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Chris on Koi

Use and Abuse of High Rate Sand Filters for the 21st Century Koi Keeper

There are only two types of koi pond filters. They can be classified as pressurised and non-pressurised. One of pressurised filters used on koi ponds are the sand filters adapted from swimming pools . As time marches relentlessly into the 21st century we review an enigma from the murky world of koi pond filters – the sand filter. This dreaded, misunderstood piece of equipment is still despised by some but utilised successfully by many.

In the 80’s koi keepers were experimenting with different filter systems and it was discovered that the freely available, pressurised swimming pool sand filter made a very good mechanical filter for a koi pond. Sand filters were freely available and easy to install. It was also discovered that as this apparatus worked efficiently the fine sand bed clogged up rapidly and the end result was clear water. The speed of clogging depended on the amount of organic material (both bacterial and algae) produced by the individual pond and the size of the sand granules. In some cases, when fine sand was used, the result was frequent backwashing every second day. Chemically treated water in swimming pools does not produce this rapid clogging as the chemicals kill the bacteria and algae. But we are dealing with life in our ponds and we have to adapt accordingly.

Another discovery, probably accidental, found that as well as being a good mechanical filter the swimming pool filter filled with sand or gravel also acted as a good biological filter (bioconverter). The fundamental principle in koi pond keeping that the bacteria in our systems is found on all surfaces supported this discovery.

Over the years experiments were done and modifications were made to improve the sand filters performance as used on koi ponds. Where possible the groves in the arms at the bottom were opened slightly. The fine sand was replaced by slightly courser gravel to overcome the slime from the bacterial biofilms which stuck the sand particles together. Some sand filters have the multiport valve, so very restrictive of flow rates, replaced with a valve system to try to improve flow rates. It was observed the bacterial load increased as the pond matured and so did the crust on top of the gravel bed. This was broken up when backwashing but it helped to turn the bed over by hand or with a spade from time to time. The influence of the billions of bacteria and billions of algae cells in our ponds became apparent.

Sand filters are pressurised filters. Many variations of this humble filter have now been produced all around the world but they all have a common characteristic – they are closed or pressurised. Certain principles apply to pressurised filters and certain principles apply to open systems. [I will publish the Classification of Koi Pond Filters when I have finished researching the subject]

With the humble sand filter we have an instant koi pond filter that can be conveniently placed anywhere and is free from the constraints of open or gravity fed systems. One of the very basic principles of koi pond keeping is to turn the pond water over in under two hours. If this can be done at a faster rate then all the better. There are very specific reasons for this. Firstly, certain things have to introduced into the pond water before we can keep koi. Oxygen - the water has to be activated with oxygen. We do this by bringing the water into contact with the atmosphere via waterfalls, streams etc. The faster this is done the more oxygen will be introduced. Secondly fresh water has to be brought into the pond to dilute the impurities that are constantly building up. If you have a mountain stream running fresh water into your pond then fine – but for the majority of us “fresh water” is brought into our ponds via recycling the water through the filter system.

As we are adding oxygen and introducing fresh water there are pollutants being added to the water. These are ammonia from our koi, algae, bacterial loads, dust etc. Not only this but our koi are removing oxygen from the water each and every time they breath. The biological filter is also removing oxygen to do its job. The micro-organisms in the pond also compete for the oxygen budget. So there has to be a constant balance between what is added and what must be removed from the pond water. This is one of the reasons why the pumps run 24 hours a day.  When the balance between what is added to and what is taken out of a koi pond is disturbed we have a host of secondary problems and more often than not - sick koi.

Some examples of factors that can disturb the balance in our ponds. (1) Koi Growth - As our koi grow they consume more oxygen and they excrete more impurities relative to their body size. Every time a koi doubles in length the body mass becomes about 8 time more – not double. So if we have a koi collection of 10 small koi and they double in size we suddenly have the equivalent of 80 small koi to contend with. This is why the planning of turnover rates are so important and one of the reasons why the faster turnover rates are critical. (2) Stocking densities – too many koi for the water volume to begin with or adding more koi with out removing some. (3) Flow rates slowing down – as pumps and filters clog with bacterial and organic material the flow rate slows down. This can result in the ambient ammonia rising.  (4) Restrictions in flow rates because of gravity fed piping. A series of vortex chambers which have 90mm piping connecting them will have the flow rate restricted to the volume of water that can be gravity fed through this pipe. Should you have a pump with a capacity greater than this flow rate the vortex system will suck dry or at least suck air. The pump will have to be throttled back to avoid this.

As a guide the basic principle of a pond turnover in under two hours is practical. This usually accommodates most pond situations. Keeping to this principle is quite difficult. Simply taking the pump specifications and dividing the pond volume is not a good way to calculate actual turnover rates. Any restriction anywhere must be factored into the calculation. Piping is restrictive. This can be over come by using larger diameter piping. 90 degree bends are restrictive. Leaves in the leaf trap of a pump are restrictive. Pressurised systems – all of them not only sand filters – are restrictive. Gravity fed systems with the incorrect diameter pipes or opening between chambers are restrictive. Different types of media have different flow rate characteristics in chambers.

