Causalities of Upscaled Urban Aquaponics

Version	Summary	Created by	Modification	Content Size	Created at	Operation
1		Gösta Baganz	+ 3764 word(s)	3764	2021-07-25 10:52:25	\|
2	format corrected.	Beatrix Zheng	+ 1264 word(s)	5028	2021-08-08 05:36:26	\|

This entry is adapted from the peer-reviewed paper 10.3390/w13152029

Aquaponics, the water-reusing production of fish and crops, is taken as an example to investigate the consequences of upscaling a nature-based solution in a circular city. We developed an upscaled-aquaponic scenario for the German metropolis of Berlin, analysed the impacts, and studied the system dynamics. To meet the annual fish, tomato, and lettuce demand of Berlin’s 3.77 million residents would require approximately 370 aquaponic facilities covering a total area of 224 hectares and the use of different combinations of fish and crops: catfish/tomato (56%), catfish/lettuce (13%), and tilapia/tomato (31%). As a predominant effect, in terms of water, aquaponic production would save about 2.0 million m3 of water compared to the baseline. On the supply-side, we identified significant causal link chains concerning the Food-Water-Energy nexus at the aquaponic facility level as well as causal relations of a production relocation to Berlin. On the demand-side, a ‘freshwater pescatarian diet’ is discussed. The new and comprehensive findings at different system levels require further investigations on this topic. Upscaled aquaponics can produce a relevant contribution to Berlin’s sustainability and to implement it, research is needed to find suitable sites for local aquaponics in Berlin, possibly inside buildings, on urban roofscape, or in peri-urban areas.

water circular city causal loop diagram CLD nature-based solutions NBS water scarcity dietary shifts aquaponic farming fish/plant harvest ratio

1. Introduction

Human activities cause significant damage to nature and the consequences are already apparent in ‘human suffering’, towering economic losses, and the accelerating erosion of life on Earth ^[1]. In order to address the most pressing global issues, the UN formulated 17 sustainable development goals (SDGs) ^[2], but increased efforts are needed to achieve these SDGs, as is evidenced by the current compliance review ^[3]. Therefore, a ‘Decade of Action’ has been declared to fulfil the 2030 Agenda ^[4]. The German Federal Government has taken up this UN demand by revising its sustainability strategy, which now contains six transformation areas including ‘circular economy’ and ‘sustainable agricultural and food systems’ ^[5]. The food sector, which addresses both transformation areas, is the most tremendous burden on the Earth’s ecosystems ^[6] and a major contributor to the transgression of the nine planetary boundaries identified by Rockström et al. ^[7]. If crossed, these boundaries could potentially endanger human existence ^[8].

Fisheries have already reached their limits ^[9] and the increase in fish consumption over the past four decades is mainly covered by aquaculture, which is the world’s fastest-growing food production industry ^[10]. This growth involves environmental problems and animal welfare risks ^[11]. In 2000, a study concluded that aquaculture needs to reduce wild fish inputs into feed ^[12]. A retrospective review 20 years later found that overall sustainability increased but dependence on marine ingredients continued ^[13]. Marine aquaculture raises environmental issues ^[14]^[15], e.g., the negative landward flux of the essential mineral phosphorus ^[16]. The alternative to marine aquaculture is freshwater aquaculture; however, freshwater aquaculture generates wastewater, especially in flow-through systems ^[17], and it is estimated that over 80% of global wastewater is not adequately treated ^[18]. Increased water use efficiency decouples economic growth from water use, e.g., by using less water in agriculture through the introduction of new technologies ^[19]. One water-efficient technology that reduces wastewater is aquaponics (AP), the coupled production of fish and crops ^[20]^[21]. Additionally, AP decreases fertiliser use and greenhouse gas emissions associated with its food production ^[22].

Cities are critical to the success of sustainable development ^[23]; thus, initiatives and proposals set out at global, European, and municipal levels to promote the transformative power of cities are aimed at the common good—e.g., the World Cities report ^[24], the New Leipzig Charter ^[25]^[26], or the roadmap to Amsterdam Circular ^[27]. The circular city (CC) is designed as a regenerative and restorative urban living system ^[28] by reducing, reusing, and recovering ^[29]. Nature is part of the transformation ^[30]: nature-based solutions (NBS) ^[31] can support closing the adaption gap ^[32] and the coupling of NBS units form a significant part of circularity in cities ^[33]^[34]. Urban agriculture, which is attracting increasing attention ^[35], contributes towards circular cities ^[36]. Strengthening urban and peri-urban food production, integrating it into city resilience plans, and applying an ecosystem approach that guides holistic land use planning and management are recommended approaches by the Milan Urban Food Policy Pact ^[37].

