Passive Treatment Technologies - University of Colorado at Boulder

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					Coexistence of mining with a healthy environment: The Use of Microbes to Reduce the Costs of Metal and Acidity Removal from Mining Industry Waste Streams
•Make Microbes and Manure Your Major Metal Wastes Management Miracle
Dr. Ronald R. Hewitt Cohen
Colorado School of Mines Golden, CO U.S.A.

AMD
Production of acid water is common to mining sites where pyrite and other metal sulfides become exposed to the atmosphere. Sufficient oxygen and water are present to initiate oxidation of pyrite. Few other natural weathering reactions produce this amount of acidity.
Pyrite  Oxygen  Water "Yellowboy" SulfuricAcid

Microbial Involvement

•

At pH 3.5 or less, bacteria such as Thiobacillus ferrooxidans accelerate the rate of conversion of Fe2+ to Fe3+. Such bacteria may accelerate reactions by orders of magnitude.

The Four Generally Accepted Reactions That Represent Acid Mine Drainage are as follows:

Pyrite  Oxygen  Water  FerrousIron  Sulfate  Acidity

FerrousIron  Oxygen  Acidity  FerricIron  Water

FerricIron  Water  FerricHydroxide( yellowboy )  Acidity

Pyrite  FerricIron  Water  FerrousIron  Sulfate  Acidity

Water contaminated with acid mine drainage may not be suitable for drinking, livestock watering, support of wildlife, irrigation or industrial use. There may be impacts associated with Cd, Cu, Pb, As, Zn, as well as antimony, beryllium, Cr, Ni, Ag, thallium, U, Ra, and Se.

Treatment of Acid Mine Drainage
There Are Two Broad Classes of Methodologies Used To Treat Acid Mine Drainage:
Active Treatment (conventional) –Mechanical addition of alkaline chemicals to the effluent is used raise pH and precipitate metals. Passive Treatment –naturally occurring chemical and biological reactions occur in a controlled microbiological – chemical reactor without powered mechanical assistance (most of the time).

Active Conventional Technologies – Chemical Precipitation

Removal of metals can be facilitated by neutralization using a hydroxide precipitate-caustic soda treatment. Often, chemical neutralization is accomplished with slaked lime or calcium carbonate added directly to the water.

ILS
There also is In-Line Aeration and Neutralization System (ILS) which incorporates the chemical treatment processes into a functionally closed system where the treatment reactions can be more closely monitored and accelerated in order to reduce the chemical reagent costs and reaction processing times.

Electro-precipitation

• Electro-precipitation processes accomplish similar results by the precipitation of metal hydroxides or by metal ion adsorption.

Active Treatment Systems:
Advantages:
 

Active treatment methods are industry proven

The regulatory community is familiar with and has a considerable understanding and comfort with active systems.

Disadvantages:
 

The chemicals employed are expensive There are extensive costs which include high overheads (e.g., operations and maintenance costs) and high disposal costs (e.g., wet metal laden sludge)

All of these active processes, both chemical and physical, are severely limited in that they:
• 1) are unable to treat the sometimes excessive sulfate concentrations associated with most acid-mine drainage, • 2) impart a high degree of hardness to the water, and • 3) produce waste sludge which requires additional treatment and/or disposal.

COSTS!

These methods can also require large capital costs, operations, and maintenance. Precipitation operations must be perpetual, and the resulting, voluminous sludge must be disposed of in increasingly limited hazardous waste repositories.

Passive Treatment Systems:
Advantages:


In theory, passive treatment systems can virtually eliminate the need for expensive chemical addition The consumptive energy needs of passive systems are negligible when compared to those of active systems Finally, the operation and maintenance requirements are significantly reduced as compared to the active systems



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Passive Treatment Systems, Continued:
Disadvantages:


The majority of the studies since 1978 have been at the bench-testing level



While full–scale implementations have shown promising results, they typically fail within 2 years or don’t remove metals to receiving water standards.


