Alternative design to dual stage NF seawater desalination using high rejection brackish water membranes

摘要

Dual stage NF membrane desalination process was proposed as an alternative approach to RO seawater desalination. Despite of being cheaper than RO desalination, dual stage NF process is not commercially applied yet due to the low overall recovery rate. In an attempt to increase the process recovery rate and to reduce the operation complicity, brackish water RO membrane was used instead of NF in the second stage. ROSA software was used in this study to verify the applicability and cost-effectiveness of the NF-BW dual stage desalination process. Similarly to dual stage NF desalination process, an NF membrane was used in the first stage and BW membrane in the second stage. Permeate from NF membrane was used as a feed into the BW membranes. The effect of membrane type and seawater salinity on the process performance was investigated. For any given recovery rate and seawater salinity, the simulation results showed that the overall cost of NF-NF was slightly lower than the NF-BW process but that was on the cost of higher permeate TDS. For instance, at 43,000 mg/1 feed salinity the difference in the specific power consumption between NF-NF and NF-BW process was 0.38 kWh/m~3. The permeate TDS was 125 mg/1 for NF-BW and 1030 mg/1 for NF-NF process. The difference in the permeate TDS between NF-NF and NF-BW process increased with increasing the feed salinity. In dual stage NF process, a low permeate was achieved at low recovery rate. For example, at 43,000 mg/1 feed salinity the permeate TDS from dual stage NF process was 359 mg/1 when the overall recovery rate was 22%. It was also found that the effect of BW membrane type on the process efficiency was insignificant. Finally, the energy requirements of NF-NF and NF-BW were compared to a single stage RO desalination process. The specific power consumption at 43,000 mg/1 feed salinity was 4.58 kWh/m~3, 4.2 kWh/m~3 and 3.86 kWh/m~3 for RO, NF-BW and NF-NF process respectively.