Water industry seeks ways to reduce energy usage at desalination plants
“Water, water everywhere, nor any drop to drink.” In this line from an oft-quoted poem, an ancient mariner laments the irony of being in the middle of the ocean without potable water. The earliest recorded reference to desalination dates back to the fourth century B.C., when Greek sailors used simple desalting techniques to avoid dying of thirst at sea. The increased interest and innovation in desalination experienced since the 1960s still springs principally from potable water needs, but desalinated water is used for other purposes as well.
“With existing freshwater resources dwindling, desalination is used today to meet growing water demands, provide drought-proof supplies, improve water quality for planned reuse and, in some instances, improve the water quality of aquatic ecosystems,” said Vasu Veerapaneni, Ph.D., Desalination Technology Leader for Black & Veatch’s global water business.
“Considering that 97.5 percent of Earth’s water is seawater, with another 1.7 percent locked up in glaciers and polar ice caps, the demand on the remaining 0.8 percent of water is tremendous. Of this remaining water, only a small part is usable freshwater in lakes and rivers, with the remaining being brackish (salty) groundwater,” said Bruce Long, Black & Veatch Water Global Practice and Technology Leader. “If you put all the water on the planet in a gallon container, usable fresh water is only two tablespoons of that. So it makes perfect sense for the planets growing population to seek out affordable and sustainable ways to desalinate seawater and brackish water.”
Bob Hulsey, Black & Veatch Director of Water Treatment Technology, noted that many regions are now looking at desalination technologies for treating impaired water sources that were previously considered unusable. “Desalination technology allows communities to treat these sources and thereby expand their water resource portfolios, especially in coastal regions.”
DESALINATION AROUND THE WORLD
With advances in technology continually improving the efficiency and reducing the environmental impact of desalination, this market continues to grow rapidly as more government and industry organizations adopt desalination to meet increased demand. The International Desalination Association (IDA) Desalination Yearbook 2010-2011 reports a global contracted capacity of desalination plants of 71.7 million cubic meters daily (18.9 billion gallons daily) in 2010, including those under construction. This reflects a three-fold increase in cumulative desalinated water capacity over the last decade. Municipal use accounts for two-thirds of worldwide installed capacity. An additional one-fourth is used for industrial purposes.
There are more than 15,000 desalination plants globally. Among the top 10 countries by total installed capacity, according to the yearbook, Saudi Arabia, United Arab Emirates, Spain, the United States, China and Algeria each have installed more than 2 million cubic meters daily (m3/d) (528 mgd) of desalination capacity since 2003.
Globally, the yearbook reports that seawater accounts for 60 percent of total feedwater capacity. Desalinated seawater is the primary source of water supply in the Middle East, where freshwater is scarce, oil is plentiful and desalination using thermal distillation traditionally has been the method of choice. In the thermal distillation process, water is heated to produce steam, which is then condensed to produce water with low salt concentration – much like what occurs in nature through solar energy.
In many other parts of the world, reverse osmosis (RO) has become more dominant due to its relative affordability. With RO, water at high pressure is forced through membranes that do not permit salt passage in order to remove salt and other impurities. Other parts of the world that have recently seen increased implementation of desalination due to drought or increased demand – including Spain, China and Algeria – have implemented RO. Even in the Middle East, there has been an increased use of RO in recent years, due to its lower cost. Electrodialysis is another, albeit less popular, membrane desalination process, which has advantages over other processes for specific water types.
Black & Veatch is currently providing engineering and procurement services to PUB, Singapores national water agency, for a 318 million liters per day (MLD) (70 million imperial gallons daily) seawater-desalination facility, which is being constructed under a Design-Build-Own-Operate (DBOO) arrangement. This facility will be commissioned in 2013 under a 25-year concession period. Black & Veatch previously served as technology partner for SingSpring Pte. Ltd. (a subsidiary of Hyflux Ltd.) in the delivery of Singapores first major desalination plant, which uses two stages of reverse osmosis.
In the United States, Black & Veatch has rapidly delivered a project through the design-build approach in Santa Monica, Calif., where new facilities that treat approximately 38 MLD (10 mgd) of brackish groundwater contaminated with MTBE meet state and federal potable water quality standards.
Also in California, Black & Veatch recently completed design of Orange Countys Groundwater Replenishment System (GWRS) expansion from 265 MLD to 379 MLD (70 to 100 mgd). Through GWRS, the county provides advanced treatment of wastewater, removing salt and other dissolved solids using the same technology applied to brackish water desalination. Half of the water is then injected into the ground to form a barrier against seawater intrusion, and the other half is allowed to percolate to recharge the aquifer that supplies potable water.
TACKLING ENERGY AND ECONOMIC ISSUES
Recent improvements in desalination technologies have resulted in lower capital and operating costs and reduced energy consumption; however, energy still accounts for approximately half the annual operating costs of thermal- and membrane-based desalination processes. Black & Veatch professionals served as principal investigators for a research study published by the Water Research Foundation in March. Launched to investigate the current status of desalination technology and find ways to improve energy efficiency, the research focused on major design and operating issues that affect energy efficiency of RO-based desalination facilities.
Veerapaneni and the other investigators on the team found that use of efficient energy recovery devices, typically limited to seawater applications, can also be used in many low-pressure RO applications such as brackish and wastewater treatment to achieve significant energy savings. Research also revealed that improved pretreatment use of newer membranes with higher permeability and improved hydraulics can significantly reduce energy consumption.
The concentrate stream, a byproduct of the desalination process, is much saltier than the feedwater being desalinated because it contains all the constituents removed. In coastal regions, the concentrate can be discharged back to the ocean in an environmentally sustainable way by mixing it with municipal effluent discharges and diffusing it over larger areas.
However, disposal of concentrate is an issue for inland desalination facilities. Black & Veatch has spearheaded Water Research Foundation projects to develop technologies for treatment of desalination concentrate to achieve desalination without any liquid discharge, commonly referred to as zero liquid discharge (ZLD) desalination. Currently, ZLD desalination is applied primarily to industrial waste streams and power plant cooling water using established thermal treatment technologies, evaporation ponds and deep well injection. But these technologies have disadvantages, such as large footprints and high capital and operating expenses, that render them prohibitively expensive for drinking water applications.
“Our research to identify and test alternative non-thermal technologies that reduce the cost for ZLD desalination recently has identified several technologies that can treat and recover concentrate from RO processes, while achieving cost savings of 50 to 60 percent compared with established ZLD technologies,” said Rick Bond, the Black & Veatch Senior Process Engineer who led the ZLD studies.
As researchers work on methods to lower energy consumption at desalination plants, forecasts clearly point toward a sharp growth in new facilities across the globe.
“Given the dire forecasts of water stress around the world and significant improvements in technology, we can expect more explosive growth of desalination implementation,” said Veerapaneni. “In addition to established technologies, new materials and processes are being developed at an increased pace. These include novel use of membranes and thermal processes such as forward osmosis, membrane distillation and ion exchange membranes. Within this decade, unprecedented research and development will result in changes in technology that address costs, energy and environmental sustainability.”
Story by Linda Bond