activated carbon filter. The use of cartridge filters provides a final barrier to water-borne
particles before the water enters the RO membrane. Cartridge filters commonly used in RO
applications are rated for removal of particles as small as 1 micron.
4
Advanced filtration technology, such as microfiltration, ultrafiltration, and nanofiltration, can
also be used as pretreatment in RO systems. Microfiltration removes particles larger than 0.1
micron; ultrafiltration removes particles larger than 0.01 micron, and nanofiltration removes
particles larger than 0.001 micron. Microfiltration can remove suspended solids and bacteria,
ultrafiltration can filter viruses, and nanofiltration can remove organic matter and some dissolved
solids.
5
These advanced systems can provide better feed water quality than other conventional
pretreatment steps; however, the cost of this equipment is still high in comparison.
2.2 Advanced Membrane Technology
There are several new advanced membrane technologies for RO systems to assist in optimal
membrane selection for specific applications. Advances in membrane technology allow more
options for higher water production rate. These advances include larger active surface areas,
higher permeable membranes, larger diameter spiral wound elements, and low fouling
membranes. These new features, if designed and implemented properly, can reduce energy
consumption and increase water recovery rates of the systems.
Larger active surface area: Spiral wound membranes with a large active surface area are
commercially available. These types of membranes enlarge the elements’ filtration area without
increasing the elements’ physical size. For example, a circa 1980 filter with an area of 320 ft
2
can now have a filtration area of 440 ft
2
with new spiral wound membranes. This increase in area
translates into a 38% increase in water production from the same pressure vessel.
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Higher permeability: Membranes with high permeability incorporate nano-particles within TFC
membranes. Nano-particles are encapsulated in the conventional RO membranes, which change
the structure of the thin-film surface, allowing more water to pass through while meeting a high
rejection rate. This type of membrane requires lower operating pressure, which reduces energy
requirements and also increases water flux and recovery rate. The membranes can cost slightly
more than conventional units; however, higher permeability can decrease operating costs because
of energy and water savings.
Larger diameter spiral wound elements: For large scale RO systems, large diameter spiral
wound modules enable significant reductions in RO plant capital cost and life-cycle cost.
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The
standard size for spiral wound elements in industrial RO systems is 8 inches. However, within
large scale RO plants, 8-inch diameter membranes are not the most efficient choice because of
the number of elements, pressure vessels, piping, and connections that must increase in direct
proportion to the increase in flow capacity. The optimum size for large scale systems is 16-inch
4
Hydranautics. Pretreatment. Accessed at http://membranes.com/docs/trc/pretreat.pdf. October 2012.
5
Safe Drinking Water Foundation. Ultrafiltration, Nanofiltration, and Reverse Osmosis. Accessed at http://safewater.org/.
October 2012.
6
Bartels C., M. Hirose, H. Fujioka. Performance Advancement in the Spiral Wound RO/NF Element Design, Oceanside, CA.
2007.
7
Hallan M.J., J.E. Johnson, M.S. Koreltz, M.H. Perry. Design, Development, and Evaluation of Sixteen Inch Diameter RO
Modules. IDA World Congress. Maspalomas, Spain. 2007.
8