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Aquaponics 101 Part 3: The System Design Continued

This is Part 3 in a series of tutorials that are going to teach you most of what you need to know about Aquaponics.

Some will argue that the standard ratio of grow bed size to fish tank size is two gallons of grow bed container capacity to one gallon of fish tank capacity instead of the one to one ratio I mentioned in Part 2. Again, the number for expanded clay is 1:1 and for gravel about 1.3:1 gallons of grow bed container capacity to fish tank capacity. The reason for this one to one ratio limitation is that the water in the fish tank goes up and down during the grow bed's flood and drain process, and too much variation in water height can stress the fish. You can use the 2:1 number only if you flood and drain some of your grow beds but not all; or if you add a sump tank to catch the water that would otherwise be returned directly to your fish tank. The increase in the ratio for gravel is that it usually displaces more water than does the expanded clay and, therefore, you need a larger grow bed in order to have room for the same amount of water.

The simplest AP system design has a low to the ground fish tank that is 24 to 30 inches high and grow beds that are up on tables high enough so that the water pumped up from the fish tank to the grow beds can gravity flow back into the fish tank from the bottom of the grow bed siphon and have it function. I prefer 24 inch high fish tanks so the grow beds don't need to be so high and, therefore, you don't need a step up to comfortably reach across them. This allows for at least six inches of extra siphon draw down below the grow bed thereby reducing the grow bed's drain time (more on this later).

The grow beds' siphons activate on their own timeline; but at some point, with multiple grow beds, the siphons arrive at nearly the same schedule. Like two or more metronomes, occasionally they all sync up and drain at the same time filling the fish tank to capacity and then they simultaneously pump water into and fill all the grow beds. With a two to one grow bed to fish tank ratio, the extra water required to fill the grow beds leaves the fish tank with a dangerously low amount of water (if none at all), which will stress the fish.

With a one to one grow bed to fish tank volume ratio, the water level in the fish tank won't go so low as to stress the fish. So, one to one is the number you want to aim at in your AP system design.

Some will argue that one way to avoid this water level in the fish tank problem created by increasing grow bed volume is to add a sump tank (mentioned above) that catches the drained water from the grow beds. The water is then pumped back to the fish tank from the sump tank after it's water reaches a certain level by way of activating a float switch connected to a submersible pump in the sump tank. This allows the sump tank to absorb the intermittent water flowing into it and helps keep the fish tank from experiencing major swings in water level during grow bed siphoning.

For a short fish tank that sits on the ground, this requires an extra pump (in addition to the one in the fish tank) and a float valve switch (mentioned above) in the sump tank, which adds extra parts, cost and potential points of failure. Turning a pump motor on and off repeatedly shortens its life, and, if either the pump motor or float valve switch were to fail (and eventually one or the other will fail), you will have water all over the place, a fish tank void of water and dead fish. In my opinion, this is not a good design. You might as well increase the size of your fish tank and keep the same number of fish, thereby saving yourself the cost of the sump tank, pump and float switch, along with its complexity and poor reliability.

Another system design raises the fish tank or has a tall fish tank, like a tall IBC tank with its top cut off, which overflows the fish tank water through a pipe into a sump tank that is positioned at a lower level. The water is pumped from the sump tank through a Tee directly into the fish tank. The pump is sized to have a good flow to the fish tank so the solids are lifted from the fish tank bottom on the return path to the sump tank. By continuously circulating a generous flow of water from the sump through the fish tank, the fish waste solids are lifted up through the overflow pipe, which starts near the fish tank bottom and exits the fish tank near the top.

The other side of the pump Tee goes to the grow beds through control valves where the water flow is individually adjusted to each grow bed for proper siphon action. The water returns from the grow beds to the sump tank, which is large enough to absorb the flood and drain action of the water coming from the grow beds while leaving the fish tank water at a constant height.

This design was created by Murray Hallam of Australian aquaponics fame and is known as a CHOP II, which stands for Constant Height One Pump, Version II. It is an elegant design because it adds only minor complexity and cost to building your system while using a single continuously running pump, and it works well.

It is very important that a float valve bringing in outside water be added to the sump tank in order to assure that there is enough water in the system because evaporation will reduce the water level in the fish tank to a level where it won't overflow, thereby keeping the fish tank water from being circulated.

