Aquaponics 101 Part Two: System Design
This is the second in a series of posts that are going to teach you everything you would ever want to know about Aquaponics. These posts are part of a book we are writing and will be selling on our website. So, if you’re curious about the most amazing food growing technology on the planet today, watch for this series of educational posts on Aquaponics and please, become interactive by making comments or asking questions. Given the state of our Union, the motto on our website is becoming more important than ever–“Time To Grow Food”.
In Part One, “The Process”, I wrote about what Aquaponics is and why it is important to Preppers (those preparing for what is about to come down the pike).
To quickly review, Aquaponics combines the raising of fish and using the fish waste as plant nutrients so we can grow vegetables. This is done year round and can provide food fish and veggies for your family.
The process includes the breakdown of fish waste into plant nutrients, the uptake of these nutrients by the plants being grown in separate grow beds and the cleaning of the water to be returned to the fish tank to be reused in an endless loop. This is all done in a continuous flow recirculating aquaculture system called Aquaponics.
I’m now going to focus on a particular system type and its design, but I’ll be referring to other designs. As mentioned in Part One, the fish tank and grow beds are separate entities. This precludes growing the majority of your vegetables directly in the fish tank. However, some green leafy plants like lettuce can be grown on rafts floating in the fish tank as long as the particular fish being raised don’t eat the plant roots. However, this is generally not done.
As I also mentioned in Part One, there is a need for a bio-filter, which is a part of the system and it is filled with a media that contains lots of surface area. Most Aquaponics farmers (we’re talking about home and backyard farmers here) choose to combine their grow beds and their bio-filters into a single unit. This simplifies the design and construction of their Aquaponics system in addition to saving cost and space. It is this combined unit I will be focusing on here.
The combined grow beds/bio-filters containers (from here on referred to as a grow beds) need to have enough volume to contain ample material with enough surface area to support the number of fish in the fish tank. It also must provide enough planting area to support the optimum amount of plants needed to uptake the nutrients without depleting the system of these nutrients, thereby causing poor plant growth. Sounds complicated, but, fortunately, many have gone before and have worked this all out, so we can build on their shoulders.
Grow beds are generally filled with media, either Hydroton (expanded clay) or smooth river stones (gravel), in order to give the needed surface area on which the bacteria thrive, while simultaneously holding the plants in place. The grow bed is filled with the media to a level near the top of the grow bed container, and the fish tank water is pumped into the grow bed filling it to a maximum level that is one inch below the top of the grow bed media. This one inch barrier is to prevent the top of the grow bed media from becoming and staying wet, thereby preventing algae growth on the top of the media; and also to help prevent the bottom plant leaves from becoming wet and moldy.
The optimum grow bed container depth should be twelve inches (when using Hydroton as a grow bed media). This will allow for eleven inches of media and ten inches of fish tank water to be placed in it, which is enough to provide for bacteria growth as well as providing ample depth for anything you wish to grow. It will also allow for some accumulation of fish solids in the bottom and give them time to break down before they over accumulate. Grow bed containers deeper than 12 inches are more costly to fill with media, heavier, transfer more water back to the fish tank (more on this later) and generally unnecessary, unless your using gravel as a grow bed media, then the extra depth can be beneficial (more on this later, also). Grow bed containers less than twelve inches deep cause your planting area to grow bed volume ratio to be less than ideal, thus creating more planting area than the bio-filter volume can support. This twelve inch number is not cast in stone; but if you build a system using it, it will work. If you use other depths, especially less, then you are on your own.
The size of the grow bed can be whatever you wish, but understand that if you go over 30 inches in width, you will have difficulty reaching across it; so you need plenty of walk around room to get to two opposite sides. 48 inches is probably the maximum width you should be considering. The length is not as important as the depth and width. It is best to have at least two grow beds in your system rather than just one. This will allow you to clean out any plant roots between planting while still having your other grow bed working to keep the fish water clean and safe for the fish.
There are two different configurations of media-filled grow beds, flood and drain (also called ebb and flow) and continuously flooded. Flood and drain are the best, regardless of the types of plants you are planting, because it provides the best distribution of incoming nutrient rich water throughout the bed as well as adding aeration to the wetted media and plant roots where the bacteria live. This also adds dissolved oxygen to the water being flooded back into the bed and then returned to the fish tanks.
You will need a way to flow the water into the grow bed. The water entry point into the grow bed depends, in part, on the shape of your grow bed and how you plan on using it. With a flood and drain grow bed, the point of entry of your water is not critical; but it should have an unrestricted opening so as not to clog with fish waste solids. Do not attempt to spray your water into the grow bed, for the spray holes will clog up in short order (experience speaking here).
The best way to drain your grow bed is with a Bell Syphon. This is a device with no moving parts that when filled with water starts to syphon out all of the water in the grow bed down to a preset level before “breaking syphon”. At that point the syphon action stops, and the grow bed starts to refill. The design of this syphon is critical in order to get it to do its job in a timely manner. I’m not going into the design and construction of Bell Syphons here, for the information can be found elsewhere on the web, as there are several different designs available.
The more often you flood and drain your grow beds, the more dissolved oxygen will be added to the water returning to your fish tank. You should count on cycling your grow beds at least four times per hour. Not only does the design of the syphon influence the cycle timing, the rate at which you flow water into your grow bed, does as well. This is one of the most critical parts of the design of an Aquaponics system. Needless to say, you will need enough pumping power to exchange most of the water in your grow beds four times an hour.
Keep in mind that once the grow bed media is in the grow bed, it will displace at least 50% (Hydroton) of the volume of the grow bed (more for gravel) leaving half or less of the original grow bed volume for water. This should be taken into account when sizing your water pump. The good news is, you may be able use a smaller pump requiring less electrical power as long as other pump sizing requirements are met.
Within the Aquaponics community you will find differing opinions on the numbers I’m about to give you, but I believe them to be the most accurate based upon what I have learned from those with years of experience in building and testing different combinations of system sizes and ratios, as well as from my own experience in verifying these numbers. You can build an Aquaponics system any size you desire, for it is the ratios of the various component sizes that really mater. So, lets start with the ratio of the fish tank size in gallons to the grow bed container size in gallons. There should be about a one to one ratio between the fish tank and grow bed container size (if you are using Hydroton as a grow bed media). This is assuming that you are using media filled grow beds.
Media filled grow beds are ones that use a growing media such as Hydroton (preferred) or gravel (smooth river stones) to fill the beds. It has been found that the amount of grow bed volume needed to support a system is more related to the pounds of fish in the tanks and how much they are fed than the amount of water in the system. If you are using Hydroton as a grow bed media (preferred), a good rule of thumb for the size of grow beds is 3.3 gallons of grow bed (minimum) for every pound of fish you plan to have in your system at maximum fish grow-out size. If you plan on a ratio of three gallons of water per pound of fish in the fish tank, then this works out to about one gallon of fish tank capacity to one gallon of grow bed container capacity when using Hydroton as a grow bed media.
For gravel media in your grow beds, you should consider increasing the size of your grow beds by about 50% or decreasing the planned fish poundage by about one third.
In Part Three, I will continue talking about system design.