5.4 Alternative Water Resources: Desalination
5.4.3 Process Technologies
No single desalination technology is considered a panacea for producing potable water. Most
technologies use either thermal or membrane processes, however other technologies exist and
many more are under development. Desalination technologies need to be chosen based on site
specific conditions including salt content, accessibility to engineering and construction services, and
the quality of water needed by the end user. Often, maintenance requirements for a given
technology will determine the type of system chosen for desalination plants.
Technology Trends
The current trends in desalination applications are dependent on source water specifics, power
availability, the date when the desalination facility was installed, and the ultimate use of product
water. Prior to the development of membrane processes, desalination was accomplished primarily
through variations of thermal distillation technologies (which include multiple stage flash
evaporation and multiple effect distillation). However, by the year 2000, membrane processes
represented 79% of the 13,600 desalination plants operating worldwide (Glueckstern 2004). The
preference for membrane systems, specifically RO, over other techniques is due in part to the
development in recent decades of membranes with higher recovery rates and lower pressure needs,
making them more efficient to operate. The application of different desalination technologies
worldwide with respect to volume of product water produced is presented below in Figure 6.
Figure 5: Desalination Technologies Capacity Worldwide (Glueckstern 2004) The source water for desalination differs from region to region based on access to the ocean, supply
of brackish groundwater, the water supplier’s ability to produce (and public acceptance of using)
recycled wastewater, and the technology available at the specific location chosen. For example,
source water for desalination processes worldwide is 56% seawater, whereas in California seawater
only represents 17% of source water for desalination, mainly because large amounts of brackish
water are readily available (Cooley et al. 2006). It is noted that the use of treated wastewater as a source
for desalination has not been considered in this study, since it is not yet generally accepted by the
public as a potable water supply.
Figure 6: Desalination Technologies Capacity in California (Cooley et al. 2006) The domination of RO in California is even more significant, as illustrated by Figure 6. Most
desalination in California occurs in the southern portion of the state, which has climatic and water
use patterns similar to Loreto’s. Similarly, in Cabo San Lucas roughly a dozen desalination plants
are now in operation, all using RO technology. As a result, it is expected RO will be the most
appropriate technology for desalination facilities developed in the Loreto region. Brief descriptions
of currently available alternative technologies, as well as technologies still under development, are
presented below, but energy demands, technical requirements, and/or uncertainties associated
with unproven performance records will likely make them unsuitable for application in the Loreto
region.
Thermal
Prior to the development of RO and Nanofiltration (NF) technologies, the majority of
desalination efforts were thermal based. The fundamental principle of thermal processes consists of
heating water beyond or near its boiling point, collecting the steam, and cooling it to produce a
clean water resource. Thermal technologies tend to be more energy intensive and less efficient than
other processes, but are suitable for applications that typically do not include municipal water
supply. The two major types of thermal technology are Multiple Stage Flash (MSF) Evaporation
and Multiple Effect Distillation (MED). The MSF Evaporator produces distilled water from
feedwater by heating it until it is ready to vaporize. The vapor is drawn to a location where it is
condensed and collected as fresh water. MED is an older technology that uses a series of chambers
exchanging heat through vapor condensation to distill water. Beca use they consume a large amount
of energy per liter of product water, these technologies are rarely used to produce a municipal
drinking water supply. However, thermal processes are still used by industries that require a very pure water supply, since they can produce water with much lower salt content than membrane
systems, typically averaging less than 25 parts per million (ppm) (U.S. Bureau of Reclamation
2003). Total dissolved solids concentrations of around 500 ppm are typically acceptable for
drinking water, so the additional removal efficiencies provided by thermal processes would not be
worth the additional operating costs for expansion of Loreto’s potable water supply.
Mechanical
In addition to thermal processes, mechanical processes have been used to desalinate seawater. The
most common process is vapor compression (VC). VC is a process where mechanical energy is
used to compress the vapor, which increases its temperature and ultimately distills water. Often
VC technology is combined with thermal technology to increase efficiencies in the thermal process.
Mechanical VC is often used in remote areas for small applications such as resorts or small
industrial processes. It is unlikely that VC technology would be an appropriate choice for
desalination facilities in the Loreto region, since operating costs are generally higher than RO and
Loreto is not considered a remote location.
