Low pressure or open filtration systems
In these systems there is a small measure of additional complexity involved. Instead of having a pump push water from start to finish throughout the filtration starting with sucking the water from the pond to pushing it back into the pond, an open or low pressure system has the pump pushing water into or pulling water out of the filter systems only.
In open systems then gravity plays a role of either feeding the filtration, or in returning the water back to the pond after it has passed through the filtration system.
In the case of swimming pool conversions though, there are almost no cases in which gravity can be used to push water into a filter system since swimming pools are not designed with large diameter bottom drains. So in almost all cases water is fed into the low pressure filters by a pump. This water then overflows from these filters back to the pond via gravity.
The significance of this is that your pump only has to get water into the filters. It therefore has less resistance to work through than a pressurised system.
Further, open systems can have oxygen added to them, allowing for an oxygen rich filtration system. It also ensures that at the end of the process you are delivering oxygen rich water back to the Koi pond.
So it's smiles all around.
Low pressure systems come in a variety of different flavours. Some use brushes, others use Japanese matting, still others use a moving bed. Our preference is for the moving bed as this is by far and away the most efficient bang for the buck with the lowest maintenance overhead.
It doesn't really matter what you use as long as the system works for you as you are the one who is going to have to live with it.
As with all Koi pond filtration the same two pronged mechanical and biological functionality is used with open or low pressure systems as it is with pressurised filtration.
Typically mechanical filtration is achieved by using any one or a combination of the media as outlined above. Open systems offer tremendous flexibility in that you can quickly and easily change the media that you are using with little modification to your existing equipment required.
Similarly your biological stage can be similarly flexible and again any or all of these differing media can be used. Don't get hung up too much on surface area - what is far more important than having masses of surface area for bacteria to grow on is having sufficient ACTIVE surface area full of the good bugs we desire to see in our filters.
Open systems can often combine mechanical and biological filtration within one common vessel. This makes for space savings and often easier maintenance.
Disadvantages of open systems are that if you have a blockage on your return water line they can overflow and you could possibly empty your pond as a result. Overflows should thus be installed as a safety net to prevent this from happening.
A further disadvantage is that the overflow back to the pond can be tricky to regulate. Siphoning can occur which means that the return pipe back to the pond suck up air with the water and you get gurgling taking place. This is overcome by returning water to the surface of your pond so that you don't end up with bubbles coming out of your return pipes. Alternately you can plumb your system in such a way as to avoid this effect.
Open systems do offer you a measure of flexibility in terms of your filtration options. You can construct them as permanent structures or you can easily customise a couple of plastic drums to get the job done for you. In other words open systems allow you a measure of flexibility that pressurised systems cannot and this is one the reasons we like them so much.
Again, the sky is the limit and the only constraint is your imagination as to how you want your system to work for you. If you have not played with filtration before, it can seem daunting but we're here to help you.