(762b) Toward Sustainability and Low Cost In Electro-Dialysis Reversal Desalination

Summary: The improvements of electro-dialysis
reversal (EDR) are discussed from the literature review. To be sustainable and
low cost desalination, the areas of design have to be further improved, and brine has to be reused in bio-energy
production are suggested in details.

Key words: acid, chemical,
electrode design, hydraulic leak, reusing concentrate from desalination.

Introduction: Electro-dialysis
reversal (EDR) desalination is known for its excellent methods to
clean membrane. Membranes are cleaned by reversing
the polarity between positive and negative and by switching the hydraulic flow between
concentrate and dilute streams in every fixed polar reversal interval. Due to
this excellence, researchers are trying to find ways to improve EDR in spacers,
membrane's ability to withstand calcium sulfate, relaxing the fouling layer in
membrane, and perm-selectivity of membrane. The spacer model was improved from
Mark III to Mark IV to promote the turbulence and to increase the utilization
area from 64 to 74%; to decrease power consumption from 0.14 to
0.10 kWh/m³; and to increase the conductivity reduction from
27 to 36% (Grebenyuk and Grebenyuk,
2002). The aliphatic anion selective membrane (AR 204 SXZL) ability to
withstand sulfate fouling was improved to a saturation level of 440% CaSO₄
to gain a calculated water recovery rate (R_c)
of 93.5% in a high level (42%) of SO₄^2- in feedwater without pretreatment but with acid and anti-scalant additions (Elyanow et
al., 1981).

The
costs of HCl and SHMP dosage in Dirab
and Labakha-hawaita, Saudi Arabia are 15 and 83 times
higher than power cost (Valcour, 1985). To avoid
these high cost and un-sustainability with the chemical, EDR was successfully
operated with low mean ion resident time in concentrate stream (MIRTc)
by maintaining the similar LSI and CaSO₄ saturation level in the
concentrate stream. For examples, EDR was successfully operated at LSI 2.29 and
358.9% CaSO4 saturation level with the higher R 79.1% (Table II) without
adding any anti-scalants and without any acids in Turek et al., 2009 lab by slowing down the velocity
in concentrate steam to gain the lower MIRTc and
lower dose in single pass without any
recirculation. Moreover, Wisniewski et al., 2001 also demonstrated to operate
ED without any recirculation to gain higher R 89.7% by decreasing the volume of concentrate for low dose and low MIRTc.
The advantages of operating EDR in lower dose and lower MIRTc are
to reduce the contact time between foulants and the
surface of membrane in the concentrate, to reduce the dose to which is not high
enough to be toxic to membrane, to increase the life of membrane, to eliminate
chemical usage.

In the classical EDR, dimensions,
flow, and velocity of dilute and concentrate are equal; LSI and CaSO₄
saturation level are used to control the scaling and fouling processes in concentrate
(AWWA, 1995) as such LSI<+2.16 for preventing CaCO₃ from fouling
and CaSO₄ saturation level<200 for averting CaSO₄ from
precipitation. If LSI is more than allowable limit, acid is added in
concentrate to keep CaCO₃ continuing dissolving (Katz, 1979); if
CaSO₄ saturation level in concentrate is more than the allowable
limit, sodium hexametaphosphate (SHMP) is added in
concentrate to maintain CaSO₄ enduring dissolving (Valcour, 1985). EDR however, was successfully modernized to operate with the higher
water recovery rate (R) without any anti-scalant and without acid; this new EDR operated LSI at
2.29 and CaSO₄ saturation level 358.9% at lower dose and lower MIRT_c.
Dose and MIRT_c are proposed to address the
controlling process. By lowering R and polar reversal interval, EDR can be
operated at MIRTc<130 min; at MIRTc<130min,
desalting cost/energy can be minimized by eliminating chemicals requirement (Myint et al., 2010a, 2010b, and 2011).

Brine disposal cost could range from 5 to 33% (Khordagui,
1997; Mohamed et al., 2005). In the case of inland sites, this minimum cost
increases to the order of 15% of the costs of desalination (Glueckstern
and Priel, 1996; Oren, et al., 2010). At present, all
the best available disposal methods are highly questionable to the
environmental concerns, and therefore Myint et al.,
2010c called for the reusing of brine concentrate wasted from desalination in bioenergy and microalgae biodiesel production to be sustainable and to dramatically reduce the cost.

Suggestion: However, water leaked
significantly from the membrane because the metered water recovery rate (R_m) 86.0% was different from calculated R_c (93.5%) (Elyanow et al., 1981). Although
membrane has the ability to withstand the saturation level of CaSO₄ to
440%, the set objective was not achieved due to the hydraulic leak. The Research
in reverse osmosis stated acid and anti-scalant
addition hinders the permeability rate (Hasson et
al., 2007). To gain the higher R_c
(equal R_m) with lower desalination cost,
researches in hydraulic leaks and acids and antiscalant
additions have to be revised in EDR. 

Moreover, ED/EDR
is generally assembled with 100 cell pairs for high salinity feed water to 700
cell pair for the brackish water (Jain and Reed, 1985) in one electrical stage
due to the current transferring area available in electrodes. The electrodes
are usually placed in the top and bottom of the cell pairs. The feed water is
supplied into influent of the cell pair and collected from the effluent of the
cell pairs. The TDS concentration gradually decreases from influent to effluent
due to the direct current supplied in the electrodes. The electrodes design has
to be updated with the higher current intensity in influent and lower current
intensity in effluent along with the TDS concentration profile. This new design
will decrease power consumption.

Conclusion: With these updating design in
electrodes, improving in hydraulic leaks, reducing in chemical usage in
concentrate stream, and reusing brine in bio-energy production, EDR will have a
capability to produce a higher water recovery rate with the low desalination
cost.

Acknowledgement: We thank and appreciate the
funding agency the Office of Naval Research (Contract # N00014-08-1-0304) from
the USA.

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