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NATIONAL DRINKING WATER CLEARINGHOUSE

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A NATIONAL DRINKING WATER CLEARINGHOUSE FACT SHEET

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SummaryDiatomaceous Earth (DE) filtration is a process that uses diatoms or diatomaceous earth—the skeletal remains of small, single-celled organisms—as the filter media. DE filtration relies upon a layer of diatomaceous earth placed on a filter element or septum and is frequently referred to as pre-coat filtration. DE filters are simple to operate and are effective in removing cysts, algae, and asbestos from water. DE has been employed in many food and beverage applications formore than 70 years and was used specifically to filter potable water during WWII. Since then, ithas been used to produce high-quality, low-cost drinking water. DE filtration is currently one ofthe U.S. Environmental Protection Agency’s (EPA) approved technologies for meeting the require-ments of the Surface Water Treatment Rule (SWTR) and is most suitable for small communitiesthat need to comply with the rule.

What is DE filtration?DE contains fossil-like skeletons of microscopicwater plants called diatoms, which are a typeof algae. These diatoms range in size from lessthan 5 micrometers to more than 100 microm-eters, and have a unique capability of extract-ing silica from water to produce their skeletalstructure. When diatoms die, their skeletonsform a diatomite deposit. In its natural state,diatomite is 85 percent inert silica. The solubleportion of diatomite is extremely low (less than1 percent). The odorless, tasteless, and chemi-cally inert characteristics make DE safe for filtering water or other liquids intended forhuman consumption.

Application and Historical BackgroundDuring WWII, the U.S. Army needed a new typeof water filter suitable for rapid, mobile militaryoperations. The U.S. Army Engineer Researchand Development Laboratories (ERDL) developeda DE filter unit that was lightweight, easily trans-ported, and able to produce pure drinking water.Later, DE filtration technology was applied to fil-tering swimming pool water and more graduallyto producing drinking water.

The earliest municipal DE filter installationwas a 75,000 gallons per day (gpd) system inCampbell Hills, Illinois, that began operating in1948. By 1977, municipalities had constructedmore than 145 plants. Today, nearly 200 DEplants are successfully operating.

How does DE filtration work?DE filtration strains particulate matter fromwater, and the process rarely uses coagulantchemicals. First, a cake of DE is placed on fil-ter leaves. A thin protective layer of diatoma-ceous earth builds up, or accumulates, on aporous filter septum (a permeable cover overinterior collection channels) or membrane.Recirculating DE slurry through the filter septum establishes this layer. The septum ismost often plastic or metallic cloth mounted on a wire mesh-covered steel frame. The DEprocess is also called pre-coat filtration becausethe solids separation at the start of a run takesplace on the built-up pre-coat layer of DE.

After the pre-coat forms on the filter leaves(usually 1/8 inch thick) raw water containing alow dose of DE, which is called body feed, isfed through the filter. Particulate solids in the

Diatomaceous Earth Filtrationfor Drinking Water

by Vipin Bhardwaj,Technical Assistance Specialist, and Mel J. Mirliss, Director, InternationalDiatomite Producers Association

product flow are separated on the pre-coat sur-face. With such separation, the unwanted partic-ulate matter actually becomes part of the filtermedia. During a filter run, removing particulatematter from raw water causes head loss to grad-ually build up in the filter. The accumulation ofDE body feed on the filter reduces the rate ofhead loss. When maximum head loss is reached,the flow of water into the filter is stopped and thefilter cake is cleaned.

High-pressure sprays, directed at the accumulat-ed cake, detach the cake and provide dilution fordraining the slurry suspension from the filter ves-sel. When cleaned, the filtration operation isrepeated, beginning with the pre-coat cycle. (See Figure 1.) Operators typically discard the DE removed from the filter leaves.

