Garden Endeavors


The facts regarding biological filter systems are:  The best systems are self cleaning drum filter systems.  They are expensive.   If you are not wealthy, forget them.  Bead/sand filter systems are also expensive.  Get one if you want and can afford it.  The cheapest actually effective systems are the submerged bio-media systems.  I have built and designed many of them, some for my own ponds (currently I have 6 in operation) and others for other ponders (the largest filter system I have designed was for a 52,000 gallon pool system).  After over 30 years experience with watergardening, I have designed and built a gravity fed submerged media biofilter that incorporates the best of my own and other’s ideas that I have distilled over the years.  This filter works.  I have proved that.  It is also cheap as compared to commercial systems of comparable capacity.  In addition, it as low maintenance as any submerged media biofilter I have seen.   I present it to you as a realistic permanent low maintenance system for your watergarden.

The plan:

A submerged media biofilter system incorporating a gravity feed side drain and skimmer with low/easy maintenance mechanical filtering, high efficiency degassing trickle filter and water polishing basket. Also including a provision for UV algae control. All vessels except the trickle filter stage incorporate cone bottom, which facilitate easy cleaning. See conepage.html .

The filter is fed by two 3” pvc lines, one from a side drain and one from a skimmer. It is designed to provide adequate filtering for a heavily stocked 2,700 gallon water garden. It could be fed by a bottom drain but in my opinion bottom drains are best used for dumping to waste rather than feeding into biofilters since the amount of organic materials coming through bottom drains will overload most submerged biofilters.

I am fortunate in the case of this particular pond in that the lot slopes away from the pond so that even with the biofilter being buried at ground level, enabling gravity feed to the filter system, the biofilter chambers can be drained to a lower part of the yard ( I would point out here that I have five other water gardens populated with koi that have submerged pumps and above ground biofilters).

Gravity fed biofilters have as an added attribute, the ability to locate the pump at the end of the filter, thus reducing pump maintenance, since the pump is working in already filtered water. Of course the filter design given here can be used in an above ground operation, simply by locating the main pump in a skimmer or flooded suction sump or even submerged in the pool. I have included herein a design for an above ground 4 vessel filter system.

Theory of operation:

Conventional wisdom assigns the responsibility of conversion of ammonia (which is the result of fish biology) to harmless nitrates to two types of bacteria, Nitrosomonas and Nitrobacters. Notwithstanding more recent research that disputes the actual identity of the bacteria responsible for the conversion, it is accepted that bacteria of some sort does in fact convert the ammonia to harmless nitrates.  At any rate, the bacteria need a place to grow and interact with the pool water in order to accomplish this task. In fact, all surfaces submerged in a water garden, even the inside of pipes and pool surfaces provide a place for such bacteria to grow. A biofilter generally provides oodles of surface area for the bacteria to flourish. This area is provided by the “media” in the biofilter. Commercial biofilter media is expensive, far more expensive, in my opinion, than it should be. The media chosen must allow free flow of water and not become easily clogged, be easy to clean when necessary, and be CHEAP. In this filter design I have used 200 psi ¾ inch pvc pipe, cut into 1 inch pieces. While cutting a 10 foot piece of PVC pipe into 120 pieces can produce a few blisters, it is a relatively cheap way to make bio media. I accomplish this task using a miter saw.  Cost is about $15.00 a cubic foot, as it takes eleven 10 foot pieces of 200 psi 3/4 pvc to make a cubic foot of media. Per my calculations, one cubic foot of such pvc pieces provides 56 square feet of bio film area.  This comes to 3.73 square feet of surface area per dollar.  For comparison with commercial bio media, Bio-Barrels are available that provide from 26 ft2 to 64 ft2 per ft3 at a cost range of from $31 to $55 (average about 1 square foot per dollar). Commercially available Bio Balls provide from 98 ft2 to 160 f2 per ft3 for a cost range from $26 to $39 (averaging about 4 square feet of surface per dollar).  The commercial bio Balls provide about the same surface area per dollar than the pvc in a smaller package, but they are harder to clean than the pvc.

