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Introduction to Membrane Processes
School of Chemical Engineering
Part 2 Water TreatmentCHEN 6071: Water & Wastewater Engineering
The objective of these slides is to cover the following information:1. What are non-traditional waters and what are the treatment
objectives for membranes in these applications2 F f
Part 4 Non-traditional waters
2. Features of
Assessment: On-line quiz questions covering your understanding of:1. Salt and water transport across semi-permeable membranes2. Pre-treatment requirements and common forms of fouling in
reverse osmosisT t i l U f ti i f t tTutorial: Use of equations on reverse osmosis from course text
(Environmental Engineering: principles & practice)
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Non-traditional waters refer to sources of water contain levels of salts and organics that are higher than surface waters or potable groundwater
Examples include;
S t (TDS 30000 /L)
Illawarra Water Reclamation Plant& Gold Coast Desalination
Seawater (TDS > 30000 mg/L)Municipal Waste (TDS 1000 mg/L)Brackish Groundwater (2000-15000 mg/L)
Water requires treatment to remove saltsAnd organics prior to use
Reverse osmosis and Nanofiltration are membrane processes that are used to remove salts
Reverse osmosis is the process where pressure is applied in excess of the osmotic gradient to reverse flow due to osmosis
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Water Transport Fw = A a Pnet
What equations describe water & salt movement across the membrane?
Salt Transport Fs = B (C)Where; a = water permeability coefficient (m3 /m2/Pa)A = membrane areaPnet = net driving pressureB = salt permeabilityC = concentration gradient
Water flux can be written as:
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Where pa (Average imposed pressure gradient) is calculated by:
Osmotic pressure gradient
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Salt passage (or flux of solute species)
Solute concentration gradient
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P
What Pressure is needed for RO?
PermeatefeednetP
2
PpPP ---=
where:Pnet = Net driving pressure Pfeed = Feed pressure Osmotic press re differential across the membrane = Osmotic pressure differential across the membrane P = Feed/Brine pressure differentialPpermeate = Permeate pressure
Typical Osmotic Pressures
Species Conc. Osmotic Press, Species Conc. (mg/L)
Osmotic Press, (kPa)
NaCl 1,000 100LiCl 1,000 160MgSO4 1,000 25Sucrose 1,000 7Seawater 35,000 2700
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Calculate permeate concentration (mg/L) at constant C across the membrane with increasing water transport
How does permeate quality change with operating conditions?
Permeate Permeate PermeatePnet Water Salt Concentration
(kPa) (l/min) (mg/min) (mg/l) 500 1 1,000 1,0001000 2 ? ?
membrane with increasing water transport
1500 3 ? ?2000 4 ? ?
RO Process ControlSystems designed to operate at constant flow
Permeate
VFDFIT
Feed
Concentrate
Permeate
FIT
Concentrate FlowControl Valve
RO Feed PumpWith VFD
Control Valve
Variable Controlling DevicePermeate Flow Feed pump speed ( P aSpeed2)Recovery Concentrate valve
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Components of an RO System
Chemical Cond.Cl i
RO
AcidScale Inhibitor
IntakeStorage &DistributionPretreatment
Chlorination/Stabilisation
Waste
Waste
Cleaning
CartridgeFiltration
In-line CoagulationDirect FiltrationMF/UF
ScreensBeach Wells
BrineWaste
Spiral Wound RO & NF Elements
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Making a Spiral Wound Membrane
Step 1 Step 2
Fold
FreshWater
SaltyWater
SaltyWater
SaltyWater
MembraneFreshWater
Permeate Spacer
Step 3
Making a Spiral Wound Membrane
Step 4
SaltyS ltS lt SaltyWater
SaltyWaterSalty
Water
SaltyWater
Permeate tubewith holes Fresh
Water SaltyWater
FreshWater
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Making a Spiral Wound Membrane
Step 5 Step 6SaltyWater
Fibreglass
SaltyWater
Fresh
Brine Spacer
SaltyWater
FreshWater
FibreglasscasingSalty
Water
SaltyWater
FreshWater
Brine Spacer
Finished RO Membranes (2, 4, 8 & 16 diameter)
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RO Element are assembled into Pressurized Vessels
Feed
Permeate
Seal
Concentrate
Permeate collection tube Permeate tube coupling
Pressure vessel
Feed
Spiral Wound RO ElementPressure Vessel Assembly
Feed
Permeate
InterconnectorO-rings Brine Seal
ConcentrateHead End Adapter R.O. ElementPressure Vessel
Retaining Ring
Head Seal Thrust Cone
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RO Skid consist of multiple pressure vessels arranged as 1, 2 or 3 staged arrays.
