Hybrid filtration method for pre-treatment of seawater reverse osmosis (SWRO)

Hybrid processes combining fibre filter with deep bed filtration process such as (i) fibre filter and sand filter, (ii) fibre filter and anthracite and (iii) fibre filter and dual media filter were investigated as pre-treatments to SWRO. Seawater was drawn from Chowder Bay, Sydney. The effect of different pretreatment hybrid systems was investigated in terms of silt density index (SDI10), modified fouling index (MFI), headloss across the filters and reduction in turbidity and dissolved organic carbon (DOC). The in-line flocculation in fibre filter improved the performance of the pretreatment hybrid system as measured by the MFI, SDI10, headloss, turbidity and DOC removal. The lowest SDI10 and MFI were found with a fibre filter operated at a filtration velocity of 40 m/h followed by dual media filter operated at a filtration velocity of 5 m/h. The lowest headloss and turbidity was found with a fibre filter operated at a filtration velocity of 40 m/hr followed by anthracite operated at a filtration velocity of 5 m/h. This system also gave an effluent with the lowest DOC of 0.64 mg/L corresponding to a removal efficiency of about 70%.

The performance of different pre-treatment hybrid systems in terms of MFI, SDI10, headloss, turbidity and DOC was investigated. (1) In this study the filtration velocity and packing density of fibre filter were kept 40 m/h and 115 kg/m3 as they were found to be optimum values. Our earlier experimental study on fibre filter revealed that the fibre filter operated at 40 m/h with a packing density of 115 kg/m3 showed comparatively good results than 20 m/h and 60 m/h filtration velocity. It is also found that in-line flocculation of influent seawater to the fibre filter gave rise to lower values of SDI10 and MFI. (2) When different pretreatment hybrid systems were operated at different filtration velocities (5 and 10 m/h), the SDI10 and MFI values ranged at 2.6–3 and 1.4–3.6 s/L2, respectively. The lowest SDI10 and MFI values of 2.6 and 1.4 s/L2 respectively were found with a fibre filter operated at a filtration velocity of 40 m/h followed by dual media filter operated at a filtration velocity of 5 m/h. (3) The headloss with and without in-line flocculation of influent seawater to the fibre filter were 6.8 and 9.5 cm, respectively. The lowest headloss was found at fibre filter operated at a filtration velocity of 40 m/h followed by an anthracite filter operated at a filtration velocity of 5 m/h. Turbidity with and without in-line flocculation decreased from 1.95 NTU to 0.35 NTU and 0.45 NTU, respectively. The lowest turbidity was found with a fibre filter operated at a filtration velocity of 40 m/h followed by a dual media filter operated at a filtration velocity of 5 m/h. (4) In-line flocculation of influent seawater together with fibre filtration and sand filtration gave a better removal of dissolved organics. The fibre filter operated at 40 m/h followed by an anthracite filter operated at 5 m/h removed 70% of DOC.