Membrane DesalinationMembrane DesalinationSystemsSystems

R. Rhodes TrussellR. Shane Trussell

http://www.http://www.trusselltechtrusselltech.com.com

The State of the Science withEmphasis on Needed Research

10/17/05

OutlineOutline

• Introduction• Today’s Successs &

Challenges• Considerations for future

research

IntroductionIntroduction

• Principle processes: RO, NF, EDand Membrane Filtration

• All are basically post-WWIIdevelopments

• First commercial ED in mid-50s• First Commercial RO in mid-60s• First Commercial NF in late-70s• Large-scale membrane Filtration

came into its own around 1990

IntroductionIntroductionPerhaps the most fundamentaldivide between membraneprocesses is in the driving force– In the case of ED the electric field»Charged species move through the

membrane and the water is retained

–With MF, UF, NF, and RO pressure»Water passes through the membrane &

the other constituents are retained

IntroductionIntroduction

Membrane processes fit into theDesalination & Reuse discussion in twoplaces:– As a means of removing particulates– As a means of removing dissolved

species

Characteristic Rejections ofCharacteristic Rejections ofPressure Driven MembranesPressure Driven Membranes

Size

excl

usio

nC

harg

eSt

eric

Eff

ects

Water

Microfiltration

ParticlesSedimentAlgaeProtozoaBacteria

UltrafiltrationSmall colloidsViruses

NanofiltrationDissolved organic matterDivalent ions (Ca2+, Mg2+)

Reverse osmosis Monovalent species (Na+, Cl-)

Dec

reas

ing

pore

size

Incr

easi

ng p

ress

ure

AA brief look at each of thesebrief look at each of these

SuccessesSuccesses

&&

ChallengesChallenges

Membrane Filtration:Membrane Filtration:SuccessesSuccesses

• The most rapidly growing of membranetechnologies

• Will displace granular media filtrationtechnologies during the next decade

• Is also revolutionizing other processes:– Solids separation for activated sludge

(MBR)– Pretreatment for RO (NF and ED?)

Membrane Filtration:Membrane Filtration:ChallengesChallenges

• Inconsistent virus removal• Inconsistent regulation• Fouling inadequately understood

A little aside on MBRA little aside on MBR

Flow Schemes for the MBR and ConventionalFlow Schemes for the MBR and ConventionalActivated Sludge ProcessActivated Sludge Process

BackwashWater

SecondaryClarifier

WASTE

TertiaryTreated

Wastewater

MicrofiltrationConventionalConventional

Aeration Basin

Primary TreatedWastewater

(Equivalent toa 1-3 mm screen) WASTE

MBR

Aeration Basin

TertiaryQuality

Wastewater

•• Benefits:Benefits:•• Excellent effluent qualityExcellent effluent quality•• A much smaller footprintA much smaller footprint•• Easily automatedEasily automated•• SludgeSludge need not settleneed not settle•• Effluent quality is notEffluent quality is not

dependent on operationsdependent on operations•• A MUCH better barrier toA MUCH better barrier to

pathogenspathogens

Why is MBR important toWhy is MBR important toReuse?Reuse?

Return to successes &Return to successes &challengeschallenges

MBR: SuccessesMBR: Successes

• Consistently superior effluent quality• Fouling and foaming are better

understood• Projects at increasingly larger scale• Costs increasingly competitive• Energy costs decreasing• Long-term success demonstrated in

numerous locations

MBR: ChallengesMBR: Challenges

• Our thinking must shift to decentralizedreuse

• Strategies for dealing with peak flowsmust mature

• MBR-specific strategies for operationmust be developed (filter, not settle)

• Membranes must be made to the sameQC standards as drinking watermembranes

RO: SuccessesRO: Successes

• Cost has dropped significantly• Membranes have improved• Energy efficiency has improved

RO: ChallengesRO: Challenges

• Particulate Fouling– The area where we’ve longest felt we

knew something (SDI, MFI, etc,)– Yet:» A high SDI has always guaranteed fouling» But» A low SDI has never guaranteed good

performance

–We need a better understanding ofthe role low levels of particulatesplay in fouling behavior

