Low energy/cost desalination: low dose and low mean ion resident time in concentrate stream of electro-dialysis reversal

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Courtesy of IWA Publishing

Species, dose, and mean ion resident time (MIRTc) in the concentrate of electro-dialysis reversal (EDR) desalination are analysed. In the classical EDR, dimensions, flow, and velocity of dilute and concentrate are equal; Langelier saturation index (LSI) and CaSO4 saturation are used to control the scaling and fouling processes in concentrate, 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 continuously dissolving; if CaSO4 saturation level in concentrate is more than the allowable limit, sodium hexametaphosphate (SHMP) is added in concentrate to maintain CaSO4 dissolving. EDR, however, was successfully modernised to operate with the higher water recovery rate (R) without any anti-scalant and without acid; this new EDR operated with LSI at 2.29 and CaSO4 saturation level 358.9% at lower dose and lower MIRTc. Dose and MIRTc are proposed to address the controlling process. Monographs for the acids and SHMP requirements, and for the desalting cost including desalting power, membrane surface area, and chemicals usage, are developed. By lowering R and polar reversal interval, EDR can be operated at MIRTc<130 min; at MIRTc<130 min, desalting cost/ energy can be minimised by eliminating chemicals requirement.

Keywords: acids, Ca2+, Mg2+, HCO-3, SO2-4, mean ion resident time, polar reversal interval, SHMP, water recovery rate

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