Reverse Osmosis
Industry:
Product:
Refining, Food and beverage, Power, Oil and Gas, Pulp and Paper, Chemical, Water
pH/ORP Meters
Background Information
Reverse osmosis (RO) is a separation process that uses
pressure to force a solution through a membrane that retains
the solute on one side and allows the pure solvent to pass to
the other side. More formally, it is the process of forcing a
solvent from a region of high solute concentration through a
membrane to a region of low solute concentration by
applying a pressure in excess of the osmotic pressure. To
illustrate, imagine a semipermeable membrane with fresh
water on one side and a concentrated aqueous solution on
the other side. If normal osmosis takes place, the fresh water
will cross the membrane to dilute the concentrated solution.
In reverse osmosis, pressure is exerted on the side with the
concentrated solution to force the water molecules across
the membrane to the fresh water side.
which the pure water flows through the membrane is
determined by water temperature and factors listed above.
By design, not all feedwater passes through the membrane.
Some feedwater is piped to flow over the membrane. This
diverted feedwater cleans away the rejected impurities in a
cross-flow filtration mode.
An RO machine, produces one purified water stream called
permeate and a second stream called concentrate, brine, or
reject from one feedwater stream. Feedwater enters the
machine is fairly low pressure. Water flows through pre-filters
to remove suspended particles, such as silt. These
replaceable pre-filters provide a cost effective method to
keep the membrane clean.
Since RO necessitates the use of pressure,
normally this pressure is provided by a
water pump that overcomes energy
differences (osmotic pressure) that drives
feedwater through a porous, semipermeable membrane. This semipermeable membrane permits pure water to
pass through, but inhibits the majority of
dissolved impurities, from passing through.
The impurities fail to pass through the
membrane for two reasons:
I.)
They are blocked due to
physical size (organics and
biologicals)
II.)
They are blocked as a result
of electrical charge repulsion
(inorganic salts).
RO membranes (Polyamide thin-film composites) are
designed to have very small pores or holes. These holes
permit the passage of water. A slight amount of impurities
the feedwater will be carried through into the purified water.
The amount of impurities found in the purified water depends
on such things as the type of membrane, condition of the
membrane (i.e. age, cleanliness) and the amount of pressure
applied (energy). The percent concentration of impurities is
normally less than 5% of feedwater content. The rate at
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RO systems are intended to run automatically. They require
only periodic operating data collection and routine
maintenance. Most modern systems are equipped with
performance monitoring equipment to call for operator
assistance or in more extreme situations, to automatically
shut down the RO system.
RO systems also have instrumentation to measure process
conditions. These include permeate flow rate and
conductivity, machine feed and discharge pressure feed pH,
and the calculable percentage recovery. All measurements
represent machine operating characteristics and are based
on the machine’s design. Consistency in each of the values
indicates the machine is operating properly.
Given the fact that RO systems use up consumable products
during operation, cartridge pre-filters are typically replaced
every two weeks. Separator (membrane elements) life,
however, is approximately three years. All other components
of a machine are considered “hardware” and should not
require replacement other than through normal mechanical
wear and tear. RO performance is optimized under a regular
maintenance program.
Activities within the program include adding grease or oil to
the high-pressure pump, changing pre-filters, sanitizing the
machine, and cleaning the separators when they become
fouled. Pretreatment equipment, such as softeners, and
instruments such as pH analyzers requires attention as well.
General Applications
Reverse Osmosis, also known as hyper-filtration, is the finest
filtration known. This process will allow the removal of
particles as small as ions from a solution. RO systems
frequently are used to reduce the levels of total dissolved
solids and suspended particles within water. These systems
remove a variety of ions and metals as well as certain
organic, inorganic and bacterial contaminants. Reverse
osmosis is used to purify water and remove salts and other
impurities in order to improve the color, taste or properties of
the fluid. Reverse osmosis is often used in commercial and
residential water filtration. It is also one of the methods used
to desalinate seawater. Sometimes reverse osmosis is used
to purify liquids in which water is an undesirable impurity
(e.g., ethanol). The RO membrane alone may not be an
effective method for total removal of these contaminants, but
properly designed system may be effective in reducing the
contaminants to safe levels.
The RO membrane’s efficiency in reducing the amount of
contaminant in the water depends on the contaminant
concentration, chemical properties of the contaminant, the
membrane type and condition, and opening conditions.
The most common use for RO is in purifying water. It is used
to remove dissolved impurities from water through the use of
separation technology and a semi-permeable membrane, to
meet the most demanding specifications that are currently in
place. Reverse osmosis is capable of rejecting bacteria,
salts, sugars, proteins, particles, dyes, and other
constituents that have a molecular weight of greater than
150-250 Daltons. The separation of ions with reverse
osmosis is aided by charged particles. This means that
dissolved ions that carry a charge, such as salts, are more
likely to be rejected by the membrane than those that are not
charged, such as organics. The larger the charge and the
larger the particle, the more likely it will be rejected.
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Pure Water Applications
Membrane rejection fluctuates regarding the following
factors:
I.)
Total Dissolved Solids (TDS)
II.)
pH Values
III.)
Cross flow rates and
IV.)
Element recovery levels
The performance of elements (membrane) in the secondpass of a reverse osmosis (RO) system can be the most
dramatically affected. These variations, while not significant
in the majority of applications, become crucial to the success
of high-purity water processing. In addition, the effect of
minor feedwater constituents, such as alkalinity and
ammonia, as seen to play a dominant role in achieving highpurity permeate.
