Mine Water

Getting to grips with acid mine drainage

South Africa has been challenged with the management and treatment of acid mine drainage (AMD) for decades. By Dr Laetitia Coetser and

Dr Andrew Wood

While there are many operational mines affected by AMD, the problem also exists for mines that have ceased operations – leaving behind long-term water quality issues. AMD is evident in a number of areas around South Africa, but particularly in Gauteng, Limpopo, KwaZulu-Natal, Mpumalanga and the North West. Numerous media articles, especially between 2000 and 2010, highlighted the possible flooding of the Johannesburg CBD and destruction of river systems that could result from the decant of mine water across this area. To address this imminent danger, the then Department of Water Affairs initiated ‘emergency measures’.

These prioritised the installation of pumping systems and the construction of high-density sludge (HDS) pretreatment facilities for mine water in the

Dr Laetitia Coetser, associate partner and principal scientist, SRK Consulting

Dr Andrew Wood, partner and principal scientist, SRK Consulting

Acid mine drainage isan increasing environmental challenge

East, Central and West Witwatersrand basins. Their primary objective was to maintain contaminated mine water levels below the environmental critical level, to prevent risks associated with the flooding of mines and decanting of AMD into the surface environment.

This intervention resulted in the spending of several billion rand, with annual operating costs reaching hundreds of millions of rand – for the ongoing treatment of nearly 200 million litres per day of AMD, for an indeterminate long term.

There is also the legacy of AMD from the coal mines in Mpumalanga, which will be the subject of a further article in the next issue of Water&Sanitation Africa.

Long-term strategy needed However, it had always been accepted that basic lime neutralisation and metals precipitation of the HDS systems did not address the high sulfate loads that have continued to discharge into the water supply systems. Further, it has always been known that the waste sludge generated by the HDS process would continue to prove highly problematic, with much being pumped back underground in the absence of alternative solutions. It was clear to all that a longer-term strategy was going to be required.

The scientific and engineering team investigating a long-term strategy for the treatment of AMD of the Witwatersrand – to which SRK contributed as one of the specialist consultancies – published its reports in 2012. It recommended that the basic lime neutralisation of the mine water implemented as the emergency measures should proceed to full-scale desalination treatment and appropriate waste sludge and brine management.

The Minister of Water Affairs at the time, in opening the East Basin HDS plant, indicated that an expected R12 billion capital cost would be required to complete the necessary treatment. While funding and authorisation to proceed with implementing the long-term strategy for AMD has been delayed for various reasons, the mining industry has implemented state-of-the-art mine water treatment facilities for individual mines and groups of mines. Building on success The Anglo American and BHP Billiton treatment plant in eMalahleni was commissioned in 2010 and continues to produce drinking water. This water is available to municipal consumers, and waste residues are used by industry and agriculture.

The project has been so successful that it has received international recognition and a number of awards. It is the world’s largest AMD treatment plant delivering water of a drinking water quality standard. Other mine water treatment plants – at mines including Middelburg, Optimum, Matla, Tweefontein, Cooke Operations and New Vaal – have since been constructed, to provide water of drinking quality while ensuring near zero liquid discharge.

Technology is available South African engineers and scientists – particularly from the universities of Cape Town and Tshwane, Mintek and the CSIR – have been leading the development and implementation of large-scale AMD treatment technologies. At the same time, South African technology providers have demonstrated their technical capability and increasing experience on local and international levels. There are also international technology providers who are interested in expanding their capacity in South Africa.

While the optimism around the eutectic freeze crystallisation (EFC) of desalination brines may not yet have realised its full potential, this technology remains under research and development. In particular, it aims to provide alternative options for the management of the monovalent brine that eludes even the latest ultra-high-pressure reverse-osmosis-based technologies.

While the latter can ensure consistent drinking water quality, research continues into alternative options. These include biological sulfate reduction, ion exchange and the potentially more environmentally acceptable passive or low-energy technologies – which do have a role to play.

Substantial costs The capital and operational costs of AMD treatment are considerable, with the ball-park capital cost at about R20 million for every million litres treated per day to drinking water quality. Ongoing operational costs are estimated at about R7 to R15 per thousand litres treated.

Looking to the future The AMD problem for South Africa is not going away. Rather, it is only now entering the rapid development and implementation phase, as its threat to scarce national water supplies is realised. To do this, the Witwatersrand Basin needs to proceed with the implementation of the proposed desalination technologies. This would require joint funding from government and the mine.

However, the Witwatersrand legacy includes hundreds of individual mines, almost all of which closed many years ago – meaning that liability cannot easily be legally directed. As with the current implementation of the emergency measures, the Department of Human Settlements, Water and Sanitation may be expected to continue to foot the bill – with water users effectively paying the cost through escalating prices of drinking and industrial water supply.

South Africa has developed worldclass technical skills in the design and operation of AMD treatment plants but, rather than treating AMD for hundreds of years, it is preferable to address the source of AMD generation where possible. Abstracting AMD to prevent uncontrolled decant to surface can reduce the long-term liability.

Also, there are opportunities to contain or stop the biochemical reactions that create the AMD in the first place. It remains a challenge, though, to find measures that can be implemented within the life-of-mine plan – which would reduce AMD generation and post-closure liabilities.

SRK Consulting has extensive experience in the evaluation and selection of appropriate mine water treatment applications and waste residue management. These solutions are based on sound engineering, economic, operational, environmental and process efficiency knowledge.