Filter Types and Sizing

Filters are essential for the health of your pond.
Filters keep the nitrogen cycle going and help keep your pond and inhabitants healthy.

The three most popular types of filter media are:

1 Mechanical

2 Biological

3 Chemical

All three types are recommended, but ponds need to have biological and mechanical filters at minimum.
Components of these media types can be incorporated in the same filter.

UV (Ultra violet) filters are great for getting rid of bacteria and cutting back on algae but still need to be used in conjunction with one of the following filters. UV filters also use quite a bit of power.

Mechanical Media
Mechanical media is available in different coarseness. The larger the pores in the mechanical media, the larger the particulate matter must be in order for the filter to extract it. Coarse media is easier to clean and reuse than finer media. The pores in the finest mechanical media strain out particles as tiny as one micron, but will clog very quickly, and may not be reusable. This media mechanically or physically strains solids from water passing through it, which is vital for the efficiency of your biological media. To prevent build-up, the filter media must be cleaned regularly. Replacement is only required when the media can no longer release all the dirt that it holds. We used re-usable (washable) furnace filters for this part.

Biological Media
Biological media houses the natural bacteria involved in the nitrogen cycle. It provides a larger surface area for beneficial bacteria to colonize, allowing water to pass over the colonies, bringing nutrients and oxygen required for the nitrogen cycle. Biological media is anything inert (e.g. lava rock) that provides housing for beneficial bacteria that break down dissolving solids to a less toxic form. Bacteria extract food and oxygen from the water passing over them. When the bio load increases, the abundance of ammonia and nitrite causes the bacteria to reproduce. Once the correct amount of bacteria have grown to overcome the ammonia spike, the excess bacteria starve and die. When this particulate matter gets into the media, (decreasing water flow) it "starves" that area of oxygen and food, causing bacteria in that area to die. This is why mechanical media should be placed before biological media. Biological media should not be replaced unless it has become to clogged to function.

Chemical Media
Chemical media is not used as often as biological or mechanical media, but can be effective for an assortment of filtering purposes. This media is available in a variety of materials that can remove one impurity or many. The chemical method of filtration removes dissolved particulates from the pond via activated carbons, resins, and other adsorbents. Chemical filtration media helps to maintain water quality as unwanted dissolved matter adheres to it. Activated carbon is filled with microscopic pores that allow certain organic or inorganic materials to stick to them.  Resins are less utilized than carbon, but are becoming more common. These work by attracting a specific molecule to adhere to them. Some attract ammonia or nitrate, and some remove dissolved organics

Sizing your Filter
The total surface area of the filter must be equal to or greater than one third of the surface area of the pond. The aim is to turn the entire pond volume over in two hours. i.e. A total pond volume of 3000 gallons including the filter and pipe work, you must have a water turn over at a rate of half the total volume or 1500 gph. In this way, you can see the entire pond volume will have passed through the filter in two hours. You can turn the pond over faster, but you will need a massive filtration system in order to get some kind of respectable residence time in the filter. With the above water volume turn over rate, this ratio of pond to filter size, you should get around 10 to 20 minutes residence (contact) time in the filter. (In order that the bacteria resident in the bio converter stage can influence the water conditions.)

This formula always seems to work out. Don't ask me why, but it does!
To check this out, lets run some simple numbers, since a bit of simple mathematics is needed. We have a pond that measures 9'x9'x3' deep and we need to find the volume. The formula to find the volume is length multiplied by the width multiplied by the depth in feet. (To give volume in cubic feet.) Then multiply this figure, by 7.5 (the number of  gallons in a cubic foot) to give total gallons. If it were a round pond we would multiply the radius by 3.142, then multiplied by the depth to give cubic feet then multiply by 7.5 to give the total volume in gallons
Our example:
9'x9' = 81 sq ft x 3' depth =245 cubic feet, 245 x 7.5 the number of  gallons to a cubic foot = 1837.5 gallons pond volume.
The given surface area of the pond in our case is 81 square feet. In order to comply with the one third surface area rule for the filter, that means we need a filter one third of this figure or 27 sq ft surface area of filter. As most filters average at least a depth of 2 ft, multiply this figure by the surface area of 27 sq ft to give, = 54 cubic feet. Then multiply this by 7.5 = 405 gallons in the filter box. Add this to the pond volume and a figure of  2,242.5 gallon total system volume is reached. This of course, would be plus the volume in the pipe runs - but minus the displacement space in a given type of media (i.e. the displacement in water volume of the media.)  (But for the benefit of this exercise we will not include this.) So, a total volume of  2,242.5 divided by 2 hours gives 1121.25, gph turn over rate. This is the speed per hour that the water is being pumped through the system. If we divide 1121.25 gallons per hour, by the filter volume (405 gallons), this gives a figure of 2.76 (the number of times this system can be divided into 60 minutes the hourly rate) 60 divided by 2.76 gives us a figure of 21.73 (the minutes of residence (contact) time is attained.)

