Chemical Filtration Ponds

Chemical filtration comes in the form of the media used inside a filter to remove impuritites within the water. In practise the chemical filtration media is used in the 2nd chamber of a non biological filter system, the first chamber will contain the strainer media used for the removal of larger solid particles. A typical combination would be filter wool or sponges and activated carbon as the chemical media. Activated carbon removes ammonia and other waste products from the water by the process of absorbtion, this means that the absorbed substances become loosely attatched to the surface area of the chemical media.
The process of activating carbon media involves heting the granuales of carbon to a very high temperature which opens up millions of pores that creat a huge surface area for chemical absorbtion. This works the same as the filter media, you are looking at the surface area to be as large as possible. Activated carbon does not last forever and is therefore a costly product to use as once the granuals have fully absorbed the chemical impurities, it cannot be reactivated. The most commonly used system for activated carbon is within aquariums and very small ponds of no more than 500 litres ( 110 gallons approximately ).
In larger ponds zeolite is more commonly used as a chemical filter medium, zeolite removes ammonia and nitrite from the pond water. Zeolite consists of hydrated silicates of calcium and aluminium, sometimes with sodium and potassium. IT looks like  off white or pale brown stone chippings and is available in various grades ( grades being the term used for sizes in the stone industry). Again as with all the filter media it is the surface area that is important and the more surface area that is exposed to the pond water will produce more effective results in the removal of the waste products.
The ideal grade to use when looking at a chemical filter media would be around 10cm, any smaller and the chippings will compact down and actually reduce the surface area exposed to the pond water. Filter systems that rely on zeolite alone require approximately 1kg per 5 litres of water, this does sound costly however the benefit of zeolite compared to activated carbon is that zeolite can be washed out, cleaned and reused. The re use of zeolite is due to the chemical reaction of ammonia attaching itself to the surface area of zeolite being very weak and can be easily be broken by adding salt to the water. As the ammonia is released the zeolite actively becomes recharged and is ready for use again.
To clean your zeolite simply remove from the filter and place into a container, add a salt solution of 6 grams of cooking salt per litre of water and leave for 24 hours. Wash the zeolite thoroughly in fresh water before returning back to the filter.
The disadvantages of using zeolite and other chemical filtration medias in your pond is that you need a large quantity of it which makes it a costly option for the larger pond owners. Also the recharging process can become a time consuming and onerous task especially during the summer when the fish are most active, usually the zeolite will need recharging once a week during this time. Finally zeolite media prevents you using salt as a treatment in your pond, simply because the ammonia and other toxic componds attached to the zeolite will be released in the salt solution and be returned directly to your pond.
Many koi keepers use zeolite in conjuction with the biological filter, simply adding the mineral media to the final chamber of the filter. This is particularly beneficial in a new pond system and during the spring, when the numbers of bacteria are low, meaning the zeolite will absorb the ammonia/ ammonium compounds keeping a safe level.