UF Pretreatment for RO Systems

Hollow fiber ultrafiltration (UF) is widely accepted today for municipal water treatment applications including production of drinking water from surface water and water reuse applications. UF is also used for industrial water treatment including pretreatment to spiral-wound reverse osmosis (RO) membranes for production of high purity water. Because of the increasing awareness of the need for adequate pretreatment, there has been significant interest in UF as pretreatment for RO for municipal applications in brackish and seawater desalination plants.

Depending on the feed water quality, extensive pretreatment may be needed to provide water that is suitable for RO feed, because the brine spacer in an RO element can be susceptible to solids plugging. UF provides excellent pretreatment to RO because it can consistently deliver filtrate with very low turbidity, regardless of feed water quality. Conventional pretreatment may not reliably produce consistent, high quality water, especially when the feed water changes. In addition, compared to conventional water treatment technologies, UF systems require less space and often have lower operating costs.

UF membranes provide a physical barrier between the feed and product water and they have pore sizes in the range of 0.01 micron. These membranes remove very small contaminants in the feed water, including essentially all suspended solids, colloidal particles, and bacteria and viruses. In general, UF filtrate turbidity is less than 0.1 NTU and has an SDI15 (Silt Density Index) of less than three. This is ideal feed to RO. High quality and consistent feed water can be expected to lead to improved RO system operation, including lower operating pressure for a given flux, less frequent membrane cleaning and less risk of plugging the element with suspended solids.

Feed water properties, site conditions, and project economics will determine if UF is an attractive option for specific applications. In this article, several examples of UF for RO pretreatment are discussed.

Blending brackish and fresh water

A system with over 100 wells containing both fresh and brackish water provides the source for a potable water treatment plant in the Caribbean. The plant uses UF to remove turbidity and microbiological contaminants and RO to reduce salinity. The maximum feed salinity is 979 mg/l total dissolved solids, and product water is approximately 340 mg/l. This system has reliably provided high quality potable water for over five years.

The UF system has three trains, each with a capacity of 250,000 GPD, and uses Koch Membrane Systems, Inc. (KMS) Model 8-48-35-PMPW membrane cartridges. Only one type of raw water, fresh or brackish, is treated at a time in each train. Adjacent trains can process different types of raw water at the same time.

The RO system has two parallel trains with a capacity of 150,000 GPD per train. Each train is configured as a 3:2 array with three KMS Model TFC®-8831 ULP Magnum® elements installed in each pressure vessel. Product water recovery is more than 80% for the combined membrane systems.

Fresh and brackish waters are always filtered through UF to reduce turbidity, bacteria and viruses. The RO system treats brackish water or a blend of fresh and brackish water from the UF system to produce water with acceptable salinity.

Operations of the membrane systems are flexible and are adjusted regularly to account for water production requirements and changes in source water properties.

Seawater with beach well intake

An RO pilot plant study was recently completed as part of an evaluation to determine if desalination could meet the water needs of a community in an arid area of Hawaii. The feed water source for the pilot was a 1,200-ft intake well. UF pilot testing was included to determine if this level of pretreatment provided any benefits for operation of the RO system. Properties of the feed water during the pilot plant study are shown in Table 1.

During the pilot plant evaluation, feed water was sent through a screen and then to UF. UF filtrate was used for RO feed. KMS Model 8-72-35-PMPW UF cartridges and Model TFC®-1820 SS RO elements were used in the pilot plant. Operating parameters for the UF are shown in Table 2. Stable operation could be achieved at 61 GFD with the addition of chlorine in the backflush three times per day.

The RO pilot operated with stable flux, salt rejection and pressures while treating feed water from the ultrafiltration pilot plant. The RO was then switched to raw well water. After a slight increase in pressure, operation stabilized. These results indicate that for this particular quality feed water, pretreatment with ultrafiltration has minimal benefits.

Seawater with open intake

Several versions of mobile water purification systems have been deployed over the past decade by the U.S. Military, and have been used to produce potable water from a variety of feed water sources, including surface, brackish and seawater. Early systems used conventional pretreatment for RO. Later, membrane technology replaced convention pretreatment. Benefits of membrane pretreatment include reduced weight, space and chemical requirements.

Extensive pilot plant work has been completed at the Naval Facilities Engineering Service Center Seawater Desalination Test Facility in Port Hueneme, Calif. MF and UF membranes from several suppliers have been evaluated. KMS Model 8-72-35-PMPW UF cartridges were recently evaluated in one of these pilot programs. The UF feed water was prescreened seawater from an open intake. The feed water TDS was 34,800 mg/l.

Stable operation of the UF system was achieved by addition of ferric chloride (FeCl3) to the feed water at the following operating parameters:

• Single pass operation;
• 40 GFD flux;
• 30 minute backflush interval with fast flush;
• No chemical addition during back flush;
• 5.0% bleed; and
• 0.75 mg/liter FeCl3 in feed water (as Fe).

The ultrafiltration pilot plant operated for over two months at these conditions without the need for membrane cleaning. The UF filtrate consistently has an SDI15 of less than one.