Flow Rates

High rate sand filters are designed for use with chemically treated water. This point is important to remember. In a swimming pool, after chemicals have been used, the algae and micro-organisms in the water die. This results in their cell structure collapsing. The collapsed cells takes up less space. Further, the dead cells do not produce the organic slime found in live ponds. The chemicals poison the organics in the water so they take days or weeks before they start to grow again, therefore, the load on the filter is minimal. A single sand filter, with fine sand will be adequate for the average swimming pool.

High Flow rates produce clearer waterIn our ponds the situation is totally different. Organic substances and micro-organisms in the water are produced and die continually. The pond is polluted from the outside continually. Therefore, we have to change the parameters for the use of high rate sand filters on koi ponds from the way they are used on swimming pools.

Single cell algae that makes the pond water green can reproduce its self up to about 30 times an hour, under ideal conditions. Many kilograms of organic material are produced from within the pond each and every week – and this is not even taking the fish into account.


Ideal Flow Rates:

It is critical to maintain flow rates in Koi ponds. Flow rates dictate the amount of oxygen you will dissolve in the water. Flow rates will dictate the speed at which the impurities are removed form the pond water. It makes no sense to work out the correct flow rates and turnover rates of a koi pond then add a massive restriction in the form of a single sand filter. The flow rate through a sand filter is greatly restricted by the nature of the fine sand or gravel in the bed and the limited surface area of the sand bed itself. The multi-port valve, that is standard equipment on the filters, also restricts flow rates. The internal arms at the bottom of sand filters are designed to have minute slits in them of less than 1mm. The reason for this is to prevent the fine sand from being forced up the arms and blocking the water flow.

Taking the above into account we have to “redesign” high rate sand filters for Koi pond applications.

Use this tool to help with calculating flow rates

In addition to the above tool, this site also have a great guide – http://www.aquaart.com/pondcalc.html

Check out this great video on a high flow pump: https://www.youtube.com/watch?v=1U6WG4rWdpw

Overcoming Restrictions:

The first step is to remove the fine sand from the filter and replace it with a SLIGHTLY larger diameter gravel. This will improve the flow rate significantly. Very fine sand clogs rapidly because of it’s efficiency at removing the solids resulting in the necessity of back-washing several times a week. But, wow! the water IS crystal clear with this fine sand. By using a 3 – 5mm gravel largely over comes this restriction. This is gravel used in aquariums. Back washing will be confined to roughly once a week in summer (various organics building up) and once every two or three weeks in winter.

A small price to pay for CLEAR water. And remember all filters need regular cleaning no matter what the design.

It must be pointed out that ANY filter on a Koi pond that has fine mechanical filtration will clog up rapidly.

Stone of 10mm or larger is used in some sand filters. This is done to increase the flow rate to the point where the filter chamber rarely needs flushing. In other words the sand filter is no longer used for it’s primary purpose – mechanical filtration. Caution must be exercised if gravel/stone of a larger diameter than 3 – 5mm is used. Larger gravel/stone will not trap the fine solids.

The arms at the bottom of the sand filter have fine slits of less than half a millimeter ( .4 mm) in diameter to prevent the very fine sand used in swimming pool applications from escaping from the filter tank. From time to time these fine grains of sand are forced into the slits by the pump pressure and clog the arms. The swimming pool filter has to be serviced by removing all the sand as well as each arm for cleaning or replacing.

Stone above 6 mm does not have the mechanical filtration effectiveness to prevent some solid particles passing through the gravel bed and clogging the arms at the bottom. Filters with larger stone will not clear the pond water to the same extent as filters with smaller stone/gravel. Remember the sand filter is under pressure and the solids are forced into or through the gravel or stone bed. A filter with stone as opposed to gravel will therefore, have to be completely cleared from time to time and the arms replaced or cleaned.

Always remember when filling a high rate sand filter with gravel or sand to half fill the chamber with water first. This cushions the weigh of the gravel as it is poured in so that the arms at the bottom are not broken or damaged. Another point to consider is the type of plastic the multi-port valve is made of. A few years in the South African sun usually destroys most plastic valves especially those that do not have UV protection. Initially planning a cover over the multi-port valve will greatly increase it’s life and prevent grit from entering the groves around the handle.

The multi-port valve is restrictive. Some sand filters are available with a direct inlet and out let controlled with 50mm valves. This system eliminates the restrictions in the multi-port valve.


Surface Area Parallel Maintaining Flow Rates:

An eighteen inch, twenty four inch or thirty inch sand filter gives you that surface area. Which, relatively speaking, is not very large. It is small when we consider the continual build up of organic and in-organic substances in the pond water that need removing. We need to maintain flow rates for oxygenation, and to lower ambient ammonia levels but still remove solids. Therefore, we must anticipate some problems if we attempt to move 20,000 liters of pond water, literally a biological soup thick with algae and microscopic life, that has not been treated chemically, through a single filter chamber with only 18, 24 or 30 inches of sand bed surface area.

A single high rate filter works very well on a small pond of perhaps less than 10,000 liters but on large ponds they are effective for a limited time. The larger the pond volume the quicker a single sand filter will clog up. A single sand filter will reduce your turnover rate significantly. This in turn means frequent maintenance by back-washing. As a matter of interest many Koi books and magazines contain recommendations that solids must be removed from the pond on a daily basis. Higher stocking densities in any pond make regular cleaning of the pond filter system essential.

Daily removal of solids and slow flow rates can be overcome by having two or more sand filters IN PARALLEL. With two or more sand filters in parallel the water moves through several multi-port valves (far less restriction) and then several sand beds at the same time. This larger volume of water passing through the sand beds slows down because of the increased surface area. In turn this is beneficial for mechanical filtration AND biological filtration. When the filters are back washed each filter must be back-washed separately. This gives maximum flushing capacity to each sand bed.

Generally speaking a single 30 inch sand filter would be adequate for a pond of between 6,000 and 8,000 liters. When we have ponds larger than 8,000 or 10,000 liters we must add another filter. Therefore, for example, a 24,000 liter pond should have three 30 inch sand filters with 3 – 5 mm gravel in parallel.

Read my conclusions here