Sand Filters Are A Bad Garden Pond Filter Choice
Why Is A Sand Filter Not A Suitable Biological Pond Filter?
The use of a sand filter in a garden pond as a biological pond filter is not a good idea, although there are those fish keepers who still use one as a means to remove solid matter.
Sand filters are used extensively in fish hatcheries, sewage treatment applications and in swimming pools (particularly in warmer climates like South Africa, Australia and the warmer southern states of the USA).
Sand filters from a cost point of view are expensive to run and haven’t been designed to remove the nitrogenous waste matter (ammonia) that needs to be removed from any garden pond or fish pond.
They have instead been designed to remove large particles of waste, although they do perform nitrification of ammonia, to a lesser extent.
The major purpose of a biological pond filter in any fish pond or water garden is to remove ammonia, which can be highly toxic to pond inhabitants in high concentrations.
The biological process responsible for removing ammonia is known as the Nitrogen Cycle and is critically important to any aquatic pond system.
The secondary function of a pond filter, which many novice pond keepers actually believe is the real purpose of a pond filter is to mechanically remove pond debris such as leaves and other organic matter such as flocculated algae (dead algae that has been destroyed by a UV sterilizer and has clumped together).
This article will hopefully help to convince you that using a sand filter in your garden pond actually has more disadvantages than advantages. I will also recommend suitable alternatives to using a sand filter for biological pond filtration purposes.
Why Are Sand Filters Bad For A Garden Pond?
Before I discuss my reasons why I believe that a sand filter has no place in a garden pond, I would quickly like to add that I am referring to a mechanical sand filter and not a fluidized sand filter, which has been designed for use as a biological filtration system for use in a pond environment.
As stated earlier in this article, sand filters are expensive to run on a continuous 24/7 basis. They also offer very little if any nitrification properties. The reasoning behind using a pond filter in the first place is to remove ammonia from your garden pond before it creates a toxic environment that is unsuitable for aquatic life, including pond fish, snails, frogs and toads etc to live in. Without the ability to perform the nitrification process (nitrogen cycle) which removes ammonia, it just does not make sense to me to use a sand filter as it doesn’t perform this critically important function.
In a pond environment containing ammonia, it is the nitrifying bacteria, Nitrosomonas and Nitrobacter that are responsible for converting ammonia (toxic), firstly into nitrite (toxic) and then into nitrate (much less harmful). These microscopic beneficial bacteria live on the pond filter media, housed in the biological pond filter chamber. In order to ensure optimum ammonia conversion, certain criteria must be met: Firstly, oxygen needs to flow through the filter media as the conversion requires lots of oxygen; secondly a pond filter media with a high SSA (specific surface area) should be used to allow much larger colonies to form. The larger the biomass (size of colony) the quicker the ammonia will be broken down. Thirdly, turbulence which throws together water, oxygen and ammonia will significantly help to increase the rate at which ammonia can be broken down.
With this statement in mind please note that:
A sand filter has a low surface area on which nitrifying bacteria can colonize.
A sand filter encourages the build up of unwanted heterotrophic bacteria. Heterotrophic bacteria (needs carbon in its diet) unlike nitrifying bacteria (autotrophic bacteria that doesn't require carbon in its diet) is particularly bad in a pond environment as they produce hydrogen sulphide (rotten egg smell), ammonia and carbon dioxide. Let's think about that! We are using a sand filter to remove ammonia but in reality it produces ammonia! There is another disadvantage of encouraging heterotrophic bacteria growth and that is the fact that this unwanted bacteria colonizes much more rapidly than beneficial bacteria. As the heterotrophic bacteria colonies grow they pollute your pond water with pathogenic organisms that cause Koi and fish health problems.
A sand filter is costly to operate due to the number of backwashes required (5 to 6 per day for approximately 5 minutes each time). The flow rate needed to perform normal filtering is the same as that needed for backwashing. Many biological pond filter systems have 2 speeds; one speed for backwashing and a second one for filtering purposes. This saves on water usage and running costs.
A sand filter is only capable of trapping solids in water that measure at least 10 microns in diameter or more; algae cells can be as little as 4 microns, meaning that they would simply pass through the sand without being trapped. As algae cells increase in number they cause algae blooms (green pond water).
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As water flows through the sand channelling starts to occur. This means that holes appear in the sand and water simply flows through, without being filtered.
As far as I am concerned the only viable solution for effective biological pond filtration in a garden pond containing fish is to use a biological pond filter or a veggie filter, although a veggie filter can be high maintenance.
What Type Of Biological Pond Filter Should I Use?
If you have a small to medium sized garden pond then the best choice would be a pressurized pond filter system. These units are far more compact than the traditional black box type pond filter and are much easier to hide from view. Due to the pressurized operation and the back flushing system they are also much easier to clean. Another advantage of pressurized bio filters is the fact that they can be situated away from the pond edge. Traditional box type filters need to be situated near to the pond, usually at the highest point, as they rely on gravity. Due to their size they are also more difficult to disguise and can be somewhat of an eyesore. Two of the best are Fishmate and Oase.
For large Koi ponds the best form of biological pond filtration is the fluidized bead biofilter (fluidized bed filter), more commonly known as a
bead filter.
Bead filters as the name suggests use tiny little beads as the filter media. In certain models there can be as many as 600,000 beads per cubic foot. This means that there is an extremely high SSA (specific surface area) on which beneficial nitrifying bacteria can colonize. These beads are suspended in a turbulent chamber of water that is constantly full of oxygen and ammonia.
The turbulent oxygen rich water, ammonia and large surface area provided by the beads ensure that ammonia is quickly converted in this type of pond filter.
Just to show the levels of oxygen needed for Nitrosomonas and Nitrobacter nitrifying bacteria to convert ammonia into nitrites and then into nitrates, take a look at the 2 equations below ...
- For Nitrosomonas: 55NH4+ + 76O2 + 109HCO3- ---> C5H7O2N + 54NO2- + 57H2O + 104H2CO3
- For Nitrobacter: 400NO2- + NH4+ + 4H2CO3 + HCO3- + 195O2 ---> C5H7O2N + 3H2O + 400NO3-
My advice to you would be to avoid installing a sand filter in your garden pond at all costs; partly due to running cost implications but mainly because of the unhealthy environment it will create. If you currently use a sand filter then remove it from your pond system ASAP.
If you can't do this then replace the sand with a biological filter media such as Fishmate Supra Biomedia and find some method of introducing high quantities of oxygen into it.