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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