Electro-Dialysis (ED)
In Electro-Dialysis desalination (ED), a direct electrical current is run through brackish water to
separate dissolved salts and minerals into positive and negative ions. These are then strained
through one of two semi-permeable membranes that allow only the positive or negative ions to pass
through, leaving desalted water behind. While ED is effective on brackish water, this technology
is still under development for use in seawater desalination. Generally, ED is not cost-effective at
removing salt concentrations above 4,000 mg/l (Pacific Ocean seawater averages approximately
35,000 mg/l), so, unless suitable low salinity brackish sources can be found, it is unlikely that ED
would be a suitable choice for a Loreto desalination facility.
Potential Technologies
A number of other technologies are in the development stages for both seawater and brackish water
desalination in an effort to reduce energy costs and minimize brine disposal problems. Notable
technologies that are suitable for desalinating seawater, yet are not completely developed for large
scale use are listed below:
- Freeze Separation – source water is frozen to separate ice crystals from salt crystals;
- Ion Exchange – source water is passed through columns of resins that remove undesirable
ions based on the specific resin’s preference for certain ions;
- Membrane Distillation – combines the concepts of thermal and membrane processes to
remove salts;
- Rapid Spray Evaporation – source water is sprayed at high velocity through vaporizing
nozzles to separate salts from water; and
- Freezing With Hydrates – a saltwater vapor/gas mixture is cooled, and the hydrates
formed are then separated from brine.
Membrane Processes - Reverse Osmosis/Nanofiltration
Reverse Osmosis and Nanofiltration (RO/NF) are similar pressure driven, membrane processes
used in the desalination of water. The NF membranes generally operate at lower pressure than RO
and are typically used for brackish water applications. RO membranes are typically used in
desalination of seawater because of these membranes’ higher salt rejection capacity than NF
membranes. The fundamental principles of both technologies consist of the separation of salt from
water when the feedwater is applied to a membrane at high pressure. Fundamentally, the process
of osmosis is reverse as water passes through a semi-permeable membrane and the salts remain on
the feedwater side (Figure 7). The water that passes through the membrane is ultra-pure while the
remaining water increases in salt concentration. The high-saline water becomes the waste stream
or “brine” and is then discharged while the product water is collected for use.
Figure 7: Reverse Osmosis Process (Courtesy of RBF Consulting) Reverse Osmosis technology is experiencing rapid growth due to extensive research and
development in recent years. The intense competition between a number of membrane
manufacturers has provoked much of this research. Operating experience with reverse osmosis
technology has improved over the past 15 years; fewer plants have had long-term operational
problems. Assuming that a properly designed and constructed unit is installed, the major
operational elements associated with the use of RO technology will be the day-to-day monitoring of
the system and a systematic program of preventive maintenance. Operation, maintenance, and
monitoring of RO plants require trained engineering staff. Staffing levels are approximately one
person for a 200 m3/day plant, increasing to three persons for a 4,000 m3/day plant.
The amount of desalinated water that can be recovered from saline water ranges between 30%-
85% of the volume of the input water, depending on the initial water quality, the quality of the
product needed, and the technology and membranes involved (Cooley et al 2006). Currently,
desalination facilities are typically defined as small if production is less than 3,700 m3/day;
medium-sized if production is between 3,700 and 37,000 m3/ day; and large if production is over
37,000 m3 day. However, the physical size of a large reverse osmosis desalination facility is small
relative to any thermal technology plant, which usually requires a boiler, power generation
facilities, and significant land area for the facility. The land areas required for multiple types of
desalination facilities is presented below in Table 2.
Table 2: Surface Area Requirements for Desalination Facilities
a. Durban, James 2006 b. Water Desalination International 1998 c. SPG Media 2006 d. SPG Media (2006)As described above, nanofiltration (NF) membranes are generally not suitable for seawater
desalination, but can function as a cost effective alternative to RO if brackish water conditions
exist. The fundamental principles of NF are the same as RO; however NF membranes have less salt
rejection capacity than RO membranes. Operating costs are less lower primarily because NF
membranes require lower operating pressures. Therefore if ideal source water conditions exist,
NF is generally preferable to RO.
Although significant advancements in technology have extended membrane life while lowering
energy requirements, overall energy consumption remains extremely high due to the very highpressure
requirements of reverse osmosis membranes. Among the more significant recent
technology advancements, the Long Beach, California Water Department has developed a twostage
Nanofiltration Process, or Long Beach Method, as it has become known. It has been
demonstrated to be 20 to 30 percent more energy efficient than RO, which is the current state-ofthe-
art technology (Long Beach Water Department 2006). The Long Beach Method technology is
not yet being applied to a municipal water scale at this time, however it demonstrates the promise
of advancements in desalination technology in the future. |
