Johannesburg — PRESSURE is mounting on the government to agree on a plan for dealing with acid mine drainage - the heavily polluted water that results from mining. But it is not clear which treatment is best.

Mariette Liefferink, CEO of the Federation for a Sustainable Environment, has warned that acid mine drainage from derelict mines on the Witwatersrand is likely to flood the centre of Johannesburg within 18 months unless treatment options are urgently considered and implemented.

Cost has been an issue, as water treatment can cost mines millions of rand a month - Grootgeluk mine, owned by Aurora, received a R5m subsidy from the government every month until recently to pump its water.

Mine owners argue they should not be forced to pay for historic problems resulting from previous mines in the area, but the government has been reluctant to commit taxpayers' money.

"Everyone has agreed this is a crisis," Ms Liefferink says, and she hopes government interest in the problem will be sustained. She says the first phase of treatment, infrastructure construction, will probably cost about R220m.

Treatment options suggested to Business Day include adding lime to the highly acidic water - which raises the pH level and helps remove heavy metals; reverse osmosis treatment plants; passive treatments such as constructing the mine shaft so less water enters; flooding the mine pits, and constructing artificial wetlands.

Acid mine drainage arises when sulphate-bearing minerals are exposed to oxygen and water moves over this exposed acid-bearing rock. It is a particular concern in Gauteng and Mpumalanga. The water can contain dissolved metals such as iron, manganese, nickel and cobalt.

Department of Water Affairs spokeswoman Linda Page said that state intervention in three geological basins - the east, west and central basins - will be co-ordinated and partial state funding is being considered.

But Mike Muller, a registered engineer and professor of public and development management, warns that the public should be wary of picking up the bill for acid mine drainage treatment as there are high financial stakes involved in pumping and treating this water. Prof Muller says that one of the mines involved valued its environmental liabilities at R1,89bn, with an outstanding liability of R764m not covered by trust funds.

He says that in some cases engineering works could modify the mine shaft design and prevent water from entering, thereby reducing treatment costs.

Dr Danie Vermeulen, a hydrogeologist based at the University of the Free State, said last week that in some cases the development of acid mine drainage could be retarded or even halted if relatively deep coal mines were allowed to flood.

But that solution depends on the mine being deeper than the surrounding topography, so that the water does not decant .

Dr Vermeulen says SA should consider passive options for acid mine drainage rather than active treatment, which is more expensive. Where mines are likely to decant into their surroundings, artificial wetlands could provide a solution, he says. The wetland filters the water, removing sediments and pollutants including heavy metals.

This method has been used by the UK Coal Authority.

Treatment has to be site-specific, says Prof Alakendra Roychoudhury of Stellenbosch University. Artificial wetlands need maintenance to prevent drying out and can result in acid soils.

The most conventional treatment is to simply add lime to the polluted water. This raises the pH, helping to neutralise the highly acidic water and remove the dissolved heavy metals, resulting in a sludge that needs to be safely disposed of. Lime treatment on its own is not usually sufficient to bring water to potable standards.

Reverse osmosis, on the other hand, involves pushing water at high pressure through fine membranes to remove large molecules and ions. The technology is highly effective and produces potable water that can be sold to local municipalities.

It is now being used at two locations: Anglo Coal's eMalehleni plant in Witbank and Optimum Coal's plant near Middelburg. Ms Liefferink says this process costs mines R10/cubic metre and the treated water can be sold for R3/cubic metre.