Garratt-Callahan is an industrial water treatment company with more than a century of expertise in serving businesses. We’re committed to effective, efficient, and safe water treatment. The following glossary is meant to aid in education about commercial water treatment and is not intended for diagnostic purposes.

— A —

— B —

Bactericide

Basin

Basin Sump

Bioaugmentation (Secondary Treatment)

Biochemical Oxygen Demand (BOD)

British Thermal Unit (BTU)

— C —

Chemical Oxygen Demand (COD)

Circulation Rate (CR)

Circulation Rate, Load Corrected (CRc)

Chlorination

Coagulation

Cold Water Basin

Cold Water Temperature (CWT)

Convection Heat Transfer

Cycles of Concentration

— D —

DBT

Deaerating or Closed Heater

Dealkalized Water

Deionized or Demineralized Water

Differential Pressure (∆P or “delta p”)

Diffused Air

Diffusion Deck

Digestion

Dissolved Air Flotation

Dissolved Oxygen (DO)

Double Flow Water Cooling Tower

Downspout

Drift Eliminator

Drift, Windage Loss, or Carryover

— E —

Equivalent Direct Radiation (EDR)

Entering Wet Bulb Temperature

Escherichia Coliform

Evaporation Loss

Evaporation Rate

Evaporative Cooling

Exhaust Wet Bulb Temperature

Exit Basin Temperature

— F —

Forced Draft Water Cooling Tower

— G —

Generating Tubes (Risers)

— H —

Heat Load or Tower Capacity

Hot Water Temperature (HWT)

Hotwell

Hyperbolic Induced Draft Water Cooling Tower

— I —

Induced Draft Water Cooling Tower

Inlet Wet Bulb Temperature

Inorganic Material

Infection

— L —

Langelier Saturation Index (LSI)

— M —

Makeup Tube, Downcomer, Supply Tube

Makeup Water

Manhole

Manifold

Mechanical Draft Water Cooling Tower

Micro-Organisms

Mixed Liquor

Mixed Liquor Suspended Solids (MLSS)

Mixed Media Gravity Filter

Mud Drum

Multiple Stage (or Concentrate Staged) System

Multiple Pass (or Permeate Staged) Systems

— N —

Natural Draft Water Cooling Tower

NDP:(Net Driving Pressure)

Nitrification

Nitrogen

Nitrogen Cycle

Nominal Tower Dimensions

Nozzle

NPF (Normalized Permeate Flow)

Nutrient

— O —

Open Feed Water Heater

Organic Matter

Organic Material

Orthophosphate

Overall Tower Dimensions

Oxic

Oxidation

Oxidation Ponds or Lagoons

— P —

Packing

Percent Load (% Load)

Permeate

pH of Saturation (pHS)

Primary Waste Treatment

PSI (psi)

Practical Scaling Index (PSI) (a.k.a. Puckorius Scaling Index)

Pressure Vessel

— R —

Radiant Heat Transfer

Return Activated Sludge

Riser (Generating Tubes)

Ryznar Stability Index (RSI)

— S —

Saturation Temperature

SDI (Silt Density Index)

Secondary Waste Treatment

Steam Boiler or Steam Generator

Steam Circulator Tubes

Steam Drum

Steam Header

Steam Separation or Steam Scrubber

— T —

Tertiary Waste Treatment

— U —

— W —

Waste Activated Sludge

Waste Treatment Sludge

Definitions

— A —

Access Tunnel

Opening in natural draft tower used for access to the cold water basin.

Activated Sludge

Is defined as a ‘suspension’ of microorganisms, both living and dead’ in a wastewater. The microorganisms are active by an input of air (oxygen) thus known as activated-sludge. Activate-sludge is that sludge which settle down in a secondary sedimentation tank after the sewage has been freely aerated and agitated for a certain time in an Aeration tank.

Activated Sludge Process

Biological wastewater treatment process used as secondary treatment to remove soluble, colloidal, and particulate (suspended) organic substances. It consists of a reactor receiving a mixture of primary treatment effluent and recycled activated sludge, which is aerated and agitated, promoting the formation of microbially-active, flocculent settleable solids. Upon sedimentation, part of this activated sludge is recycled back into the reactor.

Aeration

Mechanical process, such as mixing or agitation, by which air comes in close contact with a liquid, allowing the removal of dissolved gases, and the diffusion of atmospheric oxygen responsible for the oxidation of dissolved metals and volatile organic compounds (VOCs).

Aeration Tank

Chamber in which water is mechanically aerated to promote the diffusion of oxygen from the atmosphere.

Aerobic Bacteria

Bacteria requiring oxygen for growth and energy production, via aerobic respiration..

Airflow

Total amount of dry air and associated water vapor flowing through the tower.

Air Inlet

Opening in cooling tower through which air enters a tower.

Air Recirculation

A condition that exists when hot humid exhaust vapor finds its way back to the tower air intake. This happens mostly in forced draft towers operating in enclosed areas.

Alkalinity

A measure of the buffering capacity of a liquid solution, or the ability to neutralize acids and bases.

Algae

A low form of plant life generally requiring sunlight and air for existence.

