Potential of supercritical water desalination (SCWD) as zero liquid discharge (ZLD) technology

Abstract

A modelling and economic study was done to evaluate the suitability of supercritical water desalination (SCWD) as zero liquid discharge (ZLD) technology. ZLD was achieved with a two stage brine treatment process. The hydrothermal brine, remaining after separation of supercritical water (SCW), under supercritical conditions, was expanded in the first stage (flash-step), and the remaining brine was then expanded and dried in the second stage (flash-evaporation step) using the produced steam of the first stage expansion. A window of operation for the first and second stage pressures was determined. For the process, the optimum point of operation was at the maximum second stage pressure, where the exergy of the second stage produced steam was also at a maximum. The economic evaluation showed that the SCWD brine treatment price, for an ideal case where all the products were sold, decreased from $ 9.61 to 1.16/m3brine when increasing feed concentration from 3.5 to 20 wt% NaCl. The decrease was due to the income from the sale of salts, which increases with feed concentration. The brine treatment price was highly dependent on the brine source and it was recommended that SCWD be used for the treatment of concentrated waste streams.