1.1 Introduction

Food production relies on the availability of resources, such as land, freshwater, fossil energy and nutrients (Conijn et al. 2018), and current consumption or degradation of these resources exceeds their global regeneration rate (Van Vuuren et al. 2010). The concept of planetary boundaries (Fig. 1.1) aims to define the environmental limits within which humanity can safely operate with regard to scarce resources (Rockström et al. 2009). Biochemical flow boundaries that limit food supply are more stringent than climate change (Steffen et al. 2015). In addition to nutrient recycling, dietary changes and waste prevention are integrally necessary to transform current production (Conijn et al. 2018; Kahiluoto et al. 2014). Thus, a major global challenge is to shift the growth-based economic model towards a balanced eco-economic paradigm that replaces infinite growth with sustainable development (Manelli 2016). In order to maintain a balanced paradigm, innovative and more ecologically sound cropping systems are required, such that trade-offs between immediate human needs can be balanced whilst maintaining the capacity of the biosphere to provide the required goods and services (Ehrlich and Harte 2015).

Fig. 1.1 Current status of the control variables for seven of the planetary boundaries as described by Steffen et al. (2015). The green zone is the safe operating space, the yellow represents the zone of uncertainty (increasing risk), the red is a high-risk zone, and the grey zone boundaries are those that have not yet been quantified. The variables outlined in blue (i.e. land-system change, freshwater use and biochemical flows) indicate the planetary boundaries that aquaponics can have a positive impact on

In this context, aquaponics has been identified as a farming approach that, through nutrient and waste recycling, can aid in addressing both planetary boundaries (Fig. 1.1) and sustainable development goals, particularly for arid regions or areas with nonarable soils (Goddek and Körner 2019; Appelbaum and Kotzen 2016; Kotzen and Appelbaum 2010). Aquaponics is also proposed as a solution for using marginal lands in urban areas for food production closer to markets. At one time largely a backyard technology (Bernstein 2011), aquaponics is now growing rapidly into industrial-scale production as technical improvements in design and practice allow for significantly increased output capacities and production efficiencies. One such area of evolution is in the field of coupled vs. decoupled aquaponics systems. Traditional designs for one-loop aquaponics systems comprise both aquaculture and hydroponics units between which water recirculates. In such traditional systems, it is necessary to make compromises to the conditions of both subsystems in terms of pH, temperature and nutrient concentrations (Goddek et al. 2015; Kloas et al. 2015) (see Chap. 7). A decoupled aquaponics system, however, can reduce the need for tradeoffs by separating the components, thus allowing the conditions in each subsystem to be optimized. Utilization of sludge digesters is another key way of maximizing efficiency through the reuse of solid wastes (Emerenciano et al. 2017; Goddek et al. 2018; Monsees et al. 2015). Although many of the largest facilities worldwide are still in arid regions (i.e. Arabian Peninsula, Australia and sub-Saharan Africa), this technology is also being adopted elsewhere as design advances have increasingly made aquaponics not just a water-saving enterprise but also an efficient energy and nutrient recycling system.