Bio Filter designs and explanation


If you have read the original page on this site entitled “Bio Filters you have been introduced to there purpose.  Now, on this page I will explain more and add a couple of Frequently asked questions. .

The purpose of the device known as the bio-filter is to provide the area where the bacteria can accumulate and then have the effluent from the re-circulating system flow pass them so they can feed. Since the bacteria will stick to almost anything, the media that has been used by system designers has been everything from plastic shotgun wads to plastic hair curlers. Some companies make special coring just for this purpose. For over twenty years we at Global Aquatics have studied the various designs for biofilters, the media, the applications and water flows through the filter and past the bacteria. What we found was that although the filter would work for awhile, in almost every case, sooner or later the media became clogged with very small suspended particulate from the fish waste that was not removed prior to going to the filter, and one of two things happened. (It is almost impossible to remove 100% of the particulate prior to biofiltration). First the waste would begin building up on top of the media and sometimes along the sides of the vessel. At this point the denitrifaction bacteria started to loose clean places to live and their population began to drop. Next the waste would begin to attract aerobic bacteria which would deplete the biofilter of the oxygen the denitrification bacteria needed for survival. In time, the biofilter no longer served it’s intended purpose at all, but instead became a BOD (Bio Oxygen Demand) generator. This action not only failed to remove the original ammonia and nitrite from the water, but actually created more as it digested the particulate. The obvious answer to this problem as described by the designers was, You need to back flush it periodically to clean the waste out. The problem with this theory is that it can not be done without also removing the denitrification bacteria as well. Now you have an inactive biofilter and by the time the bacteria are able to rebuild their population back to the required numbers it is time to clean the filter again.There is only one thing that can be done to solve this problem, that is to have the best possible filter to remove even the smallest amount of solid particulate. If the only thing entering the bio filter was clean, ammonia filled water the bacteria would consume all of this with little problem. Removing all of the solids from the water column also cuts down on the ammonia load because once it is removed from the system it can no longer decompose and make even more ammonia. For this reason I tell people that the best working bio filter is one that has the best solids filter ahead of it. It was for this reason that we developed and refined our auto-cleaning drum filter. In our pre-engineered design plans package we show how to built the next best thing which is a plate filter. Ever since the first recirculating systems were built, the removal of the ammonia and nitrites has been a key issue. While the method of using bacteria to remove this was well known, the applications to make this work were a challenge. We needed two things, one was the media that would contain enough area to hold enough bacteria per to be able to consume what ever amount of ammonia we were putting in the water, and the other was the vessel to hold all of this method. After that we needed to be able to move the water past the bacteria slow enough to give them enough time to get fed. Many of the earliest bio filter where tanks filled with all sorts of different materials to hold the bacteria and the effluent from the system was trickled down from the top of the vessel and allowed to past through the media. There are some serious flaws with this concept. Unless the vessel had a small exit port so that the effluent could only escape at the same rate it came in, the water would just rush straight from the top to the bottom and not allow for enough retention time to cleanse of ammonia and nitrite. That only the first problem. The second problem came from the residual particulate that came into the vessel. These fine pieces of fish waste would accumulate near the top of the media column and over time build up to the point that the filter would clog. Because the waste material was at the top it was almost impossible to wash or flush it out in a timely manner. Another type of bio-filter was called a bead filter. This type is still used with a certain degree of success, but even here there were problems. The way this one works is, you would have a tank like vessel that was full of floating plastic beads that actually looked like BB sized ping pong balls. These beads would be the media for the bacteria to latch on to. When the tank was filled with water the beads would float to the top. Water from the effluent came in from the bottom and up through the beads and exit through the top of the vessel. Much of any suspended particulate would settle to the bottom of the vessel and could be flushed out when needed. In the early 1990’s we thought we had solved a big problem in bio-filter design with the development of the Up-Welling biofilter . Similar to the bead filter in concept, the different was, we used a new media that had been developed called PVC STRAND

Up-Whelling Bio Filter Cira 1992

A unique internal design in the bottom of this tower caused the incoming water to rotate before it started its upward flow through the media. This rotation allowed the water to stay at the bottom of the tower longer than it would in the bead filters and hence allow more time to settle to a holding flush point. Because the effluent entered through the bottom and exited through the top, the effluent was passing by the bacteria at a very low speed, giving them lots time to be in contact with the water. Also, since the effluent was not coming in from the top and there are no flat surfaces in the media, suspended solids entering the filter can only pass through, there is no where for them to settle. The tower was back flushed on a daily basis from the filter without disturbing the bacteria on the media, which was always a problem with the bead filters as the flushing would shake the bacteria loose.

