Membrane technology: Quality water in unique locations

4 min read

Improvements in RO membrane elements have provided a threefold increase in the amount of treated water per element while reducing energy costs in desalination and water purification projects around the world. Currently, desalination plants with Dow Water & Process Solutions technology enable more than 218 million gallons of freshwater to be produced every day.

The Shoabia Barge Sea Water Reverse Osmosis plant
The Shoabia Barge Sea Water Reverse Osmosis plant

Desalination projects in the Middle East are using Dow membranes to provide fresh water to Israel, Saudi Arabia, the United Arab Emirates and Jordan. Dow created the Shoaiba Barge SeaWater Reverse Osmosis (SWRO) plant in partnership with WETICO Saudi Berkefeld. Located just off the shore of Saudi Arabia, it is the world’s largest sea-based desalination plant producing more than 50,000 m3/day using Dow FilmTec membrane elements.

A combination of factors make this plant a distinct example of desalination in the Middle East. The plant is mobile, has a unique location, a five-month construction timeline - from September 2007 to February 2008 - UF pretreatment system and interlocking endcap technology for RO elements. The plant consists of two barges, each with a capacity of 26,000 m3/d.

They have been in operation since May 2008 and October 2008, feeding potable quality water into the distribution net of Saline Water Conversion Corporation (SWCC), supplying the cities of Jeddah and nearby Makkah. This project has proved so successful that after the start-up of the SWRO Shoaiba Extension and Shoaiba Phase III desalination plants in early 2009, the two barges have been moved down the coast to Shuqueiq, helping bridge water supplies during the tourist season in the Assir region.

Advancements in RO technology are key to the viability of such projects, driving down costs while increasing effectiveness. For all the pumps, piping and flow valves that are essential to the design and operation of a desalination plant, it is the RO membrane element that actually separates water molecules from dissolved salts and minerals, turning seawater into usable water. It is an energy intensive process that has been made more efficient and less costly through improvements in membrane technology. A reduction in carbon footprint is an ancillary benefit with growing importance, making efficient membrane technology a key element in community, regulatory and government approval of desalination facilities and for more widespread acceptance of seawater desalination.   FilmTec Corporation discovered and pioneered the use of polyamide composite for reverse osmosis membranes in the mid 1970s, setting the industry standard by the time The Dow Chemical Company acquired FilmTec in 1985. Under the Dow umbrella, FilmTec upgraded from hand-glued, hand-assembled construction to a fully automated system that not only drove down the cost of producing RO membranes, but also dramatically improved membrane quality, consistency and productivity.   Today, FilmTec membranes sell for half the price they did just 10 years ago and produce approximately twice as much water in desalination applications. Where the emphasis is on reducing capital costs, field tests demonstrate that advanced FilmTec membrane technology can reduce the number of elements and pressure vessels needed to produce target water capacity by 17% to 30%. At 20-30%, energy is the single largest cost factor in the operation of a seawater desalination system and much of it is expended in the RO stage to generate enough pressure to force water ions through the millions of microscopic pores in each RO membrane. Field tests demonstrate that using advanced FilmTec membrane technology can reduce pressure requirements by 2.5 to 4.5 bar to reduce energy consumption and maximize operating cost savings when compared to membrane technology available in 1996.   FilmTec RO elements are based on three distinct layers wrapped in a spiral-wound design to form a cylindrical filter. The feed channel allows the sea water to pass across the element. Pressure is applied to move the water through the elements and to overcome the osmotic pressure of the salt water, forcing it through semi-permeable polyamide layer that allow water molecules to pass through while leaving salt molecules behind. The fresh water spirals though a permeate carrier layer to the core where the desalted water moves through the product water tube, eventually finding its way to the municipal drinking supply.   Improvements in chemistry and full automation in the construction of polyamide material allow Dow to fit more membrane sheets into each RO element. Simply put, Dow is able to make each sheet thinner while maximizing durability, consistency and precision. More sheets per element translate to a larger active area. This has been a major dynamic behind recent increases in flow rate and salt rejection, two factors that figure largely into the efficiency of FilmTec membranes and desalination.   In the early 1990s, a typical FilmTec RO element had a flow rate of 4,000 gallons per day (gpd) and a salt rejection of 99.4% at standard seawater conditions. Often a second RO pass was needed to reach drinking water quality. A typical desalination plant during this time period operated at 70 bar feed pressure and 35% conversion of sea water to drinking water. By the mid 1990s, Dow had introduced elements with flow rates of 6,000 gpd and, at 99.6%, significantly higher salt rejection. This reduced the need for a second RO pass in many cases, lowering both capital and operating costs for the end-user. Another significant increase in productivity was achieved in 2003 with the introduction of FilmTec elements with flow rates of 7,500 and salt rejections of 99.75%. The following year, Dow introduced the first of several FilmTec elements with flow rates of 9,000 gpd.   These improvements are put to use in the Shoaiba plant project where raw water is taken from the Red Sea and pre-treated by ultrafiltration (UF) prior to feeding the two pass reverse osmosis (RO) system made of 5,656 eight-inch elements. The first pass of each barge consists of 12 trains, with 24 pressure vessels of seven FilmTec SW30HRLE-400i elements with nominal characteristics of 7,500 gpd and 99.75% salt rejection at standard seawater test conditions. The second pass is comprised of four trains, each of two stages, with 22 vessels in the first stage and seven vessels in the second stage. Each pressure vessel in the second pass contains seven FilmTec BW30-440i high surface area elements with nominal characteristics of 11,500 gpd and 99.50% salt rejection (at brackish water standard test conditions). The first and second passes are designed at 45and 90% respectively, thus giving an overall system recovery of 40.5 %.   The interlocking endcap, or Ilec, eliminates the need for interconnectors which have historically been a weak spot in the performance of RO systems. Each element is locked to the adjacent element in the system with no possibility of leakage and deterioration of the permeate or fresh water in the central core. Ilec interlocking endcap technology provides one of the highest mechanical integrities together with simple and fast installation; key when construction timelines are tight.   Since their start-up, both barges have provided consistently high product water quality at flows according to the specifications. At 45 to 46%t recovery, 29-32°C and 43,000-45,000 mg/L feed TDS, the permeate TDS of the first pass has been consistently below 300 mg/L.   As part of Dow’s commitment to sustainability and solving global challenges, the company has set a goal of reducing the cost of seawater desalination by 35% by 2015. Dow continues to focus on research and development to support such projects with the component technologies, making a significant contribution toward helping to enable a safe, clean and plentiful water supply even in the most water-stressed areas of the world.