Operation of the filter:

The filter system is constructed of four 55 gallon and one 30 gallon plastic drums. Water enters the first filter stage (called a vortex filter) via two 3” pvc pipes, fed by gravity from the pool. Elbows in the vessel direct the water flow to move around the vessel in a circle, or vortex. This action results in much of the suspended solids falling out of suspension into the cone bottom of the container where they can be easily drained away on a regular basis. Next, the water enters the mechanical filter funnel where additional solids are filtered out before entering the first biofilter media vessel in an upflow filtering action. The water then passes to a third vessel wherein it passes through a downflow biofilter media.  Both media vessels contain upwards of two cubic feet of bio media, providing 224 square feet of biofilm surface area.  The next stage is a collection chamber, which includes no filter media, but does allow monitoring of the water level flowing throughout the system. The main pump then moves the water to the top of the degassing trickle filter/water polishing vessel. This is a 30 gallon plastic drum that includes a cubic foot of pvc media that while remaining wet, is also bathed in oxygen, which provides not only a boosted biofilter action but also a means for nitrates to leave the pool environment. Air induction is guaranteed via the operation of a fan that constantly feeds air into the media. Degassing is accomplished via splashing as the entering water falls into the polishing basket contained inside the drum, and also as the water passes through the media.

As the water enters the trickle filter container, it is “polished”. This is accomplished through a small waste basket lined with cotton batting, which removes very small particles, leaving the pool with a “polished” quality. The design allows for the “polisher” to simply overflow into the trickle filter media when clogged, thereby causing no interruption of the pool bio filtration.  The water then passes through the aerated trickle filter media before finally exiting to the pond via a waterfall.


 Main pump protection is provided by a mercury float switch located in the collection chamber or pump reservoir that will shut off power to the main pump any time water level in the pump reservoir drops to a predetermined level.

Because this is a gravity fed filter system, special consideration is needed to provide for clogging of the mechanical filter.

If the mechanical filter in the vortex vessel should become clogged, a mechanical filter bypass pump is actuated by a liquid level switches in the second vessel. A red warning lamp is also turned on by these switches to give notice that the mechanical filter element needs cleaning.   The float level switches are wired so that when the water level in the second filter vessel drops below a predetermined level, the bypass pump is turned on, along with the signal light.  If the flow is increased enough for the water level in the same vessel rises back to normal, another float switch turns off the bypass pump.  This operation will cycle on and off until the mechanical filter is removed and cleaned.  Since the power to this circuitry is controlled by the main pump protection float switch as well, should the water level in the main pump collection chamber drop to it’s predetermined level, all pumps will be off until the water level rises back to normal.


You can build this system with modifications and still have a “state of the art” submerged media biofilter system.  The following changes are possible:

1.      Eliminate the mechanical filter bypass pump and the circuitry necessary to operate it, but keep the mechanical filter.  This modification will save some money, first on materials and second on operation, since there would be no bypass pump using electricity.  The disadvantage would be that you would have to more closely monitor the mechanical filter, since if it becomes clogged, the water in the pump resevoir will drop and the main pump will shut off, reducing or stopping flow through the biofilter.  This would not be good for the bacteria in the media.

2.      Eliminate both the bypass pump AND the mechanical filter.  Because the media is of the free flow type you would rarely have much in the way of reduced flow through the system. Disadvantage:  more gunk will make it into the biomedia, requiring more often cleaning.

3.      Add more biofilter vessels.  Adding more vessels will increase the biological filtering capacity of the system, something that you might need if you way overstock with fish.  Disadvantage: cost of the additional drum and uniseals and pvc pipe, and the need to acquire more bio media.  Also, you would need more room to accommodate more barrels. My drums cost me $24 each (used, but steam cleaned).

4.      Eliminate 1 biofilter vessel and operate with just one.  Obviously this would cost less to build and require less room.  Disadvantage: Submerged biological media must be cleaned periodically.  When cleaned, it may take several weeks for the media to effectively convert ammonia to nitrates.  If you only have one vessel, your fish may be adversely affected after you have cleaned the filter media.  The reason I have two media vessels is so that I can rotate cleaning of media by several weeks so that the system continues ammonia conversion.

