Coupled aquaponics is the integration of a recirculating aquaculture system (RAS) and a hydroponic cropping system (HCS) where treated fish culture water is used as a nutrient solution for soilless plant production. Individually, RAS and HCS are prominent components of the controlled environment agriculture (CEA) industry where protected growing conditions are strictly manipulated to provide season-independent production to reduce water usage and increase yield in a minimized, location-independent growing area (Resh, 2013; Benke and Tomkins, 2017; Timmons et al., 2018). The CEA industry is an attractive option for decentralized urban food security, and it is expected to grow as the demand for food and agricultural water are projected to increase by 56 and 60% by 2050, respectively, to meet the needs of the growing global population (Smith and Stwalley, 2018; Boretti and Rosa, 2019; van Dijk et al., 2021). Proponents of aquaponics claim that the integration of RAS and HCS into a coupled system can further optimize CEA production and result in improved environmental sustainability through minimized water and fertilizer use with increased profitability through the sale of multiple locally grown commodities (Love et al., 2014; Goddek et al., 2015; Atique et al., 2022). Additionally, urban food distribution models indicate a market for aquaponics as consumer preference is shifting to regional hubs that provide year-round fresh food production (Feldmann and Hamm, 2015; Broad et al., 2022). Despite the proposed environmental benefits and potential consumer market, commercial-scaled success has been limited.

In practitioner surveys, it was found that the majority of aquaponic systems were not profitable, and that fish production was a substantial source of financial loss (Love et al., 2014, 2015; Pattillo et al., 2022). While aquaponic systems without nutrient supplementation has demonstrated leafy green vegetable production with similar or faster growth rates than hydroponics, it has also been found that the cost of nitrogen (N) and phosphorus (P) by mass were, respectively, up to 14 and 88 times more expensive in fish feed than in inorganic fertilizer salts used in hydroponics (Colt and Schuur, 2021; Atique et al., 2022). These reports suggest that aquaponic systems where fish production is not profitable and is managed predominantly to mineralize feed that is used as the sole nutrient source for plants will likely struggle to cover production costs. A more appropriate production model would meet the fish growth and harvest rates at standards demonstrated to be profitable in the RAS industry. The potential environmental and financial benefits of integrated production could then be achieved by using excess nutrients in culture water for leafy green production commensurate with the HCS industry. This manuscript provides a coupled aquaponic system design template derived from contemporary RAS design principles. This approach focuses on a parallel unit process design for individual fish, plant, and water treatment unit hydraulic retention optimization to maximize fish growth and N production, differing from the conventionally used linear water flow design found throughout much of the published literature on coupled aquaponics.

Many self-designed aquaponic systems are adapted from a template developed at the University of Virgin Islands (UVI) where a single process flow directs nutrients produced by fish to plants (Rakocy et al., 2006; Love et al., 2014). Scalable commercial application is limited since there is little opportunity for controlling water flow rate, nutrient mass loading rates, and organic carbon accumulation as the specific unit processes are not operated under optimal conditions to meet the individual requirements for ideal fish and crop production (Baßmann et al., 2017; Knaus and Palm, 2017; Yang and Kim, 2020). Optimization of an integrated production system requires a design which utilizes hydraulic retention times (HRTs) for waste treatment and crop production unit processes (fish and plants) based on established principles in the RAS and HCS industries (Davison, 1997; Chen et al., 2006; Rusten et al., 2006; Resh, 2013; Summerfelt et al., 2016). Many of the single, in-series culture water process flow systems used in published aquaponics research did not meet the individual requirements for each unit process and therefore would be ineffective for developing scalable production systems (Baßmann et al., 2017; Knaus and Palm, 2017; Yang and Kim, 2020; Ani et al., 2021; Dusci et al., 2021) (Figure 1). Utilizing a system design approach proven to be commercially effective for aquatic food production would allow greater scalability and optimization in coupled aquaponic systems.

Applying contemporary RAS engineering principles to develop system design guidelines for consistent productivity is required to enhance economic viability of commercial coupled aquaponics. Adopting a parallel unit process design modeled from RAS engineering principles could separate fish and plant production into isolated, recirculating “loops” within one culture system that shares water treatment unit processes. This retains the benefits of integration while allowing independent unit scaling and operation at contemporary RAS and HCS industry standards (Figure 2). This manuscript provides theoretical calculations derived from existing RAS and hydroponic system design methodologies to size the primary components to estimate fish and crop production rates in an aquaponics system using a parallel unit process approach.

