Pond Construction

Installing a Sand Filter


Some Koi-keepers install a sand filter to the system after the main filter, before the water returns to the pond. They consider the sand filter to polish the water and remove a lot of the small particles, which may have passed through the filter. The installation of a sand filter is however not supported by a large contingent of purists for various reasons. As stated in various articles, the purpose of this article is not to convince other hobbyists to make use of sand filters in their systems, but to provide enough information for individuals to make an informed decision.

Basically a sand filter is a pressurised container where the water is forced through a bed of sand. Underneath the sand the water passes through a number of one millimetre slots and continues along the plumbing back to the pond. It can return to the pond either via a venturi, waterfall or jets. The sand filter may add to the surface area for beneficial bacteria but the main function is to act as a mechanical filter to remove the very fine suspended solids that may pass through the biological filter.

The following photos show the various components of a sand filter

The inside if the sand filter is fairly simple

The empty casing with all the parts removed

empty_filtr_casing       empty_filter_casing_outside_view


The collector arms with 1mm openings. Note the Bayonet type fitting of the arms to the manifold


Inside fully assembled


Easy access lid


Multi port valve to be connected to these fittings


The drawing below shows the normal installation of a sand filter.



The unit is installed right after the main pump and before other items such as UV, heater, etc. This prevents any excess pressure on these units if the back pressure should build up in the sand filter. The ingenious design of the sand filter lies in the multi-port valve. The design of the valve has been copied extensively and is basically the same for all makes of sand filter. One has to switch the pump off before directing the flow of water in one of the following directions, namely: Filter, Rinse, Bypass, Backwash, Waste, Re-circulate and Close. Although this multi-port valve restricts the water flow to an extent, it remains a very versatile way of directing the flow.


The one millimetre slots in the “fingers” or collection arms mentioned above was designed to block one millimetre sand particles, while allowing the water to pass through. The sand filter was designed to strain out dirt particles from swimming pool water. These particles have already been subjected to chlorine treatment and are therefore oxidised and easy to backwash from the sand bed. When installed in a Koi pond the particles trapped by the sand filter consists of fish waste, algae and uneaten food that are sticky, so 3 to 5 millimetre sand is normally used to prevent the filter from clogging too often.

The sticky particles that are trapped in the sand are more difficult to backwash than the particles trapped in a swimming pool filter. A true Koi pond sand filter is therefore designed with a lid that opens easily and it is necessary to stir the sand by hand while backwashing to dislodge the trapped particles. This process is not to everyone’s taste and it is one of the reasons why some Koi keepers prefer not to install a sand filter in the system. Even while backwashing and stirring the sand by hand, a sand filter is rarely clean after a backwash.

The inability to properly clean a sand filter leads to many problems, for instance channelling and the subsequent forming of pockets where anaerobic bacteria may flourish. The filter sand also needs to be replaced at intervals. Replacing sand is a tedious job that requires the hobbyist to remove the sand by hand.

The most convenient way of backwashing a sand filter is to install a Jacuzzi air blower in such a way that it assists with the backwashing process. The air blower forces a large volume of air through the collector arms. The bubbles move upwards through the water and filter sand and escape through the backwash pipe. During the upward movement through the water and sand, the air dislodges the trapped dirt particles, which will float to the top, while the heavy filter sand particles will settle at the bottom. When normal backwashing is resumed, the dirt particles are flushed from the sand filter. No replacement of sand is necessary when backwashing a sand filter this way on a regular basis. 


The drawing below shows the correct way of installing a sand filter, assisted by a Jacuzzi air blower.



This installation needs to be operated in a specific sequence. One cannot switch the multiport valve to backwash and just turn the pump and air blower on! I can assure you, some water will be pumped through the sand and the balance will be forced through the air blower! The correct sequence is important.

Valve number three is used only if the pump needs to be removed, so we can ignore it at this stage. During normal filtration, valve number one is open and valve number two is always closed. During normal backwash, valve number one is also open and valve number two is always closed.

When you want to clean the sand with the air blower, follow the following steps:

1. Switch off the pump
2. Close valve number one
3. Turn multi port valve to “backwash”
4. Switch air blower on
5. Open valve number two
6. Let the air blower run for three minutes or longer. You will notice air and water spurting from the backwash pipe.
7. After a couple of minutes, close valve number two
8. Switch off the air blower
9. Open valve number one
10. Switch the pump on. Normal backwash will now flush the dirt out of the sand filter.
11. When you are satisfied that the sand is washed clean, switch the pump off and turn the multi port valve to “filter”
12. Switch the pump on and normal filtration is resumed.

One can follow this procedure with the lid of the sand filter open or closed, as shown in the photos below.
The photos on the left show blow and backwash with open lid and the two photos on the right show blow and backwash through the waste pipe. 

blower_at_work_open   blower_at_work_closed

backwash_open  backwash_closed

Note: some owners repeat the process to make sure the sand is absolutely clean. Others will let the air blower do its work and then “rinse” first before backwashing. The only two precautions is to ensure valve number two is closed when there is any pressure that may force water through the air blower and the pump is switched off before turning the handle of the multi port valve! I would advise that you get used to the system and then experiment with “rinse” and “backwash”.


 The result: Squeaky clean sand!

The amount of sand that should be put into the sand filter will depend on the size of the filter but it is best to fill the filter up to the halfway mark. This will ensure the maximum surface area for the water to be forced through the sand. The collector arms inside the filter are easily damaged. When one dumps a bag of filter sand into the filter these arms may be damaged. It is a good idea to fill the filter with water before putting the sand into it. Most available sand is not properly washed. Before you start the filter process, the dust should be removed with a thorough backwash of the filter.

Now also read the article by Chris Neaves about The Use and Abuse of Sand Filters.

15 January 2010

Last Updated on Tuesday, 26 January 2010 23:25