High-flow, high-rejection RO membranes reduce production costs by 20%

New reverse osmosis membrane elements offer for the first time high flow rates and high salt rejection with low energy use.

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By Markus Busch

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New precision-engineered seawater elements from FilmTec are used in seawater desalination for drinking water, industrial feed and other uses. Photo by Dow Chemical Company
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Advances in technology over the past few decades have made seawater desalination an increasingly viable and important method for meeting the fresh water needs of regions with growing populations. In fact, the market for desalination plants and equipment is projected to grow from US$ 2 billion today to more than US$ 70 billion within the next 20 years.

High-flow, high-rejection, low-energy reverse osmosis (RO) membrane elements are making desalination a more economical choice than ever before by reducing the cost of produ-cing potable water by up to 20%.

System designers have begun to accept a greater role in ensuring the optimal performance of desalination plants by undertaking projects based on the BOOT (build, own, operate and transfer) model. Under BOOT, the original equipment manufacturer assumes the initial risk of designing, building and operating a plant for a certain period of time (usually several years) before transferring responsibility for the plant's ownership and operation to a governmental authority. The incentive structures of some BOOT projects have resulted in dramatic reductions in the cost of producing desalinated water, achieving levels lower than US$0.50/m3.

This quest for lower production costs has led designers and operators to identify the factors that contribute most to the cost of desalination. Energy has become the most obvious target for cost reduction because energy use is by far the single largest factor, accounting for 20% to 30% of the total cost of water.

Lower energy costs could not be achieved without sacrificing desalination performance; however these trade-offs have been eliminated with the introduction of RO membrane elements that offer for the first time high flow rates and high salt rejection with low energy use.

Low-energy elements reduce operating costs, capital expenditures or a combination of both, depending upon the specific scenario plant designers and operators choose when used in new desalination plants or existing, retrofitted plants.

New plants can reduce cost with low-energy elements using four different design scenarios, as compared with a plant using industry standard elements, such as Filmtec SW30HR-380 elements. The low-energy elements in each of the four scenarios are assumed to be Filmtec SW30-HR LE-380 elements. The properties of these elements include a permeate flow rate of 28 m3/d and typical salt rejection of 99.75%.

Design Scenario 1: Increase water production and water recovery

A plant using low-energy elements under this scenario can increase its output by 5.3% without a corresponding increase in capital or operating costs. Recovery would increase from 45% to 47.3%, allowing more water to be produced with the same feed source and pretreatment. The savings would amount to US$ .005/m3 of water with energy recovery, or US$ 0.014/m3 if energy recovery devices are not used. Over five years, the savings would be US$ 61 per element with energy recovery or US$ 187 per element without energy recovery. Additional savings would result due to smaller pretreatment requirements.

Design Scenario 2: Increase water production

Under this scenario, a plant using low-energy elements and operating with the same energy recovery and pressure as a plant with standard elements would produce up to 20% more water. Permeate flow would increase from 9500 m3/d to 11,500 m3/d, saving US$ 0.009 per cubic metre of water with or without energy recovery. The five-year savings per element would be US$ 136..Design Scenario 3: Produce the same amount of water at lower operating pressure

Under this scenario, a plant would reduce its feed pressure from 58.3 bar to 55.8 bar, resulting in savings of US$ 0.005/m3 of water with energy recovery and US$ 0.01/m3 without energy recovery. Over five years, the savings would be US$ 61 per element with energy recovery and US$128 per element without recovery.

Design Scenario 4: Produce the same amount of water with fewer elements

This scenario is similar to scenario 2, except that instead of increasing water production by 20% the same production level would be maintained while capital expenditures decrease by 20% by reducing the number of elements from 805 to 665. The savings would be identical to Scenario 2. Additional savings also would come from reductions in associated equipment costs for piping, racks, etc.

Existing plants can be retrofitted to take economic advantage of higher flow rates and lower energy requirements of the new elements. Plants using standard 22.7 m3/d, 99.70% rejection elements, such as Filmtec SW30-HR380 elements, have four options for reducing costs with the new elements. The specific retrofit steps necessary vary depending on the option chosen by plant designers and operators.

Retrofit Option 1: Produce the same amount of water at lower operating pressure

This option requires no constructional change in the pretreatment; however the full benefits of this option are achieved in different ways, depending on the type of feed pump.

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Assumptions: plants operate on a feed with 38,000 mg/L at 25ºC. The design is 115 vessels with seven elements per pressure vessel, and a vessel produces 3.45 m3/h. The overall production of the plant is 9,500 m3/d. The average flux is 14.0 L/m2h and recovery is 45%. Element operation time is five years and replacement rate is 20%. Pump efficiency is 90% and power cost is US$.08/kWh.
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For centrifugal pumps with variable speed drives and positive displacement pumps, the power can be reduced, resulting in direct energy savings.

For centrifugal pumps without variable speed drives, the pump curve needs to be considered and the pump would usually produce a higher feed flow at the lower pressure. The pump can be modified by reducing the size of the impellers to take advantage of lower energy costs. If this is not possible (or desirable), then the potential energy savings cannot be realised, and the pump must be throttled by closing the valve between the pump and the pressure vessels of the first array.

Retrofit Option 2: Increase water recovery and produce more water

This option requires no changes to intake, pretreatment and feed pumps, making it an attractive retrofit choice; however before choosing this option, plant operators should:

• Verify that the capacity of product tubing, storage and post-treatment can accommodate the higher plant productivity;
• Compare the higher expected brine concentration to the discharge concentration limits and other environmental regulations and;
• Verify that brine-control valves, product tubing and post-treatment can handle modified flow rates.

Under this option, energy recovery is increased by two to five percentage points to about 42% to 44%, based on a typical energy recovery of 40% in desalination plants. The average permeate flux also would increase slightly, and possibly the flow on the lead element would increase. However, the potential cost savings from increased water production may more than offset the higher rates of fouling associated with higher operating flux.

Retrofit Option 3: Follow the pump curve and produce more water

This option is influenced by the typical pump curve and is a combination of options 1 and 2. At lower pressure, the pump can produce more feed flow, enabling a lower energy recovery than in option 2. The lower energy recovery results in a lower net driving pressure, which enables the production of more water than in option 2. Before choosing this option, plant operators should verify that the pretreatment is capable of producing a higher amount of feed water.

Retrofit Option 4: Use same water recovery and produce more water

The benefit of this option is the ability to produce up to 20% more water with the same number of membrane elements. This ability may require a new pump as well as an increase of the pretreatment capacity. When choosing this option, plant operators should:

• Evaluate fouling due to the higher flux;
• Verify that physical stress on the elements is not increased too much as a consequence of higher feed flow and;
• Evaluate the physical stress to ensure the higher feed flow is not causing the pressure to drop.

The full range of risks and benefits of each retrofit option ultimately depends on site-specific conditions and the priority of operational versus capital costs. Whichever option is chosen, plant operators should, of course, thoroughly investigate the impact any change will have on their facility.

Author's Note
Markus Busch is the Sea Water Technology Manager for The Dow Chemical Company, based in Midland, Michigan, USA.

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