From GWI Desalination

One hundred percent of Grand Cayman Island’s public water supply is provided by seawater desalination produced at one of seven SWRO plants on the 22-mile long island. All of the plants were constructed and are operated by subsidiaries of Consolidated Water Company (CWCO), the homegrown private utility that was established on the island in 1973 and has been listed on the Nasdaq since 1995.

Unlike most SWRO plants, pretreatment is not a major issue with CWCO’s operators because all of the plants employ vertical well intakes that consistently produce feedwater with a silt density index (SDI) that rarely exceeds 1.0. Several decades of experience have proven that a 5-micron cartridge or bag filter is the only pretreatment that is required.

All of the plants operate in a partial two-pass arrangement with H₂S strippers providing post-treatment, while concentrate is discharged via onsite deepwell injection.

The relatively simple pre- and post-treatment systems allow the company to focus on its biggest challenge: how to manage energy costs on an island where electricity costs $0.40/kWh or more. Not only is the Caribbean home to some of the first SWROs, it is also the birthplace of SWRO energy recovery and remains the proving grounds for most new innovative approaches to increasing energy efficiency.

A tour of the CWCO plants reveals the evolution of the DWEER energy recovery device that was first installed on the island. Some of the early units are still in operation with the “Desalco” logo cast into the LinX valve housing.

CWCO COO John Tonner told WDR that the company continually evaluates new membranes and membrane configurations that allow it to minimize energy consumption while producing permeate with a conductivity of less than 400 microsiemens (~200 mg/L TDS). WDR has learned that the company is currently evaluating several new RO membranes and hybrid membrane configurations, including a closed circuit desal process, the details of which are confidential.

According to Casey Crowley, CWCO’s special projects engineer, the company’s North Sound Plant provides an example of how the company has taken advantage of evolving SWRO membrane technology. “The plant has two 3,000 m³/d [0.8 MGD] SWRO trains, and when the first train was installed in June 2009, it was equipped with 329 elements. We added a second train in 2007 and selected a newer, ‘high rejection, low-energy’ element with more active membrane area. It allowed us to reduce the number of required elements to 252, while still producing the same quantity and quality of water. Then, in 2009, we also refit the first train with the new elements,” said Crowley.

In the photo the first train can be seen in the background, with empty membrane racks, resulting from the reduced number of membranes.

CWCO’s facilities incorporate a variety of high-pressure pump and ERD combinations, and both Tonner and Crowley stressed the importance of reliability, availability and sustained efficiency from these key components. Tonner said that the company’s secure sites and high-quality seawater have provided ideal locations for confidential product testing and reliability trials for new components and technologies.

It seems that new component and technology testing is one way that CWCO stays abreast of technological innovation and is able to conduct its R&D; it also helps the company develop better relationships with industry OEMs.

For more on this story go to:

http://www.desalination.com/wdr/48/40/island%E2%80%99s-swros-constantly-evolving