In the lime-softening process, the pH of the water being treated is raised suciently to precipitate calcium carbonate and, if necessary, magnesium hydroxide. The normal pH of water is between 6.5–8.5. In small systems, lime softening is typically practiced by adding hydrated lime to raw water to raise the pH to approximately 10. This removes calcium carbonate, essentially limestone. If magnesium removal is also required, the pH during softening needs to be closer to 11. Lime-softening systems need not be pilot tested for small systems using groundwater sources. Jar testing to determine appropri-ate process pH and chemical doses is sucient. Doses of these chemicals should not change greatly over time unless the groundwater is subject to periodic infiltration by surface water that changes in quality. Lime-softening systems do need to be pilot tested if used on surface water sources with variable quality.
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What are the benefits of lime softening in water treatment?
Lime softening processes, with the usual filters, will reduce oxidized iron and manganese to about 0.05 and 0.01 ppm, respectively. Raw water organics (color-contributing colloids) are also reduced. Turbidity, present in most surface supplies, is reduced to about 1.0 NTU with filtration following chemical treatment.
What is the temperature of lime softening?
The warm lime softening process operates in the temperature range of 120-140°F (49-60°C). The solubilities of calcium, magnesium, and silica are reduced by increased temperature. Therefore, they are more effectively removed by warm lime softening than by cold lime softening.
How many molecules of lime are used in a softening reaction?
Lime Softening 2 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.
What is partial softening of cold lime?
Cold lime softening is also known as partial softening because the typical mini- mum effluent concentration of calcium hardness is about 35 mg/L CaCO 3. Reduction of the silica and magnesium concentrations requires the addition of Na
What is a limitation of the lime soda softening process?
Limitation of Soda Lime Process: Lime soda softening cannot produce a water at completely free of hardness because of the solubility (little) of CaCO3 and Mg(OH)2. Thus the minimum calcium hardness can be achieved is about 30 mg/L as CaCO3, and the magnesium hardness is about 10 mg/L as CaCO3.
Does lime softening increase pH?
In small systems, lime softening is typically practiced by adding hydrated lime to raw water to raise the pH to approximately 10. This removes calcium carbonate, essentially limestone.
Which chemical is often used with lime softening?
Lime Softening Chemicals normally used are lime (calcium hydroxide, Ca(OH)2) and soda ash (sodium carbonate, Na2CO3).
Does lime softening reduce TDS?
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 also be used to remove iron, manganese, radium and arsenic from water.
Does lime softening remove sulfate?
Removing sulfates with hydrated lime Many facilities prefer to remove sulfates, especially with larger concentrations, by adding hydrated lime (Ca(OH)2), which will precipitate out the sulfate as gypsum.
Does lime softening remove iron?
Lime softening can also be used to remove iron, manganese, radium and arsenic from water.
How do you reduce calcium carbonate in water?
The usual treatment for calcium carbonate precipitation is to inject acid to acidify the water, lowering the pH to 7 or less. Common acids used for this purpose include sulfuric acid, muriatic acid, and hydrochloric acid. Other acids such as citric acid and nitric acid can be used, but they are more expensive.
Does soda ash increase 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).
How can we regain the hardness of water by adding lime?
Softening - In water softening, hydrated lime is used to remove carbonate "hardness" from the water. Hardness caused by other calcium and magnesium salts, called noncarbonate hardness, is generally treated by means of the lime-soda process, which entails the precipitation of magnesium by lime.
How can we remove hardness of water?
A few methods to remove hardness from water are,Chemical Process of Boiling Hard Water.Adding Slaked Lime (Clark's Process)Adding Washing Soda.Calgon Process.Ion Exchange Process.Using Ion Exchange Resins.
Why addition of excess soda is not advisable for boiler feed water softening?
Care must be exercised in the specification of soda ash control ranges. If the softened water is to be used as boiler feedwater, hardness removal by the addition of soda ash may not be worth the cost of the resulting increase in steam condensate system corrosion.
How do you remove sodium bicarbonate from water?
Vinegar: Water dissolves sodium bicarbonate, but at a slower rate than vinegar. Alcohol: Alcohol does not dissolve sodium bicarbonate. Submerging in alcohol or using alcohol wipes to clear build-up is an exercise in futility.
What is the process of breaking the bonds between lignin and other carbohydrates?
The complex intertwined structure associated with the lignocellulosic biomass possesses a technological challenge in processing of lignin, cellulose, and hemicellulose. The process of breaking the bonds between the lignin and other carbohydrates using a chemical or thermochemical operation is called pretreatment. Pretreatment is an important step to increase the bioavailability of sugar monomers from cellulosic biomass. Several pretreatment methods like steam explosion, ammonia fiber explosion, dilute acid hydrolysis, lime treatment, carbon dioxide explosion, alkaline hydrolysis, oxidative delignification, pulsed-electric-field pretreatment, biological pretreatment, and many others have been employed in past studies [71].
Why do we use cement in soil?
Note that the increases in soil strength and stiffness values with cement treatment are significantly higher than those with lime treatment. Many agencies prefer using cement treatments for two reasons: They provide greater sustainable strength for longer periods, and there is less chemical leaching in treated soils. Many cities in North Texas are proposing combined lime–cement treatments of their subgrades, which will provide beneficial advantages of both chemical additives.
What is lime softening?
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 also be used to remove iron, manganese, radium and arsenic from water.
What happens when you add lime to water?
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 is changed into bicarbonate and then carbonate. This action causes calcium carbonate to precipitate due to exceeding the solubility product.
What is the temperature of lime softener?
The warm lime softening process operates in the temperature range of 120-140°F (49-60°C). The solubilities of calcium, magnesium, and silica are reduced by increased temperature. Therefore, they are more effectively removed by warm lime softening than by cold lime softening.
How much magnesium is reduced by lime?
The use of lime and soda ash permits hardness reduction down to 0.5 gr/gal, or about 8 ppm, as calcium carbonate. Magnesium is reduced to 2-5 ppm because of the lower solubility of magnesium hydroxide at the elevated temperatures. Silica Reduction. Hot process softening can also provide very good silica reduction.
What is the chemical that removes the hardness of water?
The water is treated with lime or a combination of lime and soda ash (carbonate ion). These chemicals react with the hardness and natural alkalinity in the water to form insoluble compounds. The compounds precipitate and are removed from the water by sedimentation and, usually, filtration.
Why is water contaminated in cold process softening?
Because the reactions in cold process softening are not complete, the water contaminant levels leaving the unit are unstable. With additional time and/or increased temperature, further precipitation will occur downstream of the unit. Frequently, acid or carbon dioxide is added to stabilize the water.
What is precipitation softening?
Precipitation softening processes are used to reduce raw water hardness, alkalinity, silica, and other constituents. This helps prepare water for direct use as cooling tower makeup or as a first-stage treatment followed by ion exchange for boiler makeup or process use. The water is treated with lime or a combination of lime and soda ash ...
Does heating reverse the solution reaction?
In effect, heating reverses the solution reaction: Reduction of noncarbonate hardness, by contrast, requires chemical addition. A combination of lime and soda ash, along with coagulant and flocculant chemicals, is added to raw water to promote a precipitation reaction. This allows softening to take place.
Does sludge reduce silica?
Good sludge contact enhances silica reduction. To ensure optimum contact, sludge is frequently recirculated back to the inlet of the unit. Cold or warm process softening is not as effective as hot process softening for silica reduction. However, added magnesium oxide and good sludge contact will improve results.
What is the pH of lime ash?
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.
What is the pH of water that removes calcium?
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).
What is the capacity of water to neutralize acids?
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.
What is the best solution for magnesium water?
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.