Due to the above, the regulatory agencies do not seem to have a warm and cozy feeling to the extent that they are limiting the available funds for further research.

Passive Treatment Systems, An Overview
Various Passive Treatment Systems Recognized Today:



Natural Wetlands
Constructed Wetlands - Aerobic - Anaerobic

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Sulfate-Reducing Bacteria Bioreactors Anoxic Limestone Drains (ALD) Successive Alkalinity Producing Systems (SAPS) Limestone Ponds

Treatment of acid mine drainage using artificial and natural wetlands is a promising new approach.

Natural Wetlands
In 1978 Huntsman et al. and later in 1982 Wieder and Lang first noted improvement in the water quality of AMD following the passage through naturally occurring Sphagnum (moss) bogs in Ohio and West Virginia. Other Studies conducted by Brooks et al. in 1985, Sencindiver and Bhumbla in 1988, and Samuel et al. in 1988 found similar results in flowing AMD through Typha (cattails) wetlands. Even though there is evidence that suggest that some wetland plants can adapt to low pH and high metal concentrations, it has been shown that AMD will eventually degrade the quality of the natural wetlands. This degradation goes completely against the laws put in place to protect the natural wetlands. (Bad Idea)

Constructed Wetlands
There Are Two Type of Constructed Wetlands “aerobic” and “anaerobic”.
Aerobic Wetlands:
 

Consist of Typha and other wetland vegetation This vegetation is planted in shallow (<30 cm) relatively impermeable sediments consisting of soil, clay or mine spoil.

Anaerobic Wetlands:
 

Consist of Typha and other wetland vegetation This vegetation is planted in deep (>30 cm) permeable sediments consisting of soil, peat moss, spent mushroom compost, sawdust, straw/manure, hay bales or a variety of other organic materials. This is often underlain with limestone or admixed with limestone.

Constructed Wetlands, Continued
Aerobic Wetlands:


For this treatment scheme the AMD must have a net alkalinity for effective treatment to occur In Aerobic wetland systems treatment occurs through the precipitation of metals by the way of oxidation reactions to form oxides and hydroxides To obtain an effective treatment in aerobic wetland systems the pH of the influent needs to be above a pH = 5.5 It is typical with this system to see upstream riffles and falls to aerate the water to promote the efficiency of the oxidation process.

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Photos of an Aerobic Wetland

Treatment System

Constructed Wetlands, Continued
Anaerobic Wetlands:


It has been reported that anaerobic wetlands can treat net acidic, low pH, high Fe, and high DO (>2 mg/L) AMD effluent This treatment capability over the aerobic wetland system is due to the ability of an anaerobic system to generate alkalinity through microbial interactions and/or the mixture of limestone in the bottom sediments. However again as with the aerobic wetland, treatment is most effective when used to treat small AMD flows with moderate water quality.

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Pictures of an Anaerobic Wetland

Treatment System

Now Called Passive Mine Drainage Treatment Systems

The focus of these Passive Mine Drainage Treatment systems (PMDTS) is to apply biogeochemical water treatment mechanisms at or near the source of the mine drainage to concentrate and immobilize metals and raise pH.

How do passive systems and wetlands ‘treat’ metals

• The PRIMARY mechanism for metal removal and pH modification is the production of sulphides and alkalinity by Sulphate Reducing Bacteria (SRB).

2CH 2 OH 2 S 2HCO3SO
H2 S Fe
2+

24

FeS ( s )

2H

+

Sulphate Reducing Bacteria
Sulphate reducing bacteria (SRB) are obligate anaerobes that decompose simple organic compounds using sulfate as the terminal electron acceptor. The result is the produc-tion of sulfide that may be given off as H2S gas or react with metals to form metal sulphides.

