Filters work in a number of different ways. Various processes remove algae, toxins, pollutants and suspended solids. These systems require different filtration materials and equipment. An efficient filter will utilise a number of different filtration methods to ensure the water emerges clear and healthy.

Mechanical filtration is the removal of suspended solid materials from the pond water. High levels of hard suspended materials, such as sand, can damage fish’s delicate gill membranes, and fish faeces, uneaten food, dead algae, etc left in the pond will gradually breakdown in a process called mineralisation. Mineralisation is the conversion, carried out by bacteria, of protein (or other organic material containing nitrogen) into ammonia. Water with high levels of suspended solids make the pond look uncared for and a build up of ammonia will put strain on the ponds biological filtration.Mechanical filtration is achieved by blocking the suspended solids, or allowing them to settle onto a surface. These solids can then be removed from the filter so that they don’t begin to mineralise, and so the pond water departs the filter cleaner. Mechanical filtration is aided with a calm flow of water into the filter. Turbulent water, or water under high pressure, will agitate the solids making them less likely to settle, and can cause settled matter to become re-suspended.There are two types of mechanical filtration; gravitational filtration, which relies on gravitational settlement to remove debris, and particle filtration, which is the physical blockage of suspended particles.

Gravitational Filtration

A settlement chamber is a simple box design, with inlet and outlet pipes positioned towards the top of the unit. The water in a settlement chamber should be quite calm so that as pond water passes through the chamber any denser solids in the water should fall out of suspension and settle at the bottom. This build up of waste can then be removed via a bottom tap or sludge drain.In a vortex, or swirl chamber the pond water enters a cylindrical chamber and is forced to spin, creating a whirlpool effect. The suspended solids are forced to the edges of the chamber where they eventually drop to the bottom and the cleaner pond water leaves the vortex through a pipe set at the top centre of the unit. Again, a bottom waste tap can be installed to remove the settled debris.

Particle Filtration

Common materials which are used as mechanical filters are brushes, Japanese matting, foam layers and polishing pads. These materials physically trap material or allow it to settle on their surface. If combinations of these materials are used in a filter they should be arranged so the courser material receives the water first, this will allow the suspended solids to be removed in order of particle size, largest first, which vastly reduces the rate at with the filter may block up.Foam layers are usually manufactured with a flat surface and a dimpled surface. The dimpled surface should face the oncoming water as this side has more surface area and will not block as quickly, again reducing the maintenance requirements of the filter.All mechanical filtration media will harbour vital bacteria. When replacing foam layers, or washing the materials in chlorinated tap water this bacteria will be lost. It is important to remember to replace these lost bacteria after cleaning the filter.