Having a series of chambers or tanks with a 50mm pipe connecting them and then sucking with a large capacity pump at the end will result in the chambers running dry as the water cannot be gravity fed fast enough. Watch what happens in a chamber system where you have brushes in the first chamber, plastic rings in the second, shade cloth in the third and a finer media such as stone or gravel in the last chamber when you are pumping the water through it. The water flows well through the first few chambers but because the flow rates through the last chamber of gravel or stone are different to the others (more restrictive) the system will back-up and over flow.

Most of this is logical but when it comes to sand filters we seem to get a metal spasm. Sand filters are very restrictive of flow rates, sand filter clog up rapidly because they work – so what? So do other mechanical filters that make the water clear. Firstly, the internal shape and size of the multi-port valve is restrictive. Secondly, the sand bed is restrictive. Thirdly, the pump has to be matched to the size of the sand bed or it will not back-wash properly.

We have often observed quite elaborate systems of open chambers flowing quite well but then suddenly a single sand filter is added onto the end to “polish” the water. You will often observe that the water is not clear. But how can this be if the sand filter is working well as a mechanical device? Simply – the single sand filter restricts the flow rate of the whole system. The solids as well as the algae builds up faster than they can be removed. This is the mechanical equivalent of ambient ammonia in a pond.

Ambient ammonia is the back ground ammonia we have in our ponds that can never be eliminated. As the filter is chemically converting ammonia to nitrite to nitrate so the fish are excreting ammonia via the gills. So there is always a little ammonia in a pond. The ambient ammonia will build up as the fish increase in size and as the number of fish increases. It will also increase as we feed more and more times a day. Ambient ammonia can be decrease by increasing the turnover rate perhaps together with a larger filter, decreasing the number of fish and decreasing the feeding quantities.

Therefore, if a sand filter is going to be used on a koi pond we have to take into account these factors. A single sand filter on a koi pond can work provided it is correctly matched up to the actual flow rates that are needed. i.e. the pond volume to the actual volume of water the pump or pumps are moving. A single sand filter is very restrictive of flow rates. Placing two or more sand filter in parallel will improve the flow rates dramatically. This applies to any mechanical filter that is capable of filtering out fine solids from the water.

Another example of restrictive flow rates is the Answer Filter. The Answer mechanical filter introduced some years ago suffers the same problem or if you want to be positive about it – the answer filter is also very efficient at filtering out solids from pond water. The Answer is a very good mechanical filter which pushes the water through a fine screen. (exactly the same as a sand filter with fine sand)The flow rate is determined by the capacity of the small pump pushing the water. The flow rate decreases after time as the slime builds on the screen. The system has to be shut down and the screen cleaned chemically or with a high pressure hose. The Answer filter has to be matched up to the turnover rate you require on your pond. Beyond a certain pond capacity you will need two or three answer filters working in parallel to maintain turnover rates. The same applies to sand filters – and all other restrictive and efficient mechanical filters.

Filter Maintenance
All filters HAVE to be maintained. The lack of maintenance by koi keepers is one of the biggest contributors to health problems. The cleaning of sand filters can be simplified. Yes the sand bed is broken up when back washing. If you have two or more sand filters on a pond you have to backwash each one individually to get the maximum push. As the organic slime produced in the pond is quite substantial you have to open the sand filters from time to time and break up the bed with your hands or a spade. Always been careful not to go too deep and break an arm at the bottom.

There is however another very simple and very effective method of cleaning sand filters – using a Jacuzzi air blower. Sand filter can be backwashed until the water is clear however, if you use a Jacuzzi air blower to agitate the bed a whole lot more trapped solids will be dislodged.

In the following picture you will see the placement of a Jacuzzi air blower. Care must be taken to make sure the valve to the air blower is closed when the pump is on. This picture was taken after the sand filter were backwashed as normal. Note the large amount of solids been dislodged for backwashing out of the chamber.

Whilst the air blower is running the valve from the pump which is used to prevent the air going back to the pump can be opened slightly. This will allow a small amount of water to enter the chamber whilst the air blower is running as push the dislodged solids out of the chamber.

Some Other Tips
Using high capacity air blowers is a very easy and effective method of cleaning sand filters and various other types of filters. They can be used on ANY filter.
Sand filters should be matched up to the pond capacity and the flow rate you want to achieve. A basic guide is to have a 30 inch sand filter for every 10,000 litres of pond capacity. A 20,000 litre pond will need two 30 inch sand filter in parallel. And so on …
When back washing the sand filter each one must be done separately.

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 = ,6kW: 24 inch = ,75kW: 30 inch = 1.1kW

Placing sand filters in parallel

Placing sand filters in parallel


Chris Neaves

Last Updated on Friday, 31 October 2008 14:25