Algaecide

A biocide used for killing and preventing the growth of algae.

Algicide

A material that will retard or prevent the growth of algae.

Ambient Atmosphere

The atmosphere adjacent to but not affected by the cooling tower.

Ambient Wet Bulb Temperature

The wet bulb temperature measured in accordance with the definition of “ambient” in Ambient Atmosphere.

Anaerobic

Refers to an environment partially or completely depleted of any form of oxygen.

Anaerobic Bacteria

Bacteria that produce energy and grow in environments depleted or deficient in dissolved oxygen (DO).

Anoxic

Refers to an environment devoid of dissolved oxygen (DO), but in which other forms of oxygen, e.g. nitrates and phosphates may be present.

Approach

The difference between the temperature of the cold water leaving the tower and the wet bulb temperature of the air entering the tower. This determines the lowest temperature to which the tower water can be cooled. Do not confuse with chiller approach—the difference between the exiting condenser water temperature and the condensed refrigerant temperature.

Array

the arrangement of multiple stages within a system
effluent from the first stage in an array enters the second stage in the array and so on
the arrays may be “permeate staged” or “concentrated staged” as described below

Atmospheric Tower

One in which air movement through the tower is dependent upon atmospheric conditions, rather than induced by mechanical fans.

— B —

Bacteria

Microscopic, unicellular prokaryotic organisms that lack nuclear membrane, divide through binary fission, and exist in different shapes, e.g. spherical, rod-shaped or spiral.

Bactericide

A material that will retard or prevent the growth of bacteria.

Basin

Also Cold Water Basin. See Distribution Basin. A shallow tank beneath the tower that receives and stores the cooled water.

Basin Sump

See SUMP.

Bioaugmentation (Secondary Treatment)

Refers to an in-situ bioremediation process through the addition of biomass from activated sludge for the enhanced degradation contaminants or pollutants.

Biochemical Oxygen Demand (BOD)

The amount of oxygen consumed by the biomass present in a unit volume of sample over five days, under aerobbic conditions, and expressed in milligrams of oxygen per liter.

Biocide

General term for algicides, bactericides, and fungicides.

Biological Oxidation

Process by which microbial activity causes the oxidation of chemical compounds, e.g. pollutants.

Blowdown or Bleed

The tower water sent to waste to control the concentration of dissolved solids in the remaining tower water.

Boiler Header

A small side wall drum, either a circular or rectangular cross section, into which tubes are rolled.

Boiler Horsepower

By definition, the generation of 34.5 lbs steam/hour from and at 212oF. By common acceptance, 10 square feet of heating surface is one rated boiler horsepower. For developed horsepower, however, use the 34.5 lbs steam/hour figure.

Boiler Scale

Those deposits precipitated from solution at the boiler tube surface.

Boiler Sludge

Solid matter from boiler water which precipitates throughout the body of water and not at the tube surface. Such deposits may drop down on the surfaces and remain, but they are not formed there.

British Thermal Unit (BTU)

BTUs are the amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit at or near 39.2oF. Calorie is a similar measure of the heat needed to raise one gram of water one degree centigrade at a pressure of one atmosphere.

British Thermal Unit (BTU)

The quantity of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit.

Bulking Sludge

Refers to sludge that fails to settle in the sedimentation tank due to the excessive growth of filamentous bacteria extending outside the flocs, thus hindering the settling process.

— C —

Capacity

The amount of water in a cooling tower system at any given time.

Carryover

Carryover is the process whereby liquid boiler water is physically entrained with the steam and passes off into the steamline. A general term applied to the result of either foaming or priming.

Casing

The vertical enclosing sidewall of a tower, excluding the louvers.

Cell

The smallest tower subdivision that can function as an independent unit with regard to airflow and water flow; it is bounded by exterior walls or partitions.

Cell Dimensions

Width: dimension perpendicular to tower longitudinal axis and usually at right angles to the louver area; Length: dimension parallel to longitudinal axis and the plane where louvers are usually placed; Height: distance from basin curb to top of fan deck, but not including fan stack. Nominal width and length are measured from and to the column centerlines.

Chemical Oxygen Demand (COD)

Refers to the amount of dissolved oxygen (DO) required to oxidize the organic matter present in a unit volume of wastewater.

Circulation Rate (CR)

The volume of water that is pumped over a tower measured in gallons per minute. It is the capacity of the

recirculating pumps.

Circulation Rate, Load Corrected (CRc)

The effective circulation rate based on the heat load when the average ∆T is not 10oF.

Chlorination

The disinfection method consisting of adding chlorine or other chlorinated compounds, e.g. chloramines, to water.

Coagulation

The process by which a positively charged chemical (coagulant) is added to water to destabilize the charge on colloids and suspended solids, thus promoting the clumping of fine flocs into larger flocs, and accelerating the settling process.

Cold Water Basin

A device beneath the tower to receive the cold water from the tower and direct its flow to the suction line or sump.

Cold Water Temperature (CWT)

Temperature of the water entering the cold water basin before addition of makeup or removal of blowdown.

