On occasion, despite the application of sound mining and reclamation principles, Acid Mine Drainage will be formed and must be treated to meet existing Federal and State standards before it is released from the minesite. Prior to opening the mine the operator should evaluate the potential for creating Acid Mine Drainage that would require treatment and should become familiar with the extent of the costs that such treatment might impose. Consideration should also be given to the possibility that treatment might have to be continued well into the future, specifically until such time that the discharge meets effluent standards without treatment.

Treatment, as normally applied to Acid Mine Drainage, involves chemical neutralization of the acidity followed by precipitation of iron and other suspended solids. Treatment systems include:

1. equipment for feeding the neutralizing agent to the Acid Mine Drainage
2. means for mixing the two streams (Acid Mine Drainage and neutralizing agent)
3. procedures for ensuring iron oxidation
4. settling ponds for removing iron, manganese, and other co-precipitates

Calcium content of limestone should be as high as possible. (Dolomitic limestones are less reactive and generally ineffective in treating Acid Mine Drainage.) Advantages of using limestone include low cost, ease of use, and formation of a dense, easily handled, sludge. Disadvantages include slow reaction time, loss in efficiency of the system because of coating of the limestone particles with iron precipitates, difficulty in treating Acid Mine Drainage with a high ferrous-ferric ratio, and ineffectiveness in removing manganese. Limestone treatment is generally not effective for acidities exceeding 50 mg/l.

Hydrated lime is normally the neutralizing agent of choice by the coal mining industry because it is easy and safe to use, effective, and relatively inexpensive. The major disadvantages are the voluminous sludge that is produced (when compared to limestone) and high initial costs that are incurred because of the size of the treatment plant.

Soda ash briquettes are especially effective for treating small Acid Mine Drainage flows in remote areas. Major disadvantages are higher reagent cost (relative to limestone) and poor settling properties of the sludge.

Caustic soda is especially effective for treating low flows in remote locations and for treating Acid Mine Drainage having a high manganese content. Major disadvantages are its high cost, the dangers involved with handling the chemical, poor sludge properties, and freezing problems in cold weather.

Anhydrous ammonia is effective in treating Acid Mine Drainage having a high ferrous iron and/or manganese content. Ammonia costs less than caustic soda and has many of the same advantages. However, ammonia is difficult and dangerous to use and can affect biological conditions downstream from the mining operation. The possible off-site impacts are toxicity to fish and other aquatic life forms, eutrophication and nitrification. Fish species generally exhibit low tolerance to unionized ammonia and toxicity levels can be affected by Ph, temperature, dissolved oxygen and other factors. A more complete review of ammonia treatment of mine drainage is given by Faulkner (1991). Ammonia use is not allowed in all States and, where permitted, additional monitoring is required.

Constructed wetlands utilize soil- and water-borne microbes associated with wetland plants to remove dissolved metals from mine drainage. Initial design and construction costs may be significant, ranging into tens of thousands of dollars. Unlike chemical treatment, however, wetlands are passive systems requiring little or no continuing maintenance. This is a relatively new treatment technology with many specific mechanisms and maintenance requirements not yet fully understood. Optimum sizing and configuration criteria are still under study. Seasonal variations in metals removal efficiency have been noted with lesser amounts removed in cold weather. Wetlands are generally more effective in removing iron than manganese. The greatest utility of wetlands appears to be in the treatment of small flows of a few gallons per minute.

The following costs for treating various Acid Mine Drainage flows and quality, using each of the neutralizing chemicals listed above except limestone, were adapted from data developed and published by Skousen, and others (1990) and Fletcher and others (1991). Limestone treatment costs were not determined because limestone is ineffective for treating Acid Mine Drainage where the acidity is above 50 mg/L.TABLE 1. Annual cost and total treatment system cost or net present value (NPV) for four chemicals used for acid drainage treatment for selected Acid Mine Drainage conditions: 1990 dollars, interest rate = 6% (Adopted from Skousen et. al., 1990, and Fletcher et.al. 1991)

Acid Mine Drainage Treatment Costs
Methods	Hydrated Lime	Soda Ash	Caustic Soda	Ammonia
Installation	25000	4000	2500	15000
Ann Repair	3300	0	0	1000
Salvage Value	10000	500	500	5000
Ann Reagent	526	3016	4289	1116
Ann Total	10038	4355	5068	6157
NPV	26832	11641	13546	16458
Scenario 1

Acid Mine Drainage Treatment Costs
Methods	Hydrated Lime	Soda Ash	Caustic Soda	Ammonia
Installation	35000	4000	2500	15000
Ann Repair	3400	0	0	1000
Salvage Value	12500	500	750	4000
Ann Reagent	2631	15079	21447	5580
Ann Total	12123	15940	22051	9432
NPV	51067	67144	94784	39731
Scenario 2

Acid Mine Drainage Treatment Costs
Methods	Hydrated Lime	Soda Ash	Caustic Soda	Ammonia
Installation	50000	N.A.	7500	15000
Ann Repair	3500		0	1000
Salvage Value	15000		1000	3000
Ann Reagent	13158		107237	27904
Ann Total	25867		108840	31932
NPV	108963		458474	134510
Scenario 3

Acid Mine Drainage Treatment Costs
Methods	Hydrated Lime	Soda Ash	Caustic Soda	Ammonia
Installation	80000	N.A.	7500	15000
Ann Repair	4000		0	1000
Salvage Value	20000		1000	2500
Ann Reagent	52634		428948	111614
Ann Total	67351		429892	114462
NPV	495712		3164040	842454
Scenario 4

* Installation costs for ammonia is based on purchasing a tank, rather than leasing. Leasing may be considerably cheaper.
** The total system cost (NPV) was calculated by taking the present value of the system (including installation cost spread over the duration, annual repair cost, annual reagent cost, and salvage value), and then summing over the duration.