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Glossary of Water Treatment Terms
Reverse Osmosis (R/O): A water treatment process utilizing a membrane to remove
dissolved minerals. The process of osmosis occurs in nature, when a pure solution is
separated from a saline solution by a semi-permeable membrane. The pure solution passes
through the membrane until a pressure differential is reached called the Osmotic Pressure.
At this pressure differential, determined by the constituents and concentrations in the
saline solution, osmosis stops. Reverse Osmosis reverses this process by applying pressure
in excess of the osmotic pressure to the saline solution, forcing water through the
membrane. As this occurs, most dissolved solids remain behind and are carried out with the
reject stream. The purified water can then be utilized as needed.
TDS (Total Dissolved Solids): This is a measure, usually given in PPM (Parts Per
Million) or in milligrams per liter (mg/l) that is used to specify the concentration of
all minerals dissolved in a water solution. This measurement is a principle factor in
determining the Osmotic Pressure of a solution and hence the operating pressure necessary
for a system to produce a reasonable amount of product water.
Osmotic Pressure: This is the pressure differential that develops as a result of a
solution containing water and a particular concentration of dissolved solids, including
minerals and salts. The osmotic pressure must be exceeded to produce purified water by
means of reverse osmosis. Tap water may have an osmotic pressure of about 10 PSI (1 Bar),
while seawater may have an osmotic pressure of around 376 PSI (27 Bar) or more. To produce
a suitable quantity of product water, reverse osmosis systems typically operate at more
than double the osmotic pressure.
PPM (Parts per Million): This measurement is a means of specifying the
concentration of any constituent in a solution of water. This is the same as mg/l or
milligrams per liter.
Feed Water: This refers to the incoming water of a water treatment system that
has not yet been treated. Also called raw water or source water.
Product Water: This is the term used to describe the treated or purified water
produced by a water treatment system, including reverse osmosis systems and others. Also
known as Permeate.
Permeate: This is a term used to refer to the water produced by a membrane process.
The permeate is the water that permeates or penetrates through the membrane as product
water.
Reject: The reject stream is the water that does not pass through the membrane. Since
minerals are left behind from the departed permeate or product water, the reject stream is
more concentrated than the source or feed water. Hence it is also called the concentrate.
Concentrate: Another term for the reject stream.
Recovery Rate: This term refers to the percentage of water recovered as product water
from a given quantity of feed water. A recovery rate of 40% would indicate that, of every
100 units of incoming water, 40% would be converted to product water or recovered for use,
and 60% would be rejected.
Barrier Layer: This refers to the active layer of membrane material that actually
separates the impurities from the product stream or permeate. This barrier layer is
supported by a micro-porous support layer, usually made from polysulfone, which is cast on
a non-woven support material.
Membrane: The term membrane generally refers, not only to the barrier layer described
above that is technically the membrane, but also to the entire engineered membrane
separation device, including the micro-porous support, the non-woven support material, the
carriers, product tube, anti-telescoping device, and the construction of the element. The
most commonly used membrane element type is a spiral wound membrane element.
Spiral Wound Membrane Element: The most common type of membrane element used in
reverse osmosis and other membrane separation technologies. The spiral wound design was
developed in California by U.S. Government funded research. The design employs a number of
layers of membrane material, folded over to create envelopes. Each envelope incorporates a
fine medium inside to facilitate the flow of the permeate or product water, and a coarse
medium or mesh outside to facilitate the flow of the reject water or concentrate. The
reject or concentrate continues its flow direction straight through the spiral wound
element, entering one end and exiting the other end. Meanwhile, the permeate or product
water, after penetrating just one layer of membrane, travels the inward spiral until it
collects in the product tube for delivery to the customer. Spiral wound elements must be
housed in pressure vessels.
Pressure Vessel: Membrane elements must be installed in a pressure vessel to provide
the means to supply feed water, remove the reject or concentrate stream, delivery product
water, and contain the pressure at which membrane systems much operate. Pressure vessels
may contain from one to eight membrane elements, arranged end to end in series.
Responsible manufacturers normally use pressure vessels with a burst pressure rating of
four times the rated operating pressure to prevent any safety issues from arising. This is
especially important with seawater systems, where operating pressures normally run over
800 PSI (56 Bar) and at times may exceed 1,000 PSI (69 Bar).
Rejection Rate: This figure, typically 80% to 99.5%, indicates the percentage of
dissolved solids that will be rejected, or prevented from passing through with the product
water. The rejection rate is often specified for a particular ion, such as the Chloride
(Cl) ion. A seawater system with a rejection rate of 99% would thus allow 1% of the
dissolved solids to pass. Hence a feed water of 35,000 PPM would result in a product TDS
of about 350 PPM at that rejection rate. Various membrane materials and designs provide
for a range of rejection rates. Typically, a higher rejection rate would require a higher
pressure to produce the same amount of permeate as would be produced by a similar membrane
system with a lower rejection rate. However, modern technology has produced "low
pressure, high rejection membranes" which provide excellent rejection at low
pressures, thus saving energy. These are available thus far for brackish water and tap
water applications.
Nano-Filtration (N/F): Nano-Filtration is a membrane based treatment method with lower
rejection rates than R/O. Nano-Filtration is often employed as a water softening device
because it typically rejects larger molecules (such as those that cause scaling and water
hardness) very well, but smaller molecules are usually allowed to pass. Nano-Filtration
systems thus usually operate at a lower pressure than similar reverse osmosis systems, and
reject less of the dissolved solids. This technology is frequently employed in waste water
treatment systems. Nano-Filtration systems can often operate at higher recovery rates than
R/O systems.
