Page 253 - Membranes for Industrial Wastewater Recovery and Re-Use

222 Membranes for Industrial Wastewater Recovery and Re-use

CO C AC C,",
Nai 36 18 18 27
CaZ+ 5 1.25 3.75 3.13
Mg2+ 5 1.25 3.75 3.13
Totals 46 20.5 25.5 33.2

From Equation (2.2 5), the current is given by:

. QFAC 500 x 96 500 x 25.5
I=- - = 30 A
-
hJ< 24 x 60 x 60 x 500 x 0.95
FromEquation (2.26):
E = i2R/OQ

where R is the ED stack resistance in ohms and can be obtained from the cell pair
area resistance, the projected membrane area and the number of cell pairs:

rN (lo+ 104/33.2) x 500
R=-= = 20.7 0
A 1 x 0.75 x 104

So, the energy demand in kWh per m3 of product at 8 5% conversion is:
30' x 20.7 x 24
E= = 1.05
0.85 x 500 x 1000

Desalination of 75% can be achieved by hydraulic staging (Section 2.4.1),
whereby the number of cell pairs is halved and the flow doubled to permit the
same current to remove half the quantity of ions as that removed at the first
stage. The energy demand for 75% desalination is thus 1.5 times that for 50%
desalination at 1.57 kWh mP3 for the transfer of ion alone. Pumping of the
water, for a pressure drop of 2.5 bar across the stack, adds an additional 0.2
kWh m-3, making a total energy demand of 1.77 kWh m-3.
It is interesting to compare the relative energy demands of the reverse osmosis
(from Section 4.3.1) and electrodialysis processes, which have been calculated
on roughly the same basis. For the examples given the specific energy demand
values for each process are about the same only if 50% desalination is acceptable,
in which case the RO energy demand could be halved by 50:50 blending with
feedwater. On the other hand, 85% recovery is readily attainable with the ED,
whereas 75% is the maximum possible from a 2-stage RO process. The
membrane process choice for light brackish waters such as these may therefore
ultimately be made on the basis of both capital cost and specific technical facets
of the process (product water purity, operability, footprint, etc.), rather than
simply on running costs.