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The Chemistry of Lime-Soda Ash

Precipitation Water SofteningUpdated on December 15, 2011

The use of lime to soften water originated around 1841 when the Thames Rier wastreated for its bicarbonate hardness! "hen lime and soda ash are added to water,

hardness causing minerals such as calcium and magnesium are precipitated out!

#odern da$ processes for water softening usuall$ treat the water in four steps to ma%e

is acceptable for human use!

1 - Pre-Treatment 

This step in the process includes aeration and presedimentation! &eration allows water 

with high concentrations of dissoled carbon dio'ide to be prepared for softening!

(resedimentation remoes some suspended solids that cause turbidit$ in the water! &

reduction in turbidit$ allows for the softening reactions to more easil$ occur!

2 - Softening Treatment 

)n this stage, lime and soda ash are added to the water to remoe the hardness causing

minerals!

3 - Recarbonation

&fter softening treatment is complete, the water is usuall$ deoid of an$ dissoled

carbon dio'ide! Recarbonation stabili*es the water+s p, brings the water to aminimum hardness leel, and prepares it for filtration!

4 - Filtration

-inall$, the treated water  is filtered through a membrane or media filter to remoe the

remaining suspended solids microorganisms!

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T$pical "ater Treatment .teps

.ource/ "anama%er 

Chemical Reactions

There are at least different chemical reactions that ta%e place during the ime3.oda

&sh water softening process!

uic% lime a67 is added to the water as a pre3treatment! )t is t$picall$ the first step

in softening! This uic% lime combines with water to form calcium h$dro'ide lime7,

which then reacts with carbon dio'ide to form calcium carbonate! This is done at an

optimum p of 10!9

Reaction/ a6 : 26 ; a672then see ne't reaction/

arbon Dio'ide in the water reacts with ime <a672= to form calcium carbonate,which will precipitate out! )f there is a lot of dissoled 6 2, the water can be

e'pensie to treat! >ecause the goal of adding ime to the water is to soften it, it ma$

 be cost effectie to aerate the water prior to dosing it with ime!

Reaction/ 62 : a672 ; a69↓ : 26

alcium >icarbonate is commonl$ remoed from water b$ adding lime! The

chemicals combine to form water and calcium carbonate which will precipitate out

 because it is insoluble!

Reaction/ a6972 : a672 ; 2a69↓ : 226

#agnesium bicarbonate reacts with lime to form magnesium carbonate! ?eep in mind

that magnesium carbonate is soluble! @ou need to add more lime to conert the

magnesium to magnesium h$dro'ide, which is insoluble and will settle out!

#agnesium precipitation reuires a p of about 11!0 to 11!9 caused b$ adding more

lime7

Reaction/ #g6972 : a672 ; #g69 : 226 A

#g69 : a672 ; #g672↓:a69↓

)f there is magnesium sulfate in the water, $ou will need two reactions to remoe it!

#agnesium sulfate reacts with lime to for calcium .ulfate and magnesium h$dro'ide!

The calcium sulfate does not precipitate out, so see the ne't reaction!

Reaction/ #g.64 : a672 ; a.64 : #g672↓

)f the hardness causing chemical is alcium sulfate, $ou can add soda ash to form

calcium carbonate, which is insoluble, and sodium sulfate, which doesn+t contribute to

hardness!

Reaction/ a.64 : Ba269 ; Ba2.64 : a69↓

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(ractical imit of ime3.oda &sh precipitation are 90 mgCl of calcium as a69 and

10 mgCl of #g672 e'pressed as a69! The following flow chart outlines the

arious reactions that occur in the water!

.ource/ "anama%er 

Determining How mch Chemicals to Add to the

Water

 How much Lime to add to a water supply:

17 onert all concentrations of constituents that will react with lime in the water to

mgCme! The table below will help $ou7!

27 The sum of the mgCme alues is the reuired amount of lime that $ou need for the

reaction!

97 onert this alue bac% to mgCl and then to lbC# to see how man$ lbs of lime per

million gallons of water $ou need! #ultipl$ mgCl b$ 8!94 to get lbC#!

 How much Soda Ash to add to a water supply:

17 onert all concentrations of constituents that will react with soda ash in the water

to mgCme this will be #agnesium sulfate andCor alcium sulfate7!

27 The sum of the mgCme alues is the reuired amount of soda ash that $ou need for 

the reaction!

97 onert this alue bac% to mgCl and then to lbC# to see how man$ lbs of .oda ash

 per million gallons of water $ou need! #ultipl$ mgCl b$ 8!94 to get lbC#!

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Data compiled from arious sources

.ource/ "anama%er 

References

&merican "ater "or%s &ssociation, and &merican .ociet$ of iil Engineers! Water Treatment Plant Design! Ed! Edward E! >aruth! -ourth ed!

 Bew @or%/ #craw3ill andboo%s, 2005!

orbit, Robert &! Standard Handbook of n!ironmental ngineering ! .econd ed! Bew @or%/ #craw3ill andboo%s, 1FFF!

(err$, Robert !, Don "! reen, and Games 6! #alone$!  Perry"s #hemical ngineers Handbook ! .eenth ed! Bew @or%/ #craw3ill 1FF!

T.+s "ater Treatment Engineering Team! Lime Softening$ U. Department of the )nterior, >ureau of Reclamation! .eptember 200F!