The first-pass feed pressure on the desalination membranes at the 139,000 gpd Cayman Brac salt water reverse osmosis (RO) plant in the Cayman Islands reached 1,024 psi. The high pressure was reducing the plant’s efficiency.

Looking for solutions, Water Authority-Cayman discussed options with NanoH2O, a manufacturer of RO membranes. Engineers suggested that the high feed pressure could be a result of scaling on the membranes. They proposed a trial of the company’s next-generation thin-film nanocomposite membranes at no cost to the authority. The parties would split any energy savings.

“Because Cayman Brac has a second-pass system and positive displacement (PD) high-pressure pump, retrofitting the system was much easier,” says Matthew Thompson, mechanical engineer for the authority. “Plants with centrifugal pumps require more work because they pump more water at lower pressure.”

After operators replaced the old membranes with QuantumFlux SW 365 ES elements, they immediately saw energy demands decrease from 13.14 kWh/1,000 gallons to 9.44 kWh/1,000 gallons. Feed pressure fell to 650 psi, while first-pass recovery increased from 37 to 40 percent.

 

Two-pass treatment

Cayman Brac serves 2,200 residents, and water sales average 70,000 gpd or nearly 50 percent of the plant’s capacity. Its production cycle is three days off and four days on. Water is stored in a 750,000-gallon tank. The authority owns and operates five tankers that deliver some 30,000 gpd to customers outside the piped distribution area.

“Our water is 100 percent salt water reverse osmosis desalination,” says Thompson. “The source water, with total dissolved solids of 31,000 ppm, is from 6-inch abstraction wells roughly 80 feet deep.”

The single-train two-pass treatment system uses five PX25 pressure exchangers from Energy Recovery, a Wheatley HP-125 PD pump, a Grundfos booster pump, 36 (6-by-6) 8-inch elements, and 36 (6-by-6) 4-inch brackish water (RO) elements. Overall permeate quality is 120 ppm total dissolved solids (TDS), versus the regulatory limit of 250 ppm.

The second-pass two-stage, 4-into-2 design has three spare 8-inch, six-element vessels. Its feed and pressure are 29 gpm/205 psi with permeate quality at 10 ppm TDS.

 

Thin-film technology

Similar to conventional thin-film composite membranes, QuantumFlux membranes are constructed with a nonwoven polyester support fabric, upon which a porous polysulfone support film is cast, followed by a dense thin-film polyamide layer. Encapsulating benign nanomaterials within the thin-film polyamide layer increases membrane permeability by 50 percent or more over conventional membranes, according to the manufacturer.

“The 8-inch elements are plug-and-play, so we didn’t have to do anything to prepare for their arrival,” says Thompson. “We did the retrofit in one day. The high-pressure PD pump kept our flows pretty much the same, but the membrane pressure dropped so low that we had to reset some alarms in the programmable logic controller.

“Replacing the original membranes with any manufacturer’s brand would have helped us save energy. Selecting NanoH2O allowed us to test a new technology and save even more.”