Cold Well (or Sump)

The lowest portion of the basin that has been deepened to accommodate submerged pumps.

Coliform Organisms

Gram-negative, oxidase negative, non-spore-forming, aerobic, or facultative anaerobic rod-shaped bacteria used as indicators of fecal pollution.

Combined Sewer

A wastewater distribution system in which rainwater runoff, domestic sewage, and industrial wastewater are combined.

Composite Sample

A sample consisting of individual grab samples collected over a specific period of time.

Concentrate

effluent with dissolved solids
high in dissolved solids
sometimes called “reject” water (if it is not the stream of interest)
may be the stream of interest (process applications, e.g., juice concentration, dairy processes, etc.)

Condensate

Condensed steam, usually recirculated back to the boiler.

Condensate Tank

A closed or open tank into which condensate returns and is stored. Condensate goes from here to a hot well, a heater, or direct to feed pump.

Convection Heat Transfer

Transfer of heat to boiler tubes by hot gases passing over the tubes.

Cooling Factor

The ratio of water being pumped over the tower to the amount of air being forced through the tower.

Cooling Range

The number of degrees water is cooled while falling through a cooling tower. Also commonly referred to as Delta T (∆T), or change in temperature.

Cooling Tower

A device for the evaporative cooling of water by contact with the air.

Crossflow Tower

A tower in which air is drawn or forced in through the air intakes by the fan, flows horizontally across the fill section and mixes perpendicularly with the falling hot water.

Counterflow Tower

A tower in which air is drawn in through the louvers (induced draft) or forced in (forced draft) at the base by the fan. It flows upward through the fill material and mixes countercurrently with the falling hot water.

CWT

See Cold Water Temperature.

Cycles of Concentration

The number of times the tower water has been concentrated. It is measured by the ratio of dissolved solids in the tower water to those in the makeup. Chlorides are measured in preference to other dissolved solids unless chloride containing chemicals are being added to the system.

— D —

DBT

See Dry Bulb Temperature.

Deaerating or Closed Heater

Feedwater heater runs above make up temperatures and is vented so that gases can escape.

Dealkalized Water

Water that has gone through a dual unit softener where all cations are exchanged for sodium and all anions exchanged for chloride and hydroxide. The total solids will be the same as before, but they will be virtually all sodium chloride. The silica content will not be greatly affected.

Deionized or Demineralized Water

Deionized or demineralized is water from which all mineral content has been removed by ion exchange.

DeltaT (∆T)

Temperature Differential (Delta T) is the cooling range or temperature drop across the tower or condenser (hot return to cold basin).

Denitrification

Microbial process consisting of the reduction of nitrates and nitrites into gaseous forms of nitrogen, mainly nitrous oxide (N₂O) and nitrogen (N₂).

Design Conditions

Defined as the combination of hot water temperature (HWT), cold water temperature (CWT), gallons per minute (gpm), and wet bulb temperature (WBT) in mechanical draft towers. In natural draft towers: HWT, CWT, gpm, and WBT, plus either dry bulb temperature (DBT) or relative humidity (RH).

Desilting Sump

The area in the cold water basin, usually at a low point, where silt can be flushed to a drain.

Desuperheater

A device for removing all or part of the superheat from steam by spraying water into it or by use of a heat

exchanger.

Differential Pressure (∆P or “delta p”)

the pressure difference between the initial feed stream and the final reject (concentrate) stream
∆P provides the driving force to move water across the membrane surfaces
if flows are constant ∆P will not change unless scaling, or fouling physically blocks the membrane
monitoring ∆P across each stage of the system can help pinpoint problems, often isolating them to a single stage, pressure vessel or element

Diffused Air

Aeration method consisting of pumping air through a pipe or tubing and releasing it though a diffuser below the water’s surface.

Diffusion Deck

A fill deck located directly under the distribution basin, or nozzle bank. The purpose of this deck is to receive water from the basin or nozzle and distribute it uniformly over the fill decks.

Digestion

Biological process of organic matter decomposition by bacteria, into simpler organic compounds, in the presence or absence of oxygen.

Dissolved Air Flotation

Water treatment process in which air and pressurized water are mixed, leading to the formation of microbubbles that adhere to coagulated flocs of suspended materials, thus increasing their buoyancy. The resulting floating bed of sludge can then be mechanically skimmed at the surface of the water to obtain a cleaner discharge water.

Dissolved Oxygen (DO)

Amount of oxygen dissolved in liquid, and measured in milligrams oer liter.

Dissolved Solids

Dissolved combined content of organic and inorganic substances within a liquid in an ionized, molecular and colloidal or suspended form.

Distribution Basin

A shallow pan-shaped basin used to distribute hot water over the tower fill.

Distribution Box

One used in conjunction with the manifold and valve assembly in a cross flow tower to disperse the hot water uniformly in all directions, thereby increasing the effectiveness of the distribution nozzles.

Distribution Nozzle

See Nozzle and next entry.

Distribution System

Those parts of a tower, beginning with the inlet connection, which distribute the hot circulating water within the tower to the points where it contacts the air. In a counterflow tower, this includes the header, laterals, and distribution nozzles. In a crossflow tower, the system includes the header or manifold, valves, distribution box, basin pan, and nozzles.

