An old man I met once told me this story:
One day – for some reason – four blind women were feeling what the old man had told them was an elephant.
Not knowing what an elephant was, the one feeling the leg said, “it feels like the base of a tree.” The one with the trunk said, “it feels like a large snake.” The one with the ear said, “it feels like a big leaf.” The one with the tusk said, “it feels like a spear.”
“No,” the old man replied. I told you it’s an elephant.
Then some ants on the tree attacked the first woman, the snake bit the second woman, the leaf gave the third poison ivy, and the fourth said “I’m not blind” and chased after the old man with the spear.
In the same way I’d like to tell you about four different types of aquaponic systems. Like the different parts of a fake elephant, each can bite you if you get bad advice.
Currently – according to my experience – two types of systems dominate the backyard aquaponics world: Constant Height One Pump (CHOP) and Deep Water Culture (DWC). Both awesome designs, they work in the cold if designed right. Some growers over the years combined the two types into a hybrid system, which combines the benefits of both.
Recently, Vertical Growing systems made some breakthroughs in their designs and began to gain some traction with commercial and some backyard growers. It offers dense growing in a small space, but works poorly in the cold.
My preferred system type, the Alternating Flood-and-Drain (AFD), resembles the CHOP design, but alternates which bed it floods. It also works as a hybrid system with DWC and Vertical Growing, and works stellar in the cold.
Each of these systems offers advantages and disadvantages. I’ll describe each of them briefly and then explain why I think AFD beats the others in the cold.
The Constant Height One Pump works like a Ronco rotisserie. You “Set it and forget it.” It consists of a fish tank, sump tank, and one or more grow beds filled with growing media.
The pump runs all the time, flooding the grow beds and cycling water back through the fish tank. The grow beds provide the nitrification. Presuming you built or bought a reliable bell siphon, the system runs automatically with little maintenance.
Unless you include a solids removal filter, every few years you have to clean out the grow beds. I’ve done this. It sucks. I recommend a solids filter.
This system allows for a small and inexpensive pump, because it moves small volumes of water constantly.
You can insulate and air seal CHOP tanks and grow beds quite well (though I haven’t seen it done). The system could allow for the addition of a controller to vary the frequency of pumping based on air temperature (though I also haven’t seen this done).
The downside? It’s a big one. You need a large sump tank. The sump tank must contain just as much water as all the grow beds, and it should not contain fish because at some point all the water will be in the grow beds at once, leaving the sump empty and the fish flopping around.
For small system, this poses little problem. For larger systems, however, especially space-constrained systems, this poses a major problem in that you must find room for the large sump tank.
From an energy perspective, this very large sump tank provides a great deal of additional surface area from which to lose heat and evaporate water, which is bad.
Summary for use in the cold: Simple and workable, but not ideal.
The Deep Water Culture system optimizes the amount of vegetables per unit of labor. Floating grow beds allow you to harvest large quantities of vegetables quickly by simply removing the whole grow bed from the water.
Originally popularized for commercial growers, this system type recently began to take hold with backyard growers, particularly in hybrid systems which also include media-filled beds.
In DWC, the plants sit in in floating beds of insulation. In some systems, such as the one in the diagram, the pumps force water into the beds which rest at a level above the fish tanks. The water then drains back into the tanks. In other cases, the fish tanks sit above the grow beds, with a small sump tank below. The pump moves water from the sump tank up to the fish tanks.
The filter typically goes between the fish tank and the grow beds, to keep the grow beds clean. Some use freshwater prawns to further clean their beds. Not that prawns are territorial, and prone to defend their honor in elaborate duels.
This system also allows for a small and inexpensive pump. It also allows you to keep your beds flooded, which avoids expelling air during flooding and sucking it back in during draining. This saves heat through enthalpy conservation.
One disadvantage to DWC is the fact that the beds floating on the water leave an air gap on all four sides where the beds meet the walls. This gap, usually about 1/2″, allows a space for water to evaporate. In the cold, this makes a much bigger difference than you would think.
You may be able to find ways to air seal this, but it would reduce the labor efficiency because the grow beds would become more difficult to remove and harvest.
The other disadvantage is that the filter requires some labor. Most DWC systems contain a three- or four-tiered filtering system with a solids filter, a dissolved particle filter, a nitrification filter, and sometimes a nitrogen removal filter. These various filters require regular (daily or ever-other day) maintenance.
Without grow beds to provide a diffuse filtering medium for providing bio-filtration and for catching dissolved solids, the filtering regime provides the only means of keeping the grow beds clean.
