Should you go for a gravity or for pump fed filtration system?

We get a lot of queries about this particular issue and it is clear that this is an issue that Koi pond novices just don't understand all that clearly.

Note that this choice only applies to low pressure filter systems rather than closed circuit high pressure systems (where it makes no difference - you can locate a high pressure filtration system just about anywhere. 

Generally low pressure systems use less electricity and are more efficient in giving you maximum water movement through your filter systems with the lowest possible energy input (a good thing!).

Low pressure systems have one drawback however in that their installation is not as easy as with a high pressure filter system. This is because one leg of a low pressure system will rely on gravity, either to feed water to the filters, or to return water back from the system to the pond.

It is thus very simple, but it takes some experience to get your head around it.

Gravity Fed Filtration

A gravity fed filter system relies on a water from the Koi pond over flowing into the filter systems. The pump that drives the system then sucks water from the final stage of the filter system and pushes it back into the pond, causing the water level in the pond to rise and to overflow and re-fill the filters.

If this pump is turned off for any reason then the pond will overflow for a while into the filters which will fill up to the same level as the Koi pond and then coming to what we call the idle state - in other words the water levels in the Koi pond and the filters are exactly the same.

The water levels in the Koi  pond are thus critical. If the water level in the pond drops for any reason you may find that the water flow rate feeding the filters slows down to the point where the pump on the other end empties the filters faster than they can be filled, resulting in the pump sucking dry and burning out on you.

If the pond is over full there shouldn't be a problem however - your filters should always be at the same height as the pond so there will be no chance of the filters overflowing. It is always a good idea to install an overflow into your pond so that that if the pond overfills - say due to rain - the excess water can be drained off preventing the filter chambers from overflowing and allowing all your ActiBioBed media to escape into your garden...

The other challenge that a gravity fed filter system presents is that unless you do your design carefully and with some forethought it is very easy to get flow rates wrong. Trust us, each pond IS different and in some instances we have seen ponds trying to feed a filter with a small 50mm pipe as the sole feed... and this on 15 000 litres of pond water. It simply isn't going to work no matter what you do!

The secret to success with a gravity fed filter system is to allow yourself as much headroom as possible - such that the filters can run with at least double the anticipated flowrate that you intend pushing through your filters. If you over specify this you can do no harm. If you under specify it you will regret it for all time.

Location of the system will depend on your situation. As with all filter systems the closer to the Koi pond the better in terms of flow rates and energy efficiencies. Try and aim at putting the pump side of the filter as closer to the Koi pond side - because if you get your flow rates correct gravity will 'pump' the water to the system to you for free. All your pump has to do it move the water a short distance back to the pond, thereby maximising your efficiency of your pump.

Pump Fed Filtration

A pump fed filtration system relies on a pump to feed the filters and then gravity to move the water back to the Koi pond. Literally the filters overflow with the intention being that the water from the filters overflows back to the pond.

The potential headache for these filters is that if the pump is capable of delivering more water to the filters than gravity is capable of returning back to the pond then the filters will overflow and you will be picking up ActiBioBed media all over again.

Gravity is not the strongest of pumps however. the higher your filters stand above your pond the stronger the gravity pump becomes - but even so there are limits. No matter what you do getting 20 000 litres per hour through a 50mm pipe is not going to happen...(!)

Also, if you have the returns from your filter system returning below water level you will find odd things happening when the system starts up from idle. The pond water creates a small resistance to the flow of water and when the system starts up this can cause the water in the filters to backup and, yes, overflow!

The solution is to use large bore pipes - and again to over specify your return pipe work by a factor of at least 2.

Surging is also an effect unique to water flowing out from a vessel via a constriction, such as a pipe. What happens is that as the vessel starts to overflow water escapes via the outlet pipe at an ever increasing rate pulling air in as it does so. Eventually if the flow rate is fast enough the air trapped in the pipe is expunged and the water velocity in the pipe increases. As it does so it can exceed the fill rate of the vessel and hence begin to suck air again. The process repeats infinitely and is referred to as surging.

In and of itself surging is not a problem. Air trapped within the water return stream is also not problematic although it does cause bubbling at the point of entry of the return water to your pond if your water return is below surface level. Surging can however result in filters overflowing if the flowrates are large enough relative to the diameter of pipe used to carry the water back to the pond.

The general rule of thumb with pump fed filters and gravity returns is to use as large a diameter pipe as possible, and then to over do it! If you return water below the surface of your pond note that you are likely to get some bubbling taking place.

In a pump fed filter system the general rule is to locate the pump as close to the Koi pond and the filters as possible to minimise the piping run it needs to deal with. Again, if you get the calculations correct gravity can 'pump' the water back to the pond for you for free.