RO: ChallengesRO: Challenges

• Inorganic Fouling (scaling)– The Good News:»We have good models to predict it»We know how to control CaCO3 and

CaSO4

– The Bad News:» For silica our principle strategy is to

reduce recovery» Scale inhibitors are available, but their

selection and use is more of an art than ascience

RO: ChallengesRO: Challenges

• Organic Fouling– The least-understood area of fouling

• Biological Fouling– Pretreatment is helpful, but– The key will be the use of

disinfectants

RO: ChallengesRO: Challenges

Water Quality and Corrosion• We are stuck with the Langelier Index• The LSI is an idea from nearly 100 years

ago that the way to prevent corrosionis keep CaCO3 supersaturated

• This strategy cannot be reconciled withUS communities who successfully usenaturally soft water: San Francisco,Oakland, Portland, Tacoma, Seattle,New York, and Boston

• We are wasting a lot of money“hardening” good quality water

RO: Challenges (reuse)RO: Challenges (reuse)• Removing Microbiological Contaminants– Little doubt that the capability is there, but– SD work showed can’t just rely on MW cut-ff– Uniform standards, specifications and

regulations are required

• Emerging Organics– New compounds will continue to show up– RO will not remove them all– It is important that we understand those that

RO does not remove and» Either establish there is no risk, or» Find other ways to remove them

RO: ChallengesRO: Challenges(seawater)(seawater)

Pretreatment– It’s unlikely that one universal treatment

will work in every application– The appropriate pretreatment depends on

the site– In some cases we’ve been lucky– Others have been less fortunate– We need to invest more in pretreatment

studies before we build projects– With an emerging technology, large-scale

failure sets back the entire industry

RO: ChallengesRO: Challenges(seawater)(seawater)

Boron• Essential for plants, perhaps for humans• Low levels have toxic effects on both• What we know about plants– Information is old– Is limited to agricultural crops– Reported toxic levels range from 0.3 to 2 mg/L– Little information on horticulture (Grandma’s roses)

• Boron is difficult to remove– Doesn’t ionize– Current technologies all increase cost

• Need better information on safe levels• Need better removal technologies

Considerations forConsiderations forfurther Researchfurther Research

Future ResearchFuture Research

Recommendation 1 - Improve FeedwaterRecovery

1. Understand scale inhibitors2. Study silica scale control (esp.)3. Study chemical pretreatment &

sidestream interventions4. Optimize element design

Recommendation 2 - Improve ourunderstanding of pretreatment forSWRO

Future ResearchFuture ResearchRecommendation 3 - Water Quality and Horticulture

1. Desktop study and field studies on commonplants

Recommendation 4 - Biofilm Control/Microbiofouling1. Understanding it better

– Reuse, seawater, brackish water– Which organisms?– Important parameters, TOC, Temp, Stagnation,

micronutrients, etc.– Effectiveness of pretreatment

2. Use of chloramines– Long-term exposure tests (>20,000 hrs)– Better documentation on the redox chemistry of

chloramines– Seawater/bromamines

Future ResearchFuture Research

• Recommendation 5 - Study OrganicFouling– We should continue to fund strong studies

in this area. We don’t understand it

• Recommendation 6 - Removal ofMicrobials– Here we have the understanding– We need standard specifications,

confirmation tests, & a design Manual

Future ResearchFuture Research

• Recommendation 7 - MBR– A huge new area– We should keep an active research

program in this area, funding only thebest proposals

• Recommendation 8 - Documentingthe rejection of emergingcontaminants– They will keep coming– We will need to maintain our database

Future ResearchFuture Research

• Recommendation 9 - Boron control– SWRO - support studies to demo

approaches– Reuse - scoping study

• Recommendation 10 - EnergyRecovery– Studies/demonstrations of new

techniques

Future ResearchFuture Research

• Recommendation 11 - Corrosion– Desktop study of natural low TDS

supplies– Testing of alternate strategies (besides

Langelier)

• Recommendation 12 - Pretreatmentfor EDR & Reuse– Strategies to prevent anion membrane

fouling– Research to understand the same

FinisFinis