Pond Construction

Sand filter cleaning with compressed air


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 previously, the purpose of this article is also 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. If one look at the objections against sand filters it is not only an objection in an attempt to join the league of purists but it is an objection because there are real concerns about the long-term effect on water quality and because it is really no a pleasant task to clean it by hand. Even if cleaned by hand, the final clean filter may not be clean enough for the owner to reap the benefits of a really clean filter and filter sand. The extensive use of Sand filters to clean koi ponds, and the difficulty in the maintenance thereof prompted me to write the article on the use of a Jacuzzi blower to ease the maintenance and increase the effectiveness of sand filters.

Initially we thought that we had a winner until a very “technically inclined” hobbyist came up with a more innovative solution to clean a sand filter. This time however, it was not by means of a blower, but through the use of compressed air. The genius of this method lies in the fact that the compressed air can be released into the sand filter during the normal backwashing process. The effect of the simultaneous use of air and water is much higher than air or water on its own.

Neville Boardman’s experience in the air filtration and engineering industry prompted him to develop this method of cleaning the “love by some and hated by others” common but effective old sand filter. This is how he described the process!

“In my humble opinion there is no filter available to the koi hobbyist that can compare with the ability of a standard swimming pool type sand filter filled with the correct media of removing those finer particles from Koi pond water. The initial cost outlay is within the ability’s of most keepers. I have found that a very high percentage of keepers use sand filters be it as a mechanical filter, a bio filter or as some call it, a polisher. It certainly gives the pond water that extra clarity (polishing) if one is prepared to spend that extra bit of time in the correct maintenance thereof. Nothing beats the look of a pond when you get that 3D impression when you are watching your fish. It is priceless to view your pond on a moonlit night and you can see the bottom of your pond as well as the shadows of you fish!

As stated, the faithful old sand filter posed a few challenges that needed attention:

Possible anaerobic activity due to difficulty of cleaning the filter

Unpleasantness and time consuming when cleaning sand by hand

Water wastage through the excessive long time of backwashing

Restricted flow of water through dirty sand

The requirement of a very powerful pump, especially when backwashing

The first option was to use a Jacuzzi blower to provide air.

This is obviously a smaller capital outlay than the compressor, but unfortunately it cannot be used while backwashing the filter, due to a lack of pressure. To use a Jacuzzi blower, the pump must be switched off. It however delivers a continuous volume of air which is convenient. The compressor on the other hand runs out of air sooner and the tank has to be filled up again.

The second option was the use of a compressor with a reservoir.

In my opinion it has the advantage that the pressure can be regulated and the pond pump can pump to its full capacity without any problem while air is released, the combined result of high pressure air and water has a major effect on the cleaning process. Air alone worked better than water alone, but with the combination of air and water, the effect is rather amazing.

To overcome the challenges stated above and utilize the combined force of air and water, a new way of cleaning a sand filter had to be found. We then designed the following adaption to the standard Koi sand filter. The whole idea behind this cleaning technique is to inject high pressure air almost straight into the collector arms of the sand filter while backwashing. This high pressure water and air mixture is able to lift the sand and simultaneously knock the accumulated dirt into suspension so it can be easily backwashed out of the filter either with the lid open or via the normal outlet piping to drain.

The design is not only very simple but also very effective. The filter is connected to the rest of the filtration system in the standard way. The modifications are done only on the filter itself through the following process:

Drill a 13 mm hole in the side of the filter casing at a convenient point and connect the following items



The items connected as seen from the outside


Items connected as seen from the inside

             inside_view_of_valve_connected    inside_view_of_valve_connected_1

Drill a 10 mm hole in the collector arm connector and glue the black PVC pipe into this hole. Tie the black PVC pipe neatly against the down pipe to keep it out of the way, and the inside installation is complete! The filter can now be filled with sand and it can operate normally.

                   inside_view_of_airline_connected_to_collector_arm_holder       inside_view_of_airline_connected_to_collector_arm_holder_2

Once you need to clean the sand filter, connect the compressor to the valve fitting on the outside by means of the compressor’s quick coupler.

       compressor    connect_to_compressor

The pond

I set about testing the installation in the filter after it was left for a period of only 14 days without any backwash. I must add that this pond is rather heavily stocked and the fish were being fed heavily at the time as it was done in March (Late summer in South Africa). The pond holds 25000 liters of water and the stocking level at the time was 65 fish averaging around 45 cm in length.