ALSO: The method of spreading the hot water uniformly over the top of the tower.

Double Flow Water Cooling Tower

A crossflow tower with two full sections and one plenum chamber common to both.

Downspout

A short, vertically placed pipe or nozzle used in a gravity distribution system to divert water from a flume or lateral to a splasher.

Drift Eliminator

An assembly of baffles installed at an angle to the direction of the air flow beyond the distribution system to remove entrained water droplets from the exhaust air.

Drift, Windage Loss, or Carryover

Water lost from the tower as liquid droplets entrained in the exhaust air. It is independent of water lost by

evaporation.

Drum Vent

A valve on top of the top drum of the boiler, used to vent air from drum on boiler startup, and opened when boiler is

cooling down to prevent a vacuum in the drum.

Dry Bulb Temperature

The temperature of the inlet or ambient air adjacent to the cooling tower, as measured by a dry bulb thermometer.

— E —

Economizer

A tube bank that is not part of the boiler circulation but is used to heat feedwater before it enters the boiler drum. Cooler gases heat it after these gases have gone through the boiler tube sections.

Effluent

The outflowing gas or liquid from a natural or man-made reactor.

Electric Boiler

A unit for production of steam by heating water either with electrical resistance elements or by passing an AC current through the water between electrodes.

Eliminator

See Drift Eliminator.

Eliminator Board

The smallest component in a wood drift eliminator assembly, usually installed in a fixed position at an angle to the direction of airflow. Also known as an eliminator baffle (blade).

Equivalent Direct Radiation (EDR)

A term used to express the amount of heat transfer per unit area expected from a heating device using a particular fluid medium.

Entering Wet Bulb Temperature

Average wet bulb temperature of the entering air. Includes any effects of recirculation.

Escherichia coli

Rod-shaped, Gram-negative bacterium in the family Enterobacteriaceae, which inhabits the lower intestines of warm-blooded animals, and is released into the environment via faeces and/or wastewater effluent.

Evaporation Loss

Water evaporated from the circulating water into the atmosphere by the cooling process.

Evaporation Rate

The volume of water being evaporated to cool the remaining water measured in gallons per minute.

Evaporative Cooling

The evaporation of a portion of the cooling tower water in order to lower the temperature of the remaining water.

Exhaust Wet Bulb Temperature

Average wet bulb temperature of the air discharged from the tower.

Exit Basin Temperature

Temperature of the circulating water leaving the cold water basin.

Exit Diameter

Diameter of the shell at the top in the hyperbolic natural draft tower.

Extended Aeration

Enhanced activated sludge process in which air is supplied by mechanical or diffused aeration to allow for the aerobic digestion of biodegradable organic wastes.

— F —

Fan

A device for moving air in a mechanical draft tower. The fan can be applied as induced draft or forced draft.

Fan Deck

The structure supporting and enclosing the fan in an induced draft tower.

Feed

water entering the membrane
Contains dissolved solids
Has (or should have) gone through extensive pretreatment

Feedwater

Feedwater is water that enters the drum from the feedline. Usually hot, it may consist of a mixture of condensate and makeup water.

Feedwater Header

Somewhat similar in its usage to the term steam header. A feed pump that supplies water to one boiler is usually called a feedwater line, but if the line branches off and supplies several boilers, it is often called a feedwater header.

Feedwater Trough

A trough in the upper drum used to distribute feedwater along the drum. Usually just above normal water level.

Fill or Packing

The narrow boards or plastic slats placed inside the tower to intercept the falling water. The resultant splashing exposes new water surfaces to the air, and thus increases the efficiency of heat and vapor transfer from water to air. That part of a crossflow, counterflow, or natural draft tower consisting of splash bars, vertical sheets of various configurations, or honeycomb assemblies, tile, or other materials, that are placed within the tower to effect heat and mass transfer between the circulating water and the air flowing through the tower.

Fill Bars

The assembly of splash bars comprising the tower filling. Fill bars intercept the downward fall of water at regular intervals, forming splash surfaces that cause water drops to break into smaller droplets, providing wetted surfaces for air-water contact.

Fill Deck

The assembly of splash bars comprising the tower filling. See Fill Bars for description of operation in a crossflow tower.

Float Valve

A valve actuated by a float, generally used to control makeup water supply.

Floc

Mass resulting from the precipitation or aggregation of suspended particles.

Flocculation

Slow stirring of the destabilized colloids and suspended solids following coagulation to promote floc growth.

Flow

may refer to feed, permeate, or reject
typical units are gallon per minute (gpm); cubic meter per hour (m3/h)
MGD (million gallon/day) is also common

Flow Control Valve

A manually controlled valve generally located in the hot water supply line.

Flux

permeate flow/membrane area
gallons per square foot per day (gfd)
liters per square meter per hour (1/m2h)

Flume

A trough that may be either totally enclosed or open at the top. Flumes are sometimes used in cooling towers to distribute the hot water over the fill.

