Northumberland County Conservation District (NCCD) and the SCRA are seeking public (e.g., DEP 319 Nonpoint Source Pollution funding) and private funding (foundations) for treatment. Proposals will be signed by the Conservation District.
• Some funds have been raised by the SCRA by soliciting businesses and citizens.
• Bucknell University has some funds which will go toward student research associated with mine drainage cleanup, and possibly some funding for construction and monitoring.
• Proposal will include in-kind matches of volunteer time and services from businesses, citizens, civic organizations, and possibly the National Guard.
• The vast majority of funds will go toward construction of treatment facilities. Other expenditures will include small portions for grant administration (by the NCCD), monitoring, and research.
• Because the passive technologies are relatively new, the research component is vital to understand how to better treat different waters, and avoid wasting money on ineffective methods.

IRON OXIDE RECOVERY - A BUSINESS OPPORTUNITY?

The iron hydroxides (yellow boy) that precipitate in pond treatment systems may be resources. The idea to recover the iron has been around at least since the turn of the century, but it may be becoming a reality now. The iron hydroxides deposited in passive pond treatment systems are consistently very high in iron content, and are being seriously proposed for use in sewage treatment, as pigment, as colorants in construction material, and other uses. The economics are being worked on now by a consultant in western PA (Bob Hedin, Hedin Environmental). Dr. Hedin is nearly ready for a large scale pilot operation and is looking for industrial partners and the appropriate mine drainage discharge.

• Treatment likely will involve an ALD and series of ponds (possible covered to keep out leaf litter and other "contaminants"
• Instead of taking the sediments to a strip pit for disposal about every 20 years, the idea is to dredge or pump the iron hydroxides (perhaps on a monthly or so basis), then dry and process them nearby (jobs?!) for sale to industry
• This idea has failed in past because the iron hydroxides from active chemical treatment facilities make a sludge that has very variable iron content and color. Passive systems have much more consistent color and composition.
  

Return to AMD Treatment Page

Return to Site 42 Page

Return to SCRA Home Page