(380b) Pilot Study of Flow Reversal RO for Ultra-High Recovery in Brackish Water Desalination

Sustainable and climate resilient water supply is a grand challenge. Increasing water recovery in municipal RO plants augments water supply and significantly reduces brine management cost. However, the recovery rate in RO is usually limited by the scaling potential of sparingly soluble minerals in the feed. In traditional steady-state design, the tail-end RO elements are exposed to the highest concentration of these scalants indefinitely, unless interrupted by a clean-in-place (CIP) or permeate flushing. Using antiscalants alone at ultra-high recoveries may not be very effective; they may only slow the precipitation, and overdosing may even cause fouling issues. One possible way to help mitigate scaling at ultra-high recoveries is transient and cyclic operation where membranes are periodically flushed. ROTEC's Flow Reversal RO (FRRO) is one such example. It is based on the conventional three-stage configuration with additional flow reversal and block rotation characteristics. These periodically reset the crystallization induction clock and reduce the imbalance of salt/foulant load across all stages. Therefore, it holds the promise of operating at higher recoveries than can be achieved with using anti-scalants alone.

This presentation reports piloting data analysis of ROTEC's FRRO at City of Santa Monica's Arcadia Water Treatment Plant. The pilot skid is comprised of six blocks arranged in a 3:2:1 array. Each block consists of six 4"x40" Toray TML10D RO elements connected in series. It takes a feed rate of 30gpm and produces about 27gpm permeate. When the system runs at its full capacity, three out of the four blocks (A, B, C, and D) form the first stage while the remaining block serves as the third stage. Blocks E and F constitute the second stage. Every time when a block from A-D rotates out to become the third stage, its feed flow direction is reversed. When the third-stage block re-joins the first stage, its feed flow direction returns to forward again. This allows the under-saturated fresh feed to periodically flush the rear-end elements in the previous step. Additionally, the feed flow direction to the second stage switches between forward and reverse at regular intervals.

The pilot study showed that it was possible to maintain a 90% recovery with a typical CIP interval over 30 days. As membrane fouls, there is a decline in the specific flux and an increase in the specific pressure drop. Nevertheless, the last stage pressure is usually no more than 200 psi. The successful pilot led to the design and commissioning of the first FRRO full-scale municipal installation in the United States.

Reference:

Li, M.; Waite, A.; Wang, S. Piloting Experience of ROTEC's Flow Reversal RO (FRRO) for 90% Recovery in Brackish Water Desalination, Desalination, 576, 117348, 2024.