Lime Soda Ash Softening
Chemical precipitation is one of the more common methods used to soften water. Chemicals normally used are lime (calcium hydroxide, Ca(OH)2) and soda ash (sodium carbonate, Na2CO3). Lime is used to remove chemicals that cause carbonate hardness. Soda ash is used to remove chemicals that cause non-carbonate hardness.
When lime and soda ash are added, hardness-causing minerals form nearly insoluble precipitates. Calcium hardness is precipitated as calcium carbonate (CaCO3). Magnesium hardness is precipitated as magnesium hydroxide (Mg(OH)2). These precipitates are then removed by conventional processes of coagulation/flocculation, sedimentation, and filtration. Because precipitates are very slightly soluble, some hardness remains in the water--usually about 50 to 85 mg/l (as CaCO3). This hardness level is desirable to prevent corrosion problems associated with water being too soft and having little or no hardness.
CO2 does not contribute to the hardness, but it reacts with the lime, and therefore uses up some lime before the lime can start removing the hardness.
CO2 = carbon dioxide, Ca(OH)2 = calcium hydroxide or hydrated lime, CaCO3 = calcium carbonate, Ca(HCO3)2 = calcium bicarbonate, Mg(HCO3)2 = magnesium bicarbonate, MgCO3 = magnesium carbonate, Mg(OH)2 = magnesium hydroxide, MgSO4 = magnesium sulfate, CaSO4 = calcium sulfate, H20 - water. Na2CO3 = sodium carbonate or soda ash
For each molecule of calcium bicarbonate hardness removed, one molecule of lime is used. For each molecule of magnesium bicarbonate hardness removed, two molecules of lime are used. For each molecule of non-carbonate calcium hardness removed, one molecule of soda ash is used. For each molecule of non-carbonate magnesium hardness removed one molecule of lime plus one molecule of soda ash is used.
CONVENTIONAL LIME-SODA ASH TREATMENT
When water has minimal magnesium hardness, only calcium needs to be removed. Only enough lime and soda ash are added to water to raise pH to between 10.3 and 10.6, and calcium hardness will be removed from the water (but minimal magnesium hardness will be removed).
EXCESS LIME TREATMENT
When magnesium hardness is more than about 40 mg/l as CaCO3, magnesium hydroxide scale deposits in household hot-water heaters operated at normal temperatures of 140 to 150° F. To reduce magnesium hardness, more lime must be added to the water. Extra lime will raise pH above 10.6 to help magnesium hydroxide precipitate out of the water.
When water contains high amounts of magnesium hardness, split treatment may be used. Approximately 80 percent of the water is treated with excess lime to remove magnesium at a pH above 11, after which it is blended with 20 percent of the source water. Split treatment can reduce the amount of carbon dioxide required to re-carbonate the water as well as offer a savings in lime feed.
Since the fraction of the water that is treated contains an excess lime dose, magnesium is almost completely removed from this portion. When this water is mixed with the water that does not undergo softening, the carbon dioxide and bicarbonate in that water re-carbonates the final blend. Split treatment reduces the amount of chemical needed to remove hardness from water by 20 to 25 percent (a significant savings).
In lime soda-ash softening plants, the softening process may be carried out by a sequence of rapid mix, flocculation, and sedimentation or in a solids contactor. In the solids contactor the rapid mix, flocculation, and sedimentation occur in a single unit. The process begins with the mixing of the chemicals into the water, followed by violent agitation, termed rapid mixing. This allows chemicals to react with, and precipitate calcium or magnesium hardness in the water.
Flocculation allows flocs to contact other flocs and grow large enough to settle in the sedimentation stage. Water is mixed gently with a small amount of energy. Most flocculators are compartmentalized, allowing for a tapered mix, so less energy must applied as the flocs grow in size.
Detention time in the flocculator is important to allow particles to come in contact with each other. The minimum time recommended is 30 minutes for conventional water softening.
Sludge returned to the head of the flocculator reduces the amount of chemical needed and provides seed flocs for the precipitation. The estimated return sludge is 10 to 25 percent of the source water.
Sedimentation follows flocculation. Settling rates for these tanks are a function of particle size and density. Detention times in the settling basins range from 1.5 hours to 3.0 hours, and they can be rectangular, square, or circular (some designs incorporate inclined tube settlers).
Sedimentation can also occur in the solids-contact unit, in which the water is mixed with chemicals and flocculated in the center of the basin, then forced down and trapped for removal in a sludge blanket in the bottom of the tank.
