Recovery of sludge produced from Ti-salt flocculation as pretreatment to seawater reverse osmosis

In this study, titanium tetrachloride (TiCl4), titanium sulfate (Ti(SO4 )2 ) and ferric chloride (FeCl3) flocculation as a pretreatment to seawater reverse osmosis (SWRO) were investigated. Their ability to remove turbidity, seawater organic matter (SWOM) and controlling the feed water quality of SWRO was evaluated. Flux decline of SWRO and membrane autopsy were studied. Titanium dioxide (TiO2) produced from sludge after Ti-salt flocculation was characterized in terms of particle structure and photocatalytic activity. A hybrid system of flocculation followed by granular activated carbon (GAC) filtration was employed to remove the suspended micro-flocs after different flocculations. The optimum dose of 10 mg/L TiCl4 and FeCl3 were used. The turbidity removal for the three coagulants was marginal at different doses at pH 6 and 8. Flocculation followed by GAC filtration significantly reduced the turbidity values from 1.5 NTU to about 0.35 NTU using TiCl4 and 0.40 using FeCl3. Silt density index (SDI15) was also decreased from 5.0 to 4.2 and 4.7, respectively. Removal of dissolved organic carbon (DOC), colour and UV absorbance after TiCl4 and FeCl3 flocculation followed by GAC filtration was found to be effective and similar. TiO2 produced from sludge showed very high photocatalytic activity under UV-irradiation. Acetaldehyde was completely removed after 45 min photocatalytic reaction.

TiCl4 flocculation as a pretreatment to SWRO was compared with FeCl3 flocculation with respect to flocculation performance, membrane autopsy of fouled RO and sludge recovery after TiCl4 flocculation. Turbidity removal was marginal for TiCl4, Ti(SO4 )2 and FeCl3 flocculation for all doses at pH 6 and 8. After flocculation followed by GAC filtration for TiCl4 and FeCl3 pretreatment, MFI and SDI15 significantly decreased with TiCl4 flocculation rather than with FeCl3. Ti-salt flocculation was able to remove 1200 Da and 950 which represents biopolymers (polysaccharides and proteins) and fulvic acids, while the small MW compounds (less than 650 Da) were marginally removed. The crystallite size and surface area of TiO2 was 40 nm and 68.1 m2/g. The anatase structure was predominant and TiO2 was doped with Si. TiO2 produced from sludge showed very high photocatalytic activity in decomposing acetaldehyde gas under UV-irradiation. Acetaldehyde was completely removed after 45 min photocatalytic reaction.