5.4 Alternative Water Resources: Desalination
5.4.4 Pre-Treatment and General Maintenance
Pretreatment is an important component of desalination systems, especially in the application of
membrane processes. Pretreatment is the process of preparing source water for the desalination
process. Thermal desalination processes require filtration and occasionally chemical treatments but
do not require the level of pre-treatment that RO membranes do. Incorporating subsurface intakes
and providing the most suitable technology to address water quality conditions in the source water
prior to desalination can drastically reduce these costs. All desalination plants require
preventive maintenance including: instrument calibration, pump adjustment, chemical feed
inspection and adjustment, leak detection and repair, and structural repair of the system on a
planned schedule.
Depending on the chemical composition of the feedwater and the method of intake to the
desalination plant, pre-treatment for RO plants can account for up to 50% of the total cost of the
facility’s operation (Pankratz 2004). RO membranes can become fouled easily by particulate
matter, scaling, and biological growth. Scaling is the deposition of minerals, caused by partially
insoluble salts in the source water, on piping materials and membranes, which can reduce process
efficiency and foul membranes. These salts precipitate out of solution and accumulate on the
membranes causing the membranes to degrade, often past repair. To reduce these effects,
membrane based desalination plants use large particulate filtration augmented by the addition of
anti-scaling chemicals and/or more refined filter technology, such as microfiltration or
ultrafiltration.
Microfiltration will remove particles generally greater than 10 microns (μm or one millionth of a
meter) and ultrafiltration will remove particles greater than 0.1 μm, both filtration processes are
pressure driven. Ultrafiltration can be used instead of adding chemicals to prevent biological
growth and scaling. If ultrafiltration is not used, chemicals such as acids are added to reduce the
effects of scaling. Unfortunately, the addition of anti-scalant chemicals can cause an increase in
biological growth on membranes which results in plugging, reduced efficiency, increased operating
costs, and potentially, actual destruction of the membrane itself. The extent of biofouling is
dependent on multiple factors, such as the amount of sunlight, the type and amount of anti-scalants
used, the pH of the feed water, and the amount of algae present in the source water. Additional
pre-treatment is required to reduce biofouling; however membranes cannot be disinfected with
chlorine.
Pre-treatment chemicals are often disposed of and discharged in the waste stream with brine. In
the Loreto region this may be problematic due to the presence of the Loreto Bay National Marine
Park. Extra mitigation efforts or advanced pretreatment technologies, such as ultrafiltration, may
be required in order to prevent the pollution of the Loreto Bay National Marine Park or other
fragile ecosystems in the region.
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