In this article the wastewater treatment specialists at WTS examine some of the environmental problems caused by legacy mine workings, looking in particular at the remediation of polluting wastewaters and treatment of acid mine drainage (AMD).
The article explains what acid mine drainage is, what causes it, and how it can lead to significant environmental problems. It goes on to highlight how AMD can be treated using physical, chemical and biological techniques. It concludes with a review of the use of eco-friendly treatment techniques including reed-beds and wetlands.
What is acid mine drainage?
We’re all becoming increasingly aware of the damage which heavy industry does to the environment. One such problem that is not immediately obvious is known is acid mine drainage, also known as AMD. This is a serious issue in countries which have an active mining industry or have had a mining industry in the past.
Acid mine drainage is the term used to describe the acid water which is produced in mining activities, which can become a serious threat to the groundwater, affecting streams, rivers and lakes, and consequently both wildlife and drinking water. In the UK, AMD is largely a historic issue – there are mines which were dug in Roman times which still cause issues with acid run-off water.
What causes acid mine drainage?
The main culprit mineral causing AMD is iron pyrites, more commonly known as fool’s gold. When the iron pyrites comes into contact with the air through mining, the chemical reaction which is created oxidises the iron.
During the period in which the mine is active, this reaction does not tend to cause problems, as water is kept out of the mine and away from any exposed iron pyrites to allow the miners to continue work. However, when the mines close, the pumping stops and the water levels underground rise to their natural level. Over time, the water which has flooded the old mine workings can eventually flow out through old tunnels and springs into nearby streams and rivers to cause serious pollution.
What problems are caused by acidic mine water?
There are several implications of having an old mine and polluting AMD seeping into the water supply. On a simple level, high levels of acidic water flowing into the water supply changes the appearance of lakes or rivers, damaging water and vegetation and making it look unattractive. High levels of metals or acidity in the water makes it unsuitable, not only for drinking, but also for agricultural or industrial uses. Acid mine drainage can also kill off fish, insects and vegetation within a river, and in serious cases, kill off all life from the point at which the polluting mine water enters the river.
Even in cases where the levels of mine water drainage are not so acute, the biodiversity of a water course can be greatly affected. This is often due to a high level of minerals suspended in the water stopping light getting through the water to the bottom of the river or lake.
The chemical reaction between the iron pyrites and the oxygen contained in the water reduces the amount of oxygen in the water for aquatic life, and heavy metals such as lead or copper in the mine water is especially damaging to fish. Deposits which sink to the bottom of rivers and streams can also affect the way in which fish eggs develop and hatch.
Treating acidic water from legacy mines
Treatment for acidic mine drainage aims to remove the metals from the water, and adjust the pH to a more normal level. There are three main ways of achieving this.
Method one – Physical treatment
Physical treatment of acidic waters uses artificial waterfalls to add turbulence into the water coming out of the mine. This helps to churn up the water and adds lots of oxygen into the mix, creating a heavy iron hydroxide sludge which can be removed and processed. Initial set-up costs for this type of physical set-up can be high, but these can often be offset by the value of the iron ore which can be reclaimed from the resulting sludge.
Method two – Chemical treatment
Acidic discharge from mines is often treated using common alkali chemicals such as limestone, quicklime, caustic soda or soda ash. The choice of alkaline chemical will depend on the metals present in the water.
Another option is to use organic material such as newspaper or even sewage, as organic matter raises the pH level of the water as it decomposes. This method is often low-cost, and has been used to great effect in parts of the United States.
Method three – Biological treatment
There are various options for using microorganisms to oxidise metals, especially iron, into insoluble oxides. Detergents can also be used to kill off the bacteria which speeds up the oxidation of the pyrites in the water.
Using reed beds to treat AMD
One of the most tried and tested passive methods of remediating AMD is with natural reed beds, currently being used in both the US and UK. Every reed bed is different, and will be designed according to the specific needs of the problem to be treated.
Each reed bed installation starts off with a storage lagoon, where the mine water can be safely contained while being treated. The first step of the process is usually to drain the acidic water through limestone, to increase the pH levels of the mine water to restore it to more natural levels. The water then flows into a reed bed or wetland area, where the sulphate chemicals in the water undergo a chemical reaction to turn them into iron.
Scientists are still researching into why reed beds in particular are so effective at treating acidic mine drainage.
Using wetlands to remediate heavy metals
Heavy metals present in the AMD can be removed by passing the water through an anaerobic wetland. This is typically an area made-up of a thick layer of organic material, often made up of manure, compost or sawdust, with a specific sulphate-reducing microorganism added. The chemical reactions in the reed bed, along with the organic material, help to convert the potentially damaging components of AMD into something far less harmful.
The practical benefits of reed beds
Reed beds also have more practical advantages over other methods involving waterfalls or the addition of chemicals. Reed beds are an eco-friendly approach to remediating acid mine water. The operating costs are low, and reed beds don’t need employees supervising or checking regularly. There is the possibility of recovering metals to offset the costs, and the pools in the reed beds can also provide a good wildlife habitat.
Are there disadvantages in using reed beds to treat acidic mine drainage?
There are some disadvantages to using reed beds to treat AMD. Reed beds tend to take up a lot of space, and it often takes longer to process the mine water in this way than by using other methods.
It’s also useful to remember that reed beds might not be suitable for all types of AMD. If concentrations of iron in the water are greater than 10 parts per million (10ppm), the contaminated water may first need some form of chemical pre-treatment before being sent to a wetland. In the UK, wetland treatment of old mine water has been used with great effect in many areas including on rivers in the Welsh Valleys, an area which saw extensive coal mining and highly polluted watercourses as a result.
Specialist wastewater treatment solutions for legacy mining sites
Water Treatment Services offer a comprehensive range of innovative water and wastewater management solutions including the remediation of mine wastewater and treatment of acid mine drainage.
Our experts can provide advice and support to help you identify the most appropriate strategies for the identification, investigation and implementation of environmentally sensitive remediation programmes for the treatment of mining related wastewaters.
With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained engineers and wastewater treatment specialists we offer cost effective environmental support solutions across the whole of the UK and internationally.
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