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A Presentation on. Dr.S. Kanmani Phone:044 – 22359027 Professor in Civil Engineering [email protected] Centre for Environmental Studies Anna University Chennai, Chennai – 600 025. Reverse Osmosis. TDS. TDS Ranges. TDS Concentrations. History of Membrane Treatment.

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A Presentation on

Dr.S. KanmaniPhone:044 – 22359027

Professor in Civil Engineering [email protected]

Centre for Environmental Studies

Anna University Chennai, Chennai – 600 025


Reverse Osmosis


TDS


TDS Ranges


TDS Concentrations


History of Membrane Treatment

  • 1748, Osmosis

  • 1948, Reverse Osmosis (RO)

  • Membrane classification MF, UF, NF & RO.


RO Removes

Ionic

Non ionic

Particulate

Microbiological


Osmosis

Reverse Osmosis


OSMOSIS & its REVERSE

Po= P

--

Po<P

--

--

Po>P

--


Reverse Osmosis

Water Flow

Product

Feed

Concentrate

Ions


TYPES OF MEMBRANES

  • Cellulose acetate

  • Polyamide hydrocarbon (PAH)

  • Sulfonated polysulfone (SPS)

    • Hollow fibre

    • Spiral bound

    • Tubular

  • RO membranes degraded by chlorine.

  • SPS membranes are resistant to Cl2 attack.

  • Chlorine-resistant membranes would eliminate the

  • need for de-chlorination of the RO feed and re-

  • chlorination of permeate.

  • Reducing the overall cost of RO.


  • MEMBRANES

    They are made of different materials

    - Cellulose acetate/triacetate

    - Cellulose acetate blends

    - Poly amides

    - Ceramics etc

    They are in different forms

    - Hollow fibre

    - Thin film composite spiral wound

    They have different characteristics -capabilities to remove impurities and operating pressures


    Hollow fibre Configuration


    Hollow-fiber membranes


    RO ELEMENT


    RO ELEMENT


    RO ELEMENT


    RO Membrane Sizes

    Recent Advancement:Extra large RO Membranes: 18 in dia x 60 in long


    Reverse Osmosis:What It Cannot Do

    Cannot concentrate to 100%

    Cannot separate to 100%

    Not always the most cost effective method


    Typical Operating PressuresPressure Driven Membrane Processes

    1 bar = 1 atm = 100 kPa = 100 kN/m2 = 1 kg/m.s2 = 10 m H2O = 760 mm Hg=14.5 psi


    Rule of Thumb

    100 mg/L TDS » 1 psi (0.07 atm) osmotic pressure

    1,000 mg/L TDS » 10 psi (0.7 atm) osmotic pressure

    35,000 mg/L TDS » 350 psi (25 atm)osmotic pressure


    SINGLE MODULE RO

    Module : Pressure vessel containing 1 -10 RO elements


    MODULES IN SINGLE ARRAY

    M-1

    Pre-

    Treatment

    M-2

    Permeate

    P

    M-3

    Reject


    MODULES IN MULTIPLE ARRAY

    M-1

    Permeate

    Pre-

    Treatment

    M-3

    P

    M-2

    Reject


    MODULES IN MULTIPLE STAGE

    M-1

    Permeate

    Pre-

    Treatment

    M-2

    P

    Reject


    Membrane Operational

    Terminology

    • Influent/Feed Water

    • Effluent/Permeate

    • Concentrate/Retentate/Reject

    • Flux – Volume of water that passes through a membrane per unit of time & per unit of surface area of the membrane (L/h/m2)

    • Recovery-%of feed that is converted to permeate

    • Contaminant removal – % of a contaminant removed from the feed stream.


    Membrane Operational

    Terminology (Contd.)

    • 7.Size exclusion – Removal of PM by sieving

    • 8. Molecular Weight Cutoff (MWCO), measured in Daltons – A Dalton is a non-SI unit of mass equal to 1/2th of mass of a carbon – 12 atom.

    • Membrane Fouling : Reduction of flux through a membrane caused by the build-up of contaminants (Macro (i.e surface) & Micro (pore) fouling, Reversible/Irreversible Fouling).

    • Transmembrane Pressure – (Average feed pr. - Permeate pr.)


    Membrane Classifications


    Terminology

    • Membrane Module : A complete unit consisting of membranes, support frame & ports (feed inlet, reject, outlet permeate)

    • Array/Train : Assembly of catridges in pressurised systems

    • Catridge: A manufactured canister of membranes with feed, permeate & reject connections.

    • Module: A collection of membranes intended to be mounted & replaced as a unit.


    Membrane Materials & Configurations

    • Physical structure of membrane – Microporous or Asymmetric

    • Pore size – uniform (isotropic), Varied (anisotropic)

    • Membrane materials – Organic (cellulose- based) & Inorganic (metals/ceramics)

    • Types of membranes – Flat sheet, Hollow fibres, Tubes, spiral-wound cylinders & Rotating flat plates


    NF & RO for Advanced Treatment

    • Fouling potential may be estimated based on SDI (<5), turbidity (<NTU).

