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Water Filtration

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Water Filtration A NATIONAL DRINKING WATER CLEARINGHOUSE FACT SHEET Filtration Summary Filtration is the process of removing suspended solids 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 human 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, membrane filtration, and cartridge filtration. Filtration Keeps Water Safe Why filter drinking water? ○○○○ 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. Flocculation: The chemically treated water is Surface water, however, may be subject to sent into a basin where the suspended particles direct animal, 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 humans, so they must be filtered by a con- times facilitate this process. structed treatment system. Sedimentation: 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 Water Treatment Rule under the stream by gravity. Once settled, the particles 1986 Safe Drinking Water Act (SDWA) Amend- combine to form a sludge that is later removed ments requires that many surface water supply from the clarified supernatant (the liquid systems and groundwater under the influence removed from settled sludge) water. of surface water filter 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 OF FOUR PAGE filter skin containing microorganisms that trap Chemical feed and rapid mix: Chemicals may be and break down algae, bacteria, 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 Diatomaceous Earth 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 turbidity. 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 “schmutzdecke” 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 turbidities 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 polymers 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 technology 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 well as
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