Residue created from lime-soda ash softening is normally very high in calcium carbonate or a mixture of calcium carbonate, and magnesium hydroxide. Calcium carbonate sludges are normally dense, stable inert, and dewater readily. Solids content in the sludge range from 5 to 30 total solids with a pH greater than 10.5.
Lime-soda ash sludges may be treated with lagooning, vacuum filtration, centrifugation, pressure filtration, recalcination, or land application. The most common method is storage of sludge in lagoons and application to farmland or landfills disposal.
There are two methods for calculating lime and soda ash dosages (conventional dosage method and conversion factor method). The conventional method, although much longer, is helpful in understanding the chemical and mathematical relationships involved in softening. The conversion factor method is simpler, quicker, and more practical for daily operations.
In both calculation methods, lime and soda ash dosages depends on carbonate and non-carbonate hardness in the water. Lime is used to remove carbonate harness, and both lime and soda ash are used to remove non-carbonate hardness. If total hardness is less than or equal to total alkalinity, there is no non-carbonate hardness (only carbonate hardness). If total hardness is greater than total alkalinity, non-carbonate hardness equals the difference between total hardness and total alkalinity (and carbonate hardness equals total alkalinity).
If total hardness is equal to or less than total alkalinity, then:
Lime Dosage = the carbon dioxide concentration [CO2] + the total hardness concentration
[Total Hardness] + the magnesium concentration [Mg] + [Excess]
Optimum chemical dosages can be evaluated with a jar test.
Alkalinity (mg/l as CaCO3) is the capacity of water to neutralize acids. This is determined by the content of carbonate, bicarbonate and hydroxide. Alkalinity is a measure of how much acid can be added to a liquid without causing any significant change in pH.
When pH is less than 8.3, all alkalinity is in the bicarbonate form and is commonly referred to as natural alkalinity. When pH is above 8.3, alkalinity may consist of bicarbonate, carbonate, and hydroxide. As pH increases the alkalinity progressively shifts to carbonate and hydroxide forms. Total alkalinity is the sum of bicarbonate, carbonate, and hydroxide alkalinity. Various chemicals effect water differently:
If hydrated lime (CaOH) is used in place of quicklime, the molecular weight of quicklime of 56 should be replaced with the weight of hydrated lime (74).
When treating water that contains non-carbonate hardness, soda ash is required. The amount of soda ash can be estimated by using the following formula:
Soda Ash (NaCO3) mg/l = mg/l Non Carbonate Hardness as CaCO3 x Na2CO3 /CaCO3
= mg/l Non-Carbonate Hardness as CaCO3 x 106/100
= mg/l Non-Carbonate Hardness as CaCO3 x 1.06
After softening, pH of the water is generally above 10. If left at this pH, water will plate filter sand and cause problems in the distribution system. Carbon dioxide (through re-carbonation), is added to lower the pH. The amount of carbon dioxide (CO2) required can be estimated:
Equivalent weight conversions required in the conventional method have been combined into single factors shown in the table below. These factors, multiplied by the concentration of the corresponding material, will give the lime or soda ash dosage needed to remove material in units of milligrams per liter or pounds per million gallons. The total dosage is the sum of all material removed from the water, such as the carbon dioxide, bicarbonate alkalinity, and the magnesium, plus the amount of excess that is required to reduce the hardness in the water. The total soda-ash dosage is found in the same manner by finding the sum of the amounts needed to remove the non-carbonate material from the water. An additional calculation is needed to adjust for the purity of the lime or soda-ash used.
After adding lime and/or soda ash, treated water will generally have a pH greater than 10. It is necessary to lower the pH to stabilize the water and prevent deposition of carbonate scale on filter sand and distribution piping. Recarbonation is the most common process used to reduce pH. This procedure adds carbon dioxide to water after softening. Generally, enough carbon dioxide is added to reduce the pH of the water to less than 8.7. The amount of carbon dioxide added is determined using a saturation index. The Langelier Index (LI) is the most common stabilization index used, but some plants instead use the Rizner Index, (reciprocal of the Langelier Index). The Langelier Index is expressed as pH of stabilization (pHs) minus actual pH measured (pHs - pH). When the Langelier Index is positive, pipes tend to become coated with scale. When it is negative, the water tends to be corrosive.
