There are no universal best practices for desalination. Best practices are determined by sitespecific
conditions. Every proposed desalination facility should be evaluated to understand the
existing constraints on sensitive environmental resources that may be affected. The practices listed
below reflect currently available and developed technologies:
Centralized/Integrated Facilities - The siting of a desalination facility should recognize the
limits of the existing infrastructure and provide for compatibility and connectivity to that
infrastructure. To avoid the problems associated with multiple private desalination facilities,
communities need to collaborate on a centralized facility. Incentives and regulations need to be
provided to encourage private developers to cooperate and commit to a regional water resource
solution. This prevents the risks associated with the unplanned applications as described above and
also allows for a single point of regulation of desalination activities.
Intake – All efforts must be made to avoid direct extraction from surface water. Therefore the
preferred method of capturing saline water in a coastal environment should consist of subsurface
intakes or beach wells. Beach wells generally have lower capacities and require subsurface
investigations such as pumping tests and test wells. If the required capacity cannot be reached using
beach wells, radial horizontal subsurface wells should be considered. These wells are generally
more expensive to construct, however, similar to the beach wells, radial horizontal wells have
minimal long-term impact on marine life. If no subsurface options are available and an open water
intake is required, the least intrusive method of salt water recovery is passive screen intakes. It
must be noted that all of the above methods will have some degree of environmental impact. These
options have been presented with a bias toward those systems that minimize both the construction
related and operating impacts. All intakes siting should be accompanied by the appropriate level of
environmental review.
Pre-Treatment – The most effective method of pre-treatment for desalination source water is
the use of sub-surface wells as filtration intakes to the system. The filtration of particles and
organisms through in-situ soils (typically sands) serves an added benefit to the system operation.
Membrane technologies such as reverse osmosis often require additional pre-treatment to minimize
fouling of the membranes. Chemicals are often used to adjust pH, act as a biocide, or to remove
partially soluble elements. The residue of these treatments ends up in the waste stream and can be
problematic when disposing of brine. The presence of pretreatment chemicals that are disposed of
in conjunction with the brine can change the classification of the waste stream to a pollutant (versus
highly concentrated seawater) and the resultant environmental impacts should be evaluated prior to
permitting this to happen. Physical separation techniques, such as ultrafiltration, should be used to avoid the use
of additional chemicals to counteract the chemical mixture added in pre-treatment. An effective
method of physical separation is ultrafiltration which provides the removal of most organisms and
particulate matter.
Process Technology – Membrane technology is currently the most widespread desalination
technology worldwide. Therefore the amount of research and development that is input into
refining reverse osmosis technology will make it the most efficient and most likely technology to be
introduced for desalinating seawater in the Loreto region. The benefits of using reverse osmosis
are reduced energy costs relative to thermal technologies, ongoing research and development
pushing higher efficiencies, and commercially available energy recovery systems. The use of
reverse osmosis technology should incorporate low pressure membranes and energy recovery
systems. The initial capital investment associated with both of these energy saving measures will
reduce energy consumption and assist in mitigating fluctuating energy costs associated with
producing a potable water supply.
Brine Disposal – There is no single best practice for brine disposal. A site-specific approach is
required when determining the appropriate method of brine disposal and often one single method
of disposal is not adequate.
A conjunctive disposal method should always be considered in the
initial site investigation. This effort will reduce the specific impact of disposal on one sector of the
environment and allow flexibility throughout operation of the plant. Critical components in
reducing the effects of brine disposal are reduction of the volume of brine that must be discharged
and minimize the adverse chemicals found in the brine.
A conjunctive approach that has the least impact on marine life would be injection of the brine into
a confined aquifer system combined with the use of evaporative ponds. Evaporative ponds are an
ideal method of disposal but can be cost prohibitive because of the large amount of land needed and
the undesirable aesthetic component of the ponds. However, evaporative ponds allow minimize
impacts to marine environments and allow for the remaining solids to be reused or disposed of
appropriately in a landfill.
Deep well injection disposes of brine underground to be diluted within an existing aquifer system.
Deep well injection requires a comprehensive hydrogeologic investigation to ensure that existing or
adjacent groundwater resources will not be contaminated and that the aquifer system has the
capacity to sustain injection indefinitely.
Open water disposal should only be considered as a last option. If open water disposal is selected,
outfalls utilizing diffusers represent the best available solution. Outfalls need to be sited with an
understanding of currents, the relative densities of the brine and seawater (brine generally has a
higher density than seawater) and the properties of any additional diluents, such as wastewater. The
effects of open water outfalls should be conceptually and numerically modeled prior to outfall
siting.
Siting a Desalination Facility in the Loreto Region
The majority of the potential coastal development in the Loreto Region is within the boundaries of
the National Marine Park. If desalination facilities are to be planned and sited within the National
Marine Park, baseline studies of the surrounding marine and estuarine environments should to be
performed to assess the ecological significance of the site and the potential impacts of the facility to
those systems.
Subsequent to the collection of baseline data, hydrogeologic investigations should be required to
determine the feasibility of subsurface intakes and deep-well injection of brine. Surface intakes and
ocean outfalls should only be considered as a last resort and only be implemented after
hydrodynamic modeling of the intake structures and dispersion modeling of the outfall structures
are complete. The results from these studies need to indicate that impact to seawater quality and
30
marine life is nominal. Given these constraints, the construction of desalination facilities within
the boundaries of the Loreto Bay National Marine Park must be very carefully evaluated.
Waste brine disposal from any site, whether in or out of the Marine Park, would likely require
some level of mitigation. If an ocean outfall is considered for applications outside of the Marine
Park, the same level of due diligence would be required as if the facility were located in the
National Marine Park. This is primarily to account for the effect of currents transporting brine or
disrupting migratory pathways of marine species in and out of the Park. Appropriate measures
should be taken to offset the negative environmental impacts of desalination regardless of the
plant’s location. |
