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ADSORPTION: Adsorption is the binding of
a molecule to a surface (solid or liquid) by non-specific physical
forces. For example, the removal of free chlorine and
chloramines by activated carbon is through the mechanism of
AERATION: A process in which air is
intimately mixed with water to remove undesirable gases, such as
carbon dioxide and hydrogen sulfide. Carbon dioxide can be more
economically removed from water by aeration than by chemical
precipitation or by the use of ion exchange resins.
ALKALINITY: Capacity of water to accept
acid, ordinarily because of its bicarbonate content. In some
cases a part of the alkalinity in water may be due to carbonate,
hydroxide, phosphate, or silicate. It is expressed in terms of
mg/l of calcium carbonate. Total alkalinity is determined by
titration with acid to a pH of 4.3. Methyl orange indicator is
frequently used as the end point, but potentiometric titration is
If the pH of the water is greater than 8.3, it is probable that
carbonate is present. Carbonate alkalinity is determined by
titration with acid to pH 8.3 (phenolphthalein indicator).
ANION: A particle of matter dissolved in
water that has a negative charge. In each system of matter the
number of anions is equal to the number of cations (positively charged
ASYMMETRIC MEMBRANES: See
BACTERIA: Bacteria are single cell
microorganisms capable of replicating on their own. They can be
divided into two broad categories: aerobic (requiring oxygen)
and anaerobic (not requiring oxygen). Bacteria can live in a
very broad range of habitats. Some, for example pseudomonads,
can thrive in environments containing very low level of nutrients.
These bacteria are frequently slime producers and are a major problem
in water treatment systems. Other bacteria, which adhere to
surfaces, secret gelatinous material which serves to protect the
bacteria from chemical disinfectants. This combination of
bacteria and their protective coating is sometimes referred to as
biofilm. The concentration of bacteria in water is commonly
given in terms of colony forming units (cfu) per ml. A colony
forming unit is a viable bacterium able to replicate to form a
whole colony when incubated in a given environment.
BICARBONATE: Bicarbonate is formed in
water by the reaction of carbon dioxide with mineral matter, such as
limestone or dolomite. The carbon dioxide is absorbed from air
or from decaying organic matter, and the bicarbonate that is formed
imparts alkalinity to water. See Alkalinity. Calcium and
magnesium bicarbonate do not exist in solid form. When water is
removed from solutions of these salts, they lose carbon dioxide and
form calcium and magnesium carbonate.
BLOWDOWN: Discharge of water containing
concentrated dissolved solids that is replaced by water containing
less concentrated dissolved solids. It is conventionally used in
connection with the operation of boilers and cooling towers. It
is also sometimes used as a term for the concentrate from the reverse
BOD: Abbreviation for Biochemical Oxygen
Demand. It is an empirical test used on wastewater in which
organisms capable of oxidizing organic matter are introduced.
The test is not precise. See COD.
BOUNDARY LAYER: A very thin liquid layer
immediately adjacent to the rejecting surface of reverse osmosis
membranes in which the concentration of dissolved solids is higher
than it is in the main body of the water being processed. The
phenomenon is known as concentration polarization.
BRACKISH WATER: Water containing between
1,000 mg/l and 15,000 mg/l of dissolved solids is generally considered
to be brackish.
BRINE: Another term for concentrate.
BRINE SEAL: A U-cup of synthetic
rubber attached to the upstream end of a membrane element. It is
activated by water pressure to form a joint with the inner surface of
the pressure vessel in which the module is placed to prevent water
from bypassing around the module.
BUFFER: A substance or combination of
substances that resist a change in pH when an acid or an alkali is
added to water in which they are dissolved.
BUFFER SOLUTION: A solution of salts that
is used to calibrate pH meters. Different combinations of salts
are used to prepare buffer solutions with different pH values.
CALCIUM BICARBONATE: A salt that is
present in most natural waters. Water containing calcium
bicarbonate loses carbon dioxide when it is evaporated or concentrated
by reverse osmosis and calcium carbonate is precipitated.
CALCIUM CARBONATE: Occurs in nature as
limestone. It dissolves in water containing carbon dioxide by
forming calcium bicarbonate.
CALCIUM SULFATE: Commonly called gypsum.