    • Pretreatments steps for RO include

      • MF/UF

      • SMBS addition

      • Chloramination

      • H2SO4addition

      • Scale inhibitor chemical addition

      • Catridge filtration.


    Design Criteria


    RO Feed Water Quality


    PRE-TREATMENT

    Pretreatment; TDS < 10,000

    No microorganisms

    No oil

    Turbidity < 1 NTU

    Particle size < 25 micron

    pH = 4.5 prevents scale formation


    PRE-TREATMENT

    • Screening of solids: prevent fouling of the membranes by fine particle or biological growth, and damage to pump.

    • Cartridge filtration : polypropylene filters to remove 1 - 5 µm particles.

    • Biocidal Treatment: biocides such as chlorine to kill bacteria.

    • De-chlorination: bisulfite dosing to remove the chlorine which can oxidize membrane.


    Factors Which Affect Performance of Membranes

    Feedwater Pressure

    Feedwater Temperature

    Feedwater Concentration

    Increased Recovery


    Fouling

    • Biological or colloidal fouling

    • Sulphate salts (CaSO4)

    • Silica fouling can be more difficult to predicted & control than other types of fouling.

    • Soluble silica is concentrated to insoluble levels in RO process

    • Effluent organics can adsorb onto membrane element surface, causing pore clogging & even a change in membrane surface charge.


    THANK YOU

    Dr.S. Kanmani, B.E., M.E.,Ph.D.

    Professor in Environmental Engineering

    Centre For Environmental Studies, Anna University

    Phone: 044-22359027

    email : [email protected]


    Membrane Technology

    • Desalination Technologies (Membrane Processes / Distillation Technologies / Chemical Processes (IX)).

    • Most common desalination technology – Membrane Processes

    • A membrane is a thin porous material that allows water molecules to pass through it, but rejects viruses, bacteria, metals, salts , etc.

    • Wide variety of polymeric materials : Cellulose, Acetate, Nylon.

    • Non – polymeric materials: Ceramics, Metals, Composites.


    • Two types of Membrane processes:

      • Pressure Driven (RO, NF, UF, MF)

      • Electrical – Driven (Electrodialysis)


    Application of Pressure – Driven Membrane Processes


    Application of Membrane Processes


    Reverse Osmosis

    • A physical process.

    • Osmotic pressure difference bt. Salt water & pure water is used.

    • A pressure greater than osmotic pressure is applied on feed water.

    • A con. Salt soln is rejected.

    • TDS upto 45,000 mg/L could be treated.

    • Pump is used to raise the pressure applied to sea water.


    • For brackish water, pump pressure ( 140 – 400 psi)

    • For sea water , pump pressure ( upto 1200 psi)

    • Two common Membrane Types (Cellulose Acetake, CA & Non – CA Membranes)

    • CA membranes are used in desalination process. They have smooth surface that is resistant to fouling.

    • Non – CA members, Thin – film Composite Membranes ( eg. Polyamide & Composite Membranes) use Organic materials (eg Polysulphone)


    • They have high Flux – Rate (Volume of fresh water per membrane surface area).

    • Non – CA membranes are stable over wide pH range than CA membranes, but susceptible to Cl2.

    • In order to meet RO feed water quality requirements , pretreatment is needed (for removal of large particles, Organics & adding Chemicals to prevent Formation of precipitates & scales).

    • CO2 & soda ash are added to increase alkalinity of treated water.

    • Recovery is the vol. of fresh water produced as a % of vol. feed water processed.

    • Typical recovery rates (30 – 80 %)


    General Membrane Operation

    & Maintenance Procedures

    • Back washing of the membrane is a common technique used with low-pressure, hollow-fiber membranes to maintain the design operating flux of the system.

    • Standard solutions of acids, bases & detergents are used to clean most membrane.

    • Cl2 in the form of sodium hypochlorite is used to remove biological foulants.

    • Critic acid (C6H8O7) is a common acid used to clean membrane is effective in removing iron scales & mineral scales (CaCO3, CaSO4, MgSO4) resulting from hard water

    • Caustic could be used to remove biological fouling when Cl2 cannot be applied

    • Other chemicals used to clean low-pressure membranes include H2O2, HCl & EDTA.


    Membrane Post Treatment System

    • Degasification to remove H2S, CO2 from permeate.

    • NaOH is used to increase pH.


    Scale Prevention

    • To prevent formation of CaCO3, pH of RO feed water is lowered by adding mineral acid.

    • Solubility of CaCO3 salt depends on pH & bicarbonate ion, thus converting bicarbonate ion to CO2 & lowering pH will increase its solubility.

    • Mineral acid can be used to control biofouling (i.e HCl, H2SO4)

    • Antiscalants are required when con. of salt exceeds its solubility.

    • Polymeric scale inhibitors (polyacrylates or poly acrylic acid) have been developed.


    Post Treatment

    • Degasification, stabilisation, Alkalinity recovery & disinfection

    • Degasification (addition of PO) to remove gases from permeate

    • NaOH or hydrated line is added to raise pH.


    Membrane Classifications


    Membranes


    Membrane Materials


    Packing Modules

    • Plate & Frame

    • Spiral Wound

    • Tubular

    • Hollow - Fiber


    RO


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