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A NATIONAL DRINKING WATER CLEARINGHOUSE FACT SHEET 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.

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Transcription of A NATIONAL DRINKING WATER CLEARINGHOUSE FACT SHEET Filtration

1 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.

2 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.

3 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.)

4 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 . combine to form a sludge that is later removed . 1986 Safe DRINKING WATER Act (SDWA) Amend- . ments requires that many surface WATER supply from the clarified supernatant (the liquid . removed from settled sludge) WATER .. systems and groundwater under the influence . of surface WATER filter their WATER supplies.. How is Filtration achieved?

5 What processes precede Filtration ? Filtration is usually a combination of physical . and chemical processes. Mechanical straining . Conventional Filtration processes are normally . preceded by coagulation, flocculation, and removes some particles by trapping them . between the grains of the filter medium (such as . sedimentation. Direct Filtration processes are . preceded by coagulation and flocculation only; sand). Adhesion is an equally important process . by which suspended particles stick to the . the floc is removed directly by the filters.

6 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.. filter skin containing microorganisms that trap . Chemical feed and rapid mix: Chemicals may be . and break down algae, bacteria, and other one . added to the WATER to improve the treatment OF FOUR. organic matter before the WATER reaches the . processes that occur later. These may include.

7 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 sedimentation.. known as precoat or diatomite .. and an underdrain system. Filtration , relies on a layer of diatoma.

8 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 pressure vessels or operated under a .. and able to achieve greater than removal and from 10 to 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.

9 Control by an operator. ADVANTAGES for Giardia removal must include .. coagulation. Often used with steel .. The filters are simple to operate and .. LIMITATIONS . effective in removing cysts, algae, pressure vessels to maintain the . and asbestos. They have been pressure in a WATER line to avoid .. It is not suitable for WATER with high .. turbidity. Filter surface requires chosen for projects with limited initial repumping after Filtration .. maintenance. Extensive land is capital, and for emergency or standby.

10 Capacity to service large seasonal .. required due to low-flow operation LIMITATIONS.. ( to gallons per minute increases in demand. Direct Filtration is only applicable for .. per square foot [gal/min/ft2] of filter .. LIMITATIONS. systems with high quality and .. bed area). seasonally consistent influent .. This filter is most suitable for WATER supplies. The influent generally .. with low bacterial counts and low .. PROCESS should have turbidity of less than 5 to .. turbidity (less than 10 nephelometric 10 NTU and color of less than 20 to Filters are operated under continu.)


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