Plant Upgrade Reduces Fouling

The industries, businesses and homes in an Iowa city rely on one Midwest generating station for electricity and steam. The reverse osmosis (RO) feedwater is taken from surface water from Cedar Rapids, Iowa. Although the conventional FILMTEC BW30-400 elements installed in 2001 have performed well, fouling has become a problem because of the issues commonly associated with the fouling tendencies of surface water. A planned plant upgrade in 2005 offered the opportunity to evaluate two new improvements: a 34-mil feed spacer and iLEC interlocking endcaps. In April 2005, FILMTEC BW30-400/34i elements featuring a 34-mil feed spacer and iLEC interlocking endcaps were installed to compare performance with a new set of standard FILMTEC BW30-400 elements installed on an adjacent train.

Feed Spacer Benefits

The function of the feed spacer is to provide an area for the feedwater to pass from the feed to the concentrate end of the element. It also provides turbulent flow against the membrane surface, which helps prevent fouling and concentration polarization. It has been frequently observed that wider feed spacers enable more turbulence and more effective membrane cleaning. Additionally, the flow dynamics and increased turbulence provided by the 34-mil feed spacer are more effective at preventing foulants from attaching to the membrane surface. FILMTEC BW30-400/34i elements use the same 34-mil feed spacer as FILMTEC BW30-365 elements.

Interlocking Endcaps Benefits

iLEC interlocking endcaps eliminate the need for multiple sliding seals between adjacent membrane elements, reducing the number of sealing surfaces per connection to a single, axially compressed O-ring. The result is a high-integrity, leak-tight seal that is lubricant- and maintenance-free for the life of the element, and reduced permeate pressure drop.

Elements using iLEC interlocking endcaps are installed in minutes with a level of effort comparable to that associated with standard interconnectors. Customers have reported that loading a pressure vessel with iLEC interlocking endcaps takes approximately 30% less time compared to loading a pressure vessel with conventional elements.

Plant Design

The total dissolved solids of the feedwater taken from Cedar Rapids is approximately 220 mg/L. Pretreatment of the RO system consists of multimedia filtration, 5-┬Ám cartridge filters, sodium bisulfite, sulfuric acid (pH reduced to 6.4) and antiscalant.

The RO plant consists of three RO units. Each unit is designed to produce 400 gal per minute of permeate at 80% recovery. The configuration of each RO unit is identical, consisting of two stages with 11:5 pressure vessel configuration. Each pressure vessel contains six membrane elements. Two RO units are equipped with FILMTEC BW30-400 membrane elements; one RO unit is equipped with FILMTEC BW30-400/34i membrane elements. The permeate is used for boiler feedwater and steam.

Plant Performance

The new installation began operation in April 2005. Differential pressure drop for the elements with the 34-mil feed spacer is about half that of the elements with the 28-mil feed spacer (Figure 1). Permeate flow rate (Figure 2) and salt passage (Figure 3) are similar for both types of elements. The train that contained the elements with the 28-mil feed spacer required cleaning before the train with the 34-mil feed spacer.

The customer preferred to eliminate the acid addition because of safety and handling reasons. In August 2005, after discussions with FilmTec Corp. technical personnel, the customer eliminated acidification and increased antiscalant dose rate. The salt passage of the membrane elements was reduced by more than 50% by operating at pH 8.1 compared to operating at pH 6.4, resulting in higher purity water. Figure 3 shows the effect of feedwater pH on the salt passage.

Conclusions

In six months of operation, the FILMTEC BW30-400/34i elements with 34-mil feed spacer and iLEC interlocking endcaps required less frequent cleaning than equivalent elements using a 28-mil feed spacer (FILMTEC BW30-400), and showed reduced feed pressure and pressure drop while maintaining flow and salt rejection. In addition, eliminating acidification resulted in rejection improvement of the RO system. This resulted in savings because of less chemical consumption in the operation of the downstream mixed-bed ion exchange beds.

Article contributed by the Dow Chemical Co. Additional information is available at 800.447.4369, or at www.dowwatersolutions.com.

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