Friday, May 2, 2014

About sand filters State-of-the-Art in Pond Filtration By David A. Dec ©2002

Note: This article is nothing but advertisement for sand filters but still has some interesting facts about the filters.

About sand filters 

State-of-the-Art in Pond Filtration

By David A. Dec ©2002

Pond filtration requires two main components; both mechanical and biofiltration. First, the mechanical filtration needs to remove both the suspended particles and the debris that has settled to the bottom so the water is clear, and we can see our fish and plants. If not removed, this organic mulm can harbor dangerous organisms and chemicals that threaten the health of our pond.

Second, the biofiltration needs to change the toxic chemicals like ammonia and nitrites, to harmless or helpful chemicals like nitrates, oxygen, and nitrogen. This is done with biologically active bacteria species through the Nitrification process.

Effective Pond Volume

A recent buzzword in pond circles is “Effective Pond Volume”. People have found that selecting a filter based on how many gallons their pond holds is not at all accurate. Since biofiltration is dependent on the ammonia load from the pond’s fish, biofilters needs to be sized according to the amount of fish wastes produced each day in the pond. This depends on the quantity of foods they consume each day.

Selecting a swimming pool filter is done by matching flow rates. However, the proper way to select a filter for a pond is to determine the maximum number of fish you are planning for the pond, and the weight of food you will feed them daily, plus the amount of algae and other natural pond food they eat. Typically the amount of food Koi eat is 1% to 3% of their weight.

How Much Food

Then you select the filter that will process that weight of food per day. However, this is not an easy task because many filter manufacturers’ feel compelled to match their competitors' exaggerated claims of their filters' capacities, which means the ability to process food to ammonia and nitrite levels of 0.0 to 0.1 parts per million (ppm). Unfortunately these manufacturers’ grossly overstate the amount of fish wastes that their filters will handle. We have done our own research into this and found most of their claims to be outlandish. Surprisingly if they were more accurate they would probably sell larger or multiple filter units to their customers, the result would be healthier fish, and everyone would be happier.

Don't forget the square area of the pond itself supplements the square area of the filter media, and can easily add 25% to 200% to the bacteria's total processing surface area.
Overfeeding the total square area for nitrification causes the ammonia and nitrite levels to increase to dangerously toxic levels, without frequent water changes. That is why many ponds are running ammonia and nitrite levels of 0.5 to 1 or more ppm, and their owners are wondering why their fish are sick and dying.

Filters can also be combined in parallel or series depending on the application or the amount of food that needs to be processed.

Surface Area Efficiency

Our own research, as well as others’, has shown that filters process about 0.1 gram of food per day per square foot of surface area in the filter media, at maximum nitrification efficiency. In other words, they convert the toxic ammonia to nitrites, and then the toxic nitrites to nitrates via the nitrification cycle. Since the bacteria live on the surface of the pond walls and filtration media, a larger surface area in the filter means there is more room for a larger bacteria colony to do the biofiltration.
The surface area in a filter is the filter media’s surface area per cubic foot, times the number of cubic feet of media. A larger total surface area allows larger colonies of nitrifying bacteria to adhere to it.

The surface area in a filter is what makes it work. The larger the surface area the greater is its biological activity.

The most important consideration in purchasing a filter system is its surface area efficiency in dollars per square feet.


The surface area per cubic foot depends on the media. For instance, very fine sand with a diameter of 0.125 mm or .005 inches has 8,000 square feet of surface area per cubic foot. So a sand filter with 2 cubic feet of very fine sand has 16,000 square feet of surface area so it can process 1,600 grams or 3.5 pounds of food per day. Sand of this diameter in a filter results in a total cost about a nickel per square foot of surface area.

Sand with a diameter of 0.5 mm or .02 inches has 2,000 square feet of surface area per cubic foot; and results in a cost of about $0.20 per square foot of surface area.

[Ed: If you read and/or have read my articles already on macroscopic thermodynamics substances part 1&2

you will have learned that numbers may look impressive on paper and sales ads but they don’t hold up under any scrutiny very well.  His assertions about very fine sand and surface area to bacterial biological filtering capabilities aren’t completely correct and are misleading information to the hobbyist. He states that sand of .125 mm in size has a square foot surface area per cubic foot of 8,000 sq.ft., and can biologically process 1,600 grams or 3.5 lbs. of food per day using two cubic feet of very fine sand which comes up to be 16,000 sq.ft. of surface area.

 Now let’s take this in perspective and compare to what’s said above to that of Activated Carbon for example. One gram (the same weight as a paper clip) of activated carbon has a 1500 m2 of surface area, which is the equivalent to 16,145.9 sq.ft. of surface area. That’s more than the 16,000 sq.ft. of surface area the authors claiming a sand filter would have using very fine sand @ two cubic foot. Lets not forget we are only talking about one gram here of activated carbon!

