Drinking Water Treatment - Filtration

Drinking Water and Human Health December 06, 2010 Print Friendly and PDF

EFFECTIVE AGAINST: Small particles and suspended solids such as ferric iron, clay, silt and sand, and some pathogens such as bacteria and viruses, and colloids (suspended matter).

NOT EFFECTIVE AGAINST: Any dissolved contaminants such as nitrate, dissolved iron, sodium, and volatile organic compounds (VOCs).

Contents

How drinking water filtration works

Filtration systems are used most often in home water treatment to remove sediment or iron, manganese, or sulfur particles. Filtration can also remove some bacteria from water. In mechanical filtration systems, water passes through a medium such as cloth or sand. Particles become trapped on the surface of or within the medium. The pore size, or space between media granules or fibers, determines what size particles a filter can remove.

Filters are rated according to the smallest particle they can trap. The filter opening size to use depends upon the material to be removed by the filter. A smaller size will satisfy removal requirements but will require more frequent cleaning or replacement of the filter. When these filters are used to pre-treat water for other water treatment devices, such as a reverse osmosis unit, follow the manufacturer’s recommendations.

Surface or screen filters remove the particles at or very near the filter surface. They function very much like a screen; particles of a certain size and larger are retained at the surface while smaller ones move through the openings.

Depth filters have a thick filter medium. Particles are retained throughout the thick filter mat. Depth filters have a gradation in the size of the filter media so that the largest particles are held at or near the filter surface, while progressively smaller particles are captured deeper in the filter where the filter media becomes smaller.

Either filter type may be used for a wide range of particles sizes.

Types of drinking water filters

There are many different types of filters used in drinking water filtration units. They differ in design, cost, and effectiveness. Before purchasing a system, verify that the treatment system you are purchasing has been tested and certified by a third party (for example, National Sanitation Foundation)to ensure manufacturer’s claims. Mechanical filtration systems include cartridge sediment filters, media and multimedia filters, and precoat filters. Which filtration method to select depends on the concentration and size of the suspended solids in the water and the rate at which water needs to be treated. Media filters such as sand filters have a greater contaminant removal capacity than other types of filtration devices. However, cartridge filters with fiber or ceramic filter material are made with a smaller and more uniform pore size and can be more reliable in removing small particles.

Cartridge sediment filters

Cartridge sediment filters remove only suspended solids. Manufacturers rate them according to the size of particles they remove. The filter material is often wound or corrugated to provide the maximum surface area for filtration. Cartridge filters can treat either at point-of-use or point-of-entry.

Cartridge filters are used as pre-filters when particles in the untreated water could reduce the effectiveness or service life of another treatment device. For example, consider a situation where the untreated water is turbid (cloudy with tiny particles of sediment or other material) and contains an organic chemical. If an activated carbon device were used to remove the organic chemical and the suspended solids (tiny particles of soil or other material) were not removed first, the suspended solids would clog the activated carbon unit, thus reducing its service life. Therefore, a cartridge filter or other mechanical filter should precede the activated carbon device to remove the suspended solids.

How cartridge sediment filters work

During filter operation, untreated water flows through the cartridge and suspended solids are retained on the filter material surface. Water flows from the outside of the cartridge to the cartridge core. If the pore size of the filter material is too small, or if the concentration of suspended solids in the untreated water is too high, the filter will become clogged easily and require frequent replacement. If the pore size is too large, suspended solids may pass through the device. As suspended solids accumulate in the filter, they assist in the filtration process and the filters effectiveness increases. Water flow, however, gradually decreases. When water flow is no longer acceptable, the cartridge or the filter material should be replaced.

Types of drinking water filters

The two basic types of cartridge filters are depth-type and pleated. Certain manufacturers offer filters that have graded densities; their openings are large at the outermost surface of the filter and decrease in size toward the center core. For these filters, some particles are trapped within the filter material instead of on the surface. Depth cartridges are usually inexpensive. Pleated cartridges increase the flow rate capacity by providing a greater surface area for filtration. The filter accumulates particles on the outermost surface, allowing a filter cake to build up and thus increasing the filtering action. Water flow usually slows as the filter cake develops. Heavy-duty pleated filters have an increased surface area to further improve flow rate and filtering capacity. Most pleated filters can be rinsed in the sink or with a garden hose and reused several times. They handle large quantities of turbidity before water pressure drops noticeably. Although they initially cost more than other cartridge types, in the long run they are less expensive. Pleated cartridge filters contain a rigid polypropylene core for support and are made of one one the following materials:

