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Conventional Filtration

Filtration systems treat water by passing it through granular media, e.g., sand, that remove the contaminants. Their effectiveness varies greatly, but these systems may be used to improve turbidity and color concerns, as well as to treat Giardia and Cryptosporidium, bacteria, and viruses.

Conventional filtration first utilizes a pretreatment chemical coagulant, such as iron or aluminum salts, which is added to the source water. The mixture is then slowly stirred to induce tiny suspended particles to aggregate to form larger and more easily removable clots, or “flocs.”

These systems next employ a sedimentation step. In this process particles in the water, including the floc created by flocculation, are allowed to settle out of the water naturally by gravity’s pull. These contaminants gather on the bottom of the system as a “sludge” that is periodically removed.

Once these processes are complete, water is passed through filters so that any remaining particles will physically attach themselves to filter material. The suspended particles are destabilized by the coagulant and thus attach more readily to the filter material.

Conventional filtration, like other filtration systems, results in significant improvement of a wide variety of source waters. It is best employed on sources with constant flow and low levels of algae—which can clog filter systems.

Coagulation chemicals require expert handling to achieve the desired results, so trained personnel are necessary to manage filtration treatment facilities.

Direct Filtration

Filtration systems treat water by passing it through granular media, e.g., sand, that remove the contaminants. Filtration effectiveness varies greatly, but these systems may be used to improve turbidity and color concerns, as well as to treat Giardia and Cryptosporidium, bacteria, and viruses.

Direct filtration first utilizes a chemical coagulant, such as iron or aluminum salts, which is added to the source water. The mixture is then slowly stirred to induce tiny suspended particles to aggregate to form larger and more easily removable clots, or “flocs.”

Once these processes are complete, source water is passed through filters so that any remaining particles attach themselves to the filter material. The suspended particles are destabilized by the coagulant and thus attach more readily to the filter.

Conventional filtration processes use sedimentation to allow particulates to settle out of water for removal. Direct filtration eliminates this step and allows the filter material itself to do the work of straining contaminants.

Direct filtration is a relatively simple filtration process, and it is economically attractive. The system results in significant improvement of source water quality—but it is best employed on relatively high quality source waters, with constant flows and low turbidity. High algae levels, in particular, may clog filtration systems.

Because all the particles are removed by filtration, direct filtration is not able to treat waters of high turbidity. A rule of thumb is that direct filtration is appropriate for source water with a turbidity below ten ntu.

Coagulation chemicals require expert handling to achieve the desired results, so trained personnel are necessary to manage filtration systems.

Diatomaceous Earth Filtration

Diatomaceous earth filtration is used to physically remove particulates, which are simply strained from source water. The process is effective at removing Giardia, Cryptosporidium, algae, and, depending on the grade, some bacteria and viruses.

This system’s filter consists of a cake of diatomaceous earth, a floury, chalky substance made of the crushed, fossilized remains of one-celled marine life forms called diatoms.

Water is passed through a diatomaceous earth filter system by pumps that either force pressurized water through the cake from the source inlet, or use vacuum suction to pull it through from the outlet side.

Unlike many other forms of filtration, coagulation chemicals are usually not used to enhance the agglomeration of contaminant particles. Because of this limitation, diatomaceous earth filtration is best-suited to higher-quality source water that is devoid of inorganic contaminants.

The process is easily scaled to fit small scale facilities. In fact, this filtration system was first developed during World War II when the U.S. Army filled a need for portable water treatment facilities. Diatomaceous earth filtration systems are easy to operate and economically attractive. These attributes make them popular for temporary crisis relief, and in communities with little funding for more expensive infrastructure.

In addition to water, this filtration system is standard in many manufacturing processes including the making of syrups, oils, antibiotics, chemicals and alcoholic beverages.

Slow Sand Filtration

Slow sand filtration was the first treatment method employed by many cities during the nineteenth century. These filters can effectively remove the microorganisms that cause waterborne disease—including protozoa like Giardia and Cryptosporidium, as well as bacteria and viruses—a capability that was first demonstrated by plunging disease rates in the European cities that pioneered the treatment.

Water treated by these systems is allowed to slowly pass through a bed of sand some two to four feet (0.6 to 1.2 meters) deep. En route, a combination of physical and biological processes filters the water and removes contaminants.

