The Perfect Filter

Note: This article was originally written to better illustrate the concepts of how biological filtration works. It was not my intent to suggest that the filter used as my model is the "perfect" filter but rather provide ideas on how to build the "perfect filter."

The Perfect Filter

The picture below depicts what may be the perfect filter system for most practical applications. Consider what is important for a properly operating and effective filter system:

First, the filter must provide two basic functions: mechanical filtration and biological filtration. In an ideal set-up, these two functions need to be distinctly separate processes that can be managed separately yet work in tandem. A two-chamber filter system is required to accomplish these functions.

Mechanical Filtration. The role of the mechanical filtration process is to remove debris of all sizes from the water being pushed through the filter. The process starts with the larger and heavier particles being "brushed" out of the water flow by the brushes suspended in the mechanical filtration chamber.

Note the water flow. The water is pumped into the aerating bar and then sprayed onto the brushes. This does two things: first, this aerates the water to improve aerobic bacteria activity for biological filtration and second, allows the flow to be broken down into multiple smaller streams so the large particles can be captured by the brushes. The water in the first chamber essentially flows around the chamber in no particular fashion and because of this movement, the heavier particles are kept from flowing into the second chamber and instead, sink to the bottom. This is a basic vortex type action. The drain in to the first chamber can be opened at anytime to drain off the large particles.

Next, the partially strained water is allowed to flow over the chamber separator and into the biological chamber. The flow of the water is significantly different in this chamber. Note the path the water takes to return to the pond. The water flows over the separator and down through three layers of filter padding. While this is basically the biological chamber, all three pads act as mechanical filters with the top and bottom layers of filter pad are course material and designed to catch the next size of particles. It is important to have both of these layers in place considering how the water moves through this chamber. The middle filter pad should be of very fine mesh as this will trap yet the finer particles. What grade of material used is solely dependent on the type of material being filtered out of the pond. Algae and larger suspended particles require a courser grade filter mat as they will clog a finer grade more easily and this will require more frequent maintenance. For smaller suspended particles, such as dirt, pollen, and the like, the finer grade is a must.

Biological Filtration. All three filter pads will provide some level of biological activity, but should not be counted on to be the key biological filtration elements in the filter. Since the filter mats are still part of the mechanical filtration process, they will need to be cleaned regularly and since spraying with the closest garden hose is the easiest route, chances are any biological filtration bacteria may be destroyed in the cleaning process.

To overcome this, one of the best ideas to enhance and protect the biological bacteria process is to add another biological bacteria media, such as bio balls into the filter. Where and how the bio balls are placed is critical to make them work right. Ideally, the bio balls need to be placed in the flow of water and not where they can get clogged by suspended particles. Many commercial filters use bio balls as a mechanical filtration process as they tend to capture particles when compressed together. This type of activity greatly hinders the aerobic bacteria activity as clogging will cut off the flow of oxygenated water to the good bacteria. So, good water flow in and around the bio balls is essential. Here are two techniques that work well when used in tandem:

1. Place a quantity of bio balls into a mesh bag and place the bag between the brushes and the chamber separator. The flow of oxygenated water through the first chamber will offer enough movement for the media to be effective. If the mesh bag configuration tends to get too clogged with algae, etc, then it will need to be moved. But if the brushes are working properly, this will not be a problem.
2. Place a small plastic container, such as a water plant pot (square/sieved) full off bio balls inside the biological chamber and right where the water flows over the chamber separator. The water should splash right into the container causing a nice stirring action for the bio balls.

The addition of the bio balls to the filter system will allow for ease of cleaning and offer a much better biological activity level.

Finally, this configuration allows for placement of the UV at either the input or discharge ends of the filter system. A properly operating UV will kill off virtually all of the suspended algae particles which results in brown suspended algae carcasses in the water that need to be filtered out mechanically. The advantage of having the UV before the filter is that you can filter out the carcasses before they are returned to the pond.

The picture below is a modified Little Sister filter system. The intake aeration tube has been modified to accommodate a greater water flow and larger streams into the brushes. Notice the mixing action of the water. The size of the aeration tube has been increased to accommodate a larger pump than the original system was designed to handle. The UV is in front of the filter. Note the position of the bio balls. In the mechanical chamber, the bio balls are in a mesh bag placed where water flow through it would be maximized, thus avoiding clogging. In the biological chamber is a basket of bio balls situated to catch the flow of water coming from the mechanical chamber. This works very nicely.

The white PVC pipe in the bio chamber is the return tube. The water actually flows up the pipe after passing through the filter mats (see the drawing). When the mats get clogged, the water will overflow into the top of the pipe, thus preventing the entire filter from overflowing and at the same time giving you a good indication of how dirty your pads might be.

Everything inside of the filter box is homemade. The box was salvaged from a pond store who sold off all of the original internal components. Filter boxes without components can be purchased from a number of sources. The brushes and filter mats have been upgraded from the original design so the filter could handle a bigger load. And the bulkhead fitting added to make the connection of the larger supply pipe far easier.

When finished, the entire filter system will be built into a "seat box" inside the pergola over the pond. This filter currently handles 800 gallons with a large fish and plant load.