Hydroponic Systems
should not be water wasters
Martin von Holdt
G
reenhouse hydroponic production
systems are efficient users of water,
and in the current situation of water
scarcity, which is likely to get
worse on a global scale over the next decade
and more, these production systems are
likely to become more and more popular.
However, what many do not realise, or ap-
preciate, is that the current “drain-to-waste”
systems in use in South Africa are not nearly
as efficient with water as they could be. Most
growers use a substrate called cocopeat, and
some use sawdust, as a rooting medium, fed
by a dripper system, allowing about 30% of
their irrigated water to drain out the bottom
of the bags on a daily basis.
With a daily application of about 60 000 litres
per hectare, a 30% over-drain amounts to
18 000 litres per day. Many growers lose this
and do not re-use it, which is a significant
waste, not only of water but also the nutrients
in that water as well.
This should be recycled and used again.
Proven commercial systems are available
which make this possible and remove the
risk of pathogenic infection. It is done, either
by sterilising it with VitaLite UV radiation or
by using closed hydroponic systems like NGS
which do not make use of growing media.
Properly designed UV water treatment systems
use low pressure UV radiation at levels up to
250 mJ/cm 2 and variable speed drives on the
pump units to control the flow rate through
the radiation tubes, based on turbidity, to
ensure there is complete radiation of all water
passing through. The low-pressure UV does
not chemically alter the water nor nutrients
as they are radiated.
Sterilised drain water is then blended with
new water at a ratio of about 1:3 by a mixing
valve ahead of the fertiliser dosing unit,
which tops up the nutrient level and adjusts
pH to spec before irrigation to the greenhouse.
In the NGS system the drain water is collected
in a sump, adjusted for EC and pH between
irrigations by the fertigation unit, and irrigated
back to the drippers in the greenhouse on a
scheduled basis. The design of the multilayer
trough system is specific in the creation of a
cool and aerated environment around the
roots, to ensure there is no proliferation of
anaerobic pathogens. No UV treatment of the
water is required under normal conditions.
In both the above systems, waste is reduced
to almost nothing. Only a small amount of
water is disposed of from time to time to
flush undesirable accumulated salts from
the system. The saving in recouped fertiliser
alone pays for the capital cost of both these
systems within three years.