In-line flocculation–filtration as pre-treatment to reverse osmosis desalination

In this paper the performance of single and dual media filters with in-line flocculation have been examined as pretreatment to seawater reverse osmosis (SWRO). A comparison of filter performance was made between single medium filter (80 cm) consisting of sand or anthracite, and dual media filter consisting of sand (40 cm at the bottom) and anthracite (40 cm on top). Short term (6 hours) experiments were conducted with in-line coagulation followed by direct filtration. Filtration velocities of 5 m/h and 10 m/h were used. The performances of these filters were assessed in terms of turbidity removal, head loss build-up, and organic compound removal in terms of molecular weight distribution (MWD). The efficiency of the filter as pretreatment was evaluated in terms of silt density index (SDI) and modified fouling index (MFI). It was found that the turbidity removal was high and all the filters produced more or less same quality water. There was a slower buildup of head loss for coarser filter medium. A post treatment of reverse osmosis after an inline-flocculation-dual media filtration showed lower normalized flux decline (J/J0) (0.35 to 0.22 during the first 20 hours operation) while, seawater without any pretreatment showed steeper flux decline (0.18 to 0.11 at first 20 hours operation) in RO.

In this study, the effectiveness of in-line flocculation–filtration as pretreatment was examined using single media and dual media filter in terms of headloss development, turbidity removal efficiency, SDI, MFI and DOC removal Anthracite, v=5 m/h Anthracite, v=10 m/h Sand, v=5 m/h Sand, v=10 m/h Dual media, v=5 m/h Dual media, v=10 m/h Headloss Development (cm) Time (h) Fig. 5. Effect of filter media and filtration velocity on headloss development (total filter depth = 80 cm, FeCl3 dose = 1 mg/l, average turbidity of seawater = 0.82 NTU). Seawater Anthracite, v=5 m/h Anthracite, v=10 m/h Sand, v=5 m/h Sand, v=10 m/h Dual media, v=5 m/h Dual media, v=10 m/h MW:390 Da 0.8 Intensity (mV) 0.6 MW:910 Da MW:1220 Da 0.4 MW:105 Da 0.2 0.0 Time (min) Fig. 6. MWD of SWOM with and without pretreatment.