Energy Recovery Devices Make Desalination A Reality

Nearly 50 years after these words were spoken by President John F. Kennedy, his vision is being realized in countries all over the world. And that’s a good thing.

“If we could produce fresh water from salt water at a low cost that would indeed be a great service to humanity and would dwarf any other scientific accomplishment.”

Nearly 50 years after these words were spoken by President John F. Kennedy, his vision is being realized in countries all over the world. And that’s a good thing.

Currently, about 885 million people globally face water scarcity, and according to the United Nations, a child dies from water-related disease every 15 seconds. Clearly, the world’s water woes have reached crisis levels.

Thankfully, though, seawater desalination has emerged as a solution, and countries like Israel, Algeria, Australia, China, India, Saudi Arabia, Mexico and even Spain rely on converting ocean water to clean, fresh water.

As the world increasingly depends on desalting seawater or brackish water—water that is saltier than fresh water, but not as salty as sea water (such as where a river meets the sea)—to provide fresh drinking water, the desalination market has grown dramatically.

When you factor in that desalination is also being used for the benefit of the tourism sector, agriculture and industry, it’s no wonder some industry analysts project the growing global desalination market to reach $30 billion by 2015.

Even here in the U.S., droughts in California have caused water costs to escalate as supply continues to dwindle. That’s why places like Sand City, a small seaside community in California's Monterey County, opened a desalination plant that produces up to 600,000 gallons of potable water daily.

Although small when compared to plants in other parts of the world, the opening of the facility marked a big moment as it was the first full-scale municipal desalination plant in California to receive permitting approval under the new regulations, setting a precedent for future seawater facilities in the state.

Historically in the U.S., desalination has been met with much controversy. Opponents think the process uses too much energy and is not good for the environment. These are antiquated arguments that were probably true 30 years ago.

Since then, technologies and processes have matured, and countries all over the world are relying on desalination for affordable, clean water.

The technology that turns sea water into drinking water is seawater reverse osmosis (SWRO), which squeezes drinking water from salt water using a semi-permeable membrane and high pressure.

SWRO has been around from many years, but has always been an expensive, energy-intensive and cost-prohibitive process. That was until the development of isobaric energy recovery devices (ERDs).

ERDs proved to be a game-changing innovation as the major technological breakthrough most responsible for reducing SWRO energy costs and enabling large-scale SWRO plants to operate all over the world.

A number of devices have been developed to recover pressure energy from the membrane reject stream and return it to the feed of the reverse osmosis (RO) process. Turbine-based, centrifugal energy recovery devices (ERDs), such as Pelton turbines or hydraulic turbochargers, have been employed since the 1980s.

Here’s how it works: The membrane concentrate is ejected at high velocity through one or more nozzles onto a turbine wheel. The turbine, coupled to the high-pressure pump shaft, assists the motor in driving the pump that pressurizes the RO system.

Because energy is converted twice—once by the turbine and once by the pump impeller—a great deal of energy is lost. The water-to-water transfer efficiency of a turbine ERD system is the product of the turbine and impeller efficiencies.

The component efficiencies range from 70 percent to a maximum of 90 percent.

Therefore, the overall efficiency of a turbine ERD is typically 50 to 80 percent. Efficient, modern seawater RO plants operating with turbine ERDs can typically produce desalinated water for less than $1 per thousand gallons.

To avoid the losses associated with the energy-conversion inherent in turbine ERDs, engineers developed positive-displacement isobaric devices for RO. These devices have been widely deployed since about 2002.

They place the RO reject and filtered feedwater in contact inside pressure-equalizing, or isobaric, chambers. There are currently two commercially available types of isobaric ERDs, including several piston-type work exchangers and the rotary PX Pressure Exchanger (PX) device from Energy Recovery, Inc. (ERI).

Piston-type devices have large chambers, pistons separating the concentrate and feed water, and valves and control systems to switch flow between the chambers and limit the travel of the pistons. The PX devices have small chambers, no pistons and no direct controls.

The energy required to operate the high-pressure portion of an RO system equipped with isobaric ERDs is the sum of the high-pressure pump and circulation pump energy consumption.

The flow rate through the high pressure pump is approximately equal to the permeate flow rate. This means that the high pressure pump only pumps the amount of water that leaves the system as permeate.

This aspect of the PX technology benefits RO operation by reducing the required size and energy consumption of the high-pressure pump and allowing a plant to operate affordably at a low conversion rate. Lower conversion rates correspond with lower membrane pressures and longer membrane life in seawater RO systems.

To give you an idea on the importance of ERDs, ERI’s PX Pressure Exchanger, for example, can reduce the amount of energy required in SWRO desalination by up to 60 percent, resulting in more economical production of drinking water and a reduced carbon footprint.

More than 8,000 PX devices have been deployed or under contract to be installed in desalination plants across the globe to support the production of drinking water for more than 25 million people. These devices save an estimated 890 MW of energy, and reduce carbon emissions by more than 4.7 million tons per year worldwide.

The world’s oceans present a virtually limitless and drought-proof supply of water. Thanks to the development of ERDs from companies like Energy Recovery, this supply of water can be turned into fresh water for millions of people around the world.

G.G. Pique is an entrepreneurial leader who has been with ERI since February 2000. As President and CEO of ERI since 2002, G.G. has driven and managed the evolution of the ERI business model and its organization. G.G. has more than 35 years’ experience in the water industry. Before joining ERI he was a Senior Vice President and Corporate officer of US Filter Corp., a Fortune 500, NYSE company focused on the acquisition, turnaround, integration, and growth management of more than 165 water treatment companies. He holds a B.S. degree in Chemical Engineering from the University of Connecticut and a Masters in Business Administration degree from the University of Hartford.

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