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In order to understand where water memory was added in the water current a number of samples were taken across the entire surface and underground water circuits. It emerged that the calcium, magnesium, chlorides and sulphates originated from the operations at the concentrator and the processing of the ore. High levels of nitrates and ammonia originated from the underground operations as a result of the use of ammonium nitrate based explosives. The surface and underground systems were linked through the surplus underground water discharged into the return water dam. The key questions that arose in an effort to reduce the water demand were as follows: • Can potable water use be reduced by more efficient water use? • If efficiency cannot be improved, can water be treated and reused to reduce potable water demand? • What is the origin of surplus underground water? • Can ground and surface water circuits be separated in order to limit the spreading of nitrates across the entire water system? It was established that the reuse of water from the return water dam was the most appropriate abstraction point for a water reclamation plant required to reduce the potable water demand. The treatment technology was determined by the water memory of the return water dam which contained remnants of the concentrating process and the underground mining process. After reassessing the water use requirements of the different equipment used in the concentrating process it was established that a number of processes do not require potable water, but could be supplied from water treated to a lower standard. The return water dam reclamation plant was therefore designed to produce two different classes of water; one to a potable standard and one to an industrial standard. It did, however, became evident that in order to achieve SANS 241 potable standards, the high level of nitrates (above 150 mg/ℓ) required a multi-stage reverse osmosis (RO) treatment process while at the same time only a single stage RO was required to remove other salts. A reduction of the nitrate source feeding the return water dam would therefore reduce the need for expensive treatment technology significantly.
the ‘fissure water’ at level 1 (closest to open cast pit) and the water memory of the open cast pit water revealed that the ‘fissure water’ originated from the open cast pit directly above the level 1 haulage. By separating the level 1 water circuit from the rest of the process water used in the mine the nitrate load to the return water dam can be reduced significantly and the cost of the reclamation plant can also be reduced significantly. Not only will the mine save in terms of treatment cost, but significant savings can result by circulating less water and at a lower water pressure up and down the shaft. The net water pumped will reduce by at least 30% and the pumping head of the ‘fissure’ water will reduce by at least 12 levels or 300 meters. In the case of the mine it was realised that water is not an infinite resource and the only option was to reuse water. As a result of significant build up memory from accumulation of minerals in the water captured from tailings dam, the advanced technology was required to erase some of the water memory in order to meet water quality requirements. Analysis of surface and ground water memory assisted in identify a significant short-circuiting in the mine water circuit and will reduce the cost of treatment and reuse.
4.3 Case 3 - Water treatment dosing strategy Water quality results and operator log sheets are often accumulated in the hope of using it productively in the future. This is exactly what was done at a large water supplier where large quantities of accurate water quality data and operator log sheets were available and the operator issued a request for proposal to understand the water memory and read the water story in order to implement a coagulant dosing control strategy. Thousands of analysis spanning several years were analysed including raw water quality, chemical dosing rates, treatment method and operational aspects in order to establish if the water memory could be used for the prediction of chemical dosing rates given the raw water quality and treatment process. After analysing the data using time series analysis no significant cyclical trends were observed. Percentile distribution analysis shows significant variations of turbidity, Chlorophyll-a, colour and faecal coliform. An artificial neural network was constructed (Naidoo & van der Walt, 2012) to read the underlying relationships of the raw water memory and the impact on chemical dosing rates. By assuming a simple relationship between turbidity, alkalinity and dosing rate it can be seen from figure 8 that the predicted chemical dosing showed similar trends to the actual dosing rates, but the accuracy was not acceptable. Additional refinement to the model improved the predictability with a prediction error of less than 1 mg/ℓ for polymer as shown in figure 4.
Figure 7 – Surface to shaft water short circuit
Figure 8 – Actual versus predicted ANN polymer dosing prediction using basic inputs
The water memory concept was applied by conducting a detailed investigation to understand the linkage between surface and underground water cycles. It was noted that during the early development of the mine (this is often the case in the Western Limb) that open cast pits have been developed some of which were rehabilitated and others left open. As these pits are not dewatered they fill with rain water. The water in these man-made aquifers now exerts significant pressure on exploratory drillings that are in some cases linked to shafts and haulages. The net effect is that rain water contained in the open cast pits are short-circuiting with the underground water circuits as shown in Figure 7. Detailed analysis of the water memory of
Figure 9 – Actual versus predicted ANN polymer dosing prediction using advanced inputs
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