What Is a Slow Sand Filter?

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  • Written By: Mary McMahon
  • Edited By: Shereen Skola
  • Last Modified Date: 13 December 2019
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A slow sand filter is a water treatment filter that can inexpensively and efficiently treat water to remove unwanted organisms and other materials. Slow sand filters have a number of advantages in settings where access to technology may be limited and communities need a reliable filtration system that will last for an extended period of time. They rely on a carefully constructed bed of rock and sand. This material is topped with a mat of beneficial microorganisms, known as a Schmutzdecke, which acts to filter water as it passes through.

These filters do not require electric pumping or chemical treatment. A community can establish an appropriately sized bed for filtration and may use gravity feeding to pump finished water to a storage tank. Slow sand filters operate continuously, and thus a method for collecting and safely storing treated water is necessary. The low cost and low technology can be suitable for environments like communities in the developing world which cannot support the infrastructure of a more complex water treatment plant.


Construction of a slow sand filter starts with drains at the base of the filter bed, topped with a layer of loose gravel. Next comes a layer of sand in which the beneficial microorganisms can grow. They create a filter cake through which water must pass to get to the drains. Operators can pour untreated water onto the top, and clean water flows out through the drains. Over time, the filter cake can eventually clog, and manual cleaning is necessary to get the slow sand filter operational again.

One disadvantage of this treatment method is implied by the name: slow sand filters work slowly. It can take a long time for water to work through the system, especially as the filter cake starts to get more dense. Communities with high water needs may need an extremely large filter bed that could become logistically impossible to construct and maintain. This can be a concern for growing communities which may strain their water supplies as they outstrip the filter's capacity to treat water.

Testing of slow sand filter systems shows that they can be highly reliable when they are installed and maintained properly. It is important to check the treated water periodically to identify potentially harmful organisms or chemicals that may be slipping through if the filter is not fully functional. If the filter fails, it may be necessary to rebuild from scratch. A consultant can evaluate the slow sand filter and test results to determine the best steps to take.


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