Gravity Sand Filter

Submitted to:

Madam Tajwar

Submitted by:

Muhammad Usman (08-chem-02-B)

Date: 30 September 2009

Department of Chemical Engineering

University of Engineering and Technology, Lahore

ContentsFilter:...........................................................................................................................................................5

Sand filter:...................................................................................................................................................5

Gravity Sand Filter:......................................................................................................................................5

Principle:......................................................................................................................................................5

Structure:....................................................................................................................................................5

Working:......................................................................................................................................................6

a. Direct collision:............................................................................................................................6

b. Cohesive forces:...........................................................................................................................6

c. Surface charge:............................................................................................................................6

Back washing:..............................................................................................................................................6

Benefits of sand filter:.................................................................................................................................7

Filter:A filter is a device (usually a membrane or layer) that is designed to physically block certain objects or substances while letting others through. Filters are often used to remove solid substances suspended in fluids.

Sand filter:A filter in which layers of sand are used as a filter media is called sand filter. There are three types of sand filters

1. rapid (gravity) sand filters 2. Up flow sand filters3. slow sand filters

Gravity Sand Filter:A sand filter which operates under the action of gravity and gravity is the basic force that is responsible for the whole filtration process.

Principle:Its working depends on the principles that gravity always pulls any material towards ground level. Using this pull of gravity this filter works.

Structure:It consists of a large vessel

Two layers of sand are there which act as a filter media in the filter.

These sand layers are supported on a layer of gravels.

1st layer of sand consists of the fine sand and 2nd layer is of coarse sand.

At the bottom of tank there is an outlet for clean water.

Working:Liquid suspension is introduced from inlet valve that is situated at the top of tank. Liquid column then passes through the sand layers and the suspended particles present in water are stuck at the surface of sand while clean water passes to the bottom of tank.

In simple gravity sand filter no mechanical power like pumps, blowers etc. are required. There are many designs for the sand filters. Some of them are following…

1. Sand filter in which upper sand layer is of fine sand.2. Sand filter in which upper sand layer is of coarse sand.3. Sand filter in which more than two layers are introduced like that of charcoal and anthracite.

Particles that are suspended in water are separated due to a no. of reasons…

a. Direct collision:Particles just strike with sand particles and are stuck.

b. Cohesive forces:Cohesive forces are building up between particles that do not allow the particles to pass through the layer.

c. Surface charge:Due to friction particles are charged. In case of negative charges particles attract each other.

Back washing:When the impurities are collected at surface of sand the clog the filter media for the process to run continuously back washing is done in which water is introduced from the bottom of the filter with high pressure. This water take up the particles from sand and filter is again usable.

Due to back washing gravity sand filter is a batch filter because when back washing is done process of filtration is stopped.

Benefits of sand filter:1. As no or less mechanical power is required so it is economical.2. Less labor is required.3. Is used in poor countries like Afghanistan due to its economically less expanses.4. Is a simple device as no complex devices are used.5. No moving parts are there.6. Filter media are cheap and easily available.

Bibliographycentra-flow filters. (n.d.). Retrieved 09 30, 2009, from www.centra-flow.com: http://www.centra-flo.com/

slow sand filtration. (n.d.). Retrieved 09 30, 2009, from www.oasisdesign.net: http://www.oasisdesign.net/water/treatment/slowsandfilter.htm

solidliquid-seperation. (2009, 09 30). Retrieved from www.solidliquid-seperation.com: http://www.solidliquid-seperation.com

water filtration. (n.d.). Retrieved from http://www.hoffland.net/src/s/8.xml: http://www.hoffland.net/src/s/8.xml

wikipedia : the free encyclopedia. (2009, 09 30). Retrieved from en.wikipedia.org: http://en.wikipedia.org/wiki/Sand_filter