Ultra-Filtration (U/F): Ultra-Filtration is another membrane based treatment system
which does not usually reject molecules except for very large ones, but rejects virtually
all particles. This technology is often used to remove suspended solids from feed streams
before feeding the water or other fluid to the reverse osmosis or nano-filtration
membranes. This can be very effective when the suspended solids load is unusually heavy or
when oil or grease is present. Ultra-Filtration can also be effectively employed as a
means to remove bacteria. U/F systems can usually achieve high recovery rates, as no
dissolved solids are rejected. U/F can be very effective in oil removal or concentration.
Micro-Filtration: This is also a membrane based fluid treatment technology, employing
all the parameters of other membrane technologies, but with a larger pore size. Operating
parameters are similar to those for U/F, but pore sizes are larger.
Sub-Micronic Filtration: Sub-micronic filtration technology is usually applied using
more conventional filtration media, but with very small pore sizes. Membranes are usually
not used for this technology. Rather, cartridge filters are employed.
Cartridge Filter: Cartridge filters are a widely used and have been utilized for water
treatment for many decades. Cartridges are usually rated in microns. 40 microns is
considered the largest particle visible to the human eye. Typical prefiltration
requirements for reverse osmosis systems are around 5 microns.
Bag Filter: Bag filters have applications similar to cartridge filters. They are
called bag filters because of their shape. Typically the contaminants are caught inside
the bag as the fluid flows down from the top. Bag filters usually have these advantages
over cartridge filters: less bulk to dispose of, lower cost. Disadvantages for most types
of bags are lower efficiency in filtration and more possibility of bypass. These
disadvantages usually mean that a cartridge filter must be used as the last step in
reverse osmosis pretreatment, but bags can often be employed, depending on system and feed
water parameters, as a means to reduce bulk of disposal and storage as well as operating
costs.
Media (or Multi-Media) Filter: Also called depth filters. A variety of media
types may be employed in this type of filtration device, depending on the objective of
this part of the water treatment system. Sand has been employed in this type of filter for
many decades. Media type filters are recommended for applications where large quantities
of suspended particles need to be removed economically. Usually the media is permanent, so
maintenance costs are very low. The device is typically designed with graded media beds,
arranged according to size. The spaces between the particles of sand or other media
determine the size of particles that will be removed. The depth of the filter base is also
a critical design criteria. Water flows downward through a distributor, through the graded
media beds, and is collected by another set of distributors installed in the bottom of the
tank. The particles are left behind in the media beds. Cleaning is accomplished by
backwashing, or reversing the flow. This flow lifts the media beds and dislodges the
particles trapped there. These are then washed away with the backwash fluid and
discharged. Media filters are often employed as one of the first parts of the pretreatment
system in membrane type water treatment systems. Usually, additional filtration afterwards
is also required, as the particle size removed by media filters is not small enough to
provide good membrane protection.
Centrifugal Filter: The centrifugal type filter is a very low maintenance device. It
can usually be effectively applied when the particles to be removed are fairly large and
relatively heavy. A feedwater sample collected in a bottle can be used to determine
whether such a device would be helpful. Shake the bottle thoroughly, then let stand for 5
minutes. Any particles that have settled to the bottom in that time would likely be
removed effectively by a centrifugal filter. For such applications, generally this type
filter will provide excellent service with almost no maintenance costs at all.
Flow Meter: Most R/O systems require instrumentation to monitor and control
performance. Flow meters monitor the rate of flow of water or other fluids, and are
typically installed to indicate the rate of flow of the permeate, reject, and feed
streams. Some systems employ a recycle stream for optimum membrane performance and higher
recovery rates. This recycle stream must be monitored to ensure proper system operation.
Pressure Gauge: The operating pressure at various stages of treatment must be
monitored. Pressure gauges indicate these pressures, and are usually installed before and
after each filtration device so that the pressure drop can be determined. This parameter
is generally required to determine the need for filter changes or cleanings. Pressure
readings are also required at pump discharges. Better systems will also provide the
pressure at the feed and reject points of the pressure tubes containing the membranes.
This permits immediate determination of the pressure drop across the membranes, needed to
determine the condition of the membrane elements and whether such may need cleaning.
TDS Monitor: This device indicates the permeate or product water’s quality in
terms of total dissolved solids. Some higher class systems may incorporate a two channel
monitor that compares the feedwater TDS with that of the product. The meter can then
calculate and read out the rejection rate. Most TDS monitor’s include user adjustable
set points that can alert the operator via an alarm or other indication of a problem with
the water quality. Some systems also include automatic diverter valves to make sure that
no water of high TDS enters the customer’s storage tanks. Knowing the permeate TDS is
also important in gauging the condition of the membranes and other key system components.
Pre-Treatment: Membrane type water treatment systems require good pretreatment to
ensure optimum system performance and acceptable membrane life. The type and extent of the
pretreatment depends on the characteristics of the feed water, the recovery the membrane
system is designed for, the use of the produced water, and other factors. Pretreatment
always includes one or more types of prefiltration, and almost always includes cartridge
filters. Some systems may employ 3 or 4 types of prefiltration devices, chemical
pretreatment, some form of sterilization, and even U/F or Nano-Filtration. The chemicals
used may include antiscalants, pH adjustment, and biological growth control or others,
depending on the application and feedwater characteristics. Sterilization may be effected
by means of Ozone or U-V. Oxidizers such as chlorine, bromine, iodine, potassium
permanganate, and others are seldom used in membrane systems because most membrane types
are seriously damaged by such oxidizers. However, some applications require such agents.
In such a case, the oxidizers usually have to be removed prior to contact with the
membranes by chemical or other means. The importance of good pretreatment in membrane
systems, and especially in R/O systems, cannot be overstated.
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