Overcoming major bottlenecks in aquaponics – A practical approach (summary)


great references on why AP is not good enough – also sectionsonnitratelevels

Aquaponics is the combination of fish production in aquaculture and hydroponic (soilless)
production of crop plants. Despite of representing already a sustainable, innovative approach
for future food production systems, aquaponics are still missing economic success and up to
date major bottlenecks were not scientifically addressed. Therefore the main aims of this
thesis were (I) to identify safe nitrate concentrations under which best growth and health
status of tilapia can be guaranteed in aquaponics and recirculating aquaculture systems (RAS),
(II) to evaluate the best design concept for an optimal combined production of fish and plants
concerning professional aquaponic applications and (III) to increase the overall system
efficiency by recycling waste water and nutrients deposited in the sludge of the mechanical
filtration unit.
The growth and health status of Nile tilapia (Oreochromis niloticus) is negatively affected by
high nitrate concentrations (> 500 mgL-1 NO3

-N) commonly reported for RAS. Specific
growth rate (SGR) of Nile tilapia decreased significantly to up to 1.1 % per day (± 0.1) and
feed conversion ratio (FCR) increased significantly to 1.1 g g-1 (± 0.2) at the highest nitrate
concentration of 1000 mgL-1 NO3

-N, confirming possible negative effects on fish production
within a realistic concentration range for RAS. Nevertheless, optimal nitrate concentrations
for plant production in aquaponic systems (~ 200 mgL-1 NO3

-N) are not affecting fish
welfare and allow for an efficient production of Nile tilapia. With increasing concentrations,
uptake of nitrate and conversion to nitrite in the stomach have been identified here as
alternative pathway mediating nitrate toxicity in fish.
A study on the optimization of aquaponics under a realistic, medium scale production
revealed that the choice of system design has a considerable influence on the overall system
performance. Decoupled aquaponics proved to be favorable for professional aquaponic
production, whereas coupled systems were suboptimal for a combined production of fish and
plants. There were no differences in fish production, whereas tomato production within the
decoupled system was considerably increased by 36 %. The advantages of decoupled
aquaponic systems were mainly attributed to the possibility of an independent regulation
(separately for fish and plants) of different productions parameters, e.g. the pH (important for
nitrification and nutrient availability) and the increased effectiveness of the supplementation /
fertilization of limited minerals, most importantly K, P.
A closer look was also taken at the improvement of the recycling efficiency in terms of
nutrient and water management. Therefore, mineralization under aerobic and anaerobic
conditions were experimentally compared. Aerobic mineralization of phosphate revealed best
phosphate recovery with only minor losses of nitrate. Within only 14 days the phosphate
concentration increased from 9.4 mgL-1 (±0.7) to 29.7 mgL-1 (±2.1) and simultaneously the
nitrate concentration was reduced by only 16 %. In contrast, anaerobic mineralization did not
result in an increase in phosphate, but nitrate concentration was up to 97 % lower. Due to a
complete loss of nitrate, the main nitrogen source in aquaponic systems and because of the
potential formation of toxic byproducts, anaerobic mineralization is more problematic for
aquaponic applications. Recycling of water sludge mixture from clarifiers resulted in a
substantial phosphor recovery, an increase in potassium and additional water savings.
Conclusively, the results of this holistic thesis clearly revealed the bottlenecks in aquaponic
technology and provided guidance in overcoming mayor obstacles in terms of optimized
nutrient and resource management to increase the overall sustainability of these systems and
improve production efficiency and profitability.