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    Reject Pretreatment

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    Visheas
    The document discusses pretreating reverse osmosis (RO) reject water before mechanical vapor compression. RO reject water typically contains high levels of hardness, ions, organics and silica. One common method for removing hardness is lime soda processing, which uses lime and soda to precipitate calcium carbonate or magnesium hydroxide and remove hardness. The lime soda process involves dosing reject water with lime, then soda, to drive reactions that form insoluble products and sediment hardness compounds out of the water. Pretreating RO reject water can help recover more water for reuse or discharge.

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    Reject Pretreatment

    Uploaded by

    Visheas
    32 views2 pages
    The document discusses pretreating reverse osmosis (RO) reject water before mechanical vapor compression. RO reject water typically contains high levels of hardness, ions, organics and silica. One common method for removing hardness is lime soda processing, which uses lime and soda to precipitate calcium carbonate or magnesium hydroxide and remove hardness. The lime soda process involves dosing reject water with lime, then soda, to drive reactions that form insoluble products and sediment hardness compounds out of the water. Pretreating RO reject water can help recover more water for reuse or discharge.

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    Ro Reject Treatement schemedme

    Original Title

    Ro Reject Pretreatment

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    The document discusses pretreating reverse osmosis (RO) reject water before mechanical vapor compression. RO reject water typically contains high levels of hardness, ions, organics and silica. One common method for removing hardness is lime soda processing, which uses lime and soda to precipitate calcium carbonate or magnesium hydroxide and remove hardness. The lime soda process involves dosing reject water with lime, then soda, to drive reactions that form insoluble products and sediment hardness compounds out of the water. Pretreating RO reject water can help recover more water for reuse or discharge.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    32 views2 pages

    Reject Pretreatment

    Uploaded by

    Visheas
    The document discusses pretreating reverse osmosis (RO) reject water before mechanical vapor compression. RO reject water typically contains high levels of hardness, ions, organics and silica. One common method for removing hardness is lime soda processing, which uses lime and soda to precipitate calcium carbonate or magnesium hydroxide and remove hardness. The lime soda process involves dosing reject water with lime, then soda, to drive reactions that form insoluble products and sediment hardness compounds out of the water. Pretreating RO reject water can help recover more water for reuse or discharge.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
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    of 2

    RO REJECT TREATMENT PRIOR TO

    MECHANICAL VAPOR COMPRESSION-(MVR)


    Typical recycling, reuse and process water systems use Reverse Osmosis (RO) technology as the
    method of decreasing the level of ions in process water. RO recovery rates normally range from
    85-90%, therefore 10-15% of the RO feed water is discharged as wastewater. With the
    increasing cost of water and waste discharge, more companies are looking to recover and reuse
    RO reject water.

    While it is possible to reuse the reject water from an MVR system to recover condensate water
    as product, the pretreatment of RO reject is recommended. Such RO reject water typically
    contains very high levels of hardness, ions, organics and silica. The hardness of reject water is
    defined as soap settling property of that water. The soap is sediment by the presence of Ca+2
    and Mg+2 ions in the water. Ions such as Fe+ 2, Zn+2, and Al+3 also help settling. However, the
    presence of high amounts of Ca+ 2 and Mg+ 2 ions in water usually leads to water hardness. This
    is the most common type of hardness in reject water. The hardness caused by these materials is
    known as total hardness. The part of the total hardness corresponding to carbonate and
    bicarbonate ions in the reject water is defined as carbonate hardness. These ions also
    determine water alkalinity. The reject is fed to Reaction Tank-1 where chemicals are added to
    enhance particle flocculation. The water is then fed to Reaction Tank -2 where additional
    chemicals are added to promote the precipitation of hardness and silica.

    Waters are classified according to hardness grades as follows.

    (mg/L) CaCO3 Degree of Hardness

    0-75 Soft

    75-150 Middle

    150-300 Hard

    300 and over Very hard

    Hardness removal (chemical softening) is a process that removes all or part of the hardness by
    adding various chemical substances into the water. The processes used for water softening are
    chemical sedimentation and ion exchange methods. Chemical sedimentation can be carried out
    following way
    Lime soda process:
    In lime-soda process, reject water is treated with lime (CaO or Ca (OH)2) firstly, after that with
    soda. In this process, the hardness is removed by sedimentation as calcium carbonate or
    magnesium hydroxide. Lime is added either as calcium hydroxide or calcium oxide, and soda is
    added as sodium carbonate. The substances form hardness in water and the reactions of lime
    and soda can be written as follows.

    Reactant Product
    Ca(OH)2 + CO2 CaCO3 + H20
    Ca(OH)2 + Ca(HCO3)2 2CaCO3 + 2 H20
    Mg(HCO3)2 + Ca(OH)2 MgCO3 + CaCO3 + 2 H20
    MgCO3 + Ca(OH)2 Mg(OH)2 + CaCO3
    MgSO4 + Ca(OH)2 Mg(OH)2 + CaSO4
    2NaHCO3 + Ca(OH)2 CaCO3 + Na2CO3 +2H20
    CaCl2+ Na2CO3 CaCO3 + NaCl
    CaSO4 + Na2CO3 CaCO3 + Na2CO3

    The usual method for lime soda processing is treatment by excessive lime amount. The pH
    should be greater than 9 for sedimentation of the magnesium hydroxide. Usually pH is between
    10-10.5. The process contains lime dosing. A small amount of alum or sodium aluminate is
    added to ensure good sedimentation. The recycling of some sediment sludge can also help to
    sediment.

    If the water contains excessive alkalinity after the softening process has been completed, there
    is a possibility that the calcium carbonate will settle in the pipes. The softened water is treated
    with H2SO4 or CO2 to remove this from the softened water. This reaction is called re-
    carbonization.

    In addition to the re-carbonization process, addition small amount of polyphosphate.(0.5-5 mg


    /L) may prevent this sedimentation after purification. If it is requested to retain permanent
    hardness in water, soda should not be used after softening. This application is defined as partial
    softening operation.

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