OPTIMIZATION OF NF PRETREATMENT OF FEED TO SEAWATER DESALINATION PLANTS*

Earlier pioneering work done at SWCC RDC demonstrated that the NF pretreatment of seawater feed to seawater desalination plants: (1) prevented SWRO membrane fouling by the removal of turbidity and bacteria, (2) prevented plants scaling (both SWRO and MSF) by removal of scale forming hardness ions, e.g., SO=4 by up to 98%, total hardness by up to 93%, and (3) lowered required pressure to operate SWRO plant by reducing TDS of seawater feed by better than 50%. The net effect of this NF pretreatment was to increase SWRO potable water yield by up to 100 % and recovery from 35 without NF pretreatment to 70 % with NF feed pretreatment. Likewise, it increased MSF distillate recovery from 35 up to 80 % without and with NF pretreatment, respectively. Moreover, it allowed for the operation of SWRO and MSF without the addition of antiscalant. It also allowed for the operation of the latter also without antifoaming agent and without scale formation at MSF top brine temperature of 120 oC. All the previous work was done employing only one type of NF membrane. Although, the NF permeate quality was excellent, nonetheless, the NF permeate water recovery ratio was modest about 50%, at an applied pressure of 30 bar. To optimize the NF pretreatment of seawater feed to seawater desalination plants, the performance behavior of a total of 11 different NF membranes, made by different membrane manufacturers, size 4 " x 40 " and or 8 " x 40", were investigated along with the influence of using NF permeate as feed on SWRO membrane performance. Although all N