If you already have a submersible pump you can place it in the pump reservoir, instead of acquiring a new pump. 

You could eliminate the forced air fan from the trickle filter, but the filters efficiency will suffer.  Such fans only cost around $12 and are very cheap to run.


Here are three bags of the filter medial made up of 1" pieces of 3/4 inch 200 psi pvc pipe. Be sure to rinse the media before installing.

This is a picture of the mechanical filter funnel showing the grid that supports the mechanical filter element.  The grid is made from ceiling panels that are made for be fluorescent lights.  They are available at Home Depot, etc. The mechanical filter element is made from the blue furnace filter material sold at many hardware suppliers such as Home Depot. This is NOT a fiberglass product, which should be avoided.

Here is the funnel with the mechanical filter element installed.



This is a side view of the funnel. The "neck" of the funnel is sized to easily fit inside a pvc coupling for 4" pvc pipe.  The cone of the funnel is made using the cone designer elsewhere in this website.

This is the vortex vessel where the mechanical filter funnel will be installed. Note the mechanical filter bypass pump (Pondmaster 1800) and the 2" pvc pipe that will carry water to the second vessel whenever the mechanical filter becomes clogged. Also note the pvc elbows that direct the incoming water to move in a circular direction around the vortex vessel. One of the 3” incoming lines originates from a side drain in the pool and the other from a skimmer.




This picture shows the mechanical filter installed in the vortex vessel.




This picture illustrates the position of the sequence main pump and the plumbing that enables its removal for cleaning.





This is how the sequence pump motor is protected with a cut-out plastic pot.  Also illustrated is the drain plumbing from the pump reservoir container.





Here is a picture of the drain system plumbing for vessels 1 through 3.  One can also see the 4 inch pvc connection between vessels 2 and 3.






On the left is the small mesh wastebasket lined with cotton/poly batting that is the “water polishing” filter.  On the right is a plastic dishpan with a mesh bottom in which the wastebasket sits inside the trickle filter container. On the top is a bag of 1” pieces of 200psi pvc pipe that will be the trickle filter media.

This is an example of a computer type cooling fan used to pump air into the trickle filter.


This is a similar fan inserted into a rubber pipe connector with pieces of pipe insulation foam inserted as air seals.





The rubber pipe connector installed on the trickle filter container with the fan inside.






This is the trickle filter container showing the plumbing for the UV filter installation in summer.  Flow through the UV is adjusted via the gate valve on the left.  In the picture above the UV has been replaced by a section of pvc pipe.  I always remove my UVs in the winter and store them.  The UV that fits in where the pvc pipe section is in the photo is a Pondmaster 20 watt unit.  Water is kept flowing through the UV plumbing via the pvc pipe section in order to prevent freezing.  The large gate valve is for total flow adjustment.





This is a shot into the bottom of the trickle filter.  At the top is the 3” pvc drain pipe that exits to the waterfall.  The long 3” pvc pipe entering from the upper right is the air induction pipe from the fan.  The other pvc pipes shown are only for support of the grid.





This is the same shot as above except that the 1 cubic foot of pvc media has been placed on the grid.





This is the support for the polishing filter.





The polishing filter, with batting installed.






The degassing/water polishing/trickle filter in operation.  Note the camouflage paint treatment of the pvc pipes.





Another view, showing the proximity of the gravity fed filter system vessels.






Looking towards the waterfall and trickle filter hidden behind the shrubbery.






The camo painted trickle filter discharge before addition of rocks.





Another view of the camouflaged trickle filter.





Overall view of the gravity fed submerged media filter system. 






Schematic of the gravity fed submerged media biological filter system.  To be gravity fed, the tops of the drums should be about 1 inch above the pond water level.





Schematic of two cone bottom designs.







This is a schematic of the circuitry used in the gravity fed filter.  The design includes operation of a mechanical filter bypass pump.  Float switches s-1 and s-2 are positioned in the second vessel with s-1 at the normal fluid level and s-2 at a lower level that would likely occur when the mechanical filter becomes clogged.  As the water level drops, s-1 would turn on but the pump would not be energized until s-2 is also turned on.  The pump will continue to run until the water level rises enough to turn off s-1.





