TREATING AMD
There are three main goals for treatment of abandoned mine drainage. First is to raise the pH. Second is to increase alkalinity. Third is to create conditions which allow the dissolved metals in the water to form as a solid and drop out of the water.
The most important condition needed to treat AMD is a substance that is able to increase alkalinity. This is usually found in limestone. Limestone provides alkalinity which buffers acidity and helps raise the pH of water.
If the pollution in a stream was due only to a low pH and high acidity, then allowing it to come in contact with limestone would be all that was needed. This would increase the pH and alkalinity. The problem with AMD water is that often there are other materials present that pollute the water. There are various metals that are dissolved in the water that kill fish and bugs.
The most important condition needed to treat AMD is a substance that is able to increase alkalinity. This is usually found in limestone. Limestone provides alkalinity which buffers acidity and helps raise the pH of water.
If the pollution in a stream was due only to a low pH and high acidity, then allowing it to come in contact with limestone would be all that was needed. This would increase the pH and alkalinity. The problem with AMD water is that often there are other materials present that pollute the water. There are various metals that are dissolved in the water that kill fish and bugs.
Iron is the most common metal that needs to be removed when treating AMD. Iron stays dissolved until two conditions are met. The water must come in contact with oxygen AND its pH must be above a 3.5 for the iron to form as a solid. To accomplish this for acidic water (acidity greater than alkalinity), the water needs to come in contact with limestone that is usually placed in a pond. The problem is that as soon as the water flows through the limestone, the iron will form as a solid and coat the rocks. In time the limestone will become clogged and the water can no longer travel through the rocks. To avoid this, the oxygen that is dissolved in the water must be removed. This is done by allowing the water to go through compost. Compost is a mixture of decaying material such as leaves or manure. When something decays it uses oxygen. Therefore if AMD affected water goes though a pond that has a layer of compost, the oxygen will be used by the material that is decaying. Then when the water comes in contact with the limestone, the pH will increase but, since there is no oxygen, the iron will not form as a solid. This will not happen until the water exits the pond and comes in contact with air. Then the iron will form as a solid and settle out. Iron will have a red-orange color.
Some discharges that contain iron are alkaline (alkalinity greater than acidity). This means the pH is high enough for the iron to form as a solid. This occurs when the water comes into contact with a limestone layer in the ground. Therefore, as soon as this water exits a mine and comes in contact with oxygen, it forms as a solid. Treating these discharges requires any method that increases the amount of oxygen water is exposed to. This includes the construction of large shallow ponds; steep rock lined channels that water can “tumble” down; and in some cases where electricity is available, aeration systems that bubble the water much like the aerator in a fish tank.
Some discharges that contain iron are alkaline (alkalinity greater than acidity). This means the pH is high enough for the iron to form as a solid. This occurs when the water comes into contact with a limestone layer in the ground. Therefore, as soon as this water exits a mine and comes in contact with oxygen, it forms as a solid. Treating these discharges requires any method that increases the amount of oxygen water is exposed to. This includes the construction of large shallow ponds; steep rock lined channels that water can “tumble” down; and in some cases where electricity is available, aeration systems that bubble the water much like the aerator in a fish tank.
Aluminum is another metal that needs to be removed when treating AMD. The only condition that aluminum needs to form as a solid is that pH of the water is above 5.0-5.5. Oxygen is not needed for this to happen. Therefore, the water only needs to come in contact with limestone. Aluminum will have a white color.
Manganese is a metal that can be dissolved in water that is polluted by AMD. The condition it requires to settle out as a solid is the pH of the water needs to be greater than 9.0. It is very difficult to reach this pH using limestone alone. Usually some type of caustic alkaline chemical needs to be added to the water in order to reach a high enough pH for manganese to form as a solid. Manganese does not have an effect on the fish and the bugs living in the water. This metal is more of a concern if the water is used as a drinking supply. It is black as a solid.
In order to take care of the above concerns in AMD, treatment systems are usually constructed near the site of the discharge. These systems can be placed into three groups: active, semi-passive or passive. Active treatment needs electricity most of the time and involves adding caustic alkaline chemicals such as lime, soda ash or ammonia. Semi-passive treatment doesn’t use electricity but requires manpower to bring in treatment materials throughout time. Passive treatment using limestone and/or compost to treat the water in wetlands does not require the addition of materials on a regular basis. These are described in more detail under Treatment Methods.