Now by what he is saying, one gram of activated carbon could biologically process 3.5 lbs. of Koi food per day…right! After all it has more surface area than 2 cubic feet of very fine .125 mm sand doesn’t it! Well the answer is no it can’t! No more than a sand filter could process 3.5 lbs. of food a day (1,277.5 lbs. of food a year or 25.5 bags of Koi food @ 50lbs. per bag.) either using two cubic feet of very fine sand. If you read my two articles above then the answer would be very apparent and simple to you that there are a lot of variables that the author is not taking into account and adding into the equation. However, by his wording he now can deceive the hobbyist into believing that sand filters are “State-of-the-Art in Pond Filtration” and just look at how efficient they are with using very little space or filtration media. But if this were the case, then why not use activated carbon instead and use a filter no bigger than that would be applied to an aquarium instead?

Okay, surface area give us a guideline only not an absolute for bacteria colonization of a filters media or capabilities. Available foodstuff, oxygen demand, what kind of bacteria is at play for processing the available foodstuff, macroscopic thermodynamics of the polymeric surfaces of the substances being used, carbon availability ect., ect., all play a role in how much bacteria will colonize a particular substrate. Other whys we would all be using activated carbon as a filter media instead of the big balky filtration system we have now.]

Plastic Bead Filters

Plastic beads with a diameter of 5 mm or 0.2 inch have a surface area of 200 square feet per cubic feet or about 40 times less than sand. So a bead filter with the same 2 cubic feet of beads has only 400 square feet of surface area, so it can only process 40 grams or 1.5 ounces of food per day; much less than that for the sand filter. For this reason when beads are used in filters the typical cost is $1.00 to $2.00 per square foot of surface area or 5x more than sand.

It is obvious that the media needs to have a very large surface area to host the nitrification bacteria. All the media up to this point have been solid, i.e. sand or plastic beads.

Hollow Media

A newer development is hollow media. If you took a miniature plastic “hollow drinking straw”, and formed internal walls inside it, you would not only have the external surface area, but also an internal surface area. They are 5 mm in diameter and vary in length from ¼” to ½”. This media has 750 square feet of surface area per cubic foot. So a filter with 2 cubic feet of media will have 1,500 square feet of surface area, and will be able to process 150 grams or 1/3 pound of food per day. This is over 4 times more efficient than bead filters, but 1/10th of the sand filter’s surface area. Its cost when used in a filter is about $0.75 per square foot of surface area or more than 3x sand.

The first disadvantage of this new media is the difficulty of manufacturing it. The units are obviously very small and have a very complicated design, which makes them very expensive at about $200/ cu ft. The second disadvantage is they are made of styrene. You can test this by putting some media in water. If it sinks it is most likely styrene. Another test is to burn it since styrene burns with a black sooty smoke. Styrene oxide as a contaminant of styrene is known to be very toxic to bacteria, which would be a disaster to bacterial colonies for nitrification.

Filter Operation

It is true that sand filters have been used in almost every swimming pool in the USA. They have been proven and constantly improved after millions of installations. They are obviously very effective, inexpensive, and are very easy to clean. In fact, they are almost self-cleaning. You just turn a valve to backwash them. This saves lots of time, inconvenience, labor, mess, and wear and tear on the pond fish, and it automatically provides the small but frequent water changes needed to remove the dissolved chemicals.
During normal operation the water flows in through the top valve, travels down through the sand or filter media where the debris is trapped, and flows into 6 to 8 perforated plastic laterals connected to a hollow stem, and then out through the top valve back to the pool. During backwash the water is re-directed down the stem and through the plastic laterals, flowing up through the sand, carrying the debris out through the top valve to waste.

One way to select a swimming pool filter is by flow rates. For instance, if you are planning a flow rate of 110 GPM or 6,600 GPH you will want one 36" diameter filter, or possibly split the flow between two 24" filters, which would be cheaper.
Filtration flow rates in gallons per hour vary with the size of the filters as follows:

Filter Diameter      Typical volume in cubic feet     Flow Rates in GPM        Flow Rates in GPH
12"    0.5     8 - 11          480 - 660
14"    0.75   12 - 18        720 - 1,080
16"    1.7     19 - 28        1,140 - 1,680
18"    2.6     29 - 34        1,740 - 2,040
20"    4       35 - 45        2,100 - 2,700
24"    5       46 - 68        2,760 - 4,080
30"    12      69 - 100      4,140 - 6,000
36"    22      101 - 165    6,060 - 9,900
42"    33      166 - 269    9,960 - 16,140
48"    47      270 - 360    16,200 - 21,600

Notice that two 36" filters have about the same performance as one 48" filter, and are a bit cheaper, but will have much less backpressure. Sand filters are typically charged with 1/2 of their cubic foot volume, with 100 pounds of sand being equivalent to 1 cubic foot.


