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Water flow is the most important part of proper system design, and the exact positioning of the grow beds has a major impact on this. Therefore it should be considered carefully and, if possible, an expert should be consulted. The grow beds should be positioned after the biofilter and before the water is recirculated to the fish tank. Consider how the water will flow from the grow bed into the fish tank. If it is by gravity, then the water level in the grow bed must be higher than the fish tank, which may mean that you have to dig the tank and connections into the ground, or that your grow beds will be so high that you would not be able to work comfortably. Usually, a sump tank with a pump is placed after the grow bed to enable water to be pumped into the fish tank. The connection between the biofilter and the grow beds should be as short as possible, and the inlet/outlet should be placed on the opposite sides of each grow bed. One of the advantages of soilless cultures is the possibility to design suitable conditions for working with plants. Ideally, the system should be designed at a height that enables you to monitor the plants easily (Figure 18).
Figure 18: Different levels of grow beds: (left) raised grow beds enable comfortable working; (right) ground level grow beds need no support construction, but are still perfect for production: easy access, lots of light, and deep enough for the roots. Also, if spraying is needed it is on perfect level to do so (Photo A. Graber, ZHAW)
As with fish tanks, the most important aspects are maximum safety for both the fish and the plants, and minimal risk of water leaks that will cause damage. Pond liners are often safe and low cost, but the risk of damage is rather high.
The diameter of the water inlets and outlets should be large enough to enable water flow quantities designed for the entire system. The inlets and outlets should preferably all be of the same diameter. Every hole is a risk for water leaks if the sealing is not appropriate. This risk should be avoided by drilling as few holes in the system as possible.
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