Kwinana Water Reclamation PlantPerth WA
Orange County Water District, CA, USAe
System recovery increases with number of stages. Recovery limited by a variety of factors including the solubility of salts in the
feed, the osmotic pressure and the maximum pressure of the vessels
3 x 2 x 1 concentrate-staged array
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A two-pass, two-staged system
RO racks have similar designRack size varies based on
plant capacity
RO Designs are Generic
plant capacityIncrease no. of pressure
vessels to increase rack capacity
Standard element size of 8 x 40 (20 cm by 100 cm)
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RO System Space RequirementsKey RO Train Dimensions
Vertical spacing450 mm
Horizontal spacing300 mm
Access for membrane replacement
Skid
SubmersiblePump
Lets return to the components of a RO System
Chemical Conditioning
AcidScale Inhibitor
IntakePretreatment RO
ChlorinationStabilization
Storage &Distribution
Cleaning
In-Line CoagulationDirect FiltrationMF/UFCartridge Filtration
BrineScreens
Beach wells
Waste
Waste
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Assessing pretreatment for RO
RO Feed tank lining failed. Tank lining between MF and RO.Cartridge filters would have reduced risk.
Pre-trearment matches the fouling tendency of the water based on the Silt Density Index (SDI)
SDI is calculated number based on filtration of a sample througha 0.45 micron filter pad. Source Water at >
30 i
SDI is calculated by following formula:100 (1 T1/T2)
SDI15 =15
Where T1 is time in seconds to filter initial 500 ml of sample andT2 is time in seconds to filter final 500 ml of sample
A d 15 i t i ll d t b t ti d l i t l 500
30 psi
Pressure Gauge
Pressure Regulator
Ball Valve
Base
TopMillipore Filter Holde
Vent (optional)
And 15 minutes is allowed to pass between timed sample intervals. 500 ml
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Cartridge filters (5-40 micron) Typically used on ground water
SDI feed water 3-5SDI d t < 3
Pre-treatment requirements
SDI product < 3
Media filters (conventional or direct) Some surface waters Some groundwaters (where Iron & aluminium removal required Seawater desalination
SDI Feedwater > 6 Total coliform < 103 cfu/100 ml
Membrane Filtration Wastewater recycling
SDI Feedwater > 6 Total coliform > 103 cfu/100 ml
Comparison of RO Pretreatment on Seawater
Slide Courtesy of Rob Huehmer, CH2M Hill
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Chemical Addition SystemsChemical Addition Systems
3 general scale types
Alkaline or carbonateeg: CaCO3
LSI/SDSI>0 is scale formingNote:
Scale Risk Assessment
Inorganic scale formation in RO
Non Alkaline or sparingly soluble salts
eg: Al(OH)3, CaF2, CaSO4, Mg3(PO4)2
LSI: TDS 10,000 mg/L
Ion Product in concentrate
(IPc)
> F x SolubilityProduct
(Ksp)F = 0.8 (w/o antiscalant)
Silica (polymerised and silicates)
Polymerisation @ 150 200 mg/L
Silicates @ pH > 9 in presence of trivalent metals
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