The reverse osmosis (RO) membrane of choice worldwide is
the polyamide (PA) thinfilm composite membrane developed
by Cadotte (1980). The PA composite membrane is made by
forming a thin PA film on the finely porous surface of a
polysulfone (PS) supporting membrane by an interfacial
reaction between the reactant pair trimesoyl chloride (TMC)
and m-phenylenediamine (MPD).
PA membranes have a surface charge that plays a role in
their separation capability. It is important to note that the
nature of this charge can be altered by the pH of the
feedwater pH. The majority of PA (RO) membranes are
negatively charged when operated on pH levels most
commonly encountered in water applications.
RO systems performance is pH sensitive. The pH at which a
protein, RO membrane, carries no net charge is called the
membranes isoelectric point. Below the isoelectric point,
which is normally between pH 4 and 5, the proteins carry a
net positive charge, above it a net negative charge. Due to a
preponderance of weakly acid residues in almost all proteins,
they are nearly all negatively charged at neutral pH. The
isoelectric point is of significance in protein purification
because it is the pH at which solubility is often minimal and
at which mobility in an electrofocusing system is zero (and
therefore the point at which the protein will accumulate). This
substantially decreases their performance when the
permeate quality is being measured by conductivity.
Acid transport across the membrane explains much of this
decrease in performance. Fortunately, this breakdown is
completely reversible when the pH is returned to nearneutral levels. The acid transport is facilitated by the
presence of un-reacted amines in the polyamide barrier
layer. Any particular membrane can respond differently to
changes in pH.
High pH levels can also reduce the rejection capabilities of
PA membranes as measured by conductivity. When values
climb above a pH of 8.5, problems can occur. Lowering the
pH via acid addition will correct this condition.
Thin-film composite Polyamide membrane performance is
also a function of the relative conductance of the feedwater.
Below a certain level of TDS, the membrane rejection will
decline with the TDS of the feed solution. This means it is
possible to have a first-pass permeate that is “too pure” for
the optimum overall performance of a two-pass RO system.
Membrane separation efficiency is affected by feedwater
chemistry. Chemistry takes the dominant role when the
desired product is high-purity water and the benchmark is
conductivity. Dissolved gases such as carbon dioxide (CO2)
can dramatically affect permeate conductivity and RO
systems cannot effectively deal with these gases by
themselves. In specific case of CO2, it is possible to force a
-2
conversion to bicarbonate (HCO3) and carbonate (CO3 )
ions by raising the feedwater pH. These ions are well
rejected by PA (RO) membranes whereas CO2 (and
carbonic acid) are not rejected at all.
By controlling the pH of the feed solution, a portion of the
CO2 present can be shifted to HCO3 and/or CO3-2 depending
on the pH level reached. These pH adjustments permit up to
98% percent of bicarbonate and carbonate being removed in
the first pass of a two-pass system.
Sometimes it is not possible to elevate the pH of the
feedwater. The same objective (removal of some of the
feedwater alkalinity and CO2) can be reached by the use of
degasifier. The degasifier can be located ahead of the first
RO machine or between the two passes. Should some
alkalinity pass into the RO permeate, it will re-equilibrate,
forming H2CO3, HCO3 and/or CO3-2 in proportion to the pH.
Ammonia is yet another water chemistry variable that plays a
significant role in achieving high purity water. Ammonia may
be present due to municipal chlorination of feedwater or from
organic contamination. Residual ammonia may also be
present in the water during the feedwater treatment by
activated carbon or ion exchange via the subsequent
liberation of ammonia.
At neutral and acidic pH values ammonia is ionized. This
means the addition of a strong alkali will produce molecular
ammonia. Should the feedwater contain ammonia, the need
to add caustic for CO2 removal must be carefully balanced
with the need to eliminate ammonia in the permeate.
Ammonia (NH3) can pass through the membrane system in
4+
either the molecular or ionic (NH ) form. Ammonium
hydroxide (NH4OH) would be the most likely ionic form to
pass through RO (thin-film composite Polyamide)
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membrane, particularly if caustic is being used to raise the
pH in the system.
Ammonium hydroxide is less conductive than ammonium
carbonate [(NH4)2CO3] so it is not uncommon to find off-line
samples or storage tank water with conductivity higher than
that of on-line readings.
The pH values will be lower. This shift in pH is due to
absorption of CO2 from the air and the formation of carbonic
acid in the water. Without the presence of ammonia, this
type of contamination of high-purity water with CO2 would
generate higher conductivity as well as the reduced pH.
Product Recommendations
Measurement System
Transmitter/Analyzer
•
•
2-wire FLXA21 pH/ ORP measurement system
4-wire PH450G pH/ORP measurement system
Option 1:
Holders
•
•
FF20 Flow-thru assembly with individual measure,
reference and temperature electrodes
FS20 Insertion assembly with individual measure,
reference and temperature electrodes
Sensors
Bellowmatic reference electrode (SR20-AC32), coupled with
the shock-proof measuring electrode (SM21-AG4) and
Pt1000 temperature electrode (SM60-T1)
Option 2:
Holder: PH8HH Flow Thru assembly
Sensor: PH8EHP Flowing reference pH Sensor for High
Purity Water