Fogging

A fog condition created when the exhaust air or plume from a cooling tower becomes supersaturated so that part of the water vapor condenses into visible liquid droplets.

Forced Draft Water Cooling Tower

A type of mechanical draft tower in which one or more fans are located at the air inlet to force air into the tower.

— G —

Generating Tubes (Risers)

Tubes that are subject to high convection or radiant heat and in which steam is generated. The water-steam mix rises in these tubes toward the top drums or steam headers.

Grease

A solid to semi-solid product of dispersion of a thickening agent in a liquid lubricant, including fats, waxes, and free fatty acids, among others.

GPM (gpm)

Gallons per minute.

Grit

Heavy mineral material found in raw, untreated wastewater, and consisting of gravel, sand, broken glass, fragments of metal and inorganic solids.

— H —

Handhole

An opening in a boiler header which permits inspection or tube rolling. The closure for this opening is called a handhole fitting.

Header

In a counterflow tower, the main pipe carrying hot water to a series of laterals for distribution over the fill

materials. In a crossflow tower, it is the main pipe carrying hot water to the distribution basin of each cell.

Heat Exchanger

A device for transferring heat from one substance to another. Heat transfer can be by direct contact (as in a cooling tower) or indirect (as in a shell and tube condenser).

Heat Load

Heat removed from the circulating water within the tower. It can be calculated from the range and the circulating water flow. Unit: BTU per hour = gpm x 500 x (HWT-CWT).

Heat Load or Tower Capacity

The amount of heat dissipated in a cooling tower, measured in BTU per minute.

Hot Water Temperature (HWT)

Temperature of circulating water entering the distribution system.

Hotwell

A closed or open tank into which the condensate returns and from which the feed pump takes suction. Makeup water normally goes into this tank if there is no heater.

Hyperbolic Induced Draft Water Cooling Tower

A cooling tower of hyperbolic shape that depends

— I —

Induced Draft Water Cooling Tower

A type of mechanical draft tower in which one or more fans are located in the air outlet to induce airflow throughthe air inlets.

Inlet Wet Bulb Temperature

The average of the wet bulb temperature obtained from several stations located on both the windward and leeward sides of the tower.

Inorganic Material

Chemical compounds with no elemental carbon, and no Carbon-Hydrogen bonds. Inorganic materials are the building blocks of non-living things, e.g. salts and metals, while organic materials are present in all living organisms.

Infection

Introduction of harmful microorganisms in drinking water systems.

— L —

Langelier Saturation Index (LSI)

An equilibrium index developed to indicate the saturation (i.e., scaling or dissolving) characteristics of calcium carbonate in water.

LSI = pH – pHS

Lateral Header

A pipe or flume distributing water from the distribution header to nozzles, or from other points of discharge to the filling area.

Louvers

The sloping boards on the outside of the tower that allow air in and prevent spray from escaping.

— M —

Makeup

Water added to the circulating water system to replace water lost from the system by evaporation, drift,

blowdown, and leakage.

Makeup Rate

The water required to replace the water lost to evaporation, drift and bleed, usually measured in gallons per minute.

Makeup Tube, Downcomer, Supply Tube

Names for tubes where the circulation is in a downward direction. These tubes are not in the hot gases, and the boiler water goes down through them to supply generating tubes. Steam is not generated in these tubes.

Makeup Water

Treated or untreated water introduced into the heater or hotwell to make up feedwater requirements not fulfilled by condensate returns.

Manhole

Opening in a drum which permits a person to crawl in for inspection.

Manifold

The main header pipe in a crossflow tower.

Mechanical Draft Water Cooling Tower

A tower through which air movement is effected by one or more fans.

Micro-Organisms

Microscopic organisms that can be bacteria, fungi, archaea or protists, and can either be a single cell (unicellular); cell clusters; or multicellular, relatively complex organisms.

Mixed Liquor

The mixture of wastewater from primary treatment and activated sludge as secondary treatment influent into an aeration basin, as part of the activated sludge process.

Mixed Liquor Suspended Solids (MLSS)

MLSS are the concentration of suspended solids in mixed liquor, usually expressed in grams per liter.

Mixed Media Gravity Filter

A filtering equipment using three or more different media in series, in contrast to single medium filter, e.g. coal and sand.

Mud Drum

The bottom drum of a water tube boiler. A blowdown line is normally taken off this drum for removal of mud and sludge and for emptying the boiler.

Multiple Stage (or Concentrate Staged) System

These arrange pressure vessels filled with membrane elements in such a way as to maximize the quantity of pure water produced. The concentrated “reject” water from the first stage becomes the feed to the second stage. The first stage usually has twice as many pressure vessels (and membrane elements) as the second stage. These are the most common types of systems and are often seen in power generation and desalination applications.

Multiple Pass (or Permeate Staged) Systems

These arrange pressure vessels filled with membrane elements in

such a way as to maximize the quality of pure water produced. The purified “permeate” water from the first pass becomes the feed to the second pass. The first stage/pass usually has twice as many pressure vessels (and membrane elements) as the second stage/pass. These systems are commonly found in the microelectronics industry where the purity of rinse water is essential to production. Three-pass systems are the most common.

