Integrated membrane systems for seawater desalination

Membrane science and technology has become nowadays the main contributor to solve water shortage problems by sea/brackish water desalination and wastewater treatment and reuse. In the present work, seven different flow sheets for seawater desalination process have been analyzed. In each of them, except for the first (FS1) constituted by the only RO unit, different membrane units have been integrated in order to reach all the important benefits in the logic of process intensification: RO operates on NF permeate in FS2; both MF and NF have been introduced for the feed water pre-treatment and load reduction to the following RO unit in FS3. Membrane crystallizer module (MCr) completes four of the seven proposed flow sheets and it has the task to increase the quantity of desalted fresh water combined to solid salts production: in FS4, MCr operates on NF brine; in FS5 on RO brine; in FS6 both on RO and NF brine; in FS7 on NF brine while MD operates on RO brine. By using the feed water flow rate, the operation conditions, the rejection values and the membrane performance reported in [1,2], the product characteristics for the proposed flow sheets have been analyzed. The results are reported in Table 1. The introduction of a MCr, on one or on both retentate streams, increases plant recovery factor so much to reach 92.8% in FS6, higher than that of a RO unit (about 40%) and much higher than that of a typical MSF (Multistage Flash) -- about 10%. For what concerns salts, the highest production was obtained when the MCr unit operates on both retentate streams.