Lime softening
Encyclopedia
Lime softening, also known as Clark's process, is a type of water treatment
used for water softening
. In the USA, it is used primarily in the Midwest, Florida
and Texas
. It utilizes the addition of lime (calcium hydroxide
) to remove hardness
(calcium
and magnesium
) ion
s by precipitation
. The process is also effective at removing a variety of microorganism
s and dissolved organic matter
.
water. The process expanded in use as the bactericidal effect of the process was discovered. Lime softening greatly expanded in use during the early 1900s as industrial water use expanded. Lime softening provides water that can, in some cases, be used more effectively for heat transfer and various other industrial uses.
in the form of Ca(OH)2
, which reacts first with CO2
to form calcium carbonate precipitate, reacts next with multivalent cations to remove carbonate hardness, then reacts with anions to replace the non-carbonate hardness due to multivalent cations with non-carbonate hardness due to calcium. The process requires recarbonation through the addition of carbon dioxide to lower the pH
which is raised during the initial softening process.
As lime is added to raw water, the pH
is raised and the equilibrium of carbonate
species in the water is shifted. Dissolved carbon dioxide
(CO2) is changed in to bicarbonate
(HCO3-) and then carbonate (CO32-). This action causes calcium carbonate
to precipitate due to exceeding the solubility product. Additionally, magnesium can be precipitated as magnesium hydroxide
in a double displacement reaction.
The process is unique in that both the calcium (and to an extent magnesium) in the raw water as well as the calcium added with the lime are both precipitated. This is in contrast to ion exchange
softening where sodium
is exchanged for calcium and magnesium ions. In lime softening, there is a substantial reduction in total dissolved solids
(TDS). In ion exchange softening (sometimes referred to as zeolite
softening), there is no significant change in the level of TDS.
Lime softening can be used to remove iron
, manganese
, radium
and arsenic
from water.
or reverse osmosis
. These processes remove calcium and magnesium ions as well as other dissolved components by way of passage through a semipermeable membrane
. Membrane processes generally produce a larger volume of waste — up to 20% of the input stream, but use substantially fewer chemicals and produce little, if any, solid waste.
Interestingly, lime softening is now often combined with newer membrane processes to reduce waste streams. Lime softening can be applied to the concentrate (or reject stream) of membrane processes, thereby providing a stream of substantially reduced hardness (and thus TDS), that may be used in the finished stream. Also, in cases with very hard source water (often the case in Midwestern ethanol
production plants), lime softening can be used to pre-treat the membrane feed water.
supplement) and can also be used as a filler material in certain cementitious materials such as low strength concrete
.
Water treatment
Water treatment describes those processes used to make water more acceptable for a desired end-use. These can include use as drinking water, industrial processes, medical and many other uses. The goal of all water treatment process is to remove existing contaminants in the water, or reduce the...
used for water softening
Water softening
Water softening is the reduction of the concentration of calcium, magnesium, and certain other metal cations in hard water. These "hardness ions" can cause a variety of undesired effects including interfering with the action of soaps, the build up of limescale, which can foul plumbing, and...
. In the USA, it is used primarily in the Midwest, Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
and Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
. It utilizes the addition of lime (calcium hydroxide
Calcium hydroxide
Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...
) to remove hardness
Hard water
Hard water is water that has high mineral content . Hard water has high concentrations of Ca2+ and Mg2+ ions. Hard water is generally not harmful to one's health but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling...
(calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
) ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s by precipitation
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
. The process is also effective at removing a variety of microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
s and dissolved organic matter
Organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
.
History
Lime softening was first used in 1841 to treat Thames RiverRiver Thames
The River Thames flows through southern England. It is the longest river entirely in England and the second longest in the United Kingdom. While it is best known because its lower reaches flow through central London, the river flows alongside several other towns and cities, including Oxford,...
water. The process expanded in use as the bactericidal effect of the process was discovered. Lime softening greatly expanded in use during the early 1900s as industrial water use expanded. Lime softening provides water that can, in some cases, be used more effectively for heat transfer and various other industrial uses.
Chemistry
Softening can be achieved by adding limeLime (mineral)
Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
in the form of Ca(OH)2
Calcium hydroxide
Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...
, which reacts first with CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
to form calcium carbonate precipitate, reacts next with multivalent cations to remove carbonate hardness, then reacts with anions to replace the non-carbonate hardness due to multivalent cations with non-carbonate hardness due to calcium. The process requires recarbonation through the addition of carbon dioxide to lower the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
which is raised during the initial softening process.
As lime is added to raw water, the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
is raised and the equilibrium of carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
species in the water is shifted. Dissolved carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(CO2) is changed in to bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
(HCO3-) and then carbonate (CO32-). This action causes calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
to precipitate due to exceeding the solubility product. Additionally, magnesium can be precipitated as magnesium hydroxide
Magnesium hydroxide
Magnesium hydroxide is an inorganic compound with the chemical formula Mg2. As a suspension in water, it is often called milk of magnesia because of its milk-like appearance. The solid mineral form of magnesium hydroxide is known as brucite....
in a double displacement reaction.
The process is unique in that both the calcium (and to an extent magnesium) in the raw water as well as the calcium added with the lime are both precipitated. This is in contrast to ion exchange
Ion exchange
Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion...
softening where sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
is exchanged for calcium and magnesium ions. In lime softening, there is a substantial reduction in total dissolved solids
Total dissolved solids
Total Dissolved Solids is a measure of the combined content of all inorganic and organic substances contained in a liquid in: molecular, ionized or micro-granular suspended form. Generally the operational definition is that the solids must be small enough to survive filtration through a sieve...
(TDS). In ion exchange softening (sometimes referred to as zeolite
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
softening), there is no significant change in the level of TDS.
Lime softening can be used to remove iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
, radium
Radium
Radium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226,...
and arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
from water.
Future Uses
While lime softening continues to be used at existing facilities, newer treatment plants often use membrane softening such as nanofiltrationNanofiltration
Nanofiltration is a relatively recent membrane filtration process used most often with low total dissolved solids water such as surface water and fresh groundwater, with the purpose of softening and removal of disinfection by-product precursors such as natural organic matter and synthetic organic...
or reverse osmosis
Reverse osmosis
Reverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...
. These processes remove calcium and magnesium ions as well as other dissolved components by way of passage through a semipermeable membrane
Semipermeable membrane
A semipermeable membrane, also termed a selectively permeable membrane, a partially permeable membrane or a differentially permeable membrane, is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion".The rate of...
. Membrane processes generally produce a larger volume of waste — up to 20% of the input stream, but use substantially fewer chemicals and produce little, if any, solid waste.
Interestingly, lime softening is now often combined with newer membrane processes to reduce waste streams. Lime softening can be applied to the concentrate (or reject stream) of membrane processes, thereby providing a stream of substantially reduced hardness (and thus TDS), that may be used in the finished stream. Also, in cases with very hard source water (often the case in Midwestern ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
production plants), lime softening can be used to pre-treat the membrane feed water.
Waste Products
Lime softening produces large volumes of calcium carbonate and magnesium hydroxide sludge. This residual stream can substantially increase costs associated with the process. The sludge can be used as an agricultural soil amendment (it is an alkalinityAlkalinity
Alkalinity or AT measures the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. The alkalinity is equal to the stoichiometric sum of the bases in solution...
supplement) and can also be used as a filler material in certain cementitious materials such as low strength concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
.