2. Analysis on Results

2.1. Berlin: Balancing Demand and Yield

German per capita (PC) consumption of fresh and processed tomatoes was 27.2 kg in 2018/19, with processed tomatoes converted to fresh weight ^[38]. Regarding the import and domestic harvest of fresh tomatoes, we concluded that fresh tomatoes had a share of 9.3 kg/PC and processed tomatoes 17.9 kg/PC in 2019 based on data from BLE ^[39]. For freshwater fish, the shares was 3 kg/pC and 0.9 kg/PC for freshwater fish products ^[40]. These data were not available for Berlin and so we assumed a similar consumption pattern to estimate the demand of metropolitan Berlin by adding the non-marketable portion, e.g., waste from fish processing. With a population of about 3.77 million in 2020 ^[41], approximately 21 kilotonnes (kt) of freshwater fish and fish products, 108 kt of fresh tomatoes and tomato products, and 27 kt of lettuce are required per year (cf. Table 1).

Table 1. Annual metropolitan Berlin demand for freshwater fish/fish products, tomatoes/tomato products (converted to fresh weight), and lettuce.

Demand	Residents Berlin 2020:			3,769,962
	Fresh/Fillet	Products	Total	Netto	Not	Brutto
	(kg/PC)	(kg/PC)	(kg/PC)	(t)	Marketable	(t)
Freshwater fish	3.0	0.9	3.9	14,703	40%	20,584
Tomato	9.3	17.9	27.2	102,543	5%	107,670
Lettuce	6.8		6.8	25,636	5%	26,918
fresh tomato only				35,061	5%	36,814

The following proportions of AP setups have been calculated: catfish/tomato at 56%, catfish/lettuce at 13%, tilapia/tomato at 31%, and tilapia/lettuce at 0% (cf. Table 2). Berlin’s F/P demand ratio was around 6.5, while the F/P ratio of AP setup AP4 (tilapia/lettuce) was 56.2; therefore, its proportion was set to zero (cf. Table 3).

Table 2. Upscaled-AP scenario: Four aquaponic setups as combinations of catfish/tilapia and tomato/lettuce and their respective share to meet the freshwater fish demand of Berlin.

Fish Demand Coverage	Aquaponic Setups (AP 1 … AP 4)
Fish Demand Coverage	Tomato		Lettuce
Catfish	AP 1	56%	AP 2	13%
Tilapia	AP 3	31%	AP 4	0%

Table 3. Upscaled-AP scenario: The proposed annual yield of catfish, tilapia, tomato, and lettuce per aquaponic setup; Number of AP facilities required to achieve this yield.

	Fish Yield (t)			Plant Yield (t)		AP Facilities
Aquaponic setup	Catfish	Tilapia	F/P*	Tomato	Lettuce
AP 1	11,527		3.3	37,508			118
AP 2	2676		10.2		27,381		27
AP 3		6381	11.1	70,912			223
AP 4 **		0	56.2		0		0
Total yield	14,203	6381	yield	108,420	27,381	required	368
fish yield share	69%	31%	demand	107,670	26,918
			delta	750	464
fish demand	20,584
fish yield	20,584			*) fish/plant harvest ratio
delta	0			**) AP 4 excluded because of the F/P ratio

2.2. Supply-Side: Impact on Berlin FWE Nexus

Food. The yield shares (fish/crop) of each AP setup within the upscaled-AP scenario to cover the city’s demand are as follows: AP1 with 11.5 kt catfish and 37.5 kt tomato, AP2 with 2.7 kt catfish and 27.3 kt lettuce, and AP3 with 6.3 kt tilapia and 70.9 kt tomato (cf. Table 3).

In order to produce a yield of 20.6 kt fish and 108.4 kt crop (tomato and lettuce) per annum, approximately 370 AP facilities are needed and this requires a total area of 224 hectares (cf. Table 3). Fish feed and fertiliser should ideally be matched to the specific AP setup. The three AP configurations should be standardised to obtain the appropriate quantities of optimised fish feed and fertiliser needed to achieve economies of scales for the upscaling-AP scenario.

Water. In the present study, we extrapolated the so-called water footprint, i.e., the LCA impact category water consumption (WCO in Table 4) from the package level to the city scale. Compared to the WCO of the German market mix for fresh tomatoes and lettuce, the aquaponic production of both fresh vegetables for Berlin would save about 2.0 million cubic metres of water.

Table 4. Upscaled-AP scenario: Reduced impact of fresh tomato and lettuce production on three LCA impact categories; author’s work and the calculation based on data from comparative-LCA ^[42].

