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Home , Filtration, Water filter

A NATIONAL DRINKING CLEARINGHOUSE FACT SHEET

Summary Filtration is the process of removing suspended from water by passing the water through a permeable fabric or porous bed of materials. Groundwater is naturally filtered as it flows through porous layers of soil. However, surface water and groundwater under the influence of surface water is subject to contamination from many sources. Some contaminants pose a threat to health, and filtration is one of the oldest and simplest methods of removing them. Federal and state laws require many water systems to filter their water. Filtration methods include slow and rapid sand filtration, diatoma- ceous earth filtration, direct filtration, packaged filtration, filtration, and cartridge filtration.

Filtration Keeps Water Safe

Why filter ? ○○○○ devices, such as baffles, and static mixers can Natural filtration removes most suspended be used to mix the water and distribute the chemicals evenly. matter from groundwater as the water passes ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ through porous layers of soil into aquifers. : The chemically treated water is Surface water, however, may be subject to sent into a basin where the suspended particles direct , human, and industrial contami- can collide and form heavier particles called nation that can cause disease or illness in floc. Gentle agitation and appropriate detention , so they must be filtered by a con- times facilitate this process. structed treatment system. : The velocity of water is de- creased so that suspended material (including What regulations govern filtration? flocculated particles) can settle out of the water The Surface Rule under the stream by gravity. Once settled, the particles 1986 Safe Drinking Water Act (SDWA) Amend- combine to form a that is later removed ments requires that many surface water supply from the clarified supernatant (the systems and groundwater under the influence removed from settled sludge) water. of surface their water supplies. How is filtration achieved? What processes precede filtration? Filtration is usually a combination of physical Conventional filtration processes are normally and chemical processes. Mechanical straining preceded by coagulation, flocculation, and removes some particles by trapping them sedimentation. Direct filtration processes are between the grains of the filter medium (such as preceded by coagulation and flocculation only; sand). Adhesion is an equally important process the floc is removed directly by the filters. by which suspended particles stick to the surface of filter grains or previously deposited Filtration processes may include one or all of material. Biological processes are also impor-

the following pretreatment procedures: tant in slow sand filters. These filters form a PAGE OF FOUR filter skin containing microorganisms that trap Chemical feed and rapid mix: Chemicals may be and break down algae, , and other added to the water to improve the treatment one organic matter before the water reaches the processes that occur later. These may include filter medium itself. pH adjusters and coagulants. A variety of