Appropriate Feed Water Quality andPerformance CapabilitiesThe use of DE filters is limited to treating sourcewaters with an upper limit of turbidity at 10 NTU.Also, filtration rates range from 0.5 to 2 gallonsper minute per square foot (gpm/ft2). The particlesize that DE filtration removes relies upon thesize distribution of the DE particles used for thepre-coat and body feed. DE filters are very effec-tive for removing Giardia and Cryptosporidiumcysts. In some cases, studies have reported up toa 6-log reduction of these cysts under routineoperating conditions. Because DE filtration usu-ally does not involve coagulation, its potential forremoving dissolved constituents, such as color, islow. Therefore, the utility or its engineer mustdetermine raw water quality before consideringDE filteration.

What are the monitoring and operatingrequirements? Monitoring requirements for DE filtration aresimpler than requirements for coagulation andfiltration because operators rarely ever usecoagulant chemicals for DE filtration. However,operators must continuously monitor raw andfiltered water turbidity. Operators also mustmonitor filter head loss so that they can determine when to backwash the filter.

In general, DE filter plant operators needmechanical skills to operate the body feedpumps, pre-coat pumps, mixers, pipes, and

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Tech Brief • Diatomaceous Earth Filtration for Drinking Water • Summer 2001

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Figure 1 Diatomaceous Earth Filter

Figure 2 Elements of a Flat Leaf Filter

Direction of Flow

Diatomaceous Earth Pre-coat DE Filter in Operation

Pre-coatMediaLayer

Pre-coatLiquid Septum/Filter

Media

Liquid to be Filtered

Septum/FilterMedia

FilteredLiquid Cake of

RemovedImpuritiesand DEParticles

Septum/FilterMedia

BackwashWater

BackwashHeader

FilterLeaf

OutletManifold

BackwashDrain

ContainmentVessel

InletManifold

BaffledInlet

Cake ofRemovedImpurities andDE Particles

Direction of Flow Direction of Flow

Source: Fulton, George P. 2000. Diatomaceous Earth Filtration for Safe Drinking Water

DE Filter in Backwashing Mode


The two basic groupings of DE filter designs areessentially defined by the hydraulic mode ofoperation, and are shown in Figure 3-A andFigure 3-B.

The principal advantages of pressure filters overthe vacuum filters are related to the significantlyhigher differential head available.

Is DE suitable for smallsystems?DE filtration is well-suitedto small systems, because it does not require chemicalcoagulation, so operators donot need to learn about thiscomplex aspect of watertreatment. In addition,installation costs for DEsystems are less than thosefor other technologies, suchas membranes. DE filtrationis currently one of theEPA’s approved technolo-gies for meeting SWTRrequirements.

valves. They also must be skilled inpreparing the body feed and pre-coat slurries. Also, keeping DE filterleaves clean is of primary impor-tance. A leaf filter that is not prop-erly cleaned at the end of a filterrun can accumulate dirt and slimeon the filter cloth, which prevents auniform pre-coat from forming whenthe filter is restored to service.

Elements of a DE filterFigure 2 shows the common elements in the manufacture of anyflat leaf filter used in treating drink-ing water. The principal elementsof a DE filter include the following:

• containment vessel,• baffled inlet,• filter leaves mounted on an

effluent manifold,• a method of cleaning the filter

leaves at the end of a run,• a drain to receive the back-

wash water,• open top or access mode, and• DE slurry preparation tank

and pump feed.

Types of DE filtrationTwo types of DE filters exist: (1) pressure filters, which have a pump or high-pressurewater source on the influent side and (2)vacuum filters, which have a pump on theeffluent side. Vacuum filters are open to theatmosphere. Pressure filters are enclosedwithin pressure vessels.

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Figure 3-A Pressure Filter

Figure 3-B Vaccuum Filter

Pressure Filters

• Operates at higher flowrates, result-ing in smaller, more compact filterunits.

• Longer filter runs, reducing the useof pre-coat material and backwashwater because of less frequentcleaning cycles.

• Less likelihood that gas bubbles willdisrupt the media.

Vacuum Filters

• Lower capital fabrication cost.

