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<Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa> 3.2 WATER QUALITY IMPROVEMENT TECHNOLOGIES 3.2.1 Electrodialysis Technical Description In this process, a direct current electrical source is connected to two electrodes immersed in saline water. The charged ions in the solution migrate towards an electrode of opposite charge. Two sets of membranes, having alternate charges, are installed. The cation membrane will allow only positively charged ions to pass through, while the anion membrane will allow only negatively charged ions to pass through (Figure 41). Desalination is thus achieved by the removal of the charged ions from the water. A strong brine solution develops in the compartments which retain a high concentration of ions. Alternate compartments contain water depleted of ions which is the processed water. For brackish mine water, pretreatment may be needed to remove suspended solids, and minerals like manganese and iron. Sodium hexameta- phosphate is added into the brine stream to prevent the precipitation of barium sulphate and resultant scale formation within the membrane stack.
Figure 41. Electrodialysis process. Extent of Use This technology is used in mining operations in South Africa (Juby and Pulles, 1990). Operation and Maintenance Monitoring of plant operations and output water is required. Operational monitoring is required to estimate chemical usage and power consumption costs. Also, regular analysis of the membrane-treated water samples is necessary to ensure effective operation of the technology. Anion and cation membrane life is estimated at 4 and 7 years respectively (Juby and Pulles, 1990). One major problem with the electrodialysis process is the fouling and scaling of the membranes, which results from the trapping of certain ions in the membrane's polymer network. This problem has been resolved by use of a "flushing" step, effected by reversing the polarity of the direct current source, thereby reversing the movement of ions, which then alters the configuration of the compartment. This innovation improves the process and is referred to as Electrodialysis Reversal (or EDR). Frequent reversals of the current (3 to 4 times per hour) are essential for effective operation. However, the product water quality deteriorates as a result of contamination when the brine and product compartments are switched after such polarity reversals. In order to avoid loss of partially desalinated water during this 30 to 60 second period, the product water is recycled back to the feed tank. The feed and processed water flow rates and stack pressures are controlled by valves. Monitoring of plant pressure stage voltages and currents is essential. Cross leakage can pollute the product stream. This is avoided by maintaining the brine loop pressure slightly below that of the dilute stream (about 450 and 480 kPa, respectively). Operation of the system also requires the continuous removal of brine and gases formed as by-products of the electrodialysis process, including hydrogen, chlorine and oxygen. Level of Involvement Implementation of this technology is generally undertaken by mining or industrial concerns as part of their production operations. Highly skilled staff are required. Costs The operational costs for a 46 l/s (4 Ml/day) plant, including labour and membrane replacement, are estimated at $0.21/m3, based upon an electricity consumption of 2.4 kWh/m3 at a cost of $0.03/kWh. The capital cost for a 46 l/s (4 Ml/d) EDR installation is estimated at $3.4 million (Juby and Pulles, 1990). Suitability Electrodialysis plants remove up to 80% of the salts in the feedwater. The product water from the EDR unit is generally better than that required for discharge in terms of most general wastewater effluent discharge standards. Environmental Benefits The processed water meets effluent disposal standards, although care must be taken in the disposal of the liquid and gaseous byproducts and, especially, the brine solution. Advantages Electrodialysis is an effective method for upgrading the quality of brackish water. The product water can be reused in the mine or, with a slight amount of further treatment, as drinking water. Removal of salts helps to protect the mine service reticulation systems from corrosion. Disadvantages This is an advance technology which does not lend itself readily to small-scale applications due to its high capital and operational costs, and requirement for highly trained human resources. Further treatment of the product water is required if it is to be used for potable purposes. Cultural Acceptability There are no known cultural problems, although the concerns over water resuse for potable purposes may apply to reused mine water. Further Development of the Technology No further technological development is anticipated. This is a fully developed technology. Information Sources Juby, G.J.G. and W. Pulles 1990. Evaluation Of Electrodialysis Reversal For Desalination Of Brackish Mine Service Water. WRC Report No. 179/1/90. Water Research Commission, Pretoria.
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