| A 16" SWRO membrane being installed into a pressure vessel |
Desalination trends and large capacity systems
The emergence of seawater desalination and its success has been driven by a number of factors: large capacity systems – 30 million gallons per day (MGD) (120,000 cubic meters/day) and larger – have helped lower the cost of water; improvements in energy recovery technology have lowered energy consumption; improvements in thermal desalination processes have resulted in greater efficiency and membrane flow and salt removal advancements have improved reliability and water quality. Prior to the year 2000, a 5MGD seawater desalination system was considered large. It was around the year 2000 that contracts were let for four desalination systems sized 25MGD and larger in Singapore, Trinidad, Fujairah and Tampa, Florida. These plants lowered the cost of water significantly, and numbers like $1.71 per 1000 gallons of potable water were reported. Cost reductions due to economy of scale were cited.
In the following years, an additional number of large-capacity desalination plants have been built or are under construction. These additional plants include Ashkelon, Israel, which is approximately 85MGD, and Maacta in Algeria, which will be 132MGD. Today, seawater desalination plants as large as 500MGD are being contemplated and some government organisations are already planning for this reality.
Energy Consumption
Seawater desalination using membranes (reverse osmosis) was first introduced in 1978. Early systems consumed approximately 10 kW-h per cubic meter of water produced (40 kW-h per 1000 gallons) and did not use energy recovery. In the 1980's pelton wheels and reverse running pumps were used to recover energy from the concentrate (brine) stream. These devices reduced energy consumption to about 6 kW-h per cubic meter.
In the late 1990s, isobaric energy recovery technology was introduced. These high efficiency systems recover over 95% of the energy in the concentrate stream. Energy usage is now down to 3 kW-h per cubic meter and, in some cases, even less. These energy recovery systems have thus lowered the energy consumption of a typical seawater desalination system (membrane) by over 70%.
The efficiency of energy recovery systems has had a major impact on the overall costs of water from a membrane desalination system. In the early 1980s, the City of Santa Barbara in California contracted for water at a cost of over $1900 per acre foot. Current water costs at sites like Carlsbad in the same state is less than $800 per acre foot. The reduction in energy consumption has lowered the overall cost of water from a typical seawater desalination plant by about 60%.
Thermal process efficiency and membrane improvements
Thermal seawater desalination consumes about two times the energy compared to a typical membrane plant. For this reason, membranes are preferred for many new and planned installations. However, improved designs in large scale multiple effect distillation (MED) plants have proven more energy efficient. When treating difficult seawater such as the Arabian Gulf where the salt content can be more than 45,000 ppm and the temperature can reach 40C, MSF and MED plants are often selected for their reliability and durability.
Membranes have improved dramatically since first being used for seawater desalination in 1978. The current thin film polyamide membranes produce water at 800 psi (instead of 1000 psi) with a flow of 9000 gallons per element per day (as opposed to 5000 gallons per day). Additionally the membrane area that can be incorporated into an 8 inch diameter by 40 inch long spiral wound element (the standard module size used in large desalination systems) has increased by over 70%.