• Lower maintenance costs.

• Tanks are open at the top, making access and observation easy.

Source: Mel J. Mirliss, Vipin Bhardwaj, and the National Drinking Water Clearinghouse

Table 1 Pressure and Vacuum Filters

DischargePressure to System

Filtrate

PumpSuction

PumpDischargePressure

DischargePressure to System

Filtrate

Pressure Difference

Pressure Difference


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Produced by the National Drinking Water Clearinghouse at West Virginia University, P.O. Box 6064, Morgantown, WV 26506-6064

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An ideal, cost-effective DE filtration applicationis for well water supplies under the influenceof surface waters, but that are otherwiseacceptable in quality. Superior cyst removalcapability makes the DE filter more advanta-geous than other alternatives. However, thereis a potential difficulty in maintaining completeand uniform thickness of DE on the filter septum.

Where can I find more information?• Fulton, George P., P.E. 2000. Diatomaceous

Earth Filtration for Safe Drinking Water,American Society of Civil Engineers,Reston, VA.

• “Precoat Filtration.” 1988. AWWA M30,Manual of Water Supply Practices.American Water Works Association,Denver, CO.

• Technologies for Upgrading or Designing NewDrinking Water Treatment Facilities,EPA/625/4-89/023. (Available from theEPA) or contact the National Drinking WaterClearinghouse.

Have you read all our Tech Briefs?

Tech Briefs drinking water treatment factsheets have been a regular feature in theNational Drinking Water Clearinghouse(NDWC) newsletter On Tap for more thanfive years.

A package of Tech Briefs is now available asa product. A three-ring binder holds all thecurrent Tech Briefs in print. New selectionscan be easily added to the package as theybecome available. To order this free product,call the NDWC at the numbers listed belowand ask for item #DWPKPE71.

Additional copies of fact sheets are also free;however, postal charges may be added.

To order, call the NDWC at (800) 624-8301or (304) 293-4191. You also may order onlineat [email protected] or down-load fact sheets from our Web site atwww.ndwc.wvu.edu.

Also, the NDWC’s Registry of EquipmentSuppliers of Treatment Technologies for SmallSystems (RESULTS) is a public reference data-base that contains information about technolo-gies used by small waters systems around thecountry. For further information about accessingor ordering RESULTS, call NDWC.

References:Fulton, George P. 2000. Diatomaceous Earth Filtration for Safe

Drinking Water, American Society of Civil Engineers, Reston, VA.

Safe Water from Every Tap, 1997. “Improving water service to smallcommunities,” National Research Council, National AcademyPress, Washington, D.C.

Schuler, P.F., M.M. Gosh, and S.N. Boutros, 1988. “Comparing theremoval of Giardia and Cryptosporidium using slow sand anddiatomaceous earth filtration,” pp. 789–805, Proceedings of 1988AWWA Conference, Denver, CO.

Ongerth, J.E., Hutton, PE, December 1997. “DE filtration to removeCryptosporidium,” Journal of American Water Works Association.American Water Works Association, Denver, CO.

Small System Compliance Technology List for the Surface WaterTreatment Rule and Total Coliform Rule, 1998. U.S.Environmental Protection Agency, 815-R-98-001.

Vipin Bhardwaj, technical assistancespecialist, has a B.S. in chemical engi-neering and has recently finished hismaster’s degree in agriculture at WestVirginia University. He expects to receivea master’s degree in civil engineeringthis fall.

Mel Mirliss has been associated with thediatomaceous earth industry for the past28 years. For the past nine years, he hasserved as the executive director of theInternational Diatomite ProducersAssociation (IDPA) where he has been

a director since the association's founding in 1987. Heholds a B.S. in chemistry from the University ofCalifornia, Berkeley.

National Drinking Water ClearinghouseWest Virginia UniversityP.O. Box 6064Morgantown, WV 26506

www.ndwc.wvu.edu

National Environmental Services Center An Equal Opportunity/Affirmative Action Institution

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