It is sparingly soluble and precipitates when water is removed from
saturated solutions of the salt.
CARBONATE HARDNESS: See Hardness.
CATION: A particle of matter dissolved in
water that has a positive charge. In each system of matter, the
number of cations is equal to the number of anions (negatively charged
COAGULATION: A process in which small
particles of suspended matter are combined by chemical means into
larger particles to effect more rapid settling or better filtration.
The most widely used coagulant is alum. Other coagulants that
are sometimes used are sodium aluminate, ferric chloride, lime and
mangnesium oxide. Polyectrolytes are frequently used as
COD: Abbreviation for Chemical Oxygen
Demand. It is a test used on wastewater in which a strong
chemical oxidizing agent react with some of the organic matter in the
water. The test is more precise than the BOD test, but it does
not measure all of the organic matter in the water. See TOC.
COLORIMETRIC TITRATION: A titration in
which a change in color is used to measure the end point.
COMPACTION: A change in the physical
structure of RO membranes caused by exposure to excessive pressure
and/or temperatures which result in a decreased ability to produce
CONCENTRATE: Water that is rejected in
the process of reverse osmosis which contains dissolved solids in the
water being processed in more concentrated form.
CONCENTRATION POLARIZATION: See "Boundary
CONDUCTIVITY: The ability of an aqueous
solution to carry electric current depends on the presence of ions in
the solution. Conductivity is a quantitative measure which
describes this ability. Solutions of inorganic ions are
relatively good conductors (and exhibit high conductivity), whereas
solutions of organic molecules are rather poor conductors (and exhibit
low conductivity). Highly purified water is also a poor
conductor. Conductivity is expressed in units of Siemen/cm
(previously known as mhos/cm). Conductivity measurements are
typically encountered in monitoring the performance of reverse osmosis
equipment. Conductivity is temperature dependent and should be
measured with a temperature-compensated meter. The usual
reference temperature is 25ºC. Conductivity measurements are
sometimes used to estimate total dissolved solids in water.
While convenient, this practice is imprecise. (See also
CONVERSION: Another term for "Recovery
DEIONIZATION (DI): Use of ion exchange
resins to demineralize water. See Ion Exchange.
DISINFECTION: Disinfection is the process
of killing micro-organisms, usually by one of a variety of chemical
agents, such as formaldehyde and sodium hypochlorite.
Disinfection lowers the number of micro-organisms without necessarily
killing all those present. Although total killing of all
organisms is virtually impossible, sterilization will reduce the
number of organisms to a safe predetermined level.
Sterilization can generally only be achieved routinely by
heat, gamma irradiation, ethylene oxide, and, in certain cases,
special filtration. Of these methods, only filtration is
suitable for mass sterilization of water and none is suitable for
sterilization of water treatment equipment used in hemodialysis
facilities. However, a proprietary chemical disinfectant
incorporating paracetic acid as the active ingredient has recently
qualified as a sterilant and this agent may be suitable for
sterilization of certain water system components.
DO: Abbreviation for Dissolved Oxygen.
DRY RESIDUE: Amount of dissolved solids
in water determined by evaporating a sample to dryness and heating.
The heating is usually done at 220°F (105°C) and at 355°F
(180°C). Dry residue is almost always not equal to total
dissolved solids because bicarbonate loses carbon dioxide in the
heating process, and some of the dry salts that are formed retain
water of hydration.
EMPTY BED CONTACT TIME: The empty bed
contact time (EBCT) is used as a measure of how much contact occurs
between particles, such as activated carbon, and water as the water
flows through a bed of the particles. As the EBCT increases, the
time available for particles to adsorb solutes from the water also
increases, as does the amount of solute removed from the water during
its transit through the bed. EBCT is calculated from:
EBCT = Vm/Q
Where: Vm is the volume of particles in the bed and
Q is the volumetric flow rate. A consistent set of units
must be used when calculating EBCT with this equation. For
example, if Vm is given in ft³, then Q must be expressed in ft³/min
for the EBCT to have units of minutes. Values of Q can be
converted from other units, such as gpm, to ft³/min using the
conversion factors given in the
flow rates table on the Conversion
FLOW VELOCITY: The flow of fluid at any
point in a water treatment system may be expressed quantitatively in
two ways, either in terms of the volume of fluid passing the point in
a given time (volumetric flow rate) or in terms of the velocity with
which fluid passes the point (flow velocity). The flow velocity
(V) depends on the geometry of the conduit through which the
fluid flows and is related to the volumetric flow (Q) by:
V = Q/A
Where: A is the cross-sectional area of the conduit.