— N —

Natural Draft Water Cooling Tower

One in which air movement is dependent upon the difference in density between the entering air and the internal air. As the heat of the water is transferred to the air passing through the tower, the warmed air tends to rise and draw in fresh air at the base of the tower. No fan is needed.

NDP:(Net Driving Pressure)

pressure that forces water through the membrane
net driving pressure is the feed pressure, minus the permeate backpressure, minus the osmotic pressure
NDP = P feed – P perm – π ,where π is the osmotic pressure of the feed solution
in systems open to the atmosphere, permeate back-pressure is negligible
for most brackish water systems, osmotic pressure (π) is also negligible
for seawater systems or other highly concentrated brines, π becomes an important factor

Nitrification

Microbial process by which reduced nitrogen compounds (primarily ammonia) are sequentially oxidized to nitrites and nitrates.

Nitrogen

One of the primary nutrients critical for the survival of all living organisms, and present in the environment as ammonia (NH₃), nitrates (NO^3-) and nitrites (NO^2-).

Nitrogen Cycle

Biogeochemical process through which nitrogen passes from the atmosphere to the soil to organisms and back into the atmosphere. It involves several processes such as nitrogen fixation, nitrification, denitrification, decay and putrefaction.

Nominal Tower Dimensions

Width and length measured from and to column centerline or walls; height measured from basin curb to top of fan deck (counterflow design) or to top of distribution basin (crossflow design). Dimensions measured in feet.

Nozzle

A device for controlled distribution of water in a cooling tower. Nozzles deliver water in either a spray pattern by pressure or in a solid stream by gravity flow.

NPF (Normalized Permeate Flow)

this mathematically corrected version of flow takes into account changes in pressure, temperature and osmotic pressure.
used to compare sets of data on an equal basis
comparing data without the normalization step can lead to poor conclusions
drop in NPF generally indicates membrane fouling or scaling
increase in NPF generally indicates membrane or system damage

Nutrient

Nutrients are elements that are essential for plant growth. They include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S) and silicon (Si). N, P and K are considered primary nutrients. N and P are the major limiting nutrients in most aquatic environments.

— O —

Open Feed Water Heater

A heater where feedwater is heated by steam, but the temperature does not go above the boiling point.

Organic Matter

Biodegradable material originally produced by living organisms (plant or animal) that is returned to the soil and goes through the decomposition process.

Organic Material

See Organic Matter.

Orthophosphate

A negatively charged ion (anion) composed of one phosphorus atom and 4 oxygen atoms (PO₄^3-), which measurement in natural waters is used to evaluate the nutrient load of the system. Also used as corrosion inhibitors in iron, copper, and lead water pipes.

Overall Tower Dimensions

Width: overall dimensions perpendicular to the tower’s longitudinal axis; Length: overall dimensions parallel to the air inlet louvers and the longitudinal axis; Total height: distance from basin curb to top of fan stack. Dimensions measured in feet.

Oxic

Refers to an environment in which oxygen is present.

Oxidation

A process involving the transfer of electrons from an oxidizing reagent to the chemical species being oxidized. In water and wastewater engineering, chemical oxidation serves the purpose of converting putrescible pollutant substances to innocuous or stabilized products.

Oxidation Ponds or Lagoons

Large, shallow ponds designed to treat wastewater through the interaction of sunlight, bacteria, and algae. Algae grow using energy from the sun and carbon dioxide and inorganic compounds released by bacteria in water.

— P —

Packing

See Fill.

Percent Load (% Load)

The percent of time that the cooling equipment is online or the percent of maximum capacity at which the equipment is operated. This can be estimated from hours of operation or average Delta T.

Permeate

purified (recovered) effluent stream
low in dissolved materials
sometimes called “product” water

pH

Defined as the logarithm of the reciprocal of the hydrogen ion concentration. pH is a measure of the acidity or alkalinity of a water solution, with 7.0 as the neutral point. This is an exponential function, so that a pH of 9.0 represents 10 times as much alkalinity as a pH of 8.0. This is also a measure of the ion concentration, not molecular concentrations.

pH of Saturation (pHS)

An equilibrium pH value used in calculating the scaling indexes (LSI, RSI, PSI). It uses the values of TDS (expressed from μmhos – to convert μmhos to a TDS value for cycled cooling water, multiply the μmhos by 0.8); Temperature (the highest temperature that the cooling water is exposed to); Calcium Hardness, and Total Alkalinity. Often the only temperature information available is the bulk water temperature which is not the highest temperature in the system. A skin temperature value should always be used when known. If the skin temperature is not known, we add a “safety factor” of 20° F to the maximum bulk water temperature (up to 115° F). If the bulk water temperature is greater than 115° F, then we use that value to determine the temperature factor. Lookup tables of factors for each of the four parameters are used to obtain the values used in the formula:

pHS = (9.3+A+B) – (C+D)

pH Value

A measure of the quantity of hydrogen ions in solution that gives an indication of the acidic or alkaline properties of a sample.

Pitot Tube

An instrument that operates on the principle of differential pressure. Its primary use in cooling towers is the measurement of circulating water rate.

