liters per day.


liters per minute.

Laminar Flow

The movement of fluid in a particular direction in smooth, continuous, nonturbulent parallel layers which do not mix with each other.


Facility in which solid waste from municipal and/or industrial sources is disposed; sanitary landfills are those that are operated in accordance with environmental protection standards.

Langelier Saturation Index

A calculated number used to predict the calcium carbonate (CaCO3) stability of a water; that is, whether a water will precipitate, dissolve, or be in equilibrium with, calcium carbonate.     It is sometimes erroneously assumed that any water that tends to dissolve CaCO3 is automatically corrosive.     Langelier saturation index = pH - pHs where pH = actual pH of the water, and pHs = pH at which the water having the same alkalinity and calcium content is just saturated with calcium carbonate.

Large Public Water System

A public water system that serves more than 50,000 persons.


linear alkylate sulfonate.


Time from the first exposure to a chemical until the appearance of a toxic effect.

Latent Heat

The amount of heat released or absorbed when a substance changes its physical phase with no change in temperature.     For example, the heat absorbed from surroundings when ice melts into liquid water at the freezing point or the heat released when a gas (steam) condenses into liquid water.     The loss or gain of latent heat is not reflected in the temperature of the melting ice or the condensing water.

Laundering weir

Sedimentation basin overflow weir. A plate with V-notches along the top to assure a uniform flow rate and avoid short-circuiting.


Sedimentation basin and filter discharge channels, consisting of overflow weir plates (in sedimentation basins) and conveying troughs.

Laundry Detergent

A product containing a surfactant and other ingredients, formulated to clean and care for the many different fabrics in the family wash.     Next to the surfactant, a builder is an important ingredient in formulated laundry detergents. Builders have a number of functions, principally inactivation of water hardness, which interferes with good cleaning. Built detergent types include granules and liquids.     Some liquid detergents are unbuilt, containing surfactants that are relatively insensitive to water hardness.     Other customary ingredients of laundry detergents include antiredeposition agents, corrosion inhibitors, fluorescent whitening agents, colorants, fragrance, and processing aids.     Optional ingredients include suds control agents, bleach, borax, enzymes, bluing, fabric softener, and soil release agent.     Some laundry detergents are denser or more concentrated than others. Density or concentration influences the amount of product recommended for the wash. Detergents also vary in sudsing characteristics, ranging from high to low suds levels.     Different suds levels are provided for reasons of compatibility with machine design and to satisfy consumer preferences.     Depending on the presence of other ingredients in the laundry detergent formulation, some products offer special benefits in addition to the expected cleaning. Thus, certain laundry detergents are especially effective at lower washing temperatures; others provide additional fabric care benefits, such as softening, static control, and wrinkle reduction.

Layered Bed

As relates to filtration, a multimedia filter bed containing, in the same vessel, several different filter media (such as anthracite, sand, and garnet) with specific gravities which differ enough to maintain different layers even after backwashing. layered bed     In ion exchange, a single exchange bed made up of two or more resins which have bead sizes and densities different enough to maintain layers after backwashing and which can be regenerated with the same regenerant.   For example, a layered bed may have a mixed bed of anion resin on top and cation resin below that is regenerated by salt brine solution.


The concentration of a chemical in air or water which is expected to cause death in 50 percent of test animals living in the air or water.


The dose of a chemical taken by mouth or absorbed by the skin which is expected to cause death in 50 percent of the test animals so treated.


To dissolve out by the action of a percolating liquid.

Leach Field

The area where the effluent from a septic tank system is distributed by horizontal underground piping designed to aid in the process of natural leaching and percolation through the soil.


Water which has percolated or filtered through soil, a filter medium, or other substance containing soluble substances so that it now contains certain amounts of these substances in solution.


The process by which soluble substances are dissolved and transported down through the soil by recharge.

Lead (Pb)

A heavy metal that is hazardous to health if breathed or swallowed. Its use in gasoline, paints, and plumbing compounds has been sharply restricted or eliminated by federal laws and regulations.

Lead Service Line

A service line made of lead which connects the water main to the building inlet and any lead pigtail, gooseneck, or other fitting which is connected to such lead line.


As relates to ion exchange, the presence in the effluent of the type of ions, present in the feedwater to be treated, which the ion exchange process was supposed to remove.     Incomplete removal of the ions may be caused by incomplete regeneration, excessive service rates, low temperatures, high concentrations of sodium, or interfering TDS in the water being treated, and other factors.     Leakage is also referred to as slippage.


Over 26 species of bacteria, such as Legionella pneumophila, which can cause the pneumonia-like illness called "Legionnaires' Disease" (after the American Legion convention at which the disease first drew attention).     These bacteria are known to thrive at about 100 degrees F and are believed to live in infected humidifiers, cooling tower water, and shower rooms.     Infection is by inhalation.


A pathological or traumatic discontinuity of tissue or loss of function of a part.


Deadly; fatal.

Level Controls

A float device (or pressure switch) which senses changes in a measured variable and opens or closes a switch in response to that change.     In its simplest form, this control might be a floating ball connected mechanically to a switch or valve such as is used to stop water flow into a toilet when the tank is full.

Lifetime Exposure

Total amount of exposure to a substance that a human would receive in a lifetime (usually assumed to be 70 years).

Lift-and-Turn Valve

A master control valve for operating and manually regenerating softeners and filters.

Lime (CaO)

A calcined chemical material, calcium oxide.     Lime is used in lime and in lime and soda ash water treatment, but must first be slaked to calcium hydroxide [Ca(OH)2]. Lime is also called burnt lime; calyx; fluxing lime; quicklime; unslaked lime.

