Exploration of efficient water, energy and nutrient use in aquaponics systems in northern latitudes
Luuk Jansen, Karel J. Keesman
Abstract
Efficient energy and resource management are essential for a more sustainable food production system. On-demand coupled multi-loop aquaponics, containing at least a fish, plant and anaerobic digester unit, has shown great potential for optimizing nutrient use. However, balancing the nutrients for both fish and crop production becomes increasingly complex at higher latitudes. In these areas also significant reductions in energy use are required. Hitherto, the effects of energy-saving strategies on the dynamics of an aquaponics system have not been explored. The objective of the study was to investigate and mathematically model an aquaculture-hydroponic (aquaponics) system with Nile tilapia (Oreochromis niloticus L.) and lettuce (Lactuca sativa L.) or tomato (Solanum lycopersicum L.), and to verify it for the Netherlands. In the mathematical model several physical and operational parameters were varied to find their influence on water and nutrient use efficiency, energy use and the growing environment, in particular in the greenhouse. The variation in crop transpiration was found to be a simple, yet effective, indicator for the influence of greenhouse settings on the performance of on-demand coupled multi-loop aquaponics systems. The correlation between water or nutrient use efficiencies and transpiration variation (0.98) is stronger than the correlation with energy use (0.78). The maximum allowed humidity (70% to 90%) and use of artificial lighting (0 – 200 W/m2) has the largest effect on both performance and energy use. Other settings, such as inclusion of thermal insulation of the cover, minimum temperature, addition of heat storage, and use of screens, have effect on the energy efficiency, but have little effect on the nutrient dynamics. To counter the negative effects of variation caused by other energy-saving strategies or climatic variations, the use of a buffer tank, placed between anaerobic digester and hydroponic system, was found a feasible option. Aquaponics is an emerging technology in sustainable food production and this research supports the introduction of on-demand coupled aquaponics systems in northern latitudes.