Part 4: Mechanical and Chemical Filtration

In this article we are going to discuss filtration. Filtration is broken down into 3 different types; mechanical, chemical and biological. I will be discussing each type in detail. This article will cover mechanical and chemical filtration with biological filtration to be covered in the next article.

To begin, we will cover mechanical filtration. Stated simply, mechanical filtration removes debris or compounds from the pond through a mechanical device. The device could be as simple as a net or as complicated as a foam fractionator. Every mechanical filter has a specific purpose. When looking at mechanical filtration it is important to first identify what you are trying to accomplish and then choose the correct product for the solution.

Most commonly on a pond we see a skimmer; which is a mechanical filter used to remove floating debris from the surface of the pond. Within the skimmer there are usually two different devices. First a weir designed to take water from the surface. Next a net or basket is used to collect debris. When looking at mechanical filters, we need to analyze how well it will do what it is intended to do. Using our skimmer example, let’s break down how it is supposed to work and determine what to look for in a skimmer.

The first thing a skimmer is supposed to do is remove debris from the surface using a weir type device. Water is intended to enter the skimmer by going over the top of the weir that floats so that only a thin layer of water goes over the top. Therefore, any water that enters the skimmer opening by not going over the top is a waste. In this way we can get a good idea of how well it will work by looking at its construction.

Most skimmers use a door that is hinged at the bottom. Yes, water will go over the top as long as the amount of water being pulled in is more than the amount of water going under the door and along the sides of the door. If the skimmer has an 8 inch weir with a ¼ inch gap around the door, the area of the gap would be approximately 5 square inches. That is almost the same amount of area as a 2 inch pipe. This is with the door shut, but as the door opens, the gaps on the sides get larger and less water is drawn from the surface. With the door shut as much as 1000 gph can go around the door before it starts to take water from the surface of the pond.

The second part of the skimmer is the net or basket that catches debris. The first thing to look at is whether the net or basket is going to catch all the debris entering the skimmer or can some of the debris get past without being caught. Also make sure the holes in the net or basket are the correct size to catch the debris you are trying to remove from the pond.

Every pond should have at least one mechanical filter and usually more than one. Not every mechanical filter is easily examined to determine if they will indeed perform the work they are intended to perform. A good example is a foam fractionator or protein skimmer. Their purpose is to remove dissolved organic materials from the water. One problem is that you can’t see these materials so how do you determine if the device is going to work? Even though you can’t see them, you can see the effect they have on the pond water. The dissolved organic materials will cause bubbles on the surface of the pond to take longer before they break. With this knowledge we can determine how well the foam fractionator is working.

Next let’s cover chemical filtration. Chemical filtration is accomplished by adding a chemical to the water to remove some substance from the water or tie it up so that it is no longer harmful. Chemical filtration has a limited use in that once the chemical is used up it no longer has an effect on the water. Chemicals used for this type of filtration can range from dechlor to ozone.

As with any filter you should first determine what you are trying to remove from the water. You then select a chemical to address that problem. In some cases it is important to know the volume of water you are trying to treat or the amount of the substance you are trying to remove. Sometimes if too much of a specific chemical is added it might poison the pond life. It is also important to know that in some cases once a chemical is added to the pond water, it stays in the water until it is removed or used up. Many chemicals do not dissipate in the water and they don’t evaporate. Because of this, when you add different chemicals in the future you may have a chemical reaction between the two that could result in undesirable or harmful conditions. Removing one chemical from the pond is not easy. It can be removed by water changes but to remove 99% of the chemical by water changes would take a change of 8.9 times the volume of the pond. For example, if you changed 5% of the pond volume once a week it would take 178 weeks to get 99% out. That is approximately 3 and a half years! The same would hold true for medications.

A word of advice; be careful of anything that you put in your pond because it might be there for a long time. Of course there are some chemicals that disappear fairly quickly. Ozone is one of these. It has a half life of 4 minutes in ideal conditions and much less in any other conditions. What, you ask, is ozone and what does it have to do with a pond?

As ozone is one of the newer ideas being used in the pond world I will explain it as simply as I can. Oxygen normally forms molecules as two atoms of oxygen (O2) but ozone is one molecule of oxygen with three atoms of oxygen (O3). Because the third atom of oxygen, ozone is always trying to get rid of that third atom. Because of this it is a very powerful oxidizer. In fact it is the second most powerful oxidizer known to man. How does that apply to a pond? It means that it can oxidize any organics in the pond. In fact it can be so efficient at oxidizing nitrite to nitrate that the bacteria that would normally take care of this can die off due to a lack of nitrite for them to eat. A word of caution, ozone can be a very dangerous compound and should not be used unless you know what you are doing.

As stated earlier, the next article will cover biological filtration.

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