Comparison of slow sand filtration and microfiltration as pretreatments for inland desalination via reverse osmosis

• MF and SSF both produced water that satisfies feed water quality requirements for RO treatment (SDI ≤ 5).
• SDI values in MF treated waters were consistently ≤ 3.0 and lower than SSF SDI values.
• Long-term RO performance was more stable following MF than SSF pretreatment.
• The economics of MF and SSF as pretreatments for RO were nearly equivalent.

A pilot study was conducted from October 2007 to November 2010 to establish the long-term feasibility of using reverse osmosis (RO) treatment to manage salt levels in Central Arizona Project water. Pretreatments consisting of microfiltration (MF) and slow sand filtration (SSF) were compared based on performance—turbidity removal, silt density index (SDI), volume treated between cleaning events and protection of downstream RO—during side-by-side operation over a yearlong period. SSF always produced feed water that was suitable for RO treatment (SDI < 5). However, MF consistently provided filtrate with SDI < 3, and long-term RO performance improved significantly with MF as pretreatment. Although the economic costs of MF and SSF pretreatments are similar; MF is preferred based on the quality of treated water and stability of downstream RO operation.