In a commercial system with staff present nearly every day, this provides no problem. In a backyard with residents who might like to go on vacation (holiday) sometimes, this ties you down.
Enter the hybrid system. This solves some of the problems of a DWC system. With grow beds to provide nitrification, you need fewer filters and thus less maintenance depending on the ratio of DWC to media beds. I love this idea, and plan to add a DWC bed to my next design.
Summary for use in the cold: Works for commercial, a nice add-on for a backyard system.
This system provides for the optimization of growing space, crucial in an urban environment or in a heated greenhouse. It achieves this by maximizing both vertical and horizontal space, filling the space with multiple rows of “towers.”
It’s really trippy to walk through one of these, especially in the dark. I’d love to see a horror movie set in a large multi-story vertical growing facility. But I digress.
It also allows the commercial grower to bring their grow beds along for the ride to the market (with the first piggy). Doing this, you can serve greens so fresh they’re still alive, or bring them back to the greenhouse for some more growing if they don’t sell.
From a cold weather perspective, because you grow more in a smaller space, it offers the benefit that you only have to heat a small space.
The vertical growing towers require a lot of runs of small tubing, a lot of surface area, and make it very difficult to provide meaningful insulation. This leads to its major disadvantage that you have to (as I discouraged you from doing in both Fundamentals #1 and Fundamentals #2) heat your greenhouse.
It also requires a sump tank, which as you may recall robs even more heat.
Similar to DWC, I think that vertical growing provides a nice add-on to a media-filled system. In the summer, that is. I would shut them off in the winter.
Summary for use in the cold: Great add-on for summer, but not for use in winter.
This brings me to my preferred method for growing in the cold, which I use personally with great and cost-effective success.
The Alternating Flood-and-Drain
This design overcomes the disadvantages of the other system types, while maintaining most of their benefits.
It allows for very tight insulation and air sealing. It does not require a sump tank or a filter (though like the CHOP system it benefits from one).
It also allows you to “set it and forget it” in that the timer or controller provides all the maintenance needed. You feed your fish, plant, and harvest produce. That’s it.
A controller can slow down the timer pumping frequency with temperature. With a timer, you can do this yourself if you expect a cold night.
In this system, you divide your growing area into four to eight grow beds. The pump floods each bed alternately using a device known as an indexing valve.
The pump supplies water to the valve which sends it to one bed. The timer or controller stops the pump, and starts it again after a short time. When the pump stops, it indexes to the next bed, and so on.
This prevents you from ever supplying all your water to all your beds at once, so you never take out more than a small portion of the water. Thus, you have no need for a sump tank and will never see fish flopping around.
The sheets of insulation holding the plants – much like those in DWC – go on top of the grow beds, providing insulation and air sealing. This also allows you to remove them and replace them with little effort at harvest time.
The primary disadvantage of this system is the need for a reliable and powerful exterior (non-submersible) pump. The indexing valve requires a significant flow rate, which can only be provided by a powerful pump. Stopping and starting the pump often also means that it must be sturdy and reliable to operate under these conditions.
The pump offered here meets both of these criteria, though it costs more than those used in DWC or CHOP. However, comparing the cost of a sump tank, a triple-or-quadruple filtering system, or a nice pump, you may find that the pump comes out ahead.
According to the manufacturer, our pumps will last a minimum of 10-15 years under these start-stop conditions. Upon failure, it is likely that replacement parts will keep them running even longer.
Another option includes the addition of a diversion valve, which allows your pump to run all the time, sometimes pumping into the beds and sometimes pumping directly back to the fish tanks. This allows you to choose a less reliable pump, though it still needs to meet the power requirements of the indexing valve. Running your pump all the time also uses more energy, which I try to avoid.
Note that the image shown does not include drain piping for simplicity’s sake (and because I just didn’t get it done in time). It shows the fish tanks (freezers in this case), buried in the ground. It is not necessary to do this, though the grow beds must rest above the level of the fish tanks. Burying them saves space.
You can take a low-res tour of the system at my YouTube channel.
In case I haven’t made this clear in all the Blog posts up until now, I’ll make it crystal clear here: Unless you grow commercially in a market which provides top dollar for your products or you have a source of very cheap energy, I highly recommend not heating your greenhouse!
The Alternating Flood-and-Drain system provides a good option for economically designing a system that doesn’t need much heat.
Also, watch out for that old man and his “elephant.”
What do you think?
Does my explanation jive with your experience? Let us know in the comments section.
The more we share, the more we learn.