**** Use & Abuse of High Rate Sand Filters on Koi Ponds ****

(as posted by Chris Neaves on the KoiVet web board, 3-26-00.)

This posting is an essay for some years ago.

A few years ago it was unheard of to use a swimming pool or industrial high rate sand filter on a koi pond. The reasons invariably given were - they clog up too quickly or "they don't work". High rate sand filters have been purpose designed to remove minute solids from water, generally, the swimming pool in domestic use. The internal dimensions, the sand size, the backwashing facility, the internal arms of this piece of equipment are all designed for use with chemically treated water.

Adapted for use on koi ponds, high rate sand filters are convenient to use as well as being very efficient. Yes, when used as mechanical filters they do clog, just like all filters - but that means they are doing their job of trapping solids and organics in an accessible place. If sand filters do not clog then they are not performing their primary function of removing solids from the water. This convenient place, the sand bed, is very easy to clean in a matter of minutes. This can be done in your dress suit if you wish to impress visiting koi enthusiasts or the neighbours or your wife. The convenience of sand filters is further enhanced by the fact that they are under pressure and can therefore, be placed literally anywhere - partly buried in the ground or placed around the corner far out of sight. Easily hidden behind shrubs, they are extremely versatile. They take up very little space. Being under pressure the return can be directed anywhere - to a waterfall or stream or to a biofilter or back to the pond. The multiport valve or open valve system makes it very easy to flush the gravel bed. The backwash is also under pressure, therefore, the expelled water can be utilised anywhere, such as on the garden or directly to waste.

Sand filters are surprisingly cost efficient. So there are numerous advantages to using high rate sand filters on koi ponds. The main benefit of high rate sand filters is that they produce truly clear pond water when used correctly. There is also a hidden advantage - the sand bed has an enormous surface area for bacterial growth when sand filters are used as biological filters.

Koi enthusiasts are using high rate sand filters AND NOTHING ELSE on their koi ponds. With great success, I may add. The regular weekly backwashing has no apparent detrimental effect on the biological efficiency of the filter.

Interestingly a number of years ago, a koi enthusiast converted his swimming pool to a koi pond. He installed four large high rate sand filters as the first step. Initially funds were limited on the whole project, so he decided to leave the high rate sand filters and then build the complex, multi-chamber biofilter, which he copied from a book at a later stage.

The result, in a very short space of time, was truly clear water with no detectable impurities although there was a reasonably high stocking density of fish. The biofilter construction was postponed several times during the course of the year. Concerned at the health of the koi the water was tested on odd occasions. He was confronted with the reality of a crystal clear pond and no measurable ammonia, nitrite or nitrate levels. However, when he discussed his filter layout with "experts" he was continually told that his system would not work and his koi would soon die if he did not install a biofilter with a multi-chamber design. I believe that to this day his system is exactly the same as the first day he installed it years ago and the 'biofilter' has never been built. Most importantly, all the koi are alive and well with excellent growth.

I had a fibreglass pond in the front garden with a capacity of 3500 litres which successfully housed 50 x 15 - 20cm high grade Japanese baby koi for over a year. The complete filtration system was a single twenty four inch sand filter (P24 inch Coliquip) with 1mm swimming pool sand. True, I did flush the filter twice a week or sometimes three times a week (for about a minute or two) and true I did occasionally open the access port and turn the sand bed with a small hand shovel as it was compacting around the sides. The backwashing resulted in a 20-25% water change each week, which by-the-way was most beneficial to the growth of the koi. This filter was used together with another sand filter on the new 20,000 litre pond I built a few years ago. After 6 months the fine sand was replaced with the next size larger that was available, namely 3 - 5 mm silica gravel as used in aquariums. Together with the two sand filters I have a single chamber biological filter. The results are there for everyone to see.

In this pond, which is only 80cm deep I have grown show quality koi to 75 cm in 4 years.

Today high rate sand filters are making an appearance everywhere. They are being used as mechanical and biological filtration systems, they are used in conjunction with specialist biological filter media, they are used in conjunction with vortex systems, they are used on timing switches to 'polish' the water once a week while the rest of the system runs continuously. They are extremely versatile and koi keepers are finding new applications and combinations.