LCA Impact Category	Abbr.	Unit	Tomato, Fresh		mio. packs	73.6	Lettuce		mio. packs	179.5	Total Reduction
LCA Impact Category	Abbr.	Unit	Mix-DE	Rooftop AP	Delta	Less Impact	Mix-DE	Rooftop AP	Delta	Less Impact	Total Reduction
Global warming potential 100	GWP100	kg CO₂ eq	0.5760	0.5261	0.0500	3679662	0.0769	0.0385	0.0383	2822799	6502	t
Water consumption	WCO	m³	0.0142	−0.0101	0.0243	1786533	0.0033	−0.0002	0.0035	261272	2,047,805	m³
Water scarcity	WSI	m³	0.0109	−0.0059	0.0168	1237314	0.0021	−0.0001	0.0022	163054	1,400,368	m³

Regarding the LCA impact category water scarcity (WSI), about 1.4 million cubic metres of water would be saved, especially in the Almeria region of Spain where the rapid development of greenhouse horticulture has dramatically affected the availability of groundwater resources ^[43].

Energy. Replacing the German market mix of tomato or lettuce (Mix-DE) with an optimised aquaponic scenario (rooftop-APDAPS-R+) would reduce the long-term CO₂ footprint (GWP100 in Table 4) by 7691 t CO₂-equivalents. This result can be significantly improved by using aquaponics-integrated microgrids (so-called smarthoods) where all FWE flows are circularly connected ^[44].

Based on data from the comparative-LCA, the relative change in environmental impact between the scenarios Mix-DE and rooftop-AP shows a reduction in the environmental footprint for all 12 LCA impact categories (cf. Figure 1).

Figure 1. Relative change in LCA impact categories for optimised rooftop-aquaponics (AP) compared to the German market mix for fresh tomatoes and lettuce (mix DE) as +/− 100%; authors work and the calculation based on data from comparative-LCA ^[42].

Of the 12 LCA impact categories, the FWE sector energy is represented by the impact categories GWP20 and GWP100, while the impact categories water consumption (WCO), water scarcity (WSP), and freshwater eutrophication (FEP) represent the sector water. The long-term CO₂ footprint (GWP100) is reduced by 9% for tomatoes and 50% for lettuce. For water consumption, the reduction is even more significant and becomes negative in the analysis of the comparative-LCA (cf. Figure 1).

2.3. Causalities: Aquaponic Variables and Production-Location Shift

Upscaling urban AP triggers two FWE interactions simultaneously: (1) local food production is increased and thus (2) the relocation of production occurs. Both processes result in interactions within the FWE nexus and the associated effects become relevant for the system as a whole. Thus, all causal dependencies take effect: on the local level, since AP internals and location issues gain importance due to local resource demand; and on the global level, since upscaling the shift in production-location impacts all sectors of the FWE nexus.

In order to understand the impact of AP on the three sectors of the FWE nexus, we identified significant AP variables and examined their causal relationships (cf. Table A1), which are often mediated by other variables and results in causal chains (cf. Figure 2). However, neither the complete functional scheme of an AP nor processes outside the AP system boundary (except for phosphorus) are considered when examining these AP-internal causal chains. For example, the AP nutrient coupling degree reduces fertiliser consumption, but the environmental impacts of the production and transport of the fertiliser are not considered in the causalities unlike in the comparative-LCA.

Figure 2. Impact of causal chains—formed by significant aquaponic variables—on FWE-Nexus (no flow chart; no functional scheme of an AP); the variables are explained in Table A1.

Table A1. Causal relations of significant aquaponic variables.