While the Up-whelling bio-filter did a great job of cleaning up the ammonia and nitrite from the fish system, we saw some things that needed some consideration. For one thing, the tower was expensive to build. Its shear size called for a lot of special bracing to allow it to hold so much water weight. We thought about changing its construction to a round vessel that would be about 4 feet in diameter and 12 feet high. The problem remained. A water tank like this is designed to lay on its side. When stood up the stress at the bottom from the weight of the water and vibrations from the flowing water would still be a problem. Plus we could not marry our special bottom to this vessel. Another problem was the vertical height of this tower. At 12 feet it could not be readily used in a building with a low ceiling. The third thing was energy use. In order to push the water upward took a 1 hp pump that ran 24/7. After the water left the tower we had to once again pump it to get it back to the fish rearing tanks. This was the same issue with the bead filter. Too much energy—there had to be a better way.After the development of the S-92 system and the Up-whelling bio-filter we started the research on a more refined system drawing from all of the things we had learned from the S-92. From the S-92 system we kept the components we liked, like the tanks, injectors, O2 monitors and PVC stand materials. We decided we had to come up with new designs for the particulate filtration and the bio-filter.We tackled both issues at the same time. We began refining a prototype drum filter we had developed earlier and redesigning the bio filter. We knew the basics of why the tower worked so well. In it we were able to make all of the water contact the bacteria for just the right amount of time to be effective. We had to duplicate this process.

200 cu ft S-09 Steel Core Bio Filter

We started by simply laying the tower on its side and going from there. After much experimenting we found that we could duplicate this in a horizontal filter by adding baffles that would make the water flow in both an upward and a downward direction throughout the new vessel. In addition to this we developed a method of using gravity flow from the directly from tanks and through the drum filter and then through the bio-filter. Not one pump is used.



This is one tough question. There are so many variables to consider. These factors include:

1. Total bio mass of fish the filter has to deal with. Biomass means the total weight of all of the fish in the tank. Understandingthat the purpose of the bio filter is to remove the ammonia and nitrites from the fish tank, the more fish you have, the more feed you put in the tank and therefore the more ammonia you will create.

2. How well does you tank clean during recirculation? Although a certain amount of the ammonia comes from Urea the bulk of it comes from the fish feces. This is the waste from the fish that must be hard filtered out. If this waste is allowed to remain in the tank for more than a few hours it will begin to break down and release the ammonia. Furthermore, the longer it stays in the water column the more it begins to dissolve creating even more problems. The smaller the piece of waste the harder it is to remove from the system. The best way to explain this is to think of a cube of sugar. If you place this cube in your coffee cup and immediately reach in and remove it, you pretty much removed all of the sugar you put in the cup. Now, if you put it back in the cup again and wait for a few minutes, and perhaps even stir it up with a spoon, what happened to it? It is still there, but now you can’t grab it anymore. Just as it completely dissolved into the water, fish waste will as well and it releases all of its ammonia which then becomes nitrite. Technically you could not build a biofilter large enough to remove all of this ammonia.So we have to grab it out before it can be released in the tank. This is why how effective your solids filtration is so important and why it can determine the size of the biofilter. One of the most effective solids filter is the Burdette Industries Drum Filter

3. What kind of feed are you using and at what rate?

Getting the right fish feed is one of the most important factors to successfully and efficiently grow fish in any tank system. Poor feed will not digest right and in turn the fish will expel much it adding a lot of waste in the tank which now turns into ammonia and nitrite Also, it is important to use floating feed.  When sinking food is used in a fish tank a certain percent of it will drop past the fish and settle in the bottom. Not only does this cause tank water pollution is is wasting you money.

Burdette Industries