In coupled aquaponics steady state conditions are recommended for optimum production.
Presenting his study at Aquaculture Europe, 20-23 September, Edinburgh, Scotland, Mr Knaus explained how he set up two identical ebb-and-flood coupled aquaponics units. Both had water temperatures of 23°C and total water volume of 3.81 m3.
Five plant boxes were used for each unit and these contained basil, parsley and marjoram.
Of the two units, one unit contained African catfish and the other Nile tilapia. The fish were fed Aller Aqua 37/10 float feed which was increased weekly from 70 to 450 grams over the 70 days.
Mr Knaus then measured the oxygen levels as an indicator for system stability. His analysis showed a three fold phase distribution.
Phase one had an increase in oxygen and was relatively stable. Phase two was the best section with a steady state and stable oxygen levels. Phase three which showed unstable conditions occurred after 28 days and saw the oxygen levels decrease rapidly due to nutrient accumulation.
As well as showing system stability, Mr Knaus was also able to deduce the optimal feed input of 150g, which was recorded at 28 days. Above this level, oxygen decreased rapidly due to the nutrient overload.
In terms of the breakdown by species, the disolved oxygen levels were higher in the catfish unit. In the tilapia unit, levels were around 9.28 per cent lower.
The catfish also reported better growth than the tilapia. However, plant yield was higher in the tilapia system.
In conclusion, Mr Knaus stated that oxygen can therefore be used as an indicator for stability.