This is a simpler submerged media filter system that could be fed by a submerged pump.  The water enters via a pipe emptying into a mechanical filter.  If the mechanical filter becomes clogged, it will simply overflow into the vortex vessel; therefore, no bypass pump is required.  Main pump protection would be accomplished by positioning a mercury float switch in the pool itself to turn off the pump if the water level in the pool should drop below a predetermined level.  As depicted in the schematic above, the drums would all sit on the ground or on a raised area that would allow for an adequate drop for a waterfall.




This picture shows the mercury switch that is used to protect the main pump from running dry as is it comes from the manufacturer.



This shows the plug removed from the mercury float switch.



These are the parts used to install the mercury float switch.



Here is the wiring necessary for the mercury float switch.




This shows the completed wiring for the mercury float switch circuit for the main pump protection circuit.



This is the liquid level switch. It is used for the mechanical filter bypass because it is more sensitive to changes in water level than the mercury float switches.



This is a picture of the circuitry for the mechanical filter bypass.





Showing the wiring of the 12 volt ac transformer and the relay.





All connections should be coated with liquid electrical tape.




This shows the installed float switches for the mechanical filter bypass pump.





A picture of the electrical services for the filter system.  Note the red warning light for the mechanical filter bypass pump.  The red lever all weather switch on the right side turns off all power to the system.  The mercury float switch is plugged into the switch controlled receptacle in the large box below the switch shown above.  The receptacle on the bottom left is controlled by the mercury float switch.  Plugged into it are the main pump and the power supply to the large box above it on the left.  The large upper box contains the 12 volt transformer and the 12 volt ac operated relay that controls the 120 volt power to a receptacle inside.  The 12 volt ac relay circuit is controlled with the two float switches as shown in the electrical schematic.  Plugged in to the inside receptacle are the Bypass pump and the red warning light.  The clamp visible on the rim of the pump reservoir drum is used to adjust the operation of the mercury float switch.






Regular maintenance is important to the operation of any watergarden.  Face it, while the goal of practically all hobby watergarden folks is to mimic mother nature and have a “natural” appearing water garden, we must understand that lined water gardens are really nothing more than outdoor aquariums.  Maintaining a healthy environment for our wet pets is our responsibility, not mother natures.


Step 1 is to monitor the screens in any skimmers in our pond system.  /fish may be caught in skimmers and will have to be freed.  Accumulated debris must also be regularly removed.


Step 2 is to monitor the mechanical filter to keep it free flowing and removing the solids it collects.  The system as described above will help you in that regard by the warning light that comes on whenever the mechanical filter bypass pump is called into action due to mechanical filter clogging.


Step 3 is to monitor the batting in the water polisher and replace it any time the wastebasket overflows.


Step 4 is to periodically open the gate valves one at a time that will drain from each filter vessel cone bottom any detritus that has accumulated therein.


Step 5 is to at least twice annually removing the pump from the system and checking to see if there is anything blocking the pump rotor such as water snail shells.


Step 6 is to annually replace the bulb in your UV light to keep any planktonian algae under control.  If you are in a zone where temperatures drop to freezing, it is best to remove the UV unit from the system during cold weather.


Step 7 is to periodically determine that the forced air fan in the trickle filter is functioning.


Step 8 is the least often requirement.  Performance of this procedure should be done at least once annually and done only on one media containing vessel at a time with an interval between vessels of at least six weeks.  Turn off power to the system.  Shut off flow from the pond by closing the knife valves feeding the system.  Remove the top grid covering the media in the selected vessel.  Reach in and grasp the bags holing the bio media and pull up and down vigorously, dislodging accumulated sediments.  Open the drain for that vessel and spray water from a hose over the media to further wash sediment away. Note: you will lose some water from other vessels in this process.  Close the drain valve and add water to the filter system until it is once again full.  Wait 1 hour to allow settling and turn power back on to the system.



Copyright 2006 JRJohns