Multi-valent ion concentration using multi-stage nanofiltration

A multi-valent ion concentration system comprising:

• a plurality of nanofiltration (NF) units arranged in series, wherein each NF unit comprises one or more feed water inlet, a membrane configured to reject multi-valent ions, one or more NF unit permeate stream outlet configured to output a NF permeate stream, and one or more NF unit reject stream outlet configured to output a NF reject stream, and
• piping configured to fluidically connect the plurality of NF units arranged in series,
• wherein a NF reject stream outlet of an upstream NF unit is fluidically connected to a feed water inlet of a downstream NF unit by piping between the upstream NF unit and the downstream NF unit and wherein a NF reject stream outlet of the downstream NF unit is fluidically connected to a feed water inlet of a further downstream NF unit by piping between the downstream NF unit and the further downstream NF unit, and
• wherein a first one of the plurality of NF units is configured to receive a saline source water at a feed water inlet, and wherein
• at least one of:
• a) piping between the upstream NF unit and the downstream NF unit and b) the downstream NF unit, and
• at least one of:
• a) piping between the downstream NF unit and the further downstream NF unit and b) the further downstream NF unit,
• are configured to individually receive a lower total dissolved salts (TDS) water stream that comprises a TDS content lower than a TDS content of the saline source water, to form a diluted NF reject stream at a volume ratio of 2:1 to 5:1 of the lower TDS water to the NF reject stream prior to contact with the membrane.