18 years ago during the "Enron era" gas-fired power plant boom in the US there were many ZLD plants installed for cooling tower blowdown treatment.  At the time, we bid both specified MVC evaporator + crystallizer combinations with an alternative using crossflow MF softening followed by RO (actually brackish RO + seawater RO in series) plus crystallizer, no evaporator.  The alternative was typically half of the CAPEX and OPEX cost of the evaporator plus crystallizer combo, but difficult to sell since it required more O&M attention (feed precipitation chemicals, sludge dewatering, membrane maintenance, operator attention).  Crossflow MF membranes work well for producing high quality pre-treated water prior to RO when chemical precipitation is required to reduce hardness and silica.  I wouldn't use anything else ahead of an RO if the feedwater is CT blowdown containing sequesterants that inhibit scale since they make operation of a conventional reactor-clarifier softener very difficult.  Crossflow MF also very efficiently uses Mg salts to remove silica than conventional reactor-clarifiers.

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18 years ago during the "Enron era" gas-fired power plant boom in the US there were many ZLD plants installed for cooling tower blowdown treatment.  At the time, we bid both specified MVC evaporator + crystallizer combinations with an alternative using crossflow MF softening followed by RO (actually brackish RO + seawater RO in series) plus crystallizer, no evaporator.  The alternative was typically half of the CAPEX and OPEX cost of the evaporator plus crystallizer combo, but difficult to sell since it required more O&M attention (feed precipitation chemicals, sludge dewatering, membrane maintenance, operator attention). 

Crossflow MF membranes work well for producing high quality pre-treated water prior to RO when chemical precipitation is required to reduce hardness and silica.  I wouldn't use anything else ahead of an RO if the feedwater is CT blowdown containing sequesterants that inhibit scale since they make operation of a conventional reactor-clarifier softener very difficult.  Crossflow MF also very efficiently uses Mg salts to remove silica than conventional reactor-clarifiers.