In its simplest iteration, iAVs has its fish tank(s) located in-ground.
The pump, located in the fish tank, moves water up to the sand bed. The water then percolates down through the sand and drains back into the fish tank.
Note: These diagrams are not to scale. (See the photo in “R411 folio prints” – Publication section – for furrow proportions).
The advantages of this arrangement are that it’s easy to understand – and similarly easy to build and operate.
Having the pump located in the fish tank affords its own advantage in that the solids are macerated as they pass through the pump which causes them to be more evenly distributed throughout the furrows.
Note: This is seen as a disadvantage in basic flood and drain media beds filled with gravel/clay pebbles but macerated solids are not an issue in iAVs because of the much greater effectiveness of sand as a filtration substrate.
There are those situations where it may not be desirable (or even possible) to put your tanks in-ground. The good news is that fish tanks can be located above-ground – and there are a couple of ways that this can happen.
- Sump Tank System
- Constant height – one pump (CHOP)
Sump Tank System
With this option, a pump is located in the fish tank. The pumping cycle is initiated by a digital timer which causes water to be pumped from the fish tank to the sand beds. The water percolates down through the sand, exits the sand bed and drains into a small sump tank.
A sump pump is located in the sump tank. A float switch triggers the sump pump once the water level reaches a certain level and the water is pumped back to the fish tank.
The benefits of this arrangement is that fish tanks can be located above-ground and only a small sump tank is required.
Note: One of the prospective concerns is the energy use involved in using two pumps but neither pump is activated for more than 2 hours per day.
The disadvantage of the method is that, in the event that the sump pump should fail, the pump in the fish tank could (following several pumping cycles) pump the fish tank dry. This risk could be mitigated by installing a magnetic float switch in the sump tank so that, once it fills beyond a certain point, it bypasses the digital timer that triggers the operation of the fish tank pump.
Constant Height – One Pump (CHOP)
If for whatever reason, you would prefer to use just one pump, then the CHOP option is for you.
With the CHOP layout, the water is pumped from the sump tank up to the fish tank. This inflow displaces water forcing it up the solids lifting outlet so that it exits the fish tank and out into the sand beds. It then percolates down through the sand and then drains back into the sump tank.
The benefit of the CHOP approach is that only one pump is required and the prospective point of failure (posed by the sump pump) is eliminated.
The downside is that the sump tank has to be large enough to contain any water that drains back to the sand beds after the pump cycle is completed.
Another possible disadvantage (yet to be confirmed) is that solid wastes are delivered to the sand beds relatively intact – with the possibility that they may build up in that part of the furrow which is closest to the inflow point.
It remains to be seen whether this is an issue in practice and may also vary by the fish species in use. The composition of fish faeces varies from one species to another. Tilapia, for example, excrete long faecal strands while the output of others is shorter or more soluble.
Mark has provided the following notes for those contemplating in-ground fish tanks:
- In the ratio studies, the slope of the tank bottom was down and away from the return inflow.
- Rectilinear shapes are not the only option. Circular tanks are also possible, and work best with a sloped (conical) bottom. I used linear aspects in the ratio studies to accommodate mirrored and adjacent tanks with common walls (to fit 16 identical volumes into the available space).
- I suggest a parabolic cross section and round or rounded rectangle (in plan) if dug into ‘soft’ ground. This increases the stability of side walls (reduces sloughing) and concentrates the solids toward the central axis of the low point (and lowest volume region) by gravity for ease of removal by pump(s). This also eliminates dead zones in corners and, when air stones are used along edges of long axis, they will vertically mix/turn-over the water, and create a pair of elliptical currents that concentrate the solids toward the pump(s) or uptake manifold. The more that the shape and currents can be employed to focus wastes into a limited region/volume for ease of extraction, the better.
Proto ’86 Layout
(click to enlarge)
Note: Approximately to scale – aisles were a bit wider.
This diagram shows the relationship between the fish tank, sump tank and sand beds.
It may help our readers to understand how Mark was able to equally distribute water over such a large layout…..using water as the levelling mechanism.
The construction of the fish tank was similarly simple…..and comprised lumber-framed tank walls lined with open mesh “snow fencing” which supported a double layer of 5 mil poly sheet. Very cheap, yet effective for its purpose.