The solids I have watched live and on my fish cam are wrapped in a clear sheath, which appears to be the slime component of the worm-like solid. By first sending the solids through a pump before they go to the media filled grow bed, they get macerated; thereby breaking them into smaller components, which allows for the heterotrophic bacteria to do a faster job of mineralization. To leave them in the sheathed form may allow them to accumulate in the grow beds, and if the water is being pumped into the top of the media filled grow beds that is where the solids accumulate. The flies love it, but it is not a pretty sight.

The simplest of systems have a submersible pump in a low fish tank that pumps ample water under pressure to the grow beds. The water can be controlled by valves at the grow beds in order to regulate their fill rate while providing some additional water under pressure to be jetted back into the fish tank in a high velocity stream for added aeration. This extra pump pressure allows for purging of the lines to the grow beds by fully opening the grow bed control valves individually for a short period of time on a weekly basis. This needs to be done periodically because the fish waste solids are heavier than water and the slow flow up to the grow beds doesn't allow for all of the solids to make it into them. This slow upward flow causes some accumulation of solids in the plumbing, which the purging alleviates.

The auto siphons return the water to the fish tank from the grow beds by gravity. This is accomplished by using a line connected from the bottom of the siphons below the grow beds to the fish tank above the water level. The grow beds are high enough without being too high, and the fish tank is low enough to allow for a good flow return from the siphons. This pretty much describes the system  designs (minus the aeration) we build and sell here at Aquaponics USA and are currently using, which are elegant in their simplicity and very cost effective. Again, to be clear, I have borrowed significantly from the tried and tested work of others.

Grow beds can be made of wood with plastic liners, but fitting them with the needed bulkhead fittings that don't leak may prove to be a challenge. However, it can be done successfully. The bulkhead fittings in the grow bed bottoms are necessary and are part of the bell siphons.

Hydroponic reservoirs make good grow beds (at least the twelve inch deep ones do). These sixty nine gallon, eleven square foot (one square meter +) reservoirs are what we use in our FGS-20 systems, and they are very sturdy. These reservoirs need bottom support when used as elevated grow bed containers because they are made to sit on the floor. We also use rectangular white reservoirs as grow beds and they have a nice form factor of 33 inches across and also need bottom support.

Animal stock tanks make good fish tanks. They are about two feet tall and, depending on the brand, are very well made from USDA approved polyethylene with UV inhibitors. You don't want to have a black fish tank because it limits the light in the tank making it hard to see the fish. Many fish species do better with more light.

As for submersible pumps, we recommend and sell magnetic drive (mag-drive) pumps because the motor is in its own sealed compartment and should never leak any oil out into the fish tank water, which would be really bad for the fish. Good ones are relatively inexpensive and have a one year warranty. The pump should be capable of turning over the total gallons in the fish tank about every thirty minutes at six feet of head pressure minimum. Be careful of advertised flow rates at zero head pressure, because that doesn't tell you what you need to know for your system (more on this below).

We use diaphragm type air pumps that require the diaphragms to be replaced after about one year of continuous use, but they are very inexpensive and easily changed. Air pumps are notoriously inefficient. They produce lots of heat and move little air, but they are absolutely essential for your AP system. You will need a pump rated at about 7 GPH of air for each gallon of fish tank capacity. With a change from 5 GPH to 8 GHP of aeration per gallon of fish tank water (a 60% increase), we saw a better than a 20% increase in DO (dissolved oxygen).

The best way we have found to put air into the tank is through cylindrical air stones. Even though they have relatively large air bubbles, they don't clog up as much as the finer bubble diffusers.

Our dissolved oxygen levels regularly measure between 6 and 7 ppm depending on water temperature. A study made with tilapia and varying amounts of dissolved oxygen (DO) in the water showed a doubling of growth rate from a DO of below 3 ppm to a DO above 6 ppm.

Air pumps need to be run 24/7, to do otherwise is to kill fish. After all, how long would you last without air?

There is what I consider misinformation regarding the size of an Aquaponics system and its biological stability. I challenge the idea that an aquaponics system has to be a minimum number of gallons to maintain biological stability. It has more to do with the fish to water ratios, regardless of the size. This assumes a constant system temperature.

The system's temperature stability is another matter. It has to do with the volume of water to the surface area of the fish tank plus the surface area of the top of the water in the fish tank. Add to that the surface area of the grow beds (all 6 sides) and the water contained therein. Then consider the amount of insulation, if any, surrounding these components as well as the varying air temperature where your system is located and you have the thermal stability.