Plenum

The enclosed space between the eliminators and the fan stack in induced draft towers or the enclosed space between the fan and the filling in forced draft towers.

Plume

Visible exhaust from a cooling tower.

Potable Water

Water that meets the required (chemical, biological and physical) quality standards at the point of supply to the users, and is considered safe for drinking, food preparation, personal hygiene and washing.

Polyelectrolytes

Polymer of electrolytes with several repeating chain units. Polyelectrolytes have properties of both electrolytes and polymers, and have a high molecular weight. Often used as coagulant aid in wastewater treatment or to reduce or prevent corrosion on metallic substrate surfaces.

Polyphosphate

Polymer of orthophosphate (PO₄^3-) of varying lengths.

Primary Waste Treatment

Process of removing solid matter from wastewater, by sedimentation, coagulation and flocculation.

PSI (psi)

Pounds per Square Inch

Practical Scaling Index (PSI) (a.k.a. Puckorius Scaling Index)

A modification of the Ryznar Stability Index developed in 1983 by Brooke and Puckorius using water analysis data from operating cooling towers (as opposed to municipal waters). While very similar to the RSI as an equilibrium index for calcium carbonate saturation, it uses a new value, pHeq, the equilibrium pH which was developed from these studies.

pHeq = 1.465 x log10 (Total Alkalinity) + 4.54

PSI = 2pHS – pHeq

Pressure Vessel

the hard sided vessel (usually stainless steel, PVC, or fiberglass) that holds membrane elements
typical pressure vessels house 3 to 7 membrane elements

— R —

Radiant Heat Transfer

Transfer of heat from flame to boiler tubes by radiation. This does not require any actual contact of gases with tubes, but requires only that the tubes see the hot luminous flame. Furnace tubes are subject to this type of heating.

Range

Difference between the hot water temperature and the cold water temperature, also known as cooling range. Range is the number of degrees water is cooled while falling through a cooling tower. Also commonly referred to as Delta T (∆T), or a change in temperature.

Raw Water

Water as received in the plant before any treatment.

Raw Wastewater

Any untreated water which quality has been adversely affected by humans and comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture.

Reactor

Delineated space in which chemical, biological and physical processes occur in nature. Lakes, segments of a river, and settling tanks in treatment plants are examples of reactors.

Receiving Waters

Bodies of water such as an ocean, stream, river, pond, lake into which treated or untreated wastewater is discharged.

Redistribution Basin

An elevated basin installed between the hot water and the cold water basins in a crossflow tower to maintain correct water distribution throughout the entire height of the fill.

Rejection

purity
Percent of salts removed by the membrane
Alternative measure: Salt Passage (SP = 100 – %Rej)
Maximum rejection of total dissolved solids (TDS) is desired

Retention Time

The time required for the tower water to fall from the top distribution system to the tower basin. (Not to be confused with the time that a slug-fed chemical remains in the system before its level is reduced to a certain point by loss due to bleed.)

Return Sludge

Settled activated sludge that is collected in the secondary clarifier or the membrane basin and returned to the aeration basin to mix with incoming raw or primary settled wastewater.

Return Activated Sludge

See ‘Return Sludge.

Riser (Generating Tubes)

Piping that connects the circulating water supply line from the level of the base of the tower to the supply header of the tower inlet connection

Ryznar Stability Index (RSI)

An equilibrium index developed to indicate the saturation (i.e. scaling or dissolving) characteristics of calcium carbonate in water. Developed from an empirical study of the Langelier Index.

RSI = 2pH – pHS

— S —

Saturation Temperature

That temperature at which water boils is fixed for each pressure and may be obtained from steam tables.

SDI (Silt Density Index)

a plugging factor used to determine the amount of particulate matter entering your RO
in an ideally operating system, the 15-minute SDI should be less than 3
poor technique can cause highly variable readings

Secondary Waste Treatment (Bioaugmentation)

Biological treatment aimed at removing fine suspended solids and dissolved biodegradable organic matter that remains in wastewater after primary treatment, and enhanced by addition of external or recycled living biomass.

Secondary Waste Treatment

Biological treatment aimed at removing fine suspended solids and dissolved biodegradable organic matter that remains in wastewater after primary treatment.

Sedimentation Tanks

A vessel where suspended particles settle out of water or wastewater as it flows slowly through the tank; also called settling tank or clarifier.

Sewage

Adversely impacted water discharge from human activities, most commonly referring to the effluent from human habitations.

Sewers

Pipe distribution systems used to collect domestic wastewater from habitations, industrial wastewater from industries, and stormwater run-off from the environment, to a point of treatment and disposal.

Sludge

Solid, semisolid, or slurry residual material that is produced as a by-product of wastewater treatment processes.

Sludge Age

Represents how long biomass remains in the system. It is defined as the ratio of mass of biomass in the aeration tank to the mass of biomass leaving the system per day.

Sludge Digestion

Biological process in which organic solids are decomposed into stable substances. Digestion reduces the total mass of solids, destroys pathogens, and makes it easier to dewater or dry the sludge.

