Conventional Filtration – Basics and requirements

• An efficient filter comprises two important stages – Mechanical and Biological. It is important to understand what parts of your filter are designed to do each job. Koi pond filtration efficiency is affected by the complete pond system design as well as the filter design itself.
• Contemporary koi pond filtration is normally based on pond systems incorporating bottom drains, which act as the primary feed for the modern gravity fed filtration system.
• Ponds incorporating pumped filter systems will never acquire the same efficiency as gravity fed systems because the pump liquidises waste and feeds this ‘oxtail soup’ to the primary stage of the filter and consequently the filter has to work much harder mechanically to clear the water and most filters are too small to provide the necessary level of settlement required for prime water quality.
• Gravity fed systems are installed so that the water level in the filter is the same as the pond (with no flow). With water flowing the level of water in each chamber will be progressively slightly lower
• Regardless of the type of filter system employed, the transfer ports between chambers in the filter must be large enough to cope with the maximum flow rates anticipated to avoid each chamber being progressively starved of water.

Mechanical filtration

• In a conventional gravity filter we have one or more bottom drains feeding the primary filter stage, normally a vortex. The vortex removes a large portion of the solids present by utilising tangential force to move the dirt to the outside wall of the chamber where friction with the outer wall slows it down and causes it to drop to the bottom of the chamber which is acutely coned shaped. A drain valve at the bottom of the chamber allows the accumulated gunge to be flushed to waste.
• As an alternative a large settlement chamber can be incorporated to allow the dirt to drop out of the water flow to the bottom of the chamber. However, vortexes are very space efficient and to get the same mechanical performance from a settlement chamber it would need to be much larger in terms of volume than a vortex.
• The 2^nd chamber of a conventional filter is normally a brush chamber which carries on where the vortex left off. i.e. It is designed to trap the remaining gunge that passed through the vortex. Brushes are actually also very efficient although they are a very old idea. They do however need fairly regular cleaning to ensure they are effective and do not block.
• The first two filter stages described above are normally the minimum requirements for efficient mechanical filtration. If space and budget permits, extra vortexes can be incorporated as can extra brush chambers for enhanced mechanical performance.
• Mechanical filtration is by far the most important element in the pond filter design as without good mechanical filtration, the biological stages of the filter can never work at full efficiency and can eventually become blocked rendering them ineffective. With poor mechanical pre-stages in a filter, the biological stages have to cope with solids, which they are not designed to do. It is a fact that most filters that fail to work correctly fail because the mechanical stages are inadequate.

Biological filtration.

• A biological filter can comprise one or many chambers each containing different media of the users choice in order to achieve the correct level of water purification.
• Biological stages of your filter work by providing an ideal environment for colonisation by bacteria, which are responsible for the nitrogen cycle.
• Firstly Nitrosammonas bacteria breed in the presence of ammonia and live on these compounds as a food source. They oxidises the ammonia and convert it into nitrites. In the presence of nitrites, Nitrobacter bacteria breed and convert the nitrites to harmless nitrates again by oxidisation.
• Both these bacteria are vital for effective pond filtration and they are aerobic bacteria. They can only thrive in an Oxygen rich environment.
• To ensure optimum biological performance we must therefore provide a constant supply of Oxygen to the filter. This is normally achieved by utilising an appropriately sized air pump and air stones within the filter system.
• The airstones can be placed within the transfer ports of the biological stages of the filter, or directly under the chosen media, but should not be place on the bottom of the filter to avoid putting any remaining debris into suspension.
• Media can be for example, Spa, Alphagrog, Japanese matting, Hortag, Sintered glass and Kaldnes.
• Free flow media works best as it does not trap gunge but allows it to pass. Therefore the best conventional media to use is open flow type medias such as Japanese matting. Alphagrog. Hortag and Spa are not free flow medias and can have a tendency to block over time and therefore need to be cleaned more often.
• Remember that matting should be placed in the filter chambers as vertical cartridges and not placed horizontally in order to avoid any possibility of dirt entrapment.
• The new media Kaldnes, is not only efficient at removing ammonia and nitrite, but is the first media to be used in a conventional filter system which actually removes nitrates as well. This give the added advantage that blanket weed growth is much reduced. It is also self cleaning and therefore requires minimal maintenance.

Additional stages.

• Additional filter add-ons can be incorporated to improve water quality further, examples of these devices are:-
• Surface skimmers - used to skim leaves and other floating debris from the pond surface and thereby avoid this material sinking and/or rotting on the pond floor.
• Protein skimming - Protein skimmers remove proteins from the water before the turn into ammonia and therefore aid bio-filtration and improve water clarity. A build up of proteins in the water makes the water look yellow and oily and causes foam to build up on the surface by water returns.
• Trickle towers - allow water to run over media in a chamber so that the media is never submersed and is therefore Oxygen rich. Trickle towers are very efficient bio-filters and also remove Nitrates from water.
• Ultra Violet lamps - designed to disrupt algae cell growth and give clear water
• Fluidised bed filtration - very efficient bio-filtration using coral sand or similar as media which is housed in a tower in such a way as to ensure that the media is constantly in motion and thus Oxygen rich.
• Ozone systems – very efficient at disinfecting the water, providing superb water clarity and providing oxygen rich water.

Sizing and installing the filter.

• The text book rule for filter sizing states that you need to turn over your entire system gallonage through the filter every 2 – 3 hours.
• Water retention time in the filter should be at least 15 minutes ideally.
• So if we use the example of a pond and filter of 5000 gallons we need a pump of 2,500 galls per hour. (This allows for some flow drop in the pipework).
• A conventional filter therefore needs to be 750 gallons to provide the necessary retention time
• Flow rate through the filter can be tested once the system is up and running to ensure the turnover rate is as expected. Back pressure on pumps can be severe and will drastically reduce flow rate if undersized piping is used in long runs with many bends. Increase pipe diameter for longer pipes runs to avoid back pressure.
• Providing the flow rate appears to be correct carry out further tests as the system matures by testing the ammonia level in the pond and in the last chamber of the filter. If the flow rate is correct, the ammonia levels will be approx. the same in both cases.
• If it is higher in the pond than the filter your flow rate is too slow.
• More recent thinking relates the filter size required in direct proportion to the volume of food being consumed and the size of the fish population as both these elements dictate how much ammonia is produced an a daily basis and therefore how large, biologically, the filter needs to be.
• Essentially your filter must be capable of supporting a large enough bacterial population to cope with the total loading of waste being produced in the pond system.
• When installing the filter, keep pipe runs as short as possible especially from bottom drain to vortex. Use 4" pipework. Always incorporate a valve to isolate pond from filter to allow for cleaning and back-flushing of bottom drain pipework.
• Ensure each chamber of filter has a separate drain to allow flushing to waste.
• Design of transfer ports or pipes between chambers should allow isolation of each chamber for cleaning
• Use one vortex for each bottom drain – ideally a separate filter system for each drain if the budget allows.