Another benefit of the "pressurized" sand filters is the ability to use the pump's suction line to operate a vacuum to clean the bottom of a pond. The vacuum hose typically plugs into a skimmer's suction line to the pump. The valve is turned to “Waste” so the vacuumed waste does not go through the filter, but goes directly to the waste line.
This same vacuum hose can also operate a mechanical robot vacuum that automatically vacuums the pool's bottom. These robots are made for either concrete or EPDM liners.


With the pressure type filter the skimmer’s strainer-basket becomes the pre-filter. We often have to clean the skimmer strainer-basket of algae, leaves, and debris twice a day, and the pump strainer-basket at least twice a week. However, it is quite easy, you just remove the basket, hose it off, and replace it. It only takes a minute or two. You can use chlorinated garden-hose water since the basket is not part of the biofilter.

Problems with Sand

There are 2 main objections to sand filters for ponds. First and most important, they can plug up if:

1.Loaded at 100% of the manufacturers’ recommendation for sand, which is about 1/2 full.
2.Not backwashed at least once per week.
3.Not backwashed with a powerful enough pump.

Second, some opponents say the water travels through it too fast to allow for effective biofiltration. They say the residence time is too short. However, they ignore the fact that the water makes many more trips through the media for a given time period, so the actual contact time per hour is about the same.

Large City Aquariums use Sand Filter

Most if not all large city Aquariums use sand filters. They know how to properly use them, and have found the efficiencies to be unsurpassed.

[Ed: Most city Aquariums use sand filters not for biological purposes but for polishing the water.]

Pea Gravel

Not knowing how to properly use a sand filter some people tried replacing the sand with pea-gravel, which has a surface area of only 100 square feet per cubic foot, or 80 times less than sand. Needless to say pea-gravel is not the answer.

Sand Diameter mm        Area in square feet per cubic foot (ft2 / ft3)
Pea Gravel   10      96
Very coarse          2       479
Coarse        1       958
Medium       0.5     1,917
Fine   0.25   3,833
Very fine     0.125          7,666
Very very fine       0.0625        15,333

Bead Filters

Somebody noticed that the plastic-bead feedstock used by plastic injection molders had diameters much smaller than pea-gravel, but larger than sand. These solid plastic beads, at 3 to 5 mm, 1/8” to 1/5", have a surface area per cubic foot of 200 to 300 sq ft / cu ft, which is better than pea-gravel, but 30 to 40 times less efficient than sand. In other words, you might need 30 to 40 bead filters to match the biological efficiency of a single sand filter.

Plastic beads diameter in mm    area in square feet per cubic foot (ft2 / ft3)
5       192
4       240
3       319
2.5     383
2       479
1.5     639
1       958

In order to get more surface area in the bead filters manufacturers simply try to put more beads into the filter; since the beads have much less surface area per cubic foot. They jam it almost full; in some cases they fill 90% of the filters' volume with beads. This leaves little room for backwash turbulence to develop.

So even with the dramatically reduced efficiency the literature shows many bead filters are still plugging up: partly because of the overfilling, and partly for the same reasons listed above for sand filters; not backwashing often enough, and not backwashing with a powerful enough pump.

So some manufacturers have added air blowers to try to reduce this plugging tendency. Unfortunately, when the beads clump up forming channels the air simply goes through the path of least resistance, the channels, which mean it has no effect. The manufacturing of “bead” filters was very simple and became quite popular. It consisted of buying standard sand filters at wholesale, dumping the sand, and inserting the polypropylene or polyethylene plastic beads, and jacking up the price 3-4X for sale to the pond industry. The drainage plugs were now referred to as “sludge removal ports”. Then some very clever bead filter manufacturers, realizing they needed to add more value, added small UV lights, and compressed airlines.

Bead Media Washout

During the backwash operation sand being heavier than water falls to the bottom of the tank, instead of flowing out through the valve to waste. However, the plastic beads being lighter than water float to the top, and since they are smaller than the valve-strainer's holes, they are washed out through the valve into the waste stream; so more and more beads are lost during each backwash operation. This limits the size of the beads being used; the smaller the beads, the greater the surface area for bacteria, but the more bead loss during backwash operations. The larger the beads the smaller the surface area for bacteria, but the backwash bead loss is reduced. So the bead filters are limited in efficiency.

Pond Sand Filter Research

Our research focused on under-loading the sand filters, and backwashing them more frequently with higher pressures and flow rates, in order to take advantage of the greater food processing surface areas, while eliminating the chance of plugging. The other advantage of the sand filters is they are more reasonably priced. We discovered a sand loading that results in a high efficiency yet doesn't plug. Other sand filter media investigated included coarser sand, porous ceramic material, and crushed lava rock. Other hollow media were also looked at.

While it is true that pressure type filters such as sand filters may require a little more electricity to operate, most pond owners are willing to spend a little electricity to replace their labor. Owners of these filters want something that will do the job better, and with less labor. In the USA, too often our “Honey-Do Lists” are too long to allow using more labor-intensive filters.

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