How do Sand Filters Work?

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First documented in ancient Sanskrit and Egyptian texts, sand filtration is one of the oldest and simplest methods of water filtration.

Sand filtration works by using multiple layers of specialized sand and gravitational fluid pressure to retain solid particles suspended in liquids, resulting in a cleaner product. As fluids pass slowly though layered sand and gravel beds, natural physical, biological, and chemical processes combine to provide treatment.

Slow sand filtration does not require chemicals or electricity to clean the water. The majority of the work is done at the top of the filtration system, where some solids are trapped on the rough surfaces of the sand grains and others are adsorbed onto the negatively charged sand surfaces. Most of the dirty work is done by biological processes, or predation.

Microorganisms are abundant in sand filters and this predatory ecosystem helps with water treatment and purification.

Sand is a widely available and affordable filtration media capable of removing suspended solids and producing high quality water with improved colour, taste, and odor, as well as removing 90-99% of pathogens.

Sand is used in the following types of filtration systems:

As always, sand selection is very important! The media selected must be washed at a suitable processing facility and have particular characteristics. These characteristics differ slightly depending on the type of system being used, but include:

  • Effective Size – If the sand is too large, particulate will not get trapped and if it is too small, particulates will accumulate on the surface of the filter bed preventing fluid flow. Effective size is determined by sieve analysis.
  • Shape – The shape of the grain should be sub-rounded to sub-angular. The more rounded the grain, the more chance it will erode and reduce the lifespan of your filter. Flat grains can be lost during cleaning (back-washing).
  • Hardness – Dense, hard, more resistant particles with a high silica content are best, as softer particles are also more susceptible to erosion.
  • Uniformity Coefficient (Cu) – Filter media should relatively uniform in size, to allow water to pass through. Optimal Water drinking systems have a sand Cu between 1.3 and 1.5, and must not exceed 1.7. The Cu can be below 1.3, but will be unnecessarily expensive and a higher Cu will be more susceptible to clogging. Septic systems require a Cu of <4. Click here for a refresher on gradations, D-values, and uniformity coefficients.

In water filtration systems, sand can be the only media used or can be used as a component in a multi-media system (combined with flocculants and other filtration materials such as charcoal) for increased effectiveness.

Drinking water systems will obviously be more elaborate than pool filtration or wastewater treatment. Filtration media for drinking water purification should be NSF certified. Forty-eight U.S. states currently have some form of legislation, regulation or policy that requires drinking water system components to comply with, or be certified to, this standard. This is in addition to the 11 Canadian provinces/territories requiring compliance to NSF/ANSI/CAN 61. Filtration media for drinking water purification should be NSF certified.

Ensure your drinking water treatment facility and filtration media provider are NSF certified; find Shaw Resources NSF certification here.

For questions or quotes on all of the filtration media available at Shaw Resources, please contact our friendly customer service representatives.