Impact of Ultrafiltration on Sulfate Removal Unit Recovery, Availability and Cleaning Frequency

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Abstract In order to demonstrate the impact of ultrafiltration (UF) on the fouling rate and cleaning intervals of nanofiltration (NF) membranes in seawater sulfate removal application, GE conducted a two-year long demonstration in Singapore. GE's ZW700B UF and SWSR NF membranes were used for this study. The evaluation was conducted in two phases. During Phase 1, pretreated seawater with SDI between 2.5 and 4.5 was used as feed. During Phase 2, water was pre-treated with the ZW700B UF membrane to yield an SDI of less than 2, before being fed to the NF sulfate removal unit. In Phase 1, the SWSR-440-ft2 NF membrane showed >99.8% sulfate removal. The membrane also showed very good resistance to fouling; validating the low-fouling properties of the SWSR membrane due to its unique three-layer construction and low surface roughness (typically 10 nm or lower). When the feed water SDI increased to 4, the feed pressure and pressure drop increased but the fouling rate remained low. The unit only needed one chemical cleaning after six months of operation. As expected, chemical cleaning at high pH temporarily reduced the sulfate rejection to 99.7%; but sulfate rejection recovered to 99.8% within two weeks of operation. The temporary increase can be explained by opening of the NF membrane pores due to high pH. In Phase 2, UF pre-treatment reduced the average SDI to 1.7 and the feed particle count by more than 96%. The NF sulfate removal unit operated more than three months without any noticeable fouling; fouling rate remained low even when the recovery was increased to 80%. Sulfate rejection remained at 99.7% to 99.8%. These results confirm that ZW700B UF pretreatment will reduce NF cleaning frequency and increase the life time of NF membranes. Lower cleaning frequency will lead to lower chemical consumption, lower chemical cost and increased system availability. UF pretreatment will also allow for higher NF Flux and recovery which will reduce the footprint and weight of SRU systems. GE's ZW700B UF system did not show any decrease in membrane permeability during the duration of the pilot. Membranes were cleaned and recovered by conducting regular backwashes as well as two chemically enhanced backwash cycles at different time intervals.