As an example, Table F.1 shows the volumetric flow rate as a function
of flow velocity in PVC schedule 80 pipes of different diameters.
(The values in Table F.1 are based on actual, rather than nominal,
pipe diameters.) Note that for such calculations, it is
essential that consistent units be employed. Thus, the term Q
would be expressed as ft³/sec and A as ft² to yield V as
Table F.1 - Volumetric Flow Rates as a function of
velocity for schedule 80 PVC pipe
Schedule 80 Nominal Pipe Size (Inches)
Volumetric Flow Rate (gpm) at
specified Flow Velocities
FLUIDIZATION: A flowing liquid impinging
on a bed of particles imparts some of its momentum to each particle.
The imparted momentum is in the direction of the fluid flow. The
particles are held to the floor of their container by gravity and to
each other by adhesive forces. If the fluid flow is upward
through the bed of particles, and if the transfer of momentum from the
fluid to the particles is sufficient to overcome both the
gravitational and the adhesive forces, the particles become suspended,
of fluidized, in the fluid stream.
FLUX: A term that is commonly used for
the flow rate of water through reverse osmosis membranes.
FOULING: Deposition of solid substances
on the surface of the reverse osmosis membrane or in ion exchange
resin particles. Fouling can be due to the presence of suspended
solids, sparingly soluble salts, or biological growth. Fouling
reverse osmosis membranes causes a decrease in both the amount of
water produced and the quality of the water. The performance of
fouled membranes can usually be restored by
appropriate cleaning procedures. Fouling of Ion Exchange
resins causes a loss of efficiency and sometimes necessitates
replacement of the resins in the beds.
FRENCH DEGREE: Sometimes used as a unit
of measure for any dissolved subtance, but it is most frequently used
as a measure of hardness. It is usually abbreviated as °F, and
it is equivalent to 10 mg/l of calcium carbonate. See Hardness.
GERMAN DEGREE: Sometimes used as a unit
of measure for any dissolved substance, but it is most frequently used
as a measure of hardness. It is usually abbreviated as °d, and
it is equivalent to 17.9 mg/l of calcium carbonate. "See
GRAINS OF HARDNESS: Although the
theoretical hardness of water is the sum of the concentrations of all
metallic ions, other than the alkali metals, it is commonly expressed
as the equivalent concentration of calcium carbonate in grains.
Ionic concentrations can be expressed in terms of their combining
potential (Eq/L), the number of moles present (mol/L), or their masses
in any of several conventions. In the English system masses are
expressed in terms of pounds (avoirdupois) which contains 7000 grains
each. Although considered outdated in most of the world, the
U.S. water purification industry continues to express hardness in
units of grains/gal expressed as calcium carbonate. Grains/gal
expressed as calcium carbonate can be converted into metric units
(mg/L) by multiplying the former by 17.1. Grains/gal expressed
as calcium carbonate can also be converted into mEq/L of a univalent
ion, such as sodium (Na+) by multiplying by 0.342.
Note: Care must be taken in using these
conversion factors to size equipment based on ion exchange principles
since the ionic content of the water will depnd on the type of ions
present as well as their total mass.
GREENSAND: See "Manganese Zeolite".
HARDNESS: Hardness was originally defined
as a measure of the ability of water to precipitate soaps made from
fatty carboxylic acids. These "soaps" precipitated in the
presence of calcium and/or magnesium ions. Today, hardness is
used to describe the total concentration of calcium and magnesium,
expressed as mg/L of calcium carbonate. It is generally
calculated from measurement of calcium and magnesium in ion
Hardness (mg CaCO3/L) =
2.497 x Ca (mg/L) + 4.118 x Mg (mg/L)
HOMOGENEOUS MEMBRANES: See
HYDROGEN SULFIDE: A gas which is more toxic
than hydrogen cyanide. it has an offensive rotten egg odor, and it
is present in some ground waters as a result of microbial action on
organic matter under anaerobic conditions. Its odor can be
detected at concentrations of a few tenths in an mg/l in water.