When low magnesium water is softened, no excess lime needs to be added. After softening, water becomes supersaturated with calcium carbonate and has a pH between 10.0 and 10.6. When carbon dioxide is added, the excess calcium carbonate is converted back to permanent hardness or calcium bicarbonate by the following formula:
Ca2+ (calcium ion) + CO32- (carbonate ion) + CO2 (carbon dioxide) + H2O (water) = 2HCO3- (bicarbonate ions)
When high magnesium water is softened, excess lime needs to be added to raise the pH above 11, and magnesium hydroxide precipitates out. After treatment, enough carbon dioxide must be added to neutralize the excess hydroxide ions, as well as convert carbonate ions to bicarbonate ions. The first stage of this reaction reduces the pH to between 10.0 and 10.5. In this range, calcium carbonate is formed and magnesium hydroxide that did not precipitate, or did not settle out, is converted to magnesium carbonate.
Ca2+ (calcium ion) + 2OH- (hydroxyl ions) + CO2 (carbon dioxide) <----> CaCO3 (calcium carbonate) + H2O (water)
Mg2+ magnesium ion) + 20H- (hydroxyl ions) + CO2 (carbon dioxide) <----> MgCO3 (magnesium carbonate) + H20 (water)
Additional carbon dioxide needs to be added to lower the pH to between 8.4 and 8.6. The previously formed calcium carbonate re-dissolves and carbonate ions are converted to bicarbonate ions as shown below:
CaCO3 (calcium carbonate) + H20 (water) + CO2 (carbon dioxide) <----> Ca2+ (calcium ion) + 2HCO3- (bicarbonate ions)
Mg2+ (magnesium ion) + CO32+ (carbonate ion) + CO2 (carbon dioxide) + H20 (water) <----> Mg2+ (magnesium ion) + 2HCO3- (bicarbonate ions)
For treatment of low magnesium water (where excess-lime addition is not required) single-stage recarbonation is used. The water is mixed with lime or soda ash in the rapid-mix basin, resulting in a pH of 10.2 to 10.5. If non-carbonate hardness removal is required, soda ash will also be added at this step. After rapid mixing, the resulting slurry is mixed gently for a period of 30 to 50 minutes to allow the solids to flocculate. After flocculation, the water is allowed to flow into a sedimentation basin where the solids will be removed by sedimentation. Following sedimentation the clear water flows to the recarbonation basin where carbon dioxide is added to reduce the pH to between 8.3 and 8.6. Any particles remaining in suspension after recarbonation are removed by filtration.
Two-stage softening is sometimes used for treatment of high magnesium water (where excess lime is required). Excess lime is added in the first stage to raise pH to 11.0 or higher for magnesium removal. Following first stage treatment, carbon dioxide is added to reduce the pH to between 10.0 and 10.5, the best value for removal of calcium carbonate. If non-carbonate hardness removal is needed, soda ash will be added at this point. After second stage treatment, the water flows to a secondary recarbonation tank, where pH is reduced to between 8.3 and 8.6.
Single-stage recarbonation is the one most commonly practiced (Because of the high capital cost for building this type of two-stage treatment train). There are some benefits to using the two-stage method, including reduced operating cost since less carbon dioxide is needed. Better finished water quality is usually obtained through the two-stage process.
Lime softening involves a relatively complicated series of chemical reactions which will be discussed in depth below. The goal of all of these reactions is to change the calcium and magnesium compounds in water into calcium carbonate and magnesium hydroxide. These are the least soluble calcium and magnesium compounds and thus will settle out of the water at the lowest concentrations. For example, calcium carbonate (which is essentially the same as limestone) will settle out of water at concentrations greater than 40 mg/L.
In order to produce calcium carbonate and magnesium hydroxide, the pH of the water must be raised by the addition of lime. Calcium compounds in water will be removed at a pH of about 9.0 to 9.5 while magnesium compounds require a pH of 10.0 to 10.5. When soda ash is used to remove noncarbonate hardness, an even higher pH is required - 10.0 to 10.5 for calcium compounds and 11.0 to 11.5 for magnesium compounds.
Carbon Dioxide Demand
The first step in lime softening is the addition of lime to water using a typical dry feeder, either volumetric or gravimetric. As in the chlorination process, lime reacts with substances in the water before it can begin softening the water. Carbon dioxide is the primary compound which creates the initial demand for lime. The following reaction occurs, using up carbon dioxide and lime and creating calcium carbonate and water:
Carbon dioxide + Lime → Calcium carbonate + Water
CO2 + Ca(OH)2→ CaCO3 + H2O
The resulting calcium carbonate precipitates out of solution.
When water, especially groundwater, has a high carbon dioxide concentration, the water is often pretreated with aeration before softening begins. Aeration removes the excess carbon dioxide and lowers the lime requirements.
