A Presentation on Dr.S. KanmaniPhone:044 – 22359027 Professor in Civil Engineering email@example.com Centre for Environmental Studies Anna University Chennai, Chennai – 600 025
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
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.)
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.
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 : firstname.lastname@example.org
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)
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.