The reasons for this type of koi pond filtration rapidly growing in popularity are easy to see. The bits and pieces are readily available and are easily fitted together by anyone. Complete units with all the joints and elbows that are ready to plug in and turn on are available at koi outlets and swimming pool dealerships. The chamber is free standing and can be plonked down anywhere above or below ground. Because of the mechanical and biological benefits they are very cost efficient. Some of the other reasons for their popularity are given above.

The sizing of sand filters is a hangover from the American measuring system and are referred to in inches across the widest part of the chamber. Some manufacturers refer to them in centimetres. Let us examine the uses and abuses of high rate sand filters on koi ponds.

High Rate Sand Filters:

High rate sand filters are designed for use with chemically treated water. This is a very important point 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. 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 will be adequate for the average swimming pool.

In 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 out side continually, especially in dry parts of South Africa.

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 it's 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.

Flow Rates:

Good flow rates / pond turn over rates are critical to good koi keeping. 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 multiport 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. It is critical to maintain flow rates in koi pond situations. (another essay on what is probably the most important point in fish keeping - flow rates is available).

Overcoming Restrictions:

Removing the fine sand from the filter and replacing it with a SLIGHTLY larger diameter gravel 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.

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. The arms at the bottom of the sand filter have fine slits of less than half a millimetre (± .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 multiport 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 multiport valve will greatly increase it's life and prevent grit from entering the groves around the handle.

The multiport 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 multiport valve.

Surface Area:

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 quickly from the pond system. Therefore, we must anticipate some problems if we attempt to move 20,000 litres 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 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. This in turn means frequent maintenance by backwashing. 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 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 multiport 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 backwashed 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 litres. When we have ponds larger than 8,000 or 10,000 litres we must add another filter. Therefore, a 24,000 litre pond should have three 30 inch sand filters with 3 - 5 mm gravel. My personal pond of 18,000 litres is running very well on a P30 inch and a P24 inch sand filter (in parallel of course). Should you decide not to run the sand filters continuously (i.e. by-passed for part of the time) the total number can be reduced. They would have to be backwashed regularly before reuse and bacterial growth would be negligible in the gravel bed.

Food For Thought:

Each pond is unique. Some ponds have more sunlight (read more algae) some ponds have greater dust and pollution settling on the pond. Some larger ponds may very well be clear and run for some time without flushing with a single filter. Calculations can be made regarding costs for different circumstances. For example - two 18 inch filters have 36 inches of surface area in their combined sand beds. The cost of two 18 inch sand filters is less than one 30 inch sand filter. We therefore, have more than double the surface area and two multiport valves as well as more gravel which results in far greater overall flow through the system, longer intervals between backwashing and an enormous surface area for bacterial growth.

Pumps and Sand Filters:

Generally speaking pump capacity is linked to the size of the filter by the manufacturers. This is not for the flow through the filter but to give adequate punch to the backwashing. Placing a ,55 kW pump on a 30 inch filter or even two 30 inch filters works very well when the water is moving through the gravel beds in the normal way. However, the backwashing is never entirely satisfactory because the smaller pump does not have the pushing capacity to completely lift and break up the gravel bed. Therefore, the sand bed will clog more readily because certain sections compact over a period of time. (see the next section - tricks? for some answers).

For this reason manufacturers give the following pumps specification to sand filter dimensions: 18 inch = ,55kW: 24 inch = ,75kW: 30 inch = 1.1kW

Tricks?

There are some 'tricks' that can be used to modify this ratio of pump volume to filter size. As discussed a smaller pump on a larger filter does not have the power to lift the weight of the gravel to break up the compacted top few centimetres. A 30 inch sand filter contains 200kg of sand for example. Opening up the filter chamber from time to time and manually turning the sand bed will break up the compacted areas of gravel that are clogging. Adding a Jacuzzi air blower at the same time as back washing (provided a port hole is open if your filer has one) will blow an enormous amount of air through the sand bed and loosen the gravel. Reducing the amount of gravel will not affect the mechanical filtration too much but will lighten the total weight and make it easier to flush completely.