Sludge Index

Process control parameter used to describe the settling characteristics of sludge in the aeration tank of an activated sludge process.

Sludge Reaeration

Aeration process intended to keep the sludge in contact with dissolved oxygen to favor biological degradation of organic matter and contaminants.

Splitter Box

Wastewater treatment facilities with a large number of primary clarifiers often employ a concrete tank with chambers and gates known as a splitter box, or distribution box, to split the influent flow into multiple streams that are routed into the bank of clarifiers. A similar device may also be used to split mixed liquor flows between secondary clarifiers.

Splash Bar

Horizontal component of a fill deck that constitutes the principle splash surface.

Softened Water

Effluent from a zeolite or lime soda softener.

Stage

a group of membrane pressure vessels running in parallel
most systems contain multiple stages arranged to provide either maximum permeate volume or maximum permeate purity

Stage Heater

Device for heating feedwater in a heat exchanger. There is no deaeration in such a heater.

Steam Boiler or Steam Generator

Any device for production of steam under pressure through firing with some form of fuel (gas, coal, oil, wood, etc.).

Steam Circulator Tubes

Tubes extending between two drums of a boiler, above the water line. These carry steam, not boiler water.

Steam Drum

The upper drum of a water tube boiler where final separation of the liquid and vapor states occurs. The steam outlet line almost always takes off from this drum.

Steam Header

This term is often applied to a large steamline which is fed by several boilers or which has several take off lines.

Steam Separation or Steam Scrubber

A device or structure at the top inside of the steam drum which is used to prevent water passing over with the steam.

Steam Trap

A device for bleeding condensate out of a steamline or steam container without allowing the steam to escape; such condensate is then normally returned to the feedwater system.

Superheater

A bank of tubes (not part of the boiler water circuit) used to heat steam above the saturation temperature. These tubes take steam from the boiler drum, heat it with combustion gases, and the superheated steam goes then to the process. Such tube banks are usually just behind the first generating tube bank.

Suspended Solids

Small solid particles which remain in suspension in water as a colloid or due to the motion of the water.

Supply Header

Portion of the water supply system from which riser or inlet connection receives the circulating water flow.

— T —

Tertiary Waste Treatment

Tertiary treatment is designed to filter out nutrients and waste particles which could negatively impact sensitive or protected ecosystems.

Thermal Capacity

The number of gallons per minute (gpm) a cooling tower will handle for a specified range, wet bulb temperature, and approach.

Total Solids

Total solids is the term applied to the material residue left in a vessel after evaporation of a sample and its subsequent drying in an oven at a defined temperature (either 103°C or 180°C) Total solids includes total suspended solids, the portion of total solids retained by a filter, and total dissolved solids, the portion that passes through a 2-um filter.

Train

the stages in a single array are sometimes called a train
Large installations have multiple, redundant trains to allow for off-line cleaning and/or repairs to one train without losing production

Trickling Filter

Aerobic treatment system consisting of microorganisms attached to a medium to remove organic matter from wastewater flowing through it.

Tube Sheet

A flat plate in a fire-tube boiler into which tubes are rolled.

Turbidity

Measure of the relative clarity of a liquid. It is a measurement of the amount of light scattered by materials in the water when a light is shined through the water sample. The higher the intensity of scattered light, the higher the turbidity.

Turnaround

A maintenance term for a regularly scheduled shutdown to clean the tower and make any necessary repairs to system equipment.

— U —

Uptake

That part of the boiler structure where the gases leave the boiler.

— W —

Wall Tube

Usually part of the furnace wall. Always a riser.

Waste Activated Sludge

Fraction of activated sludge from secondary treatment, which is discarded to preserve optimal conditions for biological degradation of contaminants in the aeration tank.

Waste Treatment Sludge

Discarded sludge.

Wastewater

Water contaminated by various effluents from human origins.

Water Cooling Tower

An enclosed, steady flow device for cooling water by evaporation through direct contact with air.

Water Loading

Circulating water flow expressed in gallons per minute (gpm) per square foot of effective horizontal wetted area of the tower. Measured in gpm per square foot.

Water Pollution

Release of substances into subsurface groundwater or into lakes, streams, rivers, estuaries, and oceans to the point where these substances interfere with the beneficial use of the water or with the natural functioning of ecosystems.

Wet Dry Tower

A wet (evaporative) cooling tower in combination with a dry (nonevaporative) heat exchanger system, used to reduce or abate cooling tower fog during cold weather by modifying the tower exhaust air condition.

Working Pressure

Allowable pressure for a given boiler. This is not necessarily the operating pressure. Operating pressure is less than or equal to the working pressure.

— Y —

Yeast & Molds

A type of fungi that can grow on various types of organic matter, including food, plants, and animals. Yeasts are unicellular organisms that typically ferment sugars and produce alcohol and carbon dioxide, while molds are multicellular organisms that grow as a network of thread-like structures called hyphae. Both yeasts and molds can be beneficial or harmful, depending on the context in which they are found.

Table of Contents

Definitions

— A —

— B —

— C —

— D —

Deaerating or Closed Heater

— E —

— F —

— G —

— H —

— I —