Let's talk about filter design. The type of filter to be discussed is a mechanical filter. This is basically removing the solid debris from the water. This can be as simple as a foam filter on the suction side of the pump. The problem with this is that it plugs up quickly. The larger the foam filter, the longer it will take to clog up. This is typically called a pre-filter. It does collect the debris so that it can be removed, but because it clogs up it reduces the water flow to the biological filter, thus starving the bacteria. This type of pre-filter can do some biological filtration and if the good bacteria are not removed when the foam is cleaned, it will continue to filter. On a small pond with a light biological load, this may be all that is necessary.

A strainer or a screen on the inlet side of the pump will act as a mechanical filter but this too will have to be cleaned frequently. The larger the opening on the screen or strainer, the less often it will have to be cleaned, but more debris will get to the biological filter. Water can be pulled through gravel such as putting a pump in a bucket and covering it with gravel. This will work but will require quite a bit of maintenance because of clogging. These are all low-tech mechanical filters.

A more high tech mechanical filter could be a vortex filter. To be effective this type of filter should be gravity fed. The ideal set-up is to set this filter outside the pond with most of the filter below ground. The top of the filter should be just above the water level of the pond. The pond should have a bottom drain that is attached to this vortex filter. There should be no pump between the drain and filter. The pump is then connected to the filter to pump water back to the pond. When water is removed from the filter, the filter pulls water from the bottom drain on the pond. This water enters the filter and is forced into a circular motion around the outside of the filter. The heavier particles fall to the bottom due to centrifugal force. A drain in the bottom of the filter can be opened up to remove these particles, thus removing the solids from the liquids. The problem with this type of filter is that it can usually not be added to an existing pond without redoing the entire pond, because it operates best when gravity fed. If a pump is used to feed it, the pump breaks down the debris and the filter doesn't operate properly.

Then there is the type of filter that is used by most sewage treatment plants, which are weirs and baffles. A weir is a flat piece of metal or plastic with teeth along the top of it. A chamber is built so the water and debris is pumped into it. The only way for the water to leave is to go over the top of the teeth on the weir. The heavier particles settle out while the clean water is skimmed off the top over the weir. This separates the solids from the water.

The important thing to look at with a mechanical filter is how well it will do its job and how much maintenance it will require.