After repeated use, the sand bed becomes host to a multitude of bacteria, algae, protozoan, rotifers, copepods, and aquatic worms. These microorganisms assist the filtration process by removing contaminants, though they may be slowed by water temperatures below ten degrees Celsius. Sand that hosts these organisms is said to be “ripened,” and is preferable to clean or new sand. It may take several weeks or months to ripen sand, depending on water contents and temperature. The process eventually clogs the sand bed and slows flow rates to the point that it must be unclogged, typically by reversing the flow, or “backwashing.”

Slow sand filtration systems may not be able to accommodate chlorinated water because chlorine can have a detrimental impact on the filter’s microbiological community. Therefore, water to be disinfected with chlorine may be treated in storage facilities after passing through the filtration process.

Storage also helps to add flexibility to a system’s water output. Slow-acting sand filter systems cannot handle increased water volumes in times of peak demand, nor should they be run at less than optimal flows during periods of lower demand.

Slow sand systems work well only on source water that is low in turbidity and algae levels, and without color contamination. These systems struggle particularly with high algae or clay content—which can clog sand beds. Nutrient-rich source water, on the other hand, may aid the cleansing action of slow sand filters by boosting their biological component.

Slow sand systems generally are simple, require little maintenance, and have low operating costs.

Bag/Cartridge Filtration

Filtration systems treat water by passing it through porous materials that remove and retain contaminants.

Bag and cartridge filters are simple and easy-to-operate systems that use a woven bag or a cartridge with a wound filament filter to physically strain microbes and sediment from source water as it is passed through the filter medium.

These systems are effective against Giardia cysts, but not are sufficient to eliminate bacteria, viruses, or chemicals. Thus, they are most appropriate for higher quality source waters and those with limited turbidity.

Bag and cartridge technology is developing rapidly and is tailored for use in small scale treatment facilities. Such systems also deliver ease of operation and maintenance, with little skill required on the part of the operator. Costs are variable depending on how often the filters must be changed.

Like many other filters, cartridges quickly become fouled by water that is high in particulates—so low turbidity water is preferred. Alternatively, “roughing filters” that use sand, mesh screens, cartridges, and other substances to physically remove larger particulates may pretreat water.

Filter materials must be changed periodically, more often when source water is high in particulates.

With repeated use of bag and cartridge systems, microbes may grow on filters, though this problem can be tempered by the use of a disinfectant. Disinfectants may also be required if water testing reveals that source water virus removal is necessary.

Ceramic Filtration

Ceramic filters have been used for water treatment for several centuries. While they are being marketed for centralized water treatment systems, most ceramic filters are now being manufactured for point of use applications. In developing countries, they are manufactured locally—sometimes as a self-financed micro-enterprise. These devices are typically shaped like a flowerpot or a bowl and are impregnated with tiny, colloidal silver particles as a disinfectant and to prevent bacterial growth in the filter. The filter sits inside a 20- to 30-liter plastic or ceramic receptacle with a spigot.

Laboratory testing has shown that, if designed and produced correctly, these devices can remove or inactivate almost all bacteria and protozoan parasites. Its effectiveness against viruses is unknown.

Cleaning and maintenance of the filter is critical; so like other low-cost point of use systems, it is best combined with an educational program about safe storage, filter cleaning, and other recommended practices.

The advantages of ceramic filters are their ease of use, long life (if not broken), and fairly low cost. Disadvantages include possible recontamination of stored water since there is no chlorine residual and a relatively low flow rate-typically one to two liters per hour.

Biosand Filtration

Slow sand systems have recently been adapted for point-of-use systems, especially in developing countries. In this context they are generally known as “biosand” filters.

Most commonly, a biosand filter takes the form of a container a little less than a meter tall and perhaps 30 cm in width and depth, filled with sand. The biologically active layer, which takes a week or two to fully develop, is maintained by keeping the water level above the top of the sand. As with slow sand filters, this bioactive layer helps to filter, adsorb, destroy, or inactivate pathogens. A porous plate is usually located above the sand to prevent disturbance to the bioactive layer when water is added. Users simply pour water into the top of the apparatus, and collect treated water from the outlet.

In laboratory and field testing, biosand filters have removed nearly all protozoa, and most bacteria. Their performance with viruses is not well established.

The apparatus can be built using concrete—a commonly available and relatively inexpensive material. Maintenance is fairly simple, usually consisting of agitating the upper surface of the sand once a month or so and manually collecting the suspended material. The cost of upkeep is quite low, since there are few or no parts to replace.

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