Coupled aquaponic management improvements are often considered from only a fish or plant optimization perspective in systems designed with a single process flow (Yang and Kim, 2020; Ani et al., 2021). In an evaluation of tilapia stocking density on water quality, Ani et al. (2021) determined that lower densities resulted in more suitable dissolved oxygen (DO), TAN, nitrite, and nitrate conditions for ideal fish health than more densely stocked systems. These results have limited applicability to a CEA industry focused on intensive production and rapid yields. Based on diagrams in Ani et al. (2021), a single process flow system was used in the study and did not allow independent control over fish tank and biofilter HRTs, both of which have established parameters for maintaining appropriate DO, TAN, nitrite, and nitrate conditions in commercial RAS at stocking densities greater than any used in the study (Rusten et al., 2006; Summerfelt et al., 2016; Timmons et al., 2018; Ani et al., 2021).

For plant optimization, Yang and Kim (2020) compared three total system HRTs (6, 9, and 17 h) to determine ideal nutrient loading rates for crops in a single process flow system. The authors acknowledged that the HRTs were chosen based on DWC flow rates and would not be ideal for intensive fish production, again limiting commercial application where data suggest the fish production is key to financial success (Love et al., 2014; Yang and Kim, 2020; Colt and Schuur, 2021). The HRTs for optimal fish waste removal rates, DWC crop nutrient uptake, and TAN conversion are all different and a single process flow system cannot meet the requirements for each component within a coupled aquaponic system (Baßmann et al., 2017; Knaus and Palm, 2017; Yang and Kim, 2020; Ani et al., 2021; Dusci et al., 2021). These results demonstrate that scalability is limited and individual unit improvement without sacrificing optimization in a different unit is difficult to achieve in a linear approach.

In contrast, this work integrates principles proven to be effective in the RAS and HCS industries to provide practitioners with a design template from a N mass balance that permits individual unit control. The isolated loops in a parallel unit process design provide water flow rate control to each component, which can then be operated at ideal conditions regardless of scale (Table 5). Additional loops with different flow rates can be added, or removed, without affecting existing units. This allows multiple hydroponic methods to increase crop diversity and the opportunity to incorporate fingerling production with reduced flow rates, while maintaining the precise HRTs identified to meet differing fish and plant requirements.

Consumer interest in locally grown CEA produce combined with the growing limitations of traditional agricultural methods to meet vegetable and protein demands indicates the potential for a successful coupled aquaponics industry (Feldmann and Hamm, 2015; van Dijk et al., 2021; Broad et al., 2022). To date, minimal economic success has been achieved through aquaponics, especially if fish production fails to be profitable (Love et al., 2014; Colt and Schuur, 2021). Although RAS is one of the most prominent aquaculture methods, much of the recently published coupled aquaponics literature does not incorporate commercial RAS designs or operating standards into experimental systems when modeling production for practitioner application (Baßmann et al., 2017; Knaus and Palm, 2017; Food Agriculture Organization of the United Nations., 2020; Yang and Kim, 2020; Ani et al., 2021; Atique et al., 2022). This manuscript provides guidance on estimating plant and fish populations, yearly production, and unit volumes based on crop growing area, daily N mass requirements for lettuce in a 35-day seed to harvest schedule, and daily N mass production from tilapia in a 35-week fry to 624 g harvest schedule. A parallel unit process design adapted from RAS to incorporate an additional HCS loop is proposed to provide greater control over individual units. Unit volumes, HRTs, and fish feed and growth rates are calculated using established RAS production guidelines.

Additional research is required to further support the commercial aquaponics industry. Production data for yield estimates and comparisons to the individual RAS and HCS industries at commercial scale for additional fish species and vegetable varieties or cash crops would further de-risk practitioner adoption. Economic analysis to determine initial investment costs and potential return on investment is also required and may vary depending on environmental control costs by region as well as the fish and crop chosen. The optimization of aquaponic system design and operation is the first step to continue advancing a potential commercial industry.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

JT: conceptualization, investigation, data curation, writing—original draft, writing—review and editing, and project administration. RF: conceptualization, validation, writing—review and editing, and visualization. TG: conceptualization, writing—review and editing, and project administration. All authors contributed to the article and approved the submitted version.