The biological system consists of a .75 Kw pump delivering the pond’s bottom water via 3 vortexes and a 3000 liter up and under bio filter. The water flows through a vegetable filter back to the pond.

The sand filter system consists of a .75 Kw pump and a 3 bag sand filter. The water is pumped from the bottom of the pond, through the sand filter and returns to the pond via a bakki shower. The flow rate is around 16000 liter per hour.

Testing the installation

The dirty Sand filter before cleaning.


Sample of the sand in the dirty filter


We did a 3 minute normal backwash and took another sample 


Note that very little or no dirt is evident from the backwash

After normal backwash



We then added air from a compressor at 2 bar pressure into the line to the sand filter while the water flow was opened fully. The multiport valve was still set on backwash.



Note the turbulent action and the amount of dirt being removed

 After air was applied.



The final result, a clean filter!


Our findings are the following.

Normal backwash does not clean fish wastes from a sand filter and is only a waste of water. (It works well on swimming pool filters, but that dirt is mainly dust and it is easy to remove, fish excretion and algae is rather “sticky” and needs something extra).

Hand stirring is a must if one does not have air available as it will remove a certain amount of dirt. But this is hard work and for this reason most koi keepers opt not to install sand filters.

The use of compressed air in cleaning a sand filter substantially increases the ability of the backwash procedure to clean the filter.

If the above step is followed and the pressure is set correctly, (Remember your system will only handle a maximum of 2.5 bar pressure. Above this pressure some of the sand will be lost, and your pump will not cope. See pressure when the lid is closed. During backwash with air) your filter will be as clean as it needs to be, to give you crystal clear water.

Our further observations are that a sand filter does not impact too seriously on the effectiveness and pressure of your pump, provided of course your filter is not allowed to clog up. Cleaning times will also depend on individual setups and each keeper should experiment to determine and find his own filter’s optimum time. The optimum time will also change during summer and winter months for obvious reasons. We have found that a backwash of 2 minutes is sufficient for a cleaner filter.

The writer feels that with the above cleaning procedures one can reduce your pump size as the pump flow rate is now not so important to clean your fitter, provided of course you still maintain recommended pond turnover rates. This, if possible will bring about considerable electricity savings.

Finally I want to say that the use of Sand filters to clean your pond water is unsurpassed. I don’t want to get involved in the Bio filtration abilities of sand filters. I am sure there are experts on that subject. I myself would like to think that the added bio value of a sand filter should be seen as a bonus and not to be relied upon.

The pressure in the sand filter during normal filtration were recorded as 0,6  to 0,7 bar. The pump operating against a closed valve delivers 1, 6 bar this is the maximum ability of a normal swimming pool type pump. Then it has run out of steam and delivers no water.

Interesting to note that your power consumption is reduced when the valve is closed and it slowly increases as you open the valve and the pump starts to deliver water.

Tests using Zeolite instead of normal sand had no negative effects on the cleaning process if any it proved to be more efficient. I can only surmise that it is because of the mass of Zeolite compared to sand. Being lighter than sand, it allows for easier lifting of the sand and one can reduce the air pressure.

As far as safety is concerned I have confirmed with both the manufacturer of fiberglass sand filters as well as the rotomoulded plastic sand filter that the drilling of the small 13mm hole through the casing, to allow for the air inlet, holds no negative results, and that no structural failure should occur. .Sand filters are apparently tested to up to 6 bar pressure.


I am of the opinion that this method will also work very effectively on the backwashing of swimming pool filters but actual tests are still pending.
The courser sand utilized in a sand filter for koi ponds (3 to 5 mm) may warrant the replacement of the collector arms with the ones below. This replacement will allow for improved flow of water during normal filtration as well as improved flow of air and water during backwash.


For technical information and further enquiries, please contact This email address is being protected from spambots. You need JavaScript enabled to view it. or on his cellular phone: 0836254640"



Last Updated on Monday, 23 August 2010 20:59