Optimizing Pretreatment

The cost of seawater desalination has played a significant role in curbing large-scale development of reverse osmosis (RO) desalination plants in the U.S., limiting construction to small facilities mainly in the nation’s water-short coastal areas. RO technology advances have improved system efficiency by reducing energy use and decreasing plant size, but an optimized pretreatment system can go a long way to further reduce a desalination plant’s life-cycle costs.

Advanced RO pretreatment

Increased performance and declining system costs are compelling many to evaluate ZeeWeed immersed ultrafiltration (UF) membranes for robust and reliable RO pretreatment. Unlike conventional granular media systems, UF membrane fibers are inherently insensitive to high turbidity or variable raw water quality. ZeeWeed membranes can consistently deliver RO feedwater with a turbidity of less than 0.1 NTU and a low silt density index (SDI15), typically less than 2.5, often less than 1.5.

This high quality feedwater protects RO systems from fouling too quickly, resulting in lower cleaning requirements, higher flux rates and extended RO membrane life. ZeeWeed currently provides RO pretreatment at dozens of industrial sites, producing high quality RO feedwater with membrane bioreactor and tertiary filtration systems.

More recently, ZeeWeed UF was selected to provide a 20-mgd UF pretreatment system for the seawater desalinationplant at the Yuhuan Power Station in China. The desalinated seawater will supply boiler feedwater and potable water for the plant; surplus will be sold to the local community.

Smaller system footprint

Raw water quality can have a significant effect on the configuration of a pretreatment system. Difficult-totreat waters can greatly increase the number of process steps for conventional systems requiring chemical coagulation and flocculation prior to filtration. These additional steps enlarge the plant footprint, and can add to capital expenditures because desalination plants are commonly located on increasingly scarce and costly coastal property.

Co-location of the desalination plant with a power generating station is one way to reduce costs because the facilities can share the same intake and outfall pipes. Coastal property values in highly populated areas such as southern California, however, can result in unaffordable land costs.

A ZeeWeed UF pretreatment system minimizes pretreatment—eliminating the need for large, chemically enhanced pretreatment systems.

This can result in a pretreatment footprint that is up to three times smaller than conventional processes, providing significant savings in land acquisition costs, preserving precious coastal property, and reducing construction traffic, thus making desalination facilities good neighbors within the community.

Lower life-cycle costs

Raw water quality and pretreatment system effectiveness also have a large impact on the overall lifecycle costs of a desalination plant.

Pilot studies conducted with UF-pretreated RO systems verify that UF membrane pretreatment provide lower life-cycle costs than conventional pretreatment systems.

The following example is derived from pilot data. It shows the operational costs of a seawater desalination plant with pretreatment provided byimmersed UF membranes as compared to conventional coagulation followed by two-stage sand filtration.

The model in Table 1 assumes the following information about the plant:

  • Double-pass RO desalination plant: 20 mgd;
  • Raw water: seawater – 35,000 ppm TDS, poor quality with highly variable turbidity, SDI15> 6;
  • Interest rate: 6.5% per plant life 25 years;
  • Power cost: $0.045 US/kW-hour; and
  • RO flux: 10 gfd with UF, 8 gfd with two-stage sand filtration.

These results assume both the conventionaland the UF pretreatment systems meet their performance requirements. Upsets to the conventional system caused by turbidity spikes from storms or seasonal variability water quality, however, can reduce feedwater quantity and quality from a granular media system.

By comparison, membranes can recover much faster from turbidity spikes, incorporating proven backpulse and chemical cleaning processes to restore permeability.

Moreover, RO feedwater quality is never compromised because the membranes physically block contaminants from entering the permeate stream, and do not rely on chemical processes or settling to remove particles.

The result is that the simple, smaller and automated pretreatment process that ZeeWeed offers for seawater desalination can produce millions of dollars in operational savings over the life of a plant.

Steve Siverns, M.E., P.E., is director of
desalination technology at Zenon
Environmental, Inc. He can be reached at
905/465-3030 or by e-mail at
ssiverns@zenon.com.

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