In recirculating systems, the nutrient solution must be regularly
discarded or ‘dumped’. This is done to maintain the balance of the
nutrient and prevent the build-up of pathogens, dirt, and harmful salts
such as sodium and chloride.
Discard at least every 2 weeks in both winter and summer. The frequency
can be less if using low salinity, rain or RO (reverse osmosis)
It may be necessary to discard more frequently (e.g. every 7 days) when
using salty make-up water because nuisance chemicals build up more rapidly
to toxic levels - especially during hot, dry weather.
Method of discarding
Poorly designed hardware can make discarding a tedious and messy task. So
consider this at the design stage. Unfortunately, most system designs are
not sympathetic to the hassles of discarding. Consider the advantages of
the following design features:
1. Install an in-line 2-way valve
between the pump and feed outlets to divert the nutrient flow to waste
2. Design a sloping floor into the
reservoir which slopes towards a sump from which the nutrient is drained
(Fig 12.1b). This will help remove the last few litres containing the bulk
of the sediment. Another simpler method can be to tilt the reservoir
towards the outlet.
3. ‘Sump’ pumps are convenient for
draining reservoirs (Fig 12.1c). They are light, portable and easy to
prime, however, they will typically only drain to a depth of around 1
inch. Hence a sloping reservoir floor or built in sump is needed for best
Where to discard
Use the remaining nutritional benefit in the ‘waste’ nutrient by placing
it on your garden or over a large area of grassland. Do NOT put it in
waterways, drains, toilets or on sand as this can cause environmental
damage, such as algae blooms and ground water contamination.