  1. Pleated paper - These filters are the most economical but are not reusable. They are sensitive to water with low or high pH (less than 6.5 or greater than 8.5). Use them only when the water contains no active bacteria, which feed on the cellulose portion of the filter.
  2. Pleated cotton/polyester – These filters are generally considered to be the most versatile because they combine the filtration ability of cotton with the strength of polyester. If the water pH is between 4 and 9, they can be cleaned and reused several times.
  3. Pleated polyester and polypropylene – These filters are the most expensive but can be rinsed repeatedly and reused. Since the fibers are rather smooth, they are not as effective in retaining particles as cotton/polyester filters. In fact, unloading can be a problem with these filters.

Capacity of drinking water filters

Base the selection of a particular cartridge filter on the flow rate produced at household water pressure, the amount of water treated before maintenance is required, and the desired water quality. Flow through the filter depends on the water pressure and the characteristics of the filter material and water.

Maintenance of water filtration systems

Regardless of the quality of the equipment purchased, it will not perform satisfactorily unless managed in accordance with the manufacturer’s recommendations for maintenance, cleaning, and part replacement. Keep a log book to record water test results, equipment maintenance, and repairs.

Replace or clean the cartridge when there is a noticeable drop in water flow through the device – usually after several weeks or months of use. Some cartridges are rated according to the number of gallons they can treat. While these ratings provide a guideline for maintenance, variations in the nature and quantity of suspended solids make it difficult to accurately predict the time between cartridge replacements.

If the cartridge requires replacement too often, a media filter may be more economical.

Special considerations for drinking water filtration

The external plastic housing for various cartridge sediment filters generally look alike. The internal filter material is what differentiates one device from another. The same plastic housing used for cartridge filters may also be used to enclose activated carbon or reverse osmosis membranes. Be sure to examine filter information completely to ensure purchasing the correct device.

Ensure the system you choose is installed and operated according to the manufacturer’s instructions. After installation, retest both the raw water (prior to treatment) and the treated water at a state certified laboratory to ensure it is working properly and removing the contaminants. You should continue to test the quality of both the untreated and treated water annually. This annual test will also help you determine how well your treatment system is working and whether maintenance or replacement of components may be necessary.

Media drinking water filters

Media filters consist of a tank, a filter medium, a support system, and an underdrain. The filter medium is usually 24 to 36 inches deep and can be silica sand, aluminum silicate, or anthracite. The tank encloses the filter medium; the support system, usually gravel, prevents the medium from being washed out of the device. Water enters the filter tank through the top and percolates through the medium, which traps any suspended solids. Treated water exits the device via the underdrain. Unlike some cartridge sediment filters, media filters can be cleaned and reused.

Uses of media drinking water filters

Media filters are point-of-entry devices. They remove particles that cause turbidity and are part of iron, manganese, and hydrogen sulfide removal systems. Dissolved iron, manganese, and hydrogen sulfide must be oxidized into solid particles via chlorination or ozonation before they can be removed by a media filter. Media filters are used as a pre-filter when suspended solids in the source water could reduce the effectiveness or service life of the primary treatment device. For example, an ultraviolet light device or a chlorination unit to disinfect water. If suspended solids are not removed prior to the UV device or chlorination unit, the solids may shield microorganisms from the killing action of light or chlorine and result in unsatisfactory treatment performance. Filters are rated by the smallest particle sizes they remove.

How media filters work

Water enters the filter under pressure and passes through the medium, which retains suspended solids. Treated water exits the filter at a slightly reduced pressure. As suspended solids accumulate on the media surface, they help filter fine particles but gradually reduce water flow. If not correctly maintained, the filter will yield increasingly less treated water.

Capacity of media filter systems

The filter tank’s diameter determines a media filter’s flow rate capacity – the greater the diameter, the greater the flow. The flow rate is also affected by the media particle size and the suspended solids concentration in the water.

Maintenance of media filtration systems

Media filters should be backwashed on a regular basis to prevent accumulated particles from clogging the device. Backwashing reverses the direction of water flow through the filter by forcing water into the bottom of the filter tank and out through the top. This flow expands the media bed and washes collected solids out of the filter. Backwash a media filter at the manufacturers recommended rate and time.

Special considerations for media filtration

Backwashing these filters will put additional wastewater into your septic system. Make sure your system can handle this. Manufacturers recommendations for backwashing rates, times, and frequencies can help estimate the increased load of backwashing that would put on the septic system.