HYDROLYSIS: Chemical degradation of a
substance by water that is usually accelerated by acids or alkalies.
ION: A charged particle of matter.
In each system of matter, the number of positive ions (cations, such
as sodium) is equal to the number of negative ions (anions, such as
ION EXCHANGE: Ion exchange is based on
the principle of electroneutrality, that is, charged species are
stable only when they exist as balanced pairs of positive and negative
charges. Ion exchange resins, the materials used to carry out
the process of ion exchange, are particles which contain fixed charges
on their surface. To maintain electroneutrality, each of these
charges has an ion of equal and opposite charge held to it; these ions
are called counter ions. The counter ions are mobile and can
leave the fixed charge if some other counter ion is available to
replace it. The replacement ion must be of the same charge as
the initial counter ion in order to maintain electroneutrality.
The initial counter ion is established by washing the resin with a
concentrated solution of the desired counter ion. For example,
softener resins are cation exchangers containing carboxylic acids on
their surfaces. If these resins are washed with strong NaCl
solutions, the predominant cation solution is Na+ and it
will become the counter ion. In use, the perfusing water will
provide competing counter ions, such as Ca2+. Because
of the preference of carboxylic acids for Ca2+ over the Na+
in dilute solutions, the water will be depleted of the Ca2+
in exchange for the Na+ initially present.
IONIC STRENGTH: Some properties of
dissolved solids, such as the solubility of calcium sulfate and
calcium carbonate, are affected by ionic strength.
No Terms Currently Listed in This Section.
LANGELIER SATURATION INDEX: The
precipitation of calcium and magnesium carbonates in water
purification systems is a serious cause of system failure. The
insolubility of these compounds is a complex function of the pH of the
water, the dissolved carbon dioxide content, the carbonate content,
the presence of other salts, and the temperature. The Langelier
Saturation Index is a method of predicting whether or not carbonate
deposits will form under given conditions. Calculation of the
Langelier Saturation Index is complex and will not normally be done by
hemodialysis personnel. Reverse Osmosis vendors may use the
index in determining the maximum recovery and rejection rates that can
be obtained from a reverse osmosis system before carbonate deposits
will seriously reduce water quality and recovery.
Note: It should be noted that the utility of
such determinations is limited to those situations in which a softener
is not used as part of the pre-treatment scheme for reverse osmosis.
MANGANESE ZEOLITE: A bed of greensand
granules used for the removal of iron and manganese from water.
The greensand is a naturally occurring mineral that has cation
exchange properties. The bed is first treated with manganous
chloride to convert it to the manganese form and then it is activated
(and subsequently regenerated in repetitive cycles) with potassium
permanganate. In use, soluble iron and manganese in water
precipitate on the granules, and these solids are backwashed out of
the bed at the end of each cycle.
MATERIALS BALANCE: The amount of
dissolved substances in the permeate plust the amount in the
concentrate must equal the amount of the feedwater. Since the
volume of the permeate is a fraction corresponding to the recovery of
the feedwater volume rate and the volume of the concentrate is a
fraction corresponding to one minus the recovery rate, materials
balance is calculated from the equation:
Materials Balance =
+ Cc (1-r)
MEMBRANES: Membranes are thin films made
with structures designed to provide selective transport of solutes.
In general, the selectivity of a membrane is based on its ability to
pass or exclude species according to their size. Membrane
structures may be homogeneous or asymmetric. Homogeneous
membranes have structures which are uniform in cross-section, at
least to a magnification of 100x (Figure M.1a). Most homogeneous
membranes have been developed for Microfiltration and hemodialysis.
Membranes reduce not only the flow of undesired solutes, but also the
flow of solvent. In order to minimize the reduction in solvent
flow, asymmetric membranes have been developed. These
membranes are made with asymmetric cross-sections, that is, they
consist of two parallel layers (Figure M.1b). The resistance to
flow of the skin layer, which gives the membrane its filtration
selectivity, is minimized by reducing its thickness. The
resistance to flow of the thicker support layer, which provides
structural strength, is minimized because of its open pore structure.
