Home\Educate\Water Reuse 101\Research Projects\Year\2013\Continuous Flow Seawater RO System for Recovery of Silica-Saturated RO Concentrate

Continuous Flow Seawater RO System for Recovery of Silica-Saturated RO Concentrate

Project: 09-12
Year Released: 2013
Type: Report

Program: Tailored Collaboration
Funding Partner: Bureau of Reclamation
Total Investment: $211,666.41 (Cash: $128,041.45, In-Kind: $83,624.96)

Principal Investigator: John E. Balliew, P.E., El Paso Water Utilities

Background

In a previous research study that was funded by the Texas Water Development Board (TWDB) and El Paso Water Utilities (EPWU), it was shown that a batch-treatment seawater reverse osmosis (SWRO) system can recover more than 85% of the water from the silica-saturated RO concentrate that is generated at the KBH desalting plant in El Paso, TX. Final feed water total dissolved solids (TDS) concentrations as high as 75,000 mg/L were routinely achieved. In this project, the pilot plant was converted from batch treatment to continuous flow.

Goals and Objectives

This project investigates how to recover most of the water from RO concentrates that are saturated with silica without fouling the RO membranes.

Research Approach

Concentrate from the KBH plant flows continuously into a feed tank that also receives the recycled concentrate from the SWRO system. A bleed valve at the bottom of the feed tank opens and closes to keep the water in the feed tank at a preset conductivity, which establishes the recovery of the system. A scale inhibitor for sulfate control was added at 5 mg/L, and the pH in the feed tank was maintained at 3.9 through the addition of sulfuric acid. Sulfuric acid was selected because of its lower cost.

Findings and Conclusions

The results showed that calcium sulfate precipitated in the feed tank at all recoveries above 35%, fouling the SWRO membrane. By comparison, a 35% recovery corresponds to a feed water TDS concentration of only approximately 15,000 mg/L. Various attempts to prevent precipitation were unsuccessful, including increasing the scale inhibitor concentration, adding a heat exchanger to the feed tank to control the temperature, decreasing the permeate flux, and reducing the volume of water in the feed tank.

At the outset of the project, it was believed that silica fouling would be the most challenging issue to resolve. However, the only membrane fouling that was observed throughout testing was due to calcium sulfate, and the cleaning solution Diamite CAL was effective in removing it from the fouled SWRO membrane. Silica precipitation was never a problem during this pilot study.

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