Filtration Technologies



Filtration is the process of removing  suspended solids from water as the water passes through a porous bed of materials (Figure 1).
Natural filtration removes most suspended matter from ground water as the water passes through porous layers of soil into aquifers (water-bearing layers under the ground).
Additionally, surface waters, are subjected to runoff and are inherently vulnerable to other sources of contamination such as sewage overflows, industrial discharges, fuel or chemical spills. Surface waters must therefore be filtered by some method in addition to being disinfected prior to use.

Conventional Filtration
Also known as rapid-sand filtration, this is the most prevalent type of water treatment technology used today. This filtration technique employes a combination of physical and chemical processes in order to achieve maximum effectiveness, as follows:
Chemical feed and rapid mix: Chemicals are added to the water in order to improve the subsequent treatment process. These may include pH adjusters and coagulants. Coagulants are chemicals, such as alum, that neutralize positive or negative charges on small particles, allowing them to stick together and to form larger particles that are more easily removed by sedimentation (settling) or filtration. A variety of devices, such as baffles, static mixers, impellers, and in-line sprays can be used to mix the water and distribute the chemicals evenly.
Flocculation: In this process, which follows the rapid mixing, the chemically treated water is sent into a basin where the suspended particles can collide, agglomerate (stick together), and form heavier particles called “flocs”. Gentle agitation of the water and appropriate detention times (the length of time water remains in the basin) help facilitate this process.
Filtration: With most of the larger particles settled out, the water now goes to the filtration process. At a rate of between 5 and 20 m³ per square meter, the water is filtered through an pproximate 1 m depth of graded sand. Anthracite coal or activated carbon may also be included in the sand to improve the filtration process, especially for the removal of organic contaminants and taste and odor problems.
Product water tank: The final step in the conventional filtration process, the product water tank provides temporary storage for the treated water. The two main purposes for this storage are to have filtered water available for backwashing the filter, and to provide detention time (or contact time) for the chlorine (or other disinfectant) to kill any microorganisms that may remain in the water.
Conventional filtration

Slow Sand Filtration
Slow sand filtration represents a technology that has been used for years in various areas and only recently has it been re-examined as a cost-effective, efficient, and relatively simple system for water filtration. In a slow sand filter, raw water passes by gravity through a bed of sand, about 1 m deep, which is supported underneath by a layer of gravel. Filtered water is collected by an under drain system which is placed within or underneath the gravel.A thin layer of biologically active microorganisms forms on top of and throughout the sand bed. These organisms filter out sediment and kill harmful bacteria. After a period of time, the filter bed will begin to clog due to trapped particulate matter. The filter is cleaned by scraping a thin layer (usually less than one inch) of sand from the top of the filter, and the filter is then put back into operation. The time between filter scrapings varies from two weeks to several months, depending on raw water quality. Eventually, the scraped sand will need to be replaced –usually when the filter bed has decreased to a depth of about 0,6 m.
As the name implies, the filtration rate of this technology is slow - from 2,2 to 4,4 m3 per day per square meter of filter area. This translates into a rather large space requirement if the system is to serve more than a few hundred people. Another consideration is the operational requirement that a double unit should be constructed, so that one filter can remain on-line while the other is off-line for scraping, sand replacement, or repairs. One obvious advantage of slow sand filtration is its applicability to small water systems with source waters of relatively good quality. Another advantage is that no coagulant chemicals are required, therefore fewer labor hours are required. This technology is simple and reliable, relatively low cost, and has proven effective at removal of Giardia cysts. Disadvantages include the space requirement mentioned above, limitations on raw water quality (ideally less than 10 NTU), and the relatively slow filtration rate.
Cartridge Filtration
Although the technology of cartridge filtration has been used for years in industries such as food and beverage, electronics, laboratories, and oil refining, it has only recently extended into the field of drinking water. Increasing public awareness of water contamination problems has led to a demand for filtration systems that are appropriate for small water systems. Cartridge filtration systems offer the benefits of filtration without the burdens of coagulant chemicals and settling basins that are associated with conventional filtration plants. The disadvantage, of course, is that cartridge filters require periodic replacement and are most appropriate for relatively low-flow situations.
Filter cartridges are manufactured from a number of materials, including cotton, nylon, polyester or polypropylene (similar to “felt”), nylon, carbon, and glass fibers. Filtration levels range from 100 microns down to 0.5 microns. Maximum flow rate is usually depending on the types of filters and the micron ratings selected.
Higher flow rates may be accommodated by placing multiple series of filters in parallel.

A common arrangement for a series of cartridge filters is to place them in the order of decreasing micron size. A roughing filter, or pre-filter, would be placed first in line to remove algae or other large debris. This filter might be in the 10 - 20 micron range. Some systems like reverse osmosis plants have used sand filters as a pre-filter, and provided backwash capability so that this filter can be periodically cleaned rather than replaced as a cartridge would be. An intermediate cartridge filter, in the 1- 10 micron range, would then be placed in line. The final filter, if one micron or smaller, should assure removal of Giardia or Cryptosporidium cysts without clogging prematurely.

One type of cartridge filter that is relatively new to the drinking water industry is the “bag” filter. These filters are mounted in housings identical to those for typical cartridge filters. The bags are usually manufactured of polyester or polypropylene felt, sewn together, and then turned inside out to avoid leakage through the threads. They are available in ratings from 50 down to 0.5 microns. Like the cartridge filters, the bags are discarded when they become clogged. Although the bags are more expensive to replace than cartridges, they will usually have a greater capacity for holding particulate material. All filters (bags or cartridges) in a cartridge filter system should have pressure gauges on each side of the filter so that head loss, or accumulation of debris, can be monitored continuously and the filters changed when necessary.

Cartridge filter

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