Removal of Carbonate Hardness
Once the carbon dioxide demand has been met, the lime is free to react with and remove carbonate hardness from the water. Calcium compounds react with lime in the reaction shown below.
bicarbonate + Lime → Calcium carbonate + Water
Ca(HCO3)2 + Ca(OH)2→ 2CaCO3 + 2H2O
We have focussed on calcium bicarbonate since it is the most common calcium compound in water, but other calcium-based hardness compounds have similar reactions. In any case, the calcium carbonate produced is able to precipitate out of solution.
Magnesium compounds have a slightly different reaction. First, magnesium bicarbonate reacts with lime and produces calcium carbonate (which precipitates out of solution) and magnesium carbonate.
bicarbonate + Lime → Calcium carbonate + Magnesium carbonate + Water
Mg(HCO3)2 + Ca(OH)2→ CaCO3 + MgCO3 + 2H2O
Then the magnesium carbonate reacts with lime and creates more calcium carbonate and magnesium hydroxide. Both of these compounds are able to precipitate out of water.
carbonate + Lime → Calcium carbonate + Magnesium hydroxide
MgCO3 + Ca(OH)2→ CaCO3 + Mg(OH)2
Removal of Noncarbonate Hardness
In many cases, only the carbonate hardness needs to be removed, requiring only the addition of lime. However, if noncarbonate hardness also needs to be removed from water, then soda ash must be added to the water along with lime.
Each noncarbonate hardness compound will have a slightly different reaction. Here, we will consider the reactions of magnesium sulfate. The lime first reacts with the magnesium sulfate, as shown below:
Magnesium sulfate + Lime → Magnesium hydroxide + Calcium sulfate
MgSO4 + Ca(OH)2→ Mg(OH)2 + CaSO4
The resulting compounds are magnesium hydroxide, which will precipitate out of solution, and calcium sulfate. The calcium sulfate then reacts with soda ash:
Calcium sulfate + Soda Ash → Calcium carbonate + Sodium sulfate
CaSO4 + Na2CO3→ CaCO3 + Na2SO4
The calcium carbonate resulting from this reaction will settle out of the water. The sodium sulfate is not a hardness-causing compound, so it can remain in the water without causing problems.
The reactions which remove carbonate and noncarbonate hardness from water require a high pH and produce water with a high concentration of dissolved lime and calcium carbonate. If allowed to enter the distribution system in this state, the high pH would cause corrosion of pipes and the excess calcium carbonate would precipitate out, causing scale. So the water must be recarbonated, which is the process of stabilizing the water by lowering the pH and precipitating out excess lime and calcium carbonate.
The goal of recarbonation is to produce stable water, which is water in chemical balance, containing the concentration of calcium carbonate in which it will neither tend to precipitate out of the water (causing scale) nor dissolve into the water (causing corrosion.) This goal is usually achieved by pumping carbon dioxide into the water. Excess lime reacts with carbon dioxide in the reaction shown below, producing calcium carbonate:
Carbon dioxide → Calcium carbonate + Water
Ca(OH)2 + CO2→ CaCO3 + H2O
Recarbonation also lowers the pH, which encourages the precipitation of calcium carbonate and magnesium hydroxide.
Recarbonation may occur in one step, in which the pH is lowered to about 10.4 and carbonate hardness is precipitated out. In some cases, a second recarbonation step is used to lower the pH to 9.8 and encourage yet more precipitation. In either case, the process must be carefully controlled since carbon dioxide can react with calcium carbonate and draw it back into solution as calcium bicarbonate, negating the softening process.
Alternatively, recarbonation can be achieved through the addition of acids such as sulfuric or hydrochloric acids or through polyphosphate addition. These types of recarbonation work differently from carbon dioxide addition.
In The Treatment Process
Lime softening uses the equipment already found in most treatment plants for turbidity removal. An overview of the lime treatment process is shown below.
Lime softening produces large quantities of sludge. In fact, for every pound of lime used, about two pounds of sludge are formed.
The softening process usually requires two sedimentation basins, each with a detention time of 1.5 to 3 hours, to deal with the large quantities of sludge. One sedimentation basin handles the sludge resulting from lime and soda ash softening and the other sedimentation basin deals with the sludge resulting from recarbonation.
Disposal of lime sludge is the same as for sedimentation basin sludge. Landfill disposal is the most common method, although sludge may sometimes be sent to sanitary sewers. Lime sludge has a high pH and has increasingly been disposed of by applying it to agricultural land to increase the pH of acidic soils.
