Nanofiltration (NF) Process

The design and operation of nanofiltration is very similar to that of reverse osmosis, with some differences. The major difference is that the nano membrane is not as "tight" as the reverse osmosis membrane. It operates at a lower feedwater pressure and it does not remove monovalent ions from the water as effectively as the RO membrane.

While a RO membrane will typically remove 98-99% of monovalent ions, such as chlorides or sodium, a nanofiltration membrane typically removes 50% to 90%, depending on the material and manufacture of the membrane. Because of its ability to effectively remove di- and trivalent ions, nanofiltration is frequently used to remove hardness from water while leaving the total dissolved solids content much less affected than would RO. For this, reason it has been called the "softening membrane". Nanofiltration is often used to filter water with low amounts of total dissolved solids, to remove organic matter and soften water.

Design Assumptions

  • Feed water of 500 mg/l TDS at 25°C, SDI < 3, Turbidity < 1
  • Negligible permeate backpressure
  • 3rd year of operation allowing for flux decline and salt passage Increase
  • Flux, minimum brine flow, and recovery in accordance with membrane manufacturers recommendations
  • 90% Salt Rejection elements (standard) provide approx. 92-97% hardness reduction
  • 80% Salt Rejection elements (optional) provide approx. 80-85% hardness reduction (motor HP is 50% of std. unit)

Available Systems

HydroTech can recommend nanofiltration systems (10GPM – 300GPM), where feed water has low TDS and the requirements to the permeate are not so stringent in order to achieve energy savings.