In addition, you will be pumping air through your fish tank in order to increase your dissolved oxygen levels. This process cools the water due to transfer of liquid water to vaporized water as the air bubbles rise through the water in the fish tank. The same thing is happening in the grow beds as you pump in and siphon out water. Air is also pumped in and out during this process as well, causing evaporation and cooling. This is known as latent heat of vaporization, or cooling as water is evaporated.

The temperature stability affects the biological stability. So, the larger system may have a better overall stability due to the larger thermal mass to surface area ratio.

Another type of siphon is the loop siphon seen in the photo below. It consists of a pipe that picks up the water from the inside bottom of the grow bed through some sort of strainer and brings it up and out (or out and up) of the grow-bed through a bulkhead in the grow bed wall. The top of the loop is set at the upper most desired waterline level in the grow bed when it is flooded. From its top, it makes a U turn downward and the water flows out the down pipe into the fish tank or sump tank. The siphoning action occurs when the grow bed is flooded and the water level starts to exceed the height of the top of the loop, and the siphon break occurs when the air enters the strainers inside the grow bed bottom after most of the water is siphoned out. We have found a properly designed loop siphon is superior to a bell siphon in its range of water flow, its ability to cycle rapidly and its leaving more grow bed space for planting.

In sizing your pump, make sure you have enough flow to fill all of your grow beds at least four times an hour. For example, let's say you have a 120 gallon fish tank and two 69 gallon hydroponic reservoirs as grow bed containers for a total of 138 gallons of grow bed capacity. The water in the grow bed will only be filled up to the 60 gallon mark, for a total of 120 gallons for two grow beds. The grow bed media will displace at least half of that so now we are looking at about 60 gallons of water total in the two grow beds. Not all of the water will drain, so let's say about a total of 50 gallons drains out before syphon break. We replace that four times an hour so we need 4 X 50 = 200 gallons per hour just to fill the grow beds. We need another 20% to jet back into the fish tank for additional aeration, which is 40 gallons per hour. The total water is 200 + 40 = 240 gallons per hour at 6 feet head pressure. This works out to be twice the fish tank volume every hour.

The rule is to always over size your pump in case you wish to add other components to your system later that require water under pressure, like vertical grow towers or an elevated small brooding tank, both of which can gravity flow/overflow back into your main fish tank. Pumps come in discrete sizes, so we use a 1000 GPH at 6 feet of head pressure pump.

The extra water pump capacity is used to rapidly move the fish tank water through the plumbing to the grow beds in order to clean out (purge) the plumbing. As I explained earlier, this is necessary as the normal flow from the fish tank to the grow beds is very slow and the fish waste solids tend to accumulate in the plumbing. Line purging is accomplished by fully opening the control valve in the line to the grow bed and allowing the full force of the pump to rapidly move the water through the line for a period of about 30 seconds, at least once a week. The water coming out during that time will be very brown. When it clears, then reduce the flow to the proper amount for your flood and drain cycle timing.

Congratulations! You've just completed Aquaponics 101, Part 3.

Now it's time to test your knowledge. Take the Part 3 Quiz here:

1.  What is the reason for my 1:1 ratio limitation regarding grow bed

    container capacity to fish tank capacity.

2.  Why does the 1:1 ratio change to 1.3:1 when you're using gravel

    media in your grow beds instead of Hydroton?

3.  What's the preferred height of the fish tank from the ground?

4.  Why are shorter fish tanks in this height range preferred?

5.  What function does the water pump serve in relation to the fish

    waste solids?

6.  How do you purge the lines to the grow beds?

7.  Why do you need to purge the lines to the grow beds and how

    often  should you do this?

8.  The auto siphon uses ________ flow to return the water from

    the grow beds to the fish tank or sump tank.

9.  What is the name of the second type of siphon other than the

    Bell Siphon?

10. What are the advantages of this second type of siphon?

11. A water pump should be capable of turning over the total

    gallons of water in the fish tank every ___ minutes at a

    minimum of ___ feet of head pressure.

12. In sizing your pump, you need to make sure you have enough

    flow to fill all of your grow beds at least ____ times an hour.

13. Tilapia grow twice as fast when the DO is above ___ ppm as

    verses below ___ ppm.

14. True or False (Circle one), thermal stability depends on only one

    element in your AP System.

FGS-20

Single Grow Bed