NATIONAL DRINKING WATER CLEARINGHOUSE Comparing Filtration Systems: ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ Slow Sand Filtration Direct Filtration The filter consists of a bed of fine Filtration Direct filtration systems are similar to sand approximately 3 to 4 feet deep Diatomaceous earth filtration, also conventional systems, but omit supported by a 1-foot layer of gravel known as precoat or diatomite sedimentation. and an underdrain system. filtration, relies on a layer of diatoma- ceous earth approximately 1/8-inch ADVANTAGES ADVANTAGES thick placed on a septum or filter Effective direct filtration performance Low cost, simple operation, reliable, element. Septums may be placed in ranges from 90 to 99 percent for virus and able to achieve greater than 99.9 pressure vessels or operated under a removal and from 10 to 99.99 percent percent Giardia cyst removal. It also vacuum in open vessels. for Giardia removal. The most does not require extensive active effective direct filtration configurations control by an operator. ADVANTAGES for Giardia removal must include The filters are simple to operate and coagulation. Often used with steel LIMITATIONS effective in removing cysts, algae, pressure vessels to maintain the It is not suitable for water with high and asbestos. They have been pressure in a water line to avoid . Filter surface requires chosen for projects with limited initial repumping after filtration. maintenance. Extensive land is capital, and for emergency or standby required due to low-flow operation capacity to service large seasonal LIMITATIONS (0.03 to 0.10 gallons per minute increases in demand. Direct filtration is only applicable for per square foot [gal/min/ft2] of filter systems with high quality and LIMITATIONS bed area). seasonally consistent influent This filter is most suitable for water supplies. The influent generally PROCESS with low bacterial counts and low should have turbidity of less than 5 to turbidity (less than 10 nephelometric Filters are operated under continu- 10 NTU and color of less than 20 to turbidity units [NTU]). Coagulant and ous, submerged conditions main- 30 units. (Water with 15 or more units filter aids are required for effective tained by adjusting a control valve of color causes aesthetic problems, virus removal. There is potential located on the discharge line from such as staining.) difficulty in maintaining complete and the underdrain system. Biological uniform thickness of diatomaceous processes and chemical/physical PROCESS earth on the filter septum. processes common to various types Direct filtration consists of several of filters occur at the surface of the combinations of treatment processes. PROCESS filter bed. A biological slime or mat It always includes coagulation and referred to as “” forms Operation and maintenance of filtration, and may require a floccula- on the surface of the bed, which traps diatomaceous earth filters require: tion tank or a pressure vessel after small particles and degrades organic preparing slurries of filter body feed the coagulation addition. material present in the raw water. (diatomaceous earth) and precoat Slow sand filters do not require diatomaceous earth; adjusting body EQUIPMENT coagulation/flocculation and may feed dosages for effective turbidity not require sedimentation. removal; periodic backwashing— Dual- and mixed-media filters are every 1 to 4 days; disposing of spent used to effectively process higher ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ influent without the use of EQUIPMENT ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ filter cake; periodically inspecting the filter septum for cleanliness and sedimentation. Small plants are typically designed damage; and verifying effluent quality. with cast-in-place concrete structures CHEMICALS with wood or concrete slab covers. EQUIPMENT Typical coagulant dosages range Piping is either cast iron or polyvinyl from less than 1 to 30 milligrams per chloride (PVC). Flow meters are used The minimum amount of filter precoat liter. Cationic often success- to monitor the output for each filter. In should be 0.2 pounds per square foot 2 fully coagulate water supplies and climates subject to freezing tempera- (lb/ft ); and minimum thickness of the assist direct filtration. Nonionic tures, filters usually must be housed, precoat should be increased from 1/8 polymers are sometimes added to the and may require heating, lighting, and to 1/5 inch to enhance Giardia cyst filtration step to increase filter ventilation. Unhoused filters in cold removal. In addition, minimum design efficiency. climates develop an ice layer that pre- criteria outlined in the Recommended vents cleaning during winter months. Standards for Water Works (better known as 10 State Standards) should be met. CHEMICALS two Water applied to slow sand filters is CHEMICALS not prechlorinated because the Use coagulant to coat the body feed

PAGE OF FOUR PAGE chlorine can destroy organisms in the

PAGE OF FOUR PAGE schmutzdecke. to improve removal rates for viruses, bacteria, and turbidity.

TWO • SEPTEMBER 1996 ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ ○○○○○○○○○○○○○○○○○○○○○○○○○

Packaged Filtration Membrane Filtration Cartridge Filtration Packaged filtration is simply all of A membrane is a thin layer of material Cartridge filters are considered an the features of filtration—chemical capable of separating substances emerging suitable for addition, flocculation, sedimentation, when a driving force is applied across removing microbes and turbidity in filtration—mounted as a unit on a the membrane. small systems. frame for simple hookup of pipes and services. It is most widely used to ADVANTAGES ADVANTAGES treat surface water supplies for Membrane filtration can be an Cartridge filters are easy to operate removal of turbidity, color, and attractive option for small systems and maintain, making them suitable coliform organisms with filtration because of its small size and for small systems with low-turbidity processes. Packaged filtration is automated operation. Membrane influent. often used to treat small community processes are increasingly employed water supplies, as as supplies in for removal of bacteria and other recreational areas, state parks, LIMITATIONS microorganisms, particulate material, construction sites, ski areas, and Cartridge filtration systems require and natural organic material, which military installations, among others. raw water with low turbidity. Polypro- can impart color, tastes, and odors to pylene cartridges become fouled ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ water. ADVANTAGES relatively quickly and must be replaced with new units. Although The four major advantages of LIMITATIONS these filter systems are operationally package plants are their compact of the is the simple, they are not automated and size, cost effectiveness, relative ease major problem preventing widespread can require relatively large operating of operation, and design for unat- application of this technology. budgets. tended operation. (Some states require that an operator be in attendance at all times. Check your PROCESS PROCESS state regulations.) Membrane filtration works by passing Cartridge filtration uses a physical water at high pressure through a thin process—straining water through LIMITATIONS membrane in the form of hollow fiber porous media. It can exclude particles or spiral-wound composite sheets. 0.2 micrometers (µm) or smaller. The When the turbidity of the raw water Organic and other contaminants are pore sizes that are suitable for varies a great deal, these package retained on the high-pressure side producing potable water range from plants require a high level of operator and frequently must be removed by 0.2 to 1.0 µm. Roughing filters, for attention and skill. reversing the flow and flushing the pretreatment prior to cartridge waste. Periodic chemical cleaning filtration, are sometimes necessary to PROCESS may be required to remove persistent remove larger suspended solids and Package plants are most appropriate contaminants. Membrane assemblies prevent the rapid fouling of the for plant sizes ranging from 0.025 to are contained in pressure vessels or cartridges. Roughing filters can be 6 million gallons per day. The most cartridges. rapid sand filters, multimedia filters, important factor to consider in fine mesh screens, or bag filters. selecting a package plant is the EQUIPMENT influent characteristics, such as The membrane are EQUIPMENT , turbidity, and color relatively simple to install and for A cartridge consists of ceramic or levels. Pilot tests might be necessary groundwater sources that do not polypropylene filter elements fitted before a final system can be selected. need pretreatment, the systems ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ into pressurized housings. require little more than a feed , EQUIPMENT a cleaning pump, the membrane CHEMICALS Package plants are assembled in a modules, and some holding tanks. A disinfectant is recommended to factory, skid mounted, and trans- prevent surface-fouling microbial ported to the treatment site, or they CHEMICALS growth on the cartridge filters and to are transported as component units Periodic backflushing and occasional reduce microbial pass-through. to the site and then assembled. chemical cleaning is necessary to Except for a disinfectant, no chemical maintain the membrane or fibers. additions are necessary. However, CHEMICALS corrosive chemicals may be required Chemical feed controls are especially for the periodic membrane cleaning important for plants without full-time process.

