6.5 The Circular Bioeconomy of Decoupled Aquaponics
By Matt Recsetar, Assistant Research Professor, Department of Biosystems Engineering, University of Arizona
Kitt Farrell-Poe, Extension Specialist & Professor, Department of Biosystems Engineering, University of Arizona
Using aquaponics and its byproducts at the University of Arizona to save water and improve plant production in hydroponics and soil.
Aquaponics is a food production method that utilizes the nutrients from fish waste to grow plants in a soilless medium. While it has been around for many years, in one form or another, only recently has it become more viable on a larger scale utilizing controlled environment agriculture techniques. Aside from using the waste stream from fish culture to grow plants, it saves on the use of hydroponic nutrients, many of which are derived from industrial processes such as mining and extraction. In other words, the carbon offset from using fish feed as the source of nutrients is significant. Recent studies by the Recsetar Lab examined cost savings in aquaponics and showed that aquaponics production not only saves water over traditional soil culture, but over hydroponics as well (Recsetar, unpublished data). In addition, when scaled to commercial production levels, aquaponic-grown plants in that study cost 85% less to grow than hydroponic-grown plants, based on current costs of the nutrient salts used in the hydroponic formulation, not considering capital costs.
In most commercial aquaponics systems, a recirculating aquaculture system (RAS) is typically run as a standalone system, which is decoupled from the hydroponic component; often referred to collectively as decoupled aquaponics. Solid wastes (comprised of fish poop and uneaten fish feed) are removed from the RAS through various filtration processes, while the filtered effluent from the RAS is used to feed the hydroponic component as needed; it does not return to the fish system, contrary to coupled aquaponics, in which it does. This allows for each system to be managed separately to optimize conditions for both fish and plants and thus maximize production of both.
The solids collected and removed through sedimentation or mechanical filtration in the RAS can be digested either aerobically or anaerobically to mobilize additional nutrients and reduce waste production (Monsees et. al., 2017). These solids would otherwise end up in landfills or emptied into the environment with or without prior treatment to meet EPA effluent guidelines for Concentrated Aquatic Animal Production (EPA, 2004). In general, aquaponics is thought of as a sustainable growing method, but externalities such as solid waste (sludge) production are often overlooked in larger systems. The Recsetar Lab demonstrated that this “sludge” can be aerobically mineralized to significantly increase the nutrient content of the effluent, but they observed in multiple systems that not all the solids were decomposed during this aerobic process. In a recent 2022 study in the Recsetar Lab, the residual solid sludge from three aquaponics systems including a commercial aquaponics farm in Tucson was collected and dewatered to create a stabilized biosolid. After lab analysis, it was found that these aquaculture biosolids contain adequate nutrient content to make an effective, organic soil amendment, exhibiting an N-P-K of 3.8-2.6-1.3 compared to Arizona wastewater biosolids which are estimated to have an N-P-K of 3.6-3.3-0.4 (Artiola, 2011), which demonstrates a valuable waste revenue stream, thus eliminating the final waste stream in aquaponics food systems.
To show the effectiveness of this circular bioeconomy, my aquaponics interns and I applied for and received a Campus Sustainability Fund Grant in fall of 2022 to develop a sustainable fruit orchard utilizing aquaponics sludge, stabilized aquaponics biosolids, and rainwater harvesting from the greenhouse roof to demonstrate this completely sustainable endeavor. Fruit grown in this sustainable orchard will be harvested by students and given to the Campus Pantry here at the University of Arizona. Through the project we will measure and show the water and fertilizer savings for producing equivalent quantities of various fruits. Results are forecasted by 2024 for plums, peaches, lemons, and oranges.
Although hydroponically grown plants may be the main profit stream in aquaponics, followed by sale of fish, additional value can be attained through sale of aquaponics effluent and aquaponics stabilized biosolids, thus improving efficiency and promoting the circular bioeconomy. Aquaponics can be utilized all over the world, to grow fish and plants together while also generating biproducts for improving soil-based agriculture and tremendous water savings. In a recent experiment, the Recsetar Lab showed that aquaponically grown plants utilized 40% less water than hydroponically grown plants to achieve the same yield (Recsetar et. al., unpublished data). Consensus says that hydroponics can achieve up to 90% water savings over soil-based agriculture (Bradley and Marulanda, 2001; Sharma et. al., 2018). While wastewater biosolids have been shown to increase soil moisture retention and improve soil aeration and organic content (Tsadilas, 2005; Artiola, 2011; Qin et. al. 2012), no studies have been done to show the effectiveness of aquaculture biosolids in soils. The possibilities for promoting resource reuse in aquaponics has created several opportunities for graduate student research projects, including nutrient modeling, microbiome manipulation and even utilization of AI to manage water and nutrients for specific crops; all of which will have a positive impact on our world.
