Suitability of supernatant of aerobic and anaerobic pikeperch (Sander lucioperca L.) sludge treatments as a water source for hydroponic production of lettuce (Lactuca sativa L. var. capitata)

Aquaponics (AP) is an integrated multi-trophic system that combines elements of recirculating aquaculture systems (RAS) and hydroponics (HP). In RAS, fish sludge rich in macronutrients and micronutrients is discharged from the system by mechanical filtration. The recovery, solubilization, and reuse of these nutrients to grow crops hydroponically are important to improve the circularity of AP systems. In this research, we investigated the effect of aerobic (AE) and anaerobic (AN) pikeperch sludge treatment supernatants, used as nutrient solutions, on the growth performance of lettuce cultured in deep flow technique (DFT) hydroponic systems. Lettuce head and root fresh weights, head shape, leaf malformations, spots on leaves, and visual nutrient deficiencies in lettuce grown in supernatant solutions were compared to those of an HP standard (Std). In a first experiment, we found P, K, and Mn deficiencies in the AE treatment that led to inferior yields (528 ± 62 g) compared to the Std treatment (675 ± 70 g). A second experiment was run with aerobic (AE-NPK) and anaerobic (AN-NPK) supernatants supplemented with the lacking nutrients. No significant differences on lettuce health and yield were found between the AE-NPK (607 ± 40 g) and Std (622 ± 41 g) treatments. The lettuce in the AN-NPK treatment showed toxicity symptoms and low head weight at harvest (209 ± 55 g). The presence of toxic factors and asphyxia of the roots due to suspended organic particles are proposed as possible reasons. The use of AE supernatant seems suitable for improving the aquaponic circularity on condition of supplementing the missing nutrients or improving the sludge treatment in such a way that all nutrients are sufficiently available.