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.
What is the difference between Lime softening and ion exchange 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 are the advantages of using warm lime-softened water in demineralizers?
The lower levels of calcium, magnesium, and especially silica reduce the ionic loading on the demineralizer when warm lime-softened water is used rather than cold lime-softened water. This may reduce both the capital and operating costs of the demineralizer.
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.
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.
What is the most commonly used coagulant in lime-soda process?
Alum, aluminium sulphate and sodium aluminate are used as coagulants in lime-soda process.
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.
Why is soda ash used in water treatment?
Soda ash, or sodium carbonate (NA2CO3) is a water soluable salt of carbonic acid. It is often used in municipal water treatment to raise the pH of the water.
Why is my TDS higher after water softener?
Softeners and filters do not affect TDS readings significantly. A softener, to illustrate, removes calcium and magnesium ions but the TDS reading will not be affected significantly because the softener adds a more-or-less equal amount of sodium in exchange.
Do water softeners reduce PPM?
Softeners may also remove as much as 5-10 ppm (parts per million; ppm is equal to milligrams per liter, or mg/L) of iron and manganese.
Is TDS and hardness the same?
TDS and hardness are water quality parameters. The main difference between TDS and hardness is that TDS include inorganic and organic substances that cannot be filtered through a filter paper whereas hardness is due to the presence of magnesium and calcium salts of carbonate, sulfate and chloride.
What is the first step in lime softening?
The first step in lime softening is the addition of lime to react with substances like CO2 in the water before it can begin softening the water. The following reaction occurs:
What is the process of lime softening water?
Lime softening water treatment process involves a relatively complicated series of chemical reactions. The goal of all of these reactions is to change the calcium and magnesium compounds in water into calcium carbonate and magnesium hydroxide, which are the least soluble compounds and thus will settle out of the water at the lowest concentrations.
What reacts with lime to form calcium carbonate?
Magnesium compounds have a slightly different reaction. First, magnesium bicarbonate reacts with lime and produces calcium carbonate (which precipitates out of solution).
How does re carbonation work?
The goal of re carbonation is to produce stable water, which is the 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:
What pH is needed to remove calcium carbonate?
Calcium compounds in water will be removed at a pH of about 9.0 to 9.5 while magnesium compounds require a pH of10.0 to 10.5. When soda ash is used to remove non carbonate hardness, an even higher pH is required; 10.0 to 10.5 for calcium compounds and 11.0 to 11.5 for magnesium compounds.
How much sludge is formed from lime?
Sludge disposal: Lime softening produces large quantities of sludge; for every kg of lime used, about two kg of sludge is formed. Landfill disposal is the most common method of sludge disposal.
Can carbon dioxide be re-carbonated?
Re carbonation process must be carefully controlled since carbon dioxide can ,ead with calcium carbonate and <haw it back into solution as calcium bicarbonate, reversing the softening process.
What is lime neutralization?
Lime neutralization technology is widely used in Canada for the treatment of acid mine drainage and other acidic effluents. In many locations, improvements to the lime neutralization process are necessary to achieve a maximum level of sludge densification and stability. Conventional lime neutralization technology effectively removes dissolved metals to below regulated limits. However, the metal hydroxide and gypsum sludge generated is voluminous and often contains less than 5% solids. Despite recent improvements in the lime neutralization technology, each year, more than 6 700 000 m 3 of sludge are generated by treatment facilities operated by the Canadian mining industry. Because lime neutralization is still seen as the best available approach for some sites, sludge production and stability are expected to remain as issues in the near future. Several treatment parameters significantly impact operating costs, effluent quality, sludge production and the geochemical stability of the sludge. Studies conducted both at CANMET and NTC have shown that through minor modifications to the treatment process, plant operators can experience a reduction in operating costs, volume of sludge generated, metal release to the environment and liability. This paper discusses how modifications in plant operation and design can reduce treatment costs and liability associated with lime treatment.
What is the purpose of calcined lime mud?
In this study, CaO-containing wastes from pulp and paper industries such as fly ash (FA) and calcined lime mud (LM) were utilized to neutralize and purify acidic wastewaters from the pickling processes in steel mills . The investigations were conducted by laboratory scale trials using four different batches of wastewaters and additions of two types of CaO-containing waste materials. Primary lime (PL), which is usually used for the neutralization, was also tested in the same experimental set up in the sake of comparison. The results show that these secondary lime sources can effectively increase the pH of the acidic wastewaters as good as the commonly used primary lime. Therefore, these secondary lime sources could be potential candidates for application in neutralization processes of industrial acidic wastewater treatment. Moreover, concentrations of metals (such as Cr, Fe, Ni, Mo and Zn) can decrease dramatically after neutralization by using secondary lime. The LM has a purification effect from the given metals, similar to the PL. Application of fly ash and calcined lime mud as neutralizing agents can reduce the amount of waste from pulp and paper mills sent to landfill and decrease the need for nature lime materials in the steel industry.
What is AMD sludge treatment?
AMD sludge treatment can include conditioning, thickening, and dewatering.Sludge disposal can be achieved by deep mine disposal, retained-in-pond disposal, ordisposal at coal refuse areas. Finally, the potential beneficial uses of AMD sludge arediscussed.
Is pickle liquor harmful to the environment?
... Due to its potential hazardous nature, the enormous quantities of spent pickle liquor are a major environmental concern for steel industry. According to many authors (Aubé and Zinck 2008; Nemerow and Dasgupta 1991;Baba et al. 2011), the most common technology used for disposal of metal sludge and spent pickling acid is neutralization with alkaline additives such as caustic soda, liquid ammonia, quicklime, limestone or slaked lime. Other methods such as solidification/stabilization (S/S) by cementations binder have also been previously reported in the literature (Mansurm et al. 2008;Weitzman 1990;Minocha et al. 2003;Wang 2009). ...
What is cold lime softening?
Precipitation softening accomplished at ambient temperatures is referred to as cold lime softening. When hydrated lime, Ca (OH) 2, is added to the water being treated, the following reactions occur:
How much calcium hardness can lime feed reduce?
If the proper chemical control is maintained on lime feed, the calcium hardness may be reduced to 35-50 ppm. Magnesium reduction is a function of the amount of hydroxyl (OH -) alkalinity excess maintained. Figures 7-1 and 7-2 show these relationships.
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.
How is precipitation softening used?
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 (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. Waters with moderate to high hardness and alkalinity concentrations (150-500 ppm as CaCO 3) are often treated in this fashion.
How does a sludge contact unit work?
In the sludge-contact unit, the water flows through a bed of sludge for additional contact. The sludge level is maintained by the proper combination of sludge bed conditioning chemicals, mechanical agitation, hydraulic suspension, and sludge blowdown. A discernible line of separation between clarified water and slurry pool should exist in a properly operated unit. Effluent turbidity is usually less than 10 NTU.
Why is magnesium reduced to 2-5 ppm?
Magnesium is reduced to 2-5 ppm because of the lower solubility of magnesium hydroxide at the elevated temperatures.
What causes hardness in water?
Hardness may also be present as a sulfate or chloride salt, referred to as noncarbonate or permanent hardness. These salts are caused by mineral acids present in rain water or the solution of naturally occurring acidic minerals.