What price water?
- Written by R.E. Robinson
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I am writing this comment at the end of the World Water Week but it might appear only in a later issue appropriate to this topic. I had the privilege of attending the Mine Water Symposium arranged by the Geological Society of South Africa in mid-March. It was an excellent presentation dealing predominately with the underground water from the gold mines on the Witwatersrand and the coalmines around Witbank and Middelburg.
The formation of acid mine drainage (AMD), its accumulation underground, and the environmental problem it gives rise to when pumped or when seeping to the surface, were discussed in some detail. Some excellent maps were presented of the central, western and southern Witwatersrand mining areas and those of the northern and southern coal areas. These areas span the continental divide and the AMD can drain into the Vaal catchment’s area, ultimately ending in the Vaal River leading to the Atlantic.
However, the area around Krugersdorp can drain into the ‘Cradle of Mankind’, now a Heritage Site, and into the Sterkfontein Caves with disastrous consequences. From the Northern coalfields, the polluted water could flow into the Olifants River and via the KNP into Moçambique and the Indian Ocean. From the southern section, the water flows through Mpumalanga past the KNP and through Moçambique into the Indian Ocean. The age old solution to the problem of AMD was to add lime to precipitate the toxic metals and dump the sludge on a slimes dam.
However, the treated effluent then contains quantities of calcium and magnesium sulphates with smaller quantities of sodium chloride which, although not toxic, represent a build-up of the total dissolved solids (tds,) which in the streams of the Vaal catchment area increase to an unacceptable level in the feed water to the domestic supply to Gauteng. Large quantities of the pristine Lesotho water then have to be used to dilute these impurities and flush them from the barrage area to flow down into the Atlantic.
There are many other problems such as the slimes dams leaking toxic metals into the rivers and there have been major campaigns to find a universal solution. Geohydrologists with the operating mines have been hard at work to reduce the quantities of AMD involved, with considerable success. It has been demonstrated that the gypsiferous effluents can be used for irrigation and indeed it is known that many soils can benefit from the addition of sulphates, but only time will tell whether this is a sustainable solution. At the symposium, some very relevant data was provided and plans for a cooperative effort were described. Amcoal, for example, had run a pilot plant using reverse osmosis to produce domestic quality water, which in due course will be made available to the Witbank municipality at considerable cost to these coalmines.
A concentrated brine and gypsum slurry still has to be disposed of elsewhere. Grootvlei mine has spent considerable amounts to treat 10 Ml/day of their AMD using the BioSure process, a patented biological reduction of sulphate to sulphides using sewage sludge as the organic food for the micro-organisms. However, a mixture of organic solids from the sewage mixed with calcium and magnesium sulphides has to be disposed of on a waste dump.
The symposium which, as a supplement to the Water Week, pointed to the urgent need to look at the water resources of Southern Africa on a holistic basis. All the stakeholders are very conscious of the need for positive action to make use of this AMD to provide sustainable solutions in line with recent thinking on future targets and challenges regarding water supplies. These are: The undertaking by Government to supply as a constitutional right all the population with domestic water. The critically urgent requirement to expand electric power production dramatically and in the next decade this means coal-fired stations with increased demand for cooling water.
Increasing demands from our neighbours who are dependent on flow of adequate water from the rivers into their territories from sources in South Africa. The use of water in agricultural irrigation is unquestionably another national priority in terms of development of rural areas and in job creation. The announced intention of the production of bio fuels, which implies an increase in agriculture and demand for water. There are many who believe that availability of water is a limiting factor in achieving the targets for economic development. The question of how we solve these problems will be debated by many experts. It is my conviction that an unrecognized top priority in water R&D is the conversion of sewage effluent into quality agricultural water for high efficiency small lot farming.
There are many worthwhile avenues to explore. Some aspects I believe relevant to the mining industry might be of interest. To explain them I propose water usage must be assigned a ‘net potential added value, (PAV). And the following fundamentals must apply: No water is to be uselessly discarded into the ocean or needlessly evaporated if it has the potential of an alternative use with a positive PAV. This does not mean, for example, that the use of water to provide evaporative cooling for power stations is banned as having a zero PAV. It can be assigned a positive PAV, since the alternative is to produce more high purity boiler feed water at a cost or maybe to install an energy consuming refrigeration cooling unit.
The cost of each of these options can easily be evaluated and the lowest of these can be translated into a PAV and assigned a priority rating. There must be a major effort devoted to assessing our water resources and alternative supplies and costs. The hydrogeologist and climatologist must play a dominant role. For example, there are countless dolomite caverns filled with water and those adjacent to mines are emptied by pumping (a negative PAV) and thrown away. They may or may not be refilled by flood water at high rainfall periods and they may have a positive PAV. Since every drop of water in SA belongs to the state by law, it is not unreasonable for the state to assign a priority level on the basis of PAVs for any proposed usage.
These over-simplified ground rules imply a large amount of investigation and calculation and a high level of wisdom in strategic planning. But I do believe some unexpected conclusions may be forthcoming. An example may illustrate the concept more clearly. Assume a farmer can grow 3 t/ha of maize worth R3 000 with 360 mm rainfall over 6 months. Assume that with an additional 180 mm centre-pivot irrigation, i.e. 1800 m3 he can grow 6.6 t/ha. The PAV for this irrigation is R2/m3.
This is about the same as for domestic usage! It also suggests that this figure is what one could afford to pay to treat waste domestic effluent (zero PAV) to provide irrigation water. If drip irrigation were to be used at six times the water efficiency of centre pivot systems, a PAV of R12/m3 is indicated. Interestingly, the farming company Tavland using dolomitic water pumped from Western Areas Mine were achieving higher PAV values using drip irrigation on land within the mine property. There are, I believe, many similar opportunities to be explored and particularly in relation to long-term sustainability. R.E. Robinson May 2006