Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for Freshwater Augumentation
3.2 WATER QUALITY IMPROVEMENT TECHNOLOGIES
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.
(a) MOVEMENT OF IONS IN A DIRECT CURRENT EOECTRIC FIELD
(b) A SIMPLIFIED ELECTRODIALYSIS PROCESS
Figure 41. Electrodialysis process.
Extent of Use
This technology is used in mining operations in South Africa (Juby and Pulles,
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
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
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).
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.
The processed water meets effluent disposal standards, although care must be
taken in the disposal of the liquid and gaseous byproducts and, especially, the
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.
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.
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
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.