Filtration System

There are many documented kinds of water filtration system for the pond. Many companies have designed water treatment products. These products usually consists of a combination of the following:

• Mechanical
  
• Biological
  
• Chemical
  
• Bead filter
  
• Vegetation/plant filter
  
• Trickle tower
  

Mechanical Filter
Aims to remove physical debris from the water. This debris would be any solids that ought to be removed from the water including fallen leaves and insects and uneaten fish food and solid fish waste. The mechanical filter is usually the first stage in a filter system, and it could be in the form of a simple mesh or combination of large brushes.

The Biological Filter
Aims to cultivate a colony of bacteria that does the actual conversion of ammonia to nitrite and from nitrite (NO2) to nitrate (NO3). The discussion often goes into what is the best media to grow these bacteria. Most widely used are filter mats and also crushed coral chips or sea-shells contained in bags made of mesh nettings. The coral chips and sea-shells served a dual purpose of buffering and controlling the pH level of the pond water. Any increased in water acidity would dissolve more carbonates in the filter and hence maintaining a constant pH.

Bead Filter
The bead filter uses tiny beads as a filter media to provide biological filtration.

Chemical filter
Most commonly used are activated carbons as an absorbent. These are sometimes used in aquariums filter but not practical for use in a pond filter.

Vegetable or plant filter
Using water plants to absorb nitrates.


Features of a Basic Filter
All filters has the common aim of passing “bad” water in and getting “good” water out; removing ammonia, nitrite and nitrates and also any other undesirable constituent in the water like parasites, fallen leaves. The following diagram shows the design of a basic filter system. In fact, this is the design used for my first pond.

This is a gravity-fed system. The pumps located at the last chamber “pushes” water out to the main pond and “pulling” water in through the filter system.

Inlet to the filter, at the first chamber, is from the main pond, through bottom drains using 4 inch (100 mm diameter) diameter pipes. There is also a small skimmer pipe (50 mm diameter) that removes surface debris from the pond to the filter.

The 3 chambers contain the filter media where the biological filtration takes place. The filter media is supported by wire mesh at the bottom of the filter chamber. The first chamber contains filter mats. The original placement of the filter mats was across the water flow. In this case, the filter mat was acting like a mechanical filter which led to problems later on. I later learnt that it is better to place the filter mats vertically, to allow water to flow along its surface.

The second and third chambers contain sacks of crushed cockle shells and coral chips.

The last chamber contains two pumps which forces the filtered water back to the main pond. A third pump serves as the drain pump, removing waste water out to a drain and is turned on only during water changes.

Each filter chamber has a bottom drain that leads to the pump chamber through a 50 mm diameter pipe. Standpipes are used to plug these drains during normal operation of the filter. During maintenance, the main pumps are turned off, the drain pump is turned on and the standpipes are removed to suck debris accumulated at the bottom of each filter chamber to the pump chamber and then out to the drain.

The common rule of thumb is that the size of filter should be one third the size of the pond. The internal dimension of my filter is 0.7 x 2.5 x 1.3 m. Although there are some shortcomings of the above design, this filter was able to maintain crystal clear water for the Koi pond (size 5.7 tons or 1500 gallon).

The above describes a basic filter system that you can build yourself. Read my later post on the Filtration System for my Third Pond if you are looking for a better design.

Many companies have designed water treatment products. Visit these online stores (Pondkeeper UK) which offers many excellent brand/models and provides information to help you choose the right one to buy.