Reducing the threat of AMD

30th April 2010 By: Carla Thomaz

Ground engineering company Golder Associates has been appointed to assist in dealing with a threat of a major decant of acid mine drainage water (AMD) to the surface waters of the Witwatersrand Goldfields and surrounding streams.

Millions of litres of AMD water is infiltrating into the old workings of the Witwatersrand Goldfields on a daily basis. The Western basin started overflowing in 2002 and the Central basin water level is rising rapidly and is expected to reach decant level by late 2011/early 2012 depending on rain levels. Golder has been appointed to develop a number of project aspects of the Witwatersrand Goldfields water reclamation project.

Golder Associates head of engineering André van Niekerk explains that, since gold-mining began on the reef in 1886, the quantity and quality of the water has been significantly affected. He adds that there are extensive underground mine workings in the Witwatersrand Goldfields area, but few remain active underground mines. The result is that water has been accumulating over time in these old defunct underground workings and, because of minerals associated with gold-bearing reef, substantial pollution potential has accumulated in the mine workings.

He adds that the mine water in the area is now typically acidic, with high salt and metal content, containing some radioactive elements, making it unsuitable for drinking purposes. When the Western basin overflowed in 2002, the mining companies that had traditionally mined in the basin took an initiative and established a collaborative company, called the Western basin Environmental Corporation, that contracted the Western Utility Corporation (WUC) to find a solution that is sustainable, technically robust, environmentally responsible and financially feasible.

Golder, which has been extensively involved in several water treatment projects over the years, was commissioned to consider the project options for collecting the water from a wide geographical area, treating it to acceptable standards and conveying it to users, and disposing of treatment wastes.

"We found that impacted mine water could be treated and transformed into a significant additional source of drinking water or discharge water in a technically and financially viable, environmentally responsible way," says Van Niekerk.

The overall project will include a treatment plant that produces 75-million litres of potable water a day, which will continue until 2018, at which time WUC envisages an increase in production to 200-million litres a day. The project aims to remove the acidity and heavy metals contained in the AMD water by adding alkalinity to the water in the form of lime to neutralise the acid content and convert all dissolved metals to insoluble oxides and hydroxides.

After this stage, the water contains elevated calcium and sulphate. By adding barium carbonate to the water, barium sulphate and calcium carbonate are formed and removed from the water. The treated water is then treated and disinfected and is then of a potable quality. Further, the sludge that generated will be treated through a thermal recovery process to convert it back to lime and barium carbonate, which is then used again in the mainstream treatment process. This then reduces the cost of having to purchase additional chemicals.

Golder is also responsible for the engineering of the required infrastructure that will be used for the collection of water from various mining sources within the area, the distribution of the treated water and disposal of waste

While WUC has started the regulatory approval process, Van Niekerk explains that a large part of the Witwatersrand mining area is no longer mined. The few remaining mines that are still in operation cannot fund and operate such a large water reclamation scheme and discussions with Government are ongoing regarding a move toward a public-private partnership and finding the best use for this water.

He adds that the reclaimed water can be discharged back into the Vaal river system, sold to a local bulk water user or transferred to the Crocodile river, where there is a growing shortage of water. That investigation is currently being conducted by Golder on behalf of the Department of Water Affairs.

Van Niekerk concludes that finalisation and implementation of the project is critical as the implications of the untreated mine decanting down streams and rivers is undesireable with human and natural aquatic life being negatively impacted.