In order to do this the sand filter chamber has to be opened from time to time. There are models on the market that are secured by bolts around the rim and there are models that have an easy access port on the side with a single butterfly release.

On the point of using a Jacuzzi air blower on a sand filter. The two sand filters on my pond (24 inch and 30 inch) run on a ,75 kW pump. This is contrary to the specifications given by the manufacturers. Once a month, in summer, the port holes are opened, the air blower is added to the inlets from the pump and the gravel bed is manually turned while the filter chambers are boiling like a kettle from the air. Care is exercised not to damage the plastic arms at the bottom. During winter this is not necessary and the filters run for long periods without backwashing. It is interesting to note that these filters can be backwashed in the normal manner until the water coming out of them is absolutely clear. Adding the air blower immediately after this dislodges an amazing amount of solids that are still be trapped in some remote corners of the gravel bed.

Points of Concern:

Sand filters are sometimes by-passed. If this is done for any longer than a day it is absolutely necessary to remember to back-wash the filter before it is used on the pond again. The water trapped in the chamber is cut off from the oxygen in the atmosphere. It will become anaerobic within a day or two. Large amounts of anaerobic bacteria are not healthy for the koi and should not be pumped to the pond. Anaerobic conditions can be detected by a very obnoxious smell.

A Bonus for Sand Filter Users?

It was mentioned earlier in this article that sand filters provide a very large surface area for bacterial growth. We know that as soon as the pond water moves there is bacterial growth on ALL the surfaces in the pond system, the walls, the floor, the piping, the waterfall. This applies to the gravel in a sand filter as well.

A cubic meter of 19 - 21 mm stone has 180 square meters of surface area for bacterial growth. A cubic meter of 10 mm stone has 360 square meters of surface area. A cubic meter of 3 - 5 mm gravel has 800 square metres of surface area.

A single 30 inch sand filter contains 5 bags of 3 - 5 mm gravel. This provides 130 - 150 square meters of surface area for bacterial growth. Therefore, two 30 inch sand filters will have considerably more surface area than a cubic meter of plastic rings for example.

Exchanging Ideas:

Because of the increased interest in using high rate sand filters on koi ponds some distributors and manufactures are testing modifications to the multiport valve systems and arms at the bottom of the chambers for increased flow rates and more efficient backwashing, making access easier, although there are sand filters already on the market with easy access ports. Lighter shells that will still be strong enough for easy use are being tested, producing sand filters with larger surface areas across their beds are but some of the developments taking place.

Summary:

High rate sand filters are very efficient mechanical filters. Efficient mechanical filters trap solids in a convenient place. The more solids that are removed from the passing water the quicker any mechanical filter will clog up. But the result will be clear water.

Koi pond filters have to deal with water conditions different from swimming pools. The pond eco-system is producing various organisms continually, the pond water is being polluted continually. In a swimming pool the living organism are killed with chemicals. Therefore, they are not growing continually.

Consideration must be given to using more than one sand filter on larger ponds. These must be in parallel so that system will benefit from improved flow rates because the restrictions from a single sand bed and a single multiport valve and restrictions in the arms will be eliminated. This results in large surface areas of mechanical and biological filtration and extended time between backwashing.

The standard fine sand is extremely efficient at removing solids. Consideration can be given to replacing this with a slightly larger gravel such as 3 - 5 mm.

Using larger gravel than 3 - 5 mm will result in the arms at the bottom of the chamber blocking. The gravel bed can be turned by hand occasionally. Easy access to the chamber will make this job more pleasant.

By-passing sand filters for any length of time allows anaerobic bacteria to grow in the chamber. The filter must be flushed before being used on the pond.

The sand or gravel in high rate sand filters has an enormous surface area for bacterial growth. The damage to the biomass by regular flushing is minimal as koi keepers have run their ponds successfully for many years on nothing else but sand filters.

Chris Neaves

Editor's note: I don't want to disagree because I've only heard from a half dozen people using Sand Filters on Koi ponds, but I take it from these people that they're backwashing every day, and even every 6-12 hours during the most algae prone times of year. (Dr. Erik Johnson)