An additional filtration option is,

This is a very important part of pond keeping and filtration. A well planted and diversified pond is important for water clarity, pond and fish health, and even fish breeding. Plant roots can trap solids while the plant itself takes up ammonia, nitrate, phosphate, and sometimes even toxins. Plants lives off ammonia, nitrates, and phosphates and thus helps to keep those chemicals from becoming overly abundant in a balanced pond. Any plant with a good root structure that grows fast and has the majority of their leaves above water are a good candidate to start with. I recommend water iris for their strong root structure, fast growth, great nutrient absorption, and a great place for baby fish to hide feed and grow. There are many other excellent plants as well such as Sweet Flag, watercress, water celery, mint, (Watercress, water celery, and mint can be harvested to eat as well) Parrots feather, Bluebells, Umbrella palm, Papyrus, & bull rush. Make sure these types of plants (plants with roots in water and leaves above) are planted in an area of good, but not strong water movement to ensure that water moves over and through the roots. I also recommend ½” -2” rock under the plants to allow the roots to better establish themselves. (Do not use sand or soil.) Other methods include separate veggie filters in containers or pools. Other plants such as lilies, water lettuce and hyacinth are excellent for nutrient removal too, but not at the rate of iris and similar plants. (These plants are useful for shade which will slow algae growth.)

Plant filtration used in conjunction with a Bio-Mechanical filter can reduce the size of bio-mechanical filter needed.
In general, to be most effective, a veggie filter should cover an area equal to at least 10% of the pond's surface area.
If you cover 40% of the pond's surface area with plant filtration, you can virtuously eliminate the need for any other filtration.
(This information is based on a fish farm that uses nothing but a veggie filter in their 70,000 gallons of pond and 1500 sq ft of veggie filter.
That's 46.6 gallons of water for every square foot of veggies. In the above filter sizing, that would be 48 sq feet or 40% of the surface area.
This surface area will be different based on the size and depth of your pond.)
I do not recommend doing away with the bio-mechanical filter. Just reduce the boi-mechanical filter size down according  to
the veggie filter size used - use them both and you will have a healthy and clean pond.

Our Ponds Filtration System
2000 sq ft of surface area, X average depth of 2 feet  = 4,000 cubic feet X 7.5 (gallons per cubic foot) = 30,000 gallons.
30,000 / by 46.6 = 644 sq feet of filtration is needed. (As compared to the math of 1/3 the surface area would by 667 a difference of 23 sq ft.)
The bio-mechanical filter accounts for 290 sq ft of filter.  So, we need 377 sq ft of veggie filter, (so 377 / 2000
gives us 18.8% of the surface area that needs to be covered.) Let's round that up to 19% for 380 sq ft, which gives us a total of  670 sq ft.
So, our bio-mechanical filter accounts for 43% of our filtration and the veggie filter 57%.
Each pod in the filter system is made up out of a 30 gallon plastic storage container. Each of these
 contain 36 sq ft of lava rock (Black & Red) and 36 sq ft of filter media (Blue) - give or take a few square inches.
As you can see, the water comes in the bottom  and exits through the top. The inlets are 1-1/4" pvc and the
exits are 3"  pvc. I would recommend  larger 4" pvc as 3" is at its limit.


There are two reasons I built this as a horizontal filter system verses vertical.
First is you have easy access to remove the filter media for cleaning, and second,
is that should it happen to become plugged up - the water can simply "over top" the filter media, continue on it's way and not cause a mess.

Should your pond get green with algae in the summer because your filter cant keep up with the growth I recommend using Barley straw. Barley straw is put into a mesh bag and set in area where it will have water flow over it, as the Barley starts to break down it produces chemicals that will starve the algae from growing.

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