If softening problems are discovered, the cause usually lies in either chemical feeder malfunctions or source water quality changes. A variety of water characteristics can influence lime-soda ash softening:
These four water characteristics should be monitored
carefully when softening water using lime. In addition, coagulants used
to remove turbidity can influence the alkalinity or pH of the water, thus affecting
the softening process. After softening, the Langelier Index of
the water should be tested to ensure that the water is not corrosive. We
will study the Langelier index and corrosive water in more depth in the next
Softening is especially well-suited to treating groundwater since groundwater characteristics tend to remain relatively constant. Changing water conditions require a great deal of manipulating the softening process to keep it efficient. In addition, the high turbidity found in surface water sometimes requires presedimentation prior to softening.
Chemicals Used in Lime Softening
Types of Lime
The lime used for softening comes in two forms - hydrated lime and quicklime. Both types of lime soften water in the same way, but the equipment required for the two types of lime is different.
Hydrated lime (Ca(OH)2) is also known as calcium hydroxide or slaked lime. Hydrated lime can be added to water as it is without requiring any special equipment, so it is a popular choice for small water treatment plants.
In contrast, quicklime (CaO), also known as calcium oxide or unslaked lime, must be slaked before it is used. Slaking is the process of converting quicklime to hydrated lime by adding water, as shown below:
oxide + Water → Hydrated lime
CaO + H2O → Ca (OH)2
Slaking requires specialized equipment. The cost of equipment and the operator time required to run the equipment usually make quicklime use uneconomical in small plants. However, since the chemical cost of quicklime is less than the cost of hydrated lime, quicklime is often used in large plants.
The slaking process can also allow a large plant to reuse a large quantity of the lime sludge produced in the softening process. First, the sludge is heated, and the calcium carbonate in the sludge produces calcium oxide:
Calcium carbonate → Calcium oxide + Carbon dioxide
CaCO3→ CaO + CO2
Then the calcium oxide can be slaked and reused in the plant. Reusing lime sludge cuts down on both chemical purchase and sludge disposal costs.
Lime Handling and Storage
Operators should observe safety procedures while handling both hydrated lime and quicklime. Lime dust can be harmful when it comes in contact with the eyes, nose, or mouth, and skin contact can cause burns. As a result, operators should wear goggles and dust masks as well as protective clothing.
Both hydrated lime and quicklime can deteriorate in quality over time while in storage. In addition, storing quicklime can cause safety problems. If quicklime comes in contact with water, it begins to slake, a process which produces a great deal of heat and can cause explosions when uncontrolled. Quicklime should never be stored with alum since the quicklime will absorb water away from the alum and cause an explosion.
Soda ash (Na2CO3) comes in only one form and does not require any treatment before it is added to the water. Safety issues resemble those for lime handling. Soda ash dust irritates the eyes and mucous membranes of the nose, so the operator should wear protective clothing, goggles, and a dust mask. In addition, areas in which soda ash is used should be equipped with a ventilation system to deal with the dust.
Caustic soda (NaOH), also known as sodium hydroxide, can replace soda ash and some of the lime in the treatment process. The treatment process using caustic soda follows the same steps as that of lime-soda ash softening.
First, carbon dioxide reacts with the caustic soda to make sodium carbonate and water.
dioxide + Caustic soda → Sodium Carbonate + Water
CO2 + 2NaOH → Na2CO3 + H2O
Then the remaining caustic soda can react with calcium bicarbonate and magnesium bicarbonate.
bicarbonate + Caustic soda → Calcium carbonate + Soda ash + Water
Ca(HCO3)2 + 2NaOH → CaCO3 + Na2CO3 + 2H2O
Magnesium bicarbonate + Caustic soda → Magnesium hydroxide + Soda ash + Water
Mg(HCO3)2 + 4NaOH → Mg(OH)2 + 2Na2CO3 + 2H2O
The caustic soda can also react with magnesium noncarbonate hardness, as shown below. Also note that the reactions between caustic soda and carbonate hardness produced soda ash, which can react with noncarbonate hardness as well.
Magnesium sulfate + Caustic soda → Magnesium hydroxide + Sodium sulfate
MgSO4 + 2NaOH → Mg(OH)2 + Na2SO4
Caustic soda has the advantages of stability in storage, lower sludge formation, and easy handling. However, safety issues still apply. Caustic soda is dangerous to the operator and can cause severe burns to the skin. As a result, rubber gloves, dusk masks, goggles, and a rubber apron should be worn while handling the chemical.