5.4 Alternative Water Resources: Desalination
5.4.2 Intake Facilities
Source water for desalination can be acquired in multiple ways. Source water is generally
comprised of either seawater or brackish groundwater. Current methods of obtaining and delivering
source water are described below.
General Issues: Siting, Cost and Environmental Concerns
Seawater desalination facilities require an intake system capable of providing an accessible, reliable
quantity of clean seawater with minimum ecological impact. To meet these objectives, it is essential
that a comprehensive evaluation of site conditions be performed. Physical characteristics,
oceanographic conditions, marine biology, and the potential effects of fouling, pollution, and
navigation must be evaluated. Intake designs are highly site specific, potentially more than any
other characteristic of the desalination facility, and can represent as much as 20% of the capital cost
of the entire facility (Pankratz 2004).
t is important to consider marine life impingement and entrainment associated with intake designs,
including hard-to-quantify constraints that may represent the most significant direct adverse
environmental impact of seawater desalination. Technologies for seawater intake facilities range
from large surface water intakes along the shore, to offshore intake structures, to screened wells
onshore. Each technology poses different challenges in the forms of design, power consumption,
and environmental considerations. However the most significant environmental concern when
designing open seawater intake facilities is the impingement and entrainment of marine life.
Impingement, which occurs when larger marine life is trapped in or against the screens, is relatively
easy to mitigate using available technologies. However entrainment is much more difficult to
control, since it involves very small and microscopic organisms (such as phytoplankton,
zooplankton, eggs and larvae) that are pulled through the screen and into the intake. This can lead
to a decrease in recruitment to the local habitat, as well as a decrease in the overall productivity of
the ecosystem, adversely affecting the commercial and recreation fishing opportunities in the region
(López 2006).
Open Water Intakes
Open water intakes extract water from the ocean or sea and can be sized to have unlimited
capacities. The main concern when designing an intake is to prevent marine life and other debris
from entering the desalination system, not only because of the impact on marine life, but also
because, as described in a subsequent section, it can foul the desalination membranes. The three
main technologies currently used to address these concerns associated with direct sweater
extraction are listed below.
- Traveling Water Screens consist of large wire mesh panels used to prevent the intake of
debris or marine organisms. Panels revolve for cleaning and can be located directly
onshore or at the end of a long channel, intake pipe, or forebay that extends beyond the
surf zone.
- Velocity Caps consist of an offshore intake in a T-shape that converts vertical flow to
horizontal flow to reduce fish impingement and entrainment.
- Passive Screens utilize slotted screens aligned on a horizontal axis with the ultimate intake
extracting water on a vertical axis as shown in (Figure 2). Often the passive screens are
constructed of significantly larger pipe than the ultimate intake pipes, to reduce flow
velocities.
Figure 2: Passive Screen Intake (Photo: Courtesy of Euroslot Industry)Subsurface Intakes
Subsurface intakes employ the concepts of groundwater extraction within a coastal environment.
Because they draw in water through saturated sand beds or other pervious, underground strata,
they generally have little or no impact on local marine life, and can provide a prefiltered water
source for the desalination process. For this reason, particularly within sensitive marine
environments like Loreto’s, subsurface wells are utilized where permitted by cost considerations
and geologic conditions. And because these wells rely on the permeability and the stability of
subsurface materials, as well as on the reliability of the subject groundwater source, all require
detailed geotechnical evaluation prior to construction. The three major types of subsurface intakes
are detailed below.
- Seawater Beach Wells – A typical beach well consists of a perforated intake pipe that
extends offshore beneath the ocean floor, as depicted in Figure 3. These systems can
usually supply desalination plants with a capacity of approximately 19,000 m3/day or
smaller (Pankratz 2004). Currently Loreto uses approximately 8,327 m3/day.
Therefore, without implementation of the previously described conservation and
infrastructure upgrade measures, 19,000 m3/day would serve approximately 36,000
Loretanos.
Figure 3: Schematic of Seawater Beach Well
- Radial Subsurface Wells – This particular design includes a large capacity sump or well
caisson that is connected to a series of horizontal wells that run along the seafloor,
depicted in Figure 4. Capacities for this subsurface intake are generally high. A single
caisson could likely serve the existing population of Loreto. Actual production rates are
dependent on the number of intakes and the underlying geologic conditions. These wells
also benefit from the natural filtration of material on the seafloor. If installing horizontal
wells is not cost effective or if the seabed material is not conducive to this application,
infiltration galleries can be constructed instead. Infiltration galleries share the same
concept of radial subsurface wells, but the horizontal wells are replaced by excavated
trenches that are backfilled with gravel or other filter material. The effects of
constructing infiltration galleries can be disruptive to marine systems and may
significantly affect the marine environment in sensitive areas such as productive reefs.
- Brackish Beach Wells –This technology is similar to that employed in seawater beach
well extraction. The primary difference is that the intake facility is typically placed
farther inland than the seawater beach well shown in Figure 3. These wells capture
brackish water with a significantly reduced salt content, typically less than 5,500 parts
per million, in comparison to seawater, typically 45,000 parts per million. Because
brackish water is essentially “cleaner,” it is easier and far less costly to remove the salts,
making it a preferred source when readily available.
Figure 4: Radial Horizontal Well Illustration |
