Excess Lime Softening
What is the estimated hydrated lime (Ca(OH)2) (90% pure) dosage required for softening (in mg/L)? (Assume 15% excess lime.) 220 Mathematics for Water/Wastewater Treatment Plant Operators Solution Determine the A to D factors: A = CO 2 (mg L ) × (74 44) C = 0 mg L = 4 mg L × (74 44) = 7 mg L D = Mg2 + (mg L ) × (74 24.3) B = Alkalinity ( mg L ) × (74 100) = 20 mg L × (74 24.3) = 175 mg L × (74 100) = 61 mg L = 130 mg L Calculate the estimated hydrated lime dosage: Hydrated lime dosage. Soda lime water softening process Carbon dioxide and carbonate hardness (calcium and Magnesium bicarbonate) are complexed by lime. In this process Calcium and Magnesium ions are precipitated by the addition of lime (Ca(OH)2) and soda ash (Na2CO3). Replicate runs for the water softening experiment evaluated parameters such as alkalinity, hardness and pH in response to varying lime dosages from 30% to 180% of the stoichiometric amount. In Run 1, the total hardness reached a minimum value of approximately 160 mg/L as CaCO 3 for a lime dosage of 90% (fig.1).
Excess lime and the absence of alkalinity caused by carbonates. The plateau between 60% and 90% of lime is due to the presence of extra alkalinity. Any excess lime added in this region converts bicarbonates to carbonates and those carbonates combine with the free calcium ions to form precipitates. Is a process in which carbon dioxide is bubbled into the water being treated to lower the pH. Recarbonation maybe carried out in two steps. The first addition of carbon dioxide would follow excess lime addition to lower pH to about 10.4 and encourage the precipitation of calcium carbonate and magnesium hydroxide.
Lime softening, also known as Clark's process,[1] is a type of water treatment used for water softening which uses the addition of limewater (calcium hydroxide) to remove hardness (calcium and magnesium) ions by precipitation. The process is also effective at removing a variety of microorganisms and dissolved organic matter by flocculation.[2]
History[edit]
Lime softening was first used in 1841 to treat Thames River 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 soft water that can, in some cases, be used more effectively for heat transfer and various other industrial uses.
Chemistry[edit]
As lime in the form of limewater 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 into bicarbonate (HCO−
3) and then carbonate (CO2-
3). 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.[3]
In the process both the calcium (and to an extent magnesium) in the raw water as well as the calcium added with the lime are 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) whereas 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. Real pool 3d free download.
Future uses[edit]
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 USA ethanol production plants), lime softening can be used to pre-treat the membrane feed water.
Waste products[edit]
Lime softening produces large volumes of a mixture of calcium carbonate and magnesium hydroxide in a very finely divided white precipitate which may also contain some organic matter flocculated out of the raw water. Processing or disposal of this material may be a cost to the process.
Excess Lime Softening Oil
References[edit]
- ^Mellor, J W, Intermediate Inorganic Chemistry, Longmans, Green & Co, London, 1941, p. 202
- ^USBR - Lime Softening fact sheetArchived 2011-06-13 at the Wayback Machine
- ^'Lime Softening'. Retrieved 4 November 2011.