These different layers may be made from the same material, as in
asymmetric cellulose acetate membranes, or from different materials,
as in thin-film composite membranes.
Membranes used in water treatment equipment are fabricated in two
forms, as flat sheets or as hollow fibers.
Figure M.1. Structure of homogeneous and
a) HOMOGENEOUS MEMBRANE
Uniform pore size throughout structure.
b) ASYMMETRIC MEMBRANE
Dense, thin "skin" overlays porous structure.
MILLIGRAM-EQUIVALENTS PER LITER (m.e./l):
Calculated by dividing mg/l by the equivalent weight of the ions.
MILLIGRAMS PER LITER (m.g./l): Equivalent
to parts per million (ppm).
METHYL ORANGE ALKALINITY: See "Alkalinity".
MHO: Unit of Conductivity. See "Conductivity".
MICROMHO: One millionth of a Mho.
MICRO-SIEMENS: Another term for micromho,
the unit of conductivity.
OHM: Unit of Resistivity. See "Resistivity".
O-RING: A ring of synthetic rubber that
is used as a seal within the connectors to join two or more modules
together in a pressure vessel.
OSMOSIS: A natural phenomenon in which
water diffuses through a membrane from a less concentrated solution of
salts to a more concentrated solution of salts.
OSMOTIC PRESSURE: When a solution, such
as salt water, is separated from pure water by a membrane which is
impermeable to the salt, a flow of water will occur from the pure
water to the salt solution. The driving force for this flow is
called the osmotic pressure and its magnitude depends on the
number of salt particles in the solution.
Note that the osmotic pressure depends on the number of particles and
not on the total mass of particles. For example, 1 g/L of a small
solute, such as sodium chloride, will exert a greater osmotic pressure
than 1 g/L of a large solute, such as protein. For water to flow from
the salt solution to the pure water this solution must be exposed to a
hydrostatic pressure greater than its osmotic pressure.
This is the principle of reverse osmosis.
PARTS PER MILLION (PPM): Equivalent to
milligrams per liter (mg/l).
PERMEATE: Water from which most of the
dissolved solids have been removed by the process of reverse osmosis.
PH: Water (H2O) can dissociate
into two ions: hydrogen (H+) and hydroxyl (OH-).
These ions can also be added to water in combination with other
oppositely charged ions. Thus, a solution of hydrochloric acid
added to the water provides both H+ and the chloride anion,
Cl-. The concentration of H+ in the water
is a measure of water's acidity and the concentration of OH-,
a measure of its alkalinity.
To simplify quantitation of H+ differences, where numbers
with a wide range of exponents are encountered, scientists devised a
logarithmic scale called pH. The pH values range from 1 to 14.
A pH value of 7 is considered neutral. Lower values of pH
indicate acidic conditions and higher pH values indicate alkaline
conditions. Because pH is a logarithmic scale, and increase of 1
pH unit corresponds to a ten-fold change in acidity.
PHS: The pH of water at which it is
theoretically in equilibrium with calcium carbonate, i.e., saturated.
POLYELECTROLYTE: A high molecular weight
water soluble polymer containing chemical groups capable of undergoing
electrolytic dissociation to give highly charged ions.
Polyelectrolytes often have a synergistic effect when used with
POTENTIOMETRIC TITRATION: A titration in
which a rapid change in pH is used to measure the end point.
PRESSURE, APPLIED: The pressure in the
feedwater supplied by the high pressure pump.
PRESSURE DROP: Expenditure of a certain
amount of energy is required for a fluid to flow through any channel,
such as a pipe, particle bed, or membrane. The pressure at any
point is a measurement of the energy content of the fluid at that
point. Since some of this energy is expended in flowing to a
second point downstream, the pressure at the downstream point is less
than at the original point. The amount of energy expended, and
hence the decrease in pressure (or pressure drop), is dependent on the
flow rate and viscosity of the fluid, and the size and shape of the
channel. Pressure drops are usually expressed in terms of lb/in2
or psi, or in the SI system, kPa (kilopascals). Pressure drop is
sometimes referred to colloquially as "delta P".
PRESSURE, NET: Net applied pressure minus
average osmotic pressure.
