levels to plummet below the minimum. Any
decomposing solid waste present in the
canal would exacerbate this problem, further
diminishing DO. Thus, aeration is required.
The simplest method is to place several small
air stones in the canals. The air stones should
release about 4 litres of air per minute, and
be arranged every 2–4 m 2 of canal area. In
addition, Venturi siphons can be added to the
water inflow pipes to aerate the water as it
enters the fish tank and on the back-pressure
line in the biofilter.
Do not add any fish into the canals that could
eat the plant roots, e.g. herbivorous fish such
as tilapia and carp. However, some small
carnivorous fish species, such as guppies,
mollies, or mosquito fish, can be used
successfully to manage mosquito larvae, which
can become a huge nuisance to workers and
neighbours in some areas.
Planting in a DWC Unit:
As mentioned previously, this method involves
suspending plants in polystyrene sheets, with
their roots hanging down into the water. The
polystyrene sheets should have a certain
number of holes drilled to fit the net cups
(or sponge cubes) used for supporting each
plant. The amount and location of the holes
is dictated by the vegetable type and the
distance desired between the plants, where
smaller plants can be spaced more closely. The
standard number is normally approximately
25 holes for every 1 m 2 .
of the raft. In Large-scale units, one should
clean the rafts with water to remove dirt and
plant residues and immediately repositioned
them with seedlings in the canals to avoid any
stress to the nitrifying bacteria.
Guide to construct the DWC Aquaponics
system:
Layout of tunnel, greenhouse, room or
outside area for the system:
It is important to plan your Aquaponics system
according to space available, enough light as
well as access to electricity and water. Wind
and insects will damage the plants and ideally,
one should try to close the system off. For
this article, the plan is to construct the system
indoors. In this case, sun would not be a
problem for this demonstration unit will make
use of red and blue LED lighting to assist with
the photosynthesis of the plants. So, in the
planning phase, it is important to consider
room for openings (doors and hallways) as
well as sufficient access to the various parts of
the system i.e. draining of waste, feeding and
general maintenance and inspection.
Seedlings can be started in a dedicated plant
nursery in soil blocks or a soil–less medium.
Once these seedlings are large enough to
handle, they can be transferred into net cups
and planted into the DWC unit. The remaining
space in the net cup should be filled with
hydroponic media, such as volcanic gravel,
Rockwool or LECA, to support the seedling.
It is also possible to simply plant a seed
straight into the net cups on top of the media.
This method is sometimes recommended
if vegetable seeds are accessible because it
avoids the transplant shock during replanting.
When harvesting, be sure to remove the whole
plant, including roots and dead leaves, from
the canal. After harvest, the rafts should be
cleaned but not left to dry to avoid killing the
nitrifying bacteria on the submerged surface
AgriKultuur |AgriCulture
14