Swro Design and Energy Recovery Part 3: Major Component Selection

The sand screens and micron filters were selected because of the durable and corrosion resistant fiberglass and PVC construction. The specific model of Eden micron filters was chosen to maintain the filter element flux at approximately 3.3 gpm/per 10" equivalent.

Due to the relative remoteness of the installation site, multistage-centrifugal, high-pressure pumps have been selected for their reliability, availability of parts, economics of operation and easy maintenance. Centrifugal pumps in general are smoother, quieter, and require less ancillary equipment (i.e. pulsation dampeners) than positive displacement pumps. Hydropro has found that positive displacement pumps are much more prone to failure and lengthily downtimes than high-quality centrifugal pumps.

The Grundfos Booster Modules were chosen for several reasons. The inline style helped conserve space and provided ease of installation, allowing everything to be mounted on the same skid (with the exception of cleaning/flush tanks, raw water booster pumps, and chemical feeds). These submersible, multi-stage centrifugal pumps were also chosen because they are very efficient and quiet, and are constructed of corrosion resistant, 904L super austenitic stainless steel.

The high pressure feed and concentrate headers were made of 2205 duplex stainless steel for superior corrosion resistance, and the structural skid was constructed of FRP for low weight and zero maintenance. ERI´s Pressure Exchanger was chosen because of its high energy efficiency, dependability, and corrosion resistant materials.

Performance
Values for the projected power consumption rates that were presented in the proposal were based on a 27ºC feed stream of 45,000 mg/l TDS and a permeate flow rate of 100,000 gpd. The membrane manufactures projection software was used to determine the system parameters at a recovery of 35%, and these parameters were subsequently used to determine the projected power consumption. The result was an anticipated feed pressure of 900 psi and a specific power consumption rate of 3.02 kWh/m³.

Once the system was installed and operating, the specific power consumption was calculated based on actual system parameters and the result was a much lower value of 2.65 kWh/m³. There were several reasons the actual value was lower, the main reason however, was the conservative design. Because of some uncertainty in the feed water quality, the SWRO system was designed with a relatively low flux (approximately 8 gpm/ft2), and a somewhat large hydraulic envelope. As it turned out, the feed water TDS was closer to 36,000 ppm and fairly stable. The lower feed TDS enabled the system to operate at a lower membrane feed pressure of 790 psi and a higher permeate flow rate of 120,000 gpd, consequently using less energy than originally projected and making higher quality permeate.

Conclusion
With most of the system assembled, the installation was fairly straightforward and went smoothly. The two units were installed, started up, tested and operator training was completed in less than three weeks. There was, however, a problem with the feed water quality and the pretreatment system, which was discovered after only 24 hours of operation. It immediately became apparent that the raw water was loaded with particulate that was quickly fouling the sand screens and the micron filters. Fortunately, the feed system could be modified to flow into an existing 250,000 gallon seawater tank from the wells, and the SWRO feed was then drawn out of this tank. This settling tank solution worked quite well and provided a feed water with a pre-filter SDI of 1.25.

There was also one other performance issue that needed to be resolved. Initially, the permeate quality was less than what was projected, and it was not clear why. The system was extensively checked ant tested for leaks, and the possibility that seawater was somehow mixing with the permeate was eventually eliminated. It was finally determined that the membranes did not meet the design rejection required to produce the projected permeate TDS. Once the membranes were replaced, the system was making plenty of high quality permeate that was well below the maximum acceptable permeate TDS.

KAJUR and the residents of Ebye have since been enjoying low-cost, high-quality water for over a year now without any noteworthy system failures. They are so pleased, in fact, that KAJUR has recently awarded Hydropro another SWRO job utilizing work exchanger energy recovery.