Utilization of runoff nutrients from recirculating aquaculture systems for hydroponic crop cultivation

Coupling of complementary and semi-closed food production systems such as recirculating aquaculture systems (RAS) and hydroponics represents a strategy for improved resource utilization. Land-based RAS facilities dominate today's production of Atlantic salmon smolt. RAS runoff water represents an environmental challenge and a rather unexploited nutrient resource. As an alternative to full integration of fish and plant cultivation, this study explored the use of RAS runoff water in downstream crop cultivation. Runoff water from the smolt department of a large-scale, commercial RAS facility for Atlantic salmon farming was characterized, illustrating a nitrate concentration of 3.5 mM towards the end of a typical 3-month production phase.

A hydroponic, deep water culture method was developed for comparing lettuce growth across different nutrient solutions with high reproducibility. A mineral nutrient solution was enriched with RAS runoff water, resulting in improved growth of lettuce by 30-34% as measured by leaf fresh weight. Compared to typical nutrient solutions, the runoff water illustrates low levels of for example K and elevated concentrations of Na and Cl, which should be accounted for in possible future applications of closed-loop hydroponics with nutrient solution enriched with RAS runoff water.

The results indicate a potential for increased resource utilization, reduced fertilizer use and reduced environmental footprint when coupling RAS to downstream crop cultivation.