Lime Scale

Hard water scale formed in pipes and vessels (generally more severe on the hot water side) containing a high percentage of calcium carbonate (CaCO3) or magnesium carbonate (MgCO3).

Lime Soap

The insoluble calcium and magnesium salts formed from the fatty acid portion of soap when it combines with minerals in hard water; it is commonly referred to as soap curd.     The use of the word lime in this term may come from the fact that limestone areas generally foster hard water, or from the fact that the words lime and calcium are closely associated. Calcium and magnesium fatty acid salts are highly insoluble and precipitate immediately on formation. Since they tend to agglomerate (cluster together), they form curd-like masses. They also tend to adhere to surfaces, thus causing filming or deposits, such as bathtub ring.     The problems lime soap causes spurred the development of mechanical water softeners, packaged water softeners, and the technology leading to new surfactants and builders and detergent products based on them.

Lime Soda Ash Softening

A water treatment process done to further reduce the hardness of lime treated water using soda ash (Na2CO3) to raise the pH of the feed stream and precipitate the hardness which is then filtered out.  The residual hardness is typically 4-5 gpg depending on the treatment design.  It does not produce soft water. SEE ALSO hot lime-soda softening, lime softening.

Lime Softening

A water treatment process done to reduce the hardness of hard water that is often used by municipalities and larger industrial water users.  Slaked lime [Ca(OH)2] is added to the water to precipitate the hardness ions as calcium and magnesium carbonate or hydroxide.  The precipitate is then filtered. The residual hardness may exceed 6-10 gpg depending on the treatment design. This does not product soft water. SEE ALSO hot lime softening; lime-soda ash softening.


A sedimentary rock composed mostly of calcium carbonate (CaCO3) and usually some magnesium carbonate (MgCO3).

Limit Stops

Several types of diaphragm valves which have adjustments used to control flow rates during various processes (backwash, fast rinse, or brine dilution) when brine is pumped in during batch regeneration of resin for portable ion exchange tanks.

Limnetic Community

The area of open water in a fresh water lake providing the habitat for fish, phytoplankton, and zooplankton.


The scientific study of conditions in freshwater lakes, ponds, and streams.


A pesticide that causes adverse health effects in domestic water supplies and also is toxic to freshwater and marine aquatic life.

Linear Alkyl Sulfonate (LAS)

Readily biodegradable form of alkylbenzene sulfonate surfactant.     This is the workhorse of the detergent industry, with sodium dodecylbenzene sulfonate being the most important single type. It is distinguished from an earlier form of alkylbenzene sulfonate, termed ABS, but is a linear (straight chain) structure, which provides its good biodegradation properties.     All LAS surfactants are anionic and high sudsing, but their sudsing may be controlled by formulation.


How closely an instrument measures actual values of a variable through its effective range; a measure used to determine the accuracy of an instrument.

Linearized multistage model

Derivation of the multistage model, where the data are assumed to be linear at low doses.


The transformation to the liquid state. This term is more commonly used to refer to the changing of gases to liquids than to refer to the melting of solids to liquids.

Liquid Chromatography

Chromatography in which the mobile phase is a liquid (i.e., the sample is introduced into a liquid solvent which then flows through a fractionating column and to a detector). Separation of the sample components is accomplished via three different modes: 1. liquid/ liquid, in which the relative solubilities of sample components in two immiscible fluids (one of which is usually water) create separation; 2. liquid/solid, in which the relative adsorption of sample components on a solid adsorptive medium surface creates separation; and 3. molecular-size, in which separations are created because of the variations in effective molecular dimensions of the sample components in solution.


A solution of one or more chemical substances (gas, solid, or liquid) in water.


The basic international metric unit of volume measure.     One liter equals 33.82 fluid ounces; 3.785 liters equals one U.S. gallon. One liter of water weighs 1,000 grams at 4 degrees Celsius at standard atmospheric pressure.

Littoral Zone

1. That portion of a body of fresh water extending from the shoreline lakeward to the limit of occupancy of rooted plants.     2. The strip of land along the shoreline between the high and low water levels.


The quantity of a substance entering the environment (soil, water, or air).


Lowest-observed-adverse-effect level; the lowest dose in an experiment which produced an observable adverse effect.


SEE magnetite


The exponent that indicates the power to which a number must be raised to produce a given number.     Also abbreviated to "log."

Longitudinal Flow

A flow pattern in which water travels from the bottom to top (or vice versa) in either a cartridge-type or loose media tank-type filtration system.     The advantages are greater contact time, higher unit capacity, more complete utilization of medium, and more uniform water quality.     Also called axial flow.


1. The plumbing network designed to continuously circulate ultrapure grade water in high purity water systems between storage and disinfection modes to maintain microbiological cleanliness.   2. A plumbing connection used to bypass water around a location designed for installation of a water treatment system or used when the treatment system is out of service for any reason.

Loose Media Filters

Those filter units which have medium products positioned in a filter bed such that the individual medium grains or particles can be repositioned or lifted relative to each other with the flow of water or backwash water; as compared to permanently fixed bed media filter or a fixed solid block of filter media.

loose medium (media)

Filter or ion exchange media (in a tank or bed) which can be expanded during backwashing and rinsing.  "Loose" is used to differentiate from a contained or "fixed medium" in a tank or the fixed or compressed media layer in a cartridge filter.


SEE sodium hydroxide


To undergo lysis


A device for measuring the percolation and drainage of water through soils.


A process of disintegration or destruction of bacteria or microbiological cells by chemically breaking them down into their component parts.

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Updated on Mon, 22 Jun 2020 by Jonathan

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