PAGE OF FOUR operators or with variable influent PAGE OF FOUR characteristics. Even with these automated devices, however, the three operator needs to be properly trained and well acquainted with the process and control system.

NATIONAL DRINKING WATER CLEARINGHOUSE How does one select the appropriate ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ Cartridge Filter filtration system? With replaceable inner package First, review all raw data to outlet establish the requirements for the alternatives. end seal Once the potential alternatives are selected, determine the necessity of pilot or bench-scale tests. If the desired performance of one or more pleated of the alternatives is in doubt, testing is appro- membrane priate. (Testing is always useful if time and budget allow).

radial Otherwise, literature surveys, bench-scale raw water studies, or pilot-test results can be used to flow derive each alternative’s performance charac- teristics and design considerations. Following cartridge element this initial selection, the basic process concerns for the various alternatives should be identified housing (opaque) and evaluated, including: • turbidity removal performance, If these publications are no longer available from • Giardia removal performance, the EPA, call the National Drinking Water Clear- inghouse (NDWC). Environmental Pollution • color removal performance, Control Alternatives: Drinking Water Treatment • cleaning cycle frequency, for Small Communities, item #DWBKGN09, an • necessary chemicals/chemical dosages, 82-page publication, costs $11.82. Technologies • applicable regulatory standards, for Upgrading Existing or Designing New - ing Water Treatment Facilities, item • required operational skills, and #DWBKDM04, a 209-page book, costs $30.05. • necessary sludge management. Shipping and handling charges apply.

Where can I find more information? Also, the NDWC’s Registry of Equipment Suppli- Information in this fact sheet was primarily ers of Treatment Technologies for Small Systems obtained from two sources: Environmental (RESULTS) is a public reference database that Pollution Control Alternatives: Drinking Water contains information about technologies in use at Treatment for Small Communities, EPA/625/5- small water systems around the country. For 90/025; and Technologies for Upgrading Exist- further information on accessing or ordering ing or Designing New Drinking Water Treatment RESULTS, call the NDWC at (800) 624-8301. Facilities, EPA/625/4-89/023. Both can be For additional copies of “Tech Brief: Filtration,” ordered free from the U.S. Environmental Protec- item #DWBRPE5O, or a copy of “Tech Brief: tion Agency Office of Research and Development Disinfection,” item #DWBRPE47, call NDWC at at (513) 569-7562. the number above. Slow Filter Effluent flow Clearwell Inlet control structure Raw water baffle source

Manhole Valve

Raw water Ground level

Filtered water Sand filter bed To drain four

Support gravel

PAGE OF FOUR PAGE Telescoping valve Under drain To supply

TWO • SEPTEMBER 1996