Treatment of acid mine drainage and acidic effluents

Abstract

The scarcity of water in South Africa is exacerbated by pollution of the surface- and ground- water resources. Typical pollutants of the aquatic environment include acid mine drainage and industrial effluents. AMD and acidic effluents can have detrimental effects on mining infrastructure, water reuse options and environmental discharge. As a result, some form of treatment is required at many mine sites. Unless treated, acid water cannot be discharged into public water courses. It is therefore important to treat mine drainage and wastewater for recycling in industrial facilities. Wastewater treatment represents one of the last frontiers left to maintain our fresh water supplies. Measures to control acid mine drainage include the treatment of acidic effluents. Acid and sulphate rich waters can effectively be treated for re-use or discharge by applying the integrated limestone technology followed by desalination with the DesEl process. This study investigated the chemical aspects of the limestone neutralisation process. One conclusion was that the limestone particle size is an important parameter in the process. It was shown that the smaller the particle size, the faster is the rate of neutralisation. Finer limestone particle size also resulted in faster settling rates and lower sludge volumes. Due to the capital and running cost associated with poor sludge settling, the production of high quality sludge is another important parameter of limestone process. It was shown that the sludge settling rate is significantly influenced by: the feed water acid- and sludge concentration as a result of gypsum precipitation, the way the limestone is added to the acid water, . the addition of a flocculant Neutralisation is generally the first step in the treatment of acid mine water. With the limestone neutralisation process, acidity is removed and only partial sulphate (up to 1200 mg/L) and metal removal are achieved. Further treatment for sulphate (to less than 400 gm/L) and metal removal are needed to make the limestone neutralised water suitable for re-use or discharge into waterways. Encouraging results obtained from laboratory studies showed that the DesEl process can be used effectively to lower sulphates from 3700 mg/L to less than 400 mg/L from limestone neutralised acid water. Operating costs amounts to 1.29 R/m³ feed water treated. Traditionally acid mine water is neutralised with lime. Limestone is a cheaper alternative for such applications. A case study showed that limestone can be used effectively to replace lime for the neutralization of arsenic rich acid water. The cost of limestone treatment is 45.8% less than that of lime. The acidity can be removed from 33.5 to 0.06 g/l (as CaC03). The study also showed no significant differences in the TCLP characteristics of the resultant sludge when water is treated with lime or with limestone. Sludge from the limestone treatment process can be disposed of on a non-hazardous landfill site.