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Particle and Microfiltration

 

Introduction
Operation and Construction
Filter Rating
Filter Selection
Filter Cost

Introduction
Particle filtration is a process that removes small amounts of suspended particles, ranging in size from sand to clay, from water. It can be used alone or ahead of other water treatment devices. Home filters are not intended to filter large amounts of particles. Instead, sedimentation, or sand filters, are used to filter and remove particles from large volumes of water (see below). Microfiltration may also be used to remove some bacteria and large pathogens, like cysts such as Giardia and Chryptosporidium. Note that microfiltration should not be relied on to disinfect water with high concentrations of bacteria and viruses; chemical disinfection should be used instead. Other forms of filtration include ultrafiltration and reverse osmosis.

Operation and Construction
Filters function in two general modes:

Surface or screen filters remove the particles at or very near the filter surface. Ceramic filters are porous ceramic cylinders that filter at the surface. They are expensive but long lasting, may be cleaned, and provide precise filtration. Resin-bonded filters look like ceramic filters but are produced by bonding particles with resin rather than heat.

Depth filters have a thick filter medium. Particles are retained throughout the thick filter mat and these filters may be used for a wide range of particle sizes. There are several types of depth filters. String-wound filters are easily recognized by the criss-cross pattern of the string (which may be made of cotton or synthetic materials such as polypropylene and nylon). Spun-fiber filters look like a fuzzy fiber tube are usually are constructed from synthetic fibers (such as polypropylene and nylon) or natural fibers (such as cellulose). Pleated-fiber filters are constructed either of individual fibers pressed and bonded together or of a continuous sheet or membrane with very small openings.

Precautions on use:
-- Synthetic filters (made out of plastic fibers or resins) are a possible source of chemical contamination in themselves.
-- Misuse of these devices, including overuse and fast or inadequate flushing, may prevent or reduce filtration of contaminants or may release large amounts of contaminants back into the water (initial flush effect).
-- Filters should be used regularly. Long idle periods may lead to excessive bacterial growth, early clogging, and the possible release of high concentrations of potentially harmful bacteria when flow is restarted.
--Replace filters at manufacturers' prescribed intervals.

Filtration guide. Source: midified from Filtration Application Guide, Water Quality Improvement Center.

click to enlarge

Filter Rating
Particle filters have two types of ratings:
--Average or nominal particle size implies that a range of particles pass through different sized openings within the filter.
--Absolute particle size implies that no particle larger than the stated size may pass through the filter.

Filter Selection
Filters are rated by the smallest size particle they
will remove, stated in microns. A micron is
approximately 0.00004 inches (some common
particles sizes are shown in the Filtration Guide).

If no colloidal materials or pathogens are present, the filter with the largest rating size that will work is recommended as it will require less maintenance. If the filter must be very fine, such as for removing pathogens, two filters are often recommended. A larger opening depth filter might be selected as the first filter and an absolute rated surface filter could be used as the second filter to ensure removal of the organisms.

 

particle filtration process

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Filter Cost
Particle filters and microfilters range in price from a few dollars for a small self-contained specialty type to $150 or more for a large ceramic filter system. Replacement cartridges range from a few dollars to $100 or more for ceramic cartridges. Total costs are highly variable depending on requirements and particulate load guidelines that determine the cartridge’s service interval.

(Portions of this text are adapted from Powell, G.M., and R.D. Black, "Microfiltration," Manhattan: Kansas State University Cooperative Extension Service, MF-882, 1987.)

Go to other treatment methods:
Activated Carbon Filters
Reverse Osmosis
Distillation
Ion Exchange Water Softening
Disinfection of Drinking Water
Other Treatment Methods

 


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