Certain arid regions of the United States are considering banning backwashing because it increases water consumption. Before purchasing equipment that requires backwashing, consult a local health department or Cooperative Extension office for information on any local restrictions.

Multimedia drinking water filters

Multimedia filters have the same basic construction as media filters except the filter tank encloses three or four layers of different media. The filter media can be backwashed and reused. A multimedia filter has a greater filtering capacity than a single-media filter with the same tank diameter. Multimedia filters also operate at a higher flow rate and longer maintenance interval than single-media filters, so operating expenses tend to be less.

Uses of multimedia filters

Multimedia filters are point-of-entry (POE) devices. They remove particles that cause turbidity and are part of iron, manganese, and hydrogen sulfide removal system. Dissolved iron, manganese, and hydrogen sulfide must be oxidized into solid particles via chlorination or ozonation before they can be removed by a multimedia filter.

How multimedia filters work

In a multimedia filter, the coarsest media layer is on top. Each successively lower layer is finer. One example of layering in a multimedia filter is bituminous coal or plastic beads on top, followed by layers of anthracite coal, sand, and garnet. A media support system and an underdrain are located beneath the media bed. The media support system is usually gravel and prevents the media from washing into the underdrain. Water enters the top of the tank under pressure and flows through the media layers. Suspended solids pass through the top layer and are trapped by lower layers.

Single-media filters retain particles on the top of the media column. In multimedia filters, the entire depth of the media column traps particles, thereby allowing a longer maintenance interval. The particle sizes of the media layers determine what sizes of suspended solids the filter can remove.

Capacity of multimedia filters

The filter tank diameter, media types, and media bed depth determine the service flow rate. In general, the greater the tank diameter, the greater the service flow rate. The water flow rate available for backwashing limits the tank size unless a water treatment equipment dealer provides a backwashing service. Manufacturers or water treatment equipment dealers can provide information concerning flow rates for various-diameter multimedia filters. Select the maximum tank diameter appropriate for the available backwash rate.

As with single-media filters, if the available backwash flow rate limits the size of the filter, use additional tanks. Each filter is backwashed separately since all the available flow is necessary to achieve the backwash flow rate of a single device. To determine the number of tanks needed, divide the desired flow rate by the flow rate for the chosen tank diameter.

Maintenance of multimedia filtration systems

To maintain a consistent quantity and quality of treated water, backwash the filter media regularly. Backwashing reverses the water flow through the device, suspending media particles and carrying away any accumulated solids. The physical characteristics of the filter media allow the layers to resettle in their proper order after backwashing.

Multimedia filters can fail because of an inadequate backwash flow rate, an irregular backwashing schedule, or insufficient time allowed for backwashing. If backwashing is done improperly, untreated water may eventually channel through the media, resulting in a clogged underdrain or incomplete treatment. For households with an inadequate backwash flow rate, some water treatment equipment dealers offer a backwashing service.

Regardless of the quality of the equipment purchased, it will not perform satisfactorily unless managed in accordance with the manufacturer’s recommendations for maintenance, cleaning, and part replacement. Keep a log book to record water test results, equipment maintenance, and repairs.

Microfiltration is generally a low-cost, safe treatment process that is nearly self monitoring. When the filter becomes clogged, the pressure drop across the filter increases and water flow decreases. This indicates that the filter needs service. As long as the filter remains in place and there are no leaks through the filter or the seals, the process works safely with little attention.

Other considerations for multimedia filtration

Ensure the system you choose is installed and operated according to the manufacturer’s instructions. After installation, retest both the raw water (prior to treatment) and the treated water at a state certified laboratory to ensure it is working properly and removing the contaminants. You should continue to test the quality of both the untreated and treated water annually. This annual test will also help you determine how well your treatment system is working and whether maintenance or replacement of components may be necessary.

Bacteria can grow on the filter, so it is important that you change the filter as necessary.

Questions to ask before you buy

Before purchasing a water treatment device, have your water tested at a state certified laboratory to determine the contaminants present. This will help you determine if filtration is an effective treatment method for your situation. See Questions to Ask Before You Buy A Water Treatment System for more information.


Adapted from: Wagenet,L., K. Mancl, and M. Sailus. (1995). Home Water Treatment. Northeast Regional Agricultural Engineering Service, Cooperative Extension. NRAES-48. Ithaca, NY.

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.