Oxygen consumption in recirculating nutrient film technique in aquaponics

Abstract

Due to root respiration, a minimum oxygen concentration in the root zone of plants is necessary. To keep the plant healthy and fully efficient the concentration should not fall below a plant specific critical value. Especially in nutrient film technique, the oxygen concentration can deplete heavily during the daytime. Besides the root respiration also microorganisms, living in the root zone of the plants, consume oxygen and have to be considered. In double recirculating aquaponic systems (DRAPS) fish waste water is used for preparation of nutrient solution for plant production in hydroponics. The fish waste water, delivered by a recirculating aquaculture system (RAS), probably contains high amounts of microorganism which can compete with plants for oxygen.The present study was conducted to investigate the oxygen concentration of nutrient solutions used in both, conventional hydroponics and in DRAPS, respectively. Therefore, the oxygen concentration within the cultivation trenches of conventional hydroponics (control; nutrient solution prepared with fresh water, electrical conductivity (EC) 1.8 dS m−1) and DRAPS (nutrient solution prepared with fish waste water (AP), EC 1.8 dS m−1 (low) and EC 3.0 dS m−1 (high)) were investigated. To evaluate whether the differences in oxygen depletion might be almost due to the used process water, the oxygen consumption of the prepared nutrient solutions was investigated separately without plants.The oxygen concentration within the cultivation trenches followed a typical daily fluctuation pattern with a decreasing oxygen concentration during the day. As time passed from April to June the depletion during the day increased and was strongest in AP high. In June the oxygen concentration dropped partly to zero in AP high, while it dropped only to nearly 150 μmol L−1 and 60 μmol L−1 in control and AP low, respectively. The oxygen consumption of pure nutrient solution (without plants) was significantly different between AP high and control, which also affects the oxygen concentration within the cultivation trenches.