PRESSURE, NET APPLIED: The average of the
pressure in the feedwater and the pressure in the concentrate upstream
from the pressure control valve.
PRESSURE, EFFECT ON OUTPUT: RO output
varies directly with Net Pressure.
No terms currently listed in this section.
RECOVERY RATE: The percentage of water
being processed by reverse osmosis that is produced as permeate.
REJECTION RATE: When hydraulic pressure
is applied to water in contact with reverse osmosis membranes, water
diffuses through the membranes and the dissolved solids that were in
the water are repelled. The degree to which they are repelled is
the rejection rate. The rejection rate decreases as the
feedwater flows through a reverse osmosis unit because the dissolved
solids in it are becoming progressively more concentrated. The
net result is that the overall rejection rate depends upon the average
concentration of dissolved solids in the entire unit. It is
calculated from the equation:
% Rejection = 100 (1.0 -
However, the average feed
concentration is not the arithmetic average of the feed and the
concentrate, but rather it is the integrated average composition,
which is calculated from the equation:
F.C. x 1n
Integrated average concentration
Natural (Naperian) logarithm
Recovery rate, as a decimal
RESISTIVITY: Resistivity is a measure of
the ability of a substance, such as water, to carry an electric
current. It is expressed in units of ohm-cm and is the
reciprocal of conductivity. Resistivity measurements are
commonly used to asses the quality of water produced by deionizers.
Like conductivity, resistivity is temperature dependent and should be
measured with a temperature-compensated meter. The
usual reference temperature is 25ºC.
SILT DENSITY INDEX: The silt density
index (SDI) is a measure of the ability of water to foul a membrane or
plug a filter. SDI is measured using an apparatus which
typically consists of an inlet pressure regulator and pressure gauge
followed by a filter holder containing a 0.45 µm microporous membrane
filter. Commercial test kits, complete with instructions on how
to calculate the index, are available.
Here for More Information on Silt Density Index Products!.
SUPERFICIAL VELOCITY: The velocity of a
fluid flowing through a tan containing a bed of particles is described
in terms of the superficial velocity. The superficial velocity
is defined as the velocity which would be achieved by the fluid if it
flowed at the same volumetric flow rate through the tank when it was
empty of particles. Mathematically, the superficial velocity (Vs)
is given by:
Where: Q is the volumetric flow rate and A is the
cross-sectional area of the empty tank. For example, if an ion
exchange tank has a cross-sectional area of 640 cm² and water is
pumped through it at a rate of 64 cm³/sec, the superficial velocity is
64/640 or 0.1 cm/sec. The actual velocity at the surface of any
particle may be greater or less than the superficial velocity because
flow will not be uniform throughout the bed and because the particles
occupy some of the cross-section. Nevertheless, engineers have
found it useful to calculate superficial velocities in this manner to
aid in estimating mass transfer between a flowing liquid and the
stationary surfaces of a particle. (see also Empty
Bed Contact Time.)
TITRATABLE ALKALINITY: When certain
anions, such as carbonate (CO3=), are dissolved
in water, they bind hydrogen ions (H+) and thus shift the
water equilibrium (see definition of pH) to produce free hydroxyl ions
(OH-). This excess concentration of OH- is
termed alkalinity. Titratable alkalinity can be measured by
determining the amount of H+
(in mEq or mmol) which must be added to water to restore the pH to
7.0, the condition of neutrality where [H+] = [OH-].
TOTAL ORGANIC CARBON: Organic compounds
dissolved in water are characterized by their carbon content.
Total organic carbon is the mass of carbon present in a water sample,
excluding the carbon present as CO2 and/or carbonates.
The values are determined by catalytically oxidizing (burning) all
dissolved carbon (after CO2/CO3=
removal by acidification) to CO2. The resulting CO2
may be measured directly by infra-red absorption, or it may be reduced
in a furnace with hydrogen to form methane, which is measured by flame
TURBIDITY: Turbidity is a measure of the
presence of colloidal matter in the water that remains suspended.
Suspended matter in a water sample, such as clay, silt, or finely
divided organic and/or inorganic matter will scatter the light from an
incident light beam. The extent of scattering is expressed in
Jackson or Nephelometric turbidity units (JTU and NTU, respectively).
No Terms currently listed in this section.