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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