Rates of Sulfate Reduction in nanomoles of S2- per cm3 of substrate per day
ISOTOPIC METHOD 35S
Location sample taken in Reactor Date Rate Standard Deviation (number of samples)

Surface
Bottom Surface Control

11/95
11/95 11/95 11/95

600
440 750 12.2

10.9 (4)
10.4 (3) 8.66 (6) 29 (3)

AVS Method Surface 11/95 670 35 (6)

How Effective Can a PMDTS Be?
Cohen and Staub used composted cow manure and hay in a 4 to 1 by volume ratio. The substrate used by Cohen and Staub was able to efficiently (98-100% metal removal) treat between 4 and 8 times the mine drainage flow rates of previous systems used in the Rocky Mountain Region.

The Organic Substrate

The organic matter in the manure encourages decomposition and generation of low redox potentials and serves as a SRB nutrient source. Thus, ideal conditions are presented to the SRB until they can modify their own microenvironments.

Constructed Wetlands, Continued
Problems Associated with Constructed Wetlands:


The long term retention of iron sulfides and iron hydroxides in wetlands is still not well understood. Wetland plant species very greatly in their abilities to accumulate metals and some species have no ability at all. Wetlands are prone to hydrological failure or clogging which leads to channeling and greatly reduces the overall effectiveness of treatment of the AMD. Finally, by following the suggested sizing equations found throughout the literature, wetlands take a lot of area. Due to the location of a site, this may well lead to choosing alternative treatment systems.

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Anoxic Limestone Drains


First described by the Tennessee Division of Water Pollution Control (TDWPC) in 1990. The TVA later observed AMD being pretreated as it flowed through an old haul road buried under a refuse dam, which contained a high amount of limestone. were typically used as a pretreatment for water flowing into constructed wetlands.

 ALDs

 A maximum producible alkalinity of

300 mg/L as calcium carbonate has been reported in the correct conditions.



It has been reported that if high levels of dissolved ferric iron are present in the AMD, clogging of the limestone pores by the iron hydroxides will occur. This in turn reduces the generation of alkalinity.

A Picture of an Anoxic Limestone Drain Under Construction – Perhaps Assoc. with Jim Gusek?

Successive Alkalinity Producing Systems


Successive Alkalinity Producing Systems are comprised of an anoxic limestone drain and an organic substrate. DO concentration is often a design constraint if >1mg/L when dealing with the ALD, however in a SAPS the acid water is ponded from 1 to 3 m over a layer 0.2m to 0.3m of organic compound, this in turn is over a 0.5m to 1.0m layer of limestone which contains a network of permeated pipes. hydraulic head of the pond drives the water through the organic compost to strip it of oxygen and reduce ferric iron to ferrous iron. The water then flows out into an aerobic pond where it precipitates out. relatively new, it is anticipated that by putting them in series treatment of AMD with high iron loads is feasible.

 The

 The

 Although these systems are

Successive Alkalinity Producing System
Top – Under Construction
Bottom - Post Construction

Limestone Ponds
 The

limestone pond is new idea built on the upwelling of an AMD seep or underground water discharge point.



Limestone is placed at the bottom of the pond in a layer 1 to 3 meters deep. The water is then forced to flow up through the limestone with a retention time in the pond of 1 to 2 days. with an ALD system it is recommended for use with a water containing a very low DO content and no Fe(III) or Al(III).
main advantage over the ALD is that the operator can see if coating of the limestone starts to occur. If coating does occur the operator can stir the limestone with a backhoe to help remove it periodically.

 As

 The

A Picture of a
Limestone Pond Under Construction

In Closing


Passive treatment can offer several advantages over the more expensive alternatives, however further pilot scale studies need to occur in order to convince regulators that this is a viable alternative



It will also take investors willing to take some risk to back the engineers and scientist that in order to show that these methods are viable



Site topology, geography, hydrology and chemistry must be in the front
end of any passive treatment project because as you can see, this is not a one size fits all solution



One final note: All stake holders must come to terms with one simple fact, any man made system will eventually or routinely require some degree of maintenance

Thank You


				
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