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Americas Authority in Membrane Treatment

Improving Americas Waters Through Membrane Treatment and Desalting

Water utilities nationwide are turning todvanced filtration to meet more

tringent federal drinking wateregulations in order to removeurbidity, precursors, and disinfectantolerant micro-organisms from both

roundwater and surface water supplies.

Low pressure microfiltration (MF) andltrafiltration (UF) membrane filtration

echnology have emerged as viableptions for addressing the current anduture drinking water regulations

elated to the treatment of surfacewater, groundwater under thenfluence, and water reuse applicationsor microbial and turbidity removal.

Full-scale facilities have demonstratedhe efficient performance of both MFnd UF as feasible treatment

lternatives to conventional granularmedia processes. Both MF and UFave been shown to exceed the removalfficiencies identified in the Surface

Water Treatment Rule and related rules,uch as Cryptosporidiumoocyst,Giardia

yst, and turbidity.

MF and UF membrane systemsenerally use hollow fibers that can beperated in the outside-in or inside-out

irection of flow. Pressure (5 to 35 psi)r vacuum (-3 to -12 psi for outside-inmembranes only) can be used as theriving force across the membrane.

Typical flux (rate of finished waterermeate per unit membrane surfacerea) at 20 degrees C for MF and UFanges between 50 and 100 gallons per

quare foot per day (gfd).

ince both processes have relativelymall membrane pore sizes, membrane

ouling, caused by the deposition of

organic and inorganic compounds onthe membrane, may occur at unaccept-able levels if the system is not properly

selected, designed, and operated.Automatedperiodic backwashing andchemical washing processes are used tomaintain the rate of membrane fouling

within acceptable limits. Chemicalcleaning is employed once a maximumtransmembrane pressure differential hasbeen reached. Some systems utilize air/

liquid backwash. Typical cleaning agentsutilized include acids, caustic, surfactants,enzymes, and certain oxidants,

depending upon membrane materialand foulants encountered. Chemicalsused for cleaning, and the method usedin the cleaning process, must be accept-

able to the membrane manufacturer.

Overall treatment requirements anddisinfection credits must be discussed

with and approved by the reviewingauthority. Disinfection is recommendedafter membrane filtration as a secondarypathogen control barrier and distribution

system protection.

MF and UF membranes are mostcommonly made from various orga

polymers such as different cellulosederivatives, polysulfones, polypropyland polyvinylidene fluoride (PVDFPhysical configurations include holl

fiber, spiral wound, cartridge, andtubular. MF membranes are capab

of removing particles with sizes doto 0.1- 0.2 microns. Some UF

processes have a lower cutoff ratinof 0.005-0.01 microns. Pressure orvacuum may be used as the driving

force to transport water across themembrane surface.

Membrane filtration is also becomi

popular for conventional plant retroreplacing sand media, for enhancedwater quality and capacity increase.

Membrane Filtration (MF/UF)

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When Selecting MF/UF Systems, the

Following Should be Considered:

1. A review of historical sourceraw water quality and variability

data, including turbidity, algae,particle counts, seasonalchanges, organic contents,microbial activity, andtemperature as well as otherinorganic and physical parametersis critical to determine theoverall cost of the system.The degree of pretreatment, ifany, should also be ascertained.Design considerations andmembrane selection at this

phase must also address theissue of target removal efficienciesand system recovery versusacceptable membrane foulingrate. At a minimum on surfacewater supplies, pre-screening isrequired.

2. The life expectancy of aparticular membrane underconsideration should beevaluated (typically 7-10 years).Membrane replacement

frequency is a significant factorin operation and maintenancecost comparisons in theselection of the process.Warranties offered by manufac-turers vary significantly andshould be considered closely.

3. Somemembrane materials areincompatible with certainoxidants such as chlorine. Ifthe system must rely on

pretreatment oxidants for otherpurposes, for example, zebramussel control, taste and odorcontrol, or iron and manganeseoxidation, the selection of themembrane material becomes asignificant design consideration.

4. The source water temperaturecan significantly impact the fluxof the membrane underconsideration. At low watertemperatures, the flux can be

reduced appreciably (due to

higher water viscosity andresistance of membrane topermeate), possibly impactingprocess economics by thenumber of membrane unitsrequired for a full-scale facility.System capacity must beselected for the expecteddemand under seasonal (coldand warm water temperature)conditions.

5. Backwashing waste volumes canrange from 4 to 15 percent ofthe permeate flow, dependingupon the source water quality,membrane flux, frequency of

backwashing, and the type ofpotential fouling.

6. Membrane systems used fordrinking water productionshould be provided with anappropriate level of finishedwater monitoring and a directintegrity test feature. Monitoringoptions may include laserturbidimeters, particle counters,and manual and/or automatedintegrity testing using pressure

decay or air diffusion tests. TheUSEPA has recently published amembrane filteration guidancemanual (EPA 815-R-06-009).

7. Cross-connection controlconsiderations must beincorporated into the systemdesign, particularly with regardto the introduction and dis-charge of chemicals and wastepiping. Membrane systems thatuse chemical washing processeswith harsh chemicals requireadditional consideration.

8. Redundancy of criticalcomponents and controlfeatures should be consideredin the final design.

9. Other post-membranetreatment requirements such ascorrosion control and secondarydisinfection must be evaluatedin the final design.

10. Other contaminants of consuch as color and disinfectioby-product precursors shoualso be addressed.

11. Prior to initiating the designan MF or UF treatment facthe state reviewing authorityshould be contacted to detemine the disinfection creditsavailable for the membraneprocess, and whether a pilotplant study will be required.most cases a pilot plant studwill be necessary to determithe best membrane to use,particulate/organism remov

efficiencies, cold and warmwater flux, the need for pretreatment, fouling potential,operating and transmembranpressure, and other designconsiderations. The statereviewing authority should bcontacted prior to conductinthe pilot study to establish thprotocol to be followed.

This material has been prepared as an

educational tool by the American MembTechnology Association (AMTA). It is

designed for dissemination to the public

further the understanding of the contri

tion that membrane water treatment tech

nologies can make toward improving the

quality of water supplies in the US and

throughout the world.

For more information, please contact:

American Membrane TechnologyAssociation (AMTA)

2409 SE Dixie HighwayStuart, Florida 34996Phone: (772) 463-0820

Fax: (772) 463-0860Email: admin@amtaorg.com

or visit our website at:

www.amtaorg.com