Page 1 of 3 Form No. 45-D01580-en, Rev. 10
March 2022
Tech Manual Excerpt
System Design
Introduction
Introduction
An entire reverse osmosis (RO)/nanofiltration (NF) water treatment system consists
of pretreatment, the membrane process, and posttreatment. Pretreatment processes
are discussed in Chapter 2 in the FilmTec™ Reverse Osmosis Membranes Technical
Manual (FormNo.45-D01504-en). Posttreatment is employed to achieve the
required product quality. In seawater desalination, this is usually pH adjustment,
rehardening, and disinfection. In the production of water for the most stringent
applications, such as boiler feedwater for power plants or ultrapure water (UPW) for
semiconductor manufacturing, the permeate is usually posttreated by polishing ion
exchange demineralization or electrodeionization (EDI) and a polishing filter.
In this section, the membrane system is addressed. The system includes a set of
membrane elements, housed in pressure vessels that are arranged in a certain
manner. A pump is used to feed the pressure vessels. Instrumentation, spare parts,
and tools for servicing the system are added as required. A clean-in-place (CIP)
system facilitates cleaning of the membranes; this is described in Section 6 in the
FilmTec™ Reverse Osmosis Membranes Technical Manual (FormNo.45-D01504-en).
The membrane system is a complete plant with an inlet for feedwater and outlets
for permeate and concentrate. RO/NF system performance is typically characterized
by two parameters: permeate (or product) flow and permeate quality. These
parameters should always be referenced to a given feedwater analysis, temperature,
feed pressure, and recovery to normalize for fluctuations in the operating conditions.
The typical goal of the designer of an RO/NF system for a certain required permeate
flow is to minimize feed pressure and membrane costs while maximizing permeate
quality and recovery. In some cases, optimizing for operational costs takes
precedence over capital cost optimization.
The optimal design depends on the relative importance of these aspects. The
recovery of brackish water systems is limited by the solubility of sparingly soluble
salts (see Scaling Calculations (FormNo.45-D01551-en) )—90% is about the
maximum. In seawater desalination, the limit of about 50% recovery is dictated by
the osmotic pressure of the concentrate stream, which approaches the physical
pressure limit of the FilmTec™ Seawater Element.
Introduction (cont.)
Membrane elements for a particular system are selected according to:
l
Application (i.e., standard elements or fullfit)
l
System capacity (i.e., element diameter)
l
Feedwater TDS (i.e., NF, BW, or SW)
l
Feedwater fouling potential (i.e., fouling-resistant elements and/or wider feed
spacers)
l
Required product water quality and energy requirements (i.e., low-energy or
high-rejection element type)
Obtaining the required salt rejection is mainly a matter of membrane selection.
Nanofiltration (NF), brackish water (BW), and seawater (SW) membrane chemistries
have increasing salt rejections in this order. Various grades of membranes also exist
within each of these three main categories, prioritizing energy requirements at one
end of the spectrum and salt rejection at the other. Therefore, the NF to low-energy
BW membrane is typically applied to feedwaters up to 2,000 mg/L total dissolved
solids (TDS), standard BW is typically used up to 10,000 mg/L, and SW membrane is
applied to high-salinity feedwaters up to 50,000 mg/L. For given operating
conditions, the permeate quality can be calculated.
The feed pressure needed to produce the required permeate flow for a given
membrane depends on the designed permeate flux (permeate flowrate per unit
membrane area). The higher the permeate flow per unit of active membrane area,
the higher the feed pressure. In seawater systems the permeate flux is relatively low
even at maximum allowed pressure. However, the permeate flux could be very high
in brackish water systems without reaching the limit of 41 bar (600 psi) for brackish
water elements. Caution should be exercised to not increase the permeate flux with
the goal of decreasing the cost for membrane elements because the flux has to be
limited to minimize fouling.
From experience, the flux limit to be used in system design depends on the fouling
tendency of the feedwater. A system designed with high permeate flux rates is likely
to experience higher fouling rates, requiring more frequent chemical cleaning. Only
experience can set the limits on permeate flux for different types of waters. When
designing a membrane system for a specific feedwater, it is advantageous to know
the performance of other membrane systems operating on the same water.
However, quite often there are no other membrane systems for comparison. Then
the system design suggestions in FilmTec™ Design Guidelines for multiple-element
systems of 8-inch elements (FormNo.45-D01695-en) and FilmTec™ Design
Guidelines for multiple-element systems of midsize elements
(FormNo.45-D01588-en) could be followed.
Further information required to design a system is best collected by using the forms
of and . The more complete this information, the better the system design can be
optimized towards the customer’s needs.
Page 2 of 3 Form No. 45-D01580-en, Rev. 10
March 2022
Excerpt from FilmTec™ Reverse Osmosis Membranes Technical Manual (FormNo.45-D01504-en), Chapter 3, "System Design."
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Page 3 of 3 Form No. 45-D01580-en, Rev. 10
March 2022