Aquaponic Variables	Selected Causal Relationships
Cooling	Cooling lowers the greenhouse temperature, which requires energy and generates excess heat depending on the technology.
Coupling degree—energy	Thermal connections between the AP units can reduce the total energy demand of the AP.
Coupling degree—nutrient	In a well-balanced AP, a high nutrient coupling degree reduces fertiliser consumption to a minimum.
Coupling degree—water	The double use of water is at the core of the aquaponic principle and a high water coupling degree is the objective of a well-balanced AP. It reduces both the external water consumption of the HP and the wastewater generation of the facility.
Electricity	Electricity is mainly used for pumps, control systems, lighting, and heating of RAS process water. Thus, these components directly affect the energy sector of the nexus through their operating times and energetic efficiency.
Feed conversion rate	Feed conversion rate (FCR) describes the conversion of feed into biomass.
Fertiliser	Fertiliser is essential for optimal plant growth; over-fertilisation defects are not considered here.
Freshwater fish	Fish production contributes to the food sector; its farming generates sludge. The amount of wastewater should be as low as possible, but zero is a difficult goal to achieve. If the production of freshwater fish in RAS were to replace marine fish production in net-cages then the phosphorus flux into the sea could be reduced.
Fish feed	Fish feed is the prerequisite for fish growth and the type and quality of feed also affect FCR.
Fish species	Freshwater fish can be divided into three groups according to their temperature requirements: tropical, warm water, and cold-water fish, which determines the water temperature of the aquaculture unit. For different fish species, different stocking densities are allowed: e.g., tilapia max. 100 kg/m³ or catfish with up to 400 kg/m³. In addition, the species influences the FCR.
Fish-free feed	Fish feed without fish meal and fish oil reduces phosphorus removal from the oceans by wild fisheries among other positive environmental aspects ^[45]; insects can be part of fish diets ^[46]; and the impact on the quality of fish feed is case-specific.
Gas: CO₂ and O₂	O₂ is used in RAS to increase yield and ensure the minimum oxygen content in the water in critical situations. CO₂ is used in HP greenhouse production to increase yield. The gases can be exchanged between both AP units ^[47].
Greenhouse temperature	Greenhouse temperature influences plant growth with positive link polarity.
Heating	Heating is needed for tropical fish and greenhouses, especially in the colder season.
Lighting	Greenhouse lighting requires electrical energy; it can also contribute to heating if, e.g., heat-emitting sodium vapour lamps are used. LED lamps do not emit long-wave heat and contribute to greenhouse heating to a lesser extent.
Plant productionwinter break	In the winter season, plant production in the greenhouse can be suspended, which saves energy for lighting and heating, but at the same time reduces the yield of crop production.
Plant species	The plant species affects the type and quantity of fertiliser needed, the required greenhouse temperature, the water uptake, the harvest yield, and their dynamics.
Plants	Increased harvest contributes positively to the food sector. Plants take up water, transpire it, and the water vapour can be regained in modern greenhouse systems. Depending on the irrigation method, wastewater is produced, e.g., for flushing the plant troughs.
Sludge	The quantity and composition of the sludge determine how much of it can be recycled.
Sludge recycling	Sludge removal and mineralisation can save fertiliser and thus reduce the use of phosphorous as a supplemental fertiliser.
Stocking density	Stocking density affects both FCR and the amount of fish that can be harvested and the requirements for additional oxygen or improved water treatment.
Wastewater	Wastewater is the water leaving the facility. All internal water flows are not included. In particular, the nutrient water is not considered wastewater, as suggested by Baganz et al. ^[48].
Water regain for reflux	The more plants are cultivated, the more energy is needed to regain the evapotranspired water in the greenhouse, which in turn saves the water needed in the aquaculture unit.
Water temperature	Fish are poikilothermic; unlike homoeothermic animals, they do not use their metabolisms to heat or cool themselves. They can therefore invest more energy into growth, resulting in a higher FCR. However, in a temperate climate zone, the water for tropical fish must be heated, which means that the energy saved internally by fish must be supplied externally.

The variables listed in Table A1 influence the three sectors of the FWE nexus directly or via causal chains. These are the variables through which the designer/operator of an AP can influence the environmental impacts. General factors for increasing energy efficiencies such as solar panels, low-energy greenhouses, or energy-efficient pumps are not included in the scope of this consideration but must be taken into account as part of an overall concept. Ideally, this concept then considers future GHG attributions next to the current ones. Future GHG emission changes are expected to result from the process of decarbonisation, such as a changed electricity mix or biogas-fuelled combined heat and power unit (CHP). Figure 2 is a graphical representation of the causal chains formed by the AP variables.

The connectors in Figure 2 are causal links, but can easily be confused with flows. For example, the variable ‘plants’ affects the variable ‘water’ in that water demand increases with the number of ‘plants’, but the water needed flows from RAS to HP. We adopted the syntax of causal loop diagrams and extended it by adding case-specific considerations (ambiguous) that can results in positive or negative link polarity. The FWE nexus influences the AP parameters and creates causal loops, but these are beyond the scope of this study.

As local food production increases, the location of production simultaneously shifts across national borders. This fact touches on the problem of domestic and imported resource use and is, thus, a system boundary problem.

For example, the emission of greenhouse gases (GHG), for which its impact is indicated as GWP in the LCA approach, is an essential indicator for measuring climate sustainability. However, a country-specific CO₂ balance has some weaknesses: Germany emitted an estimated total of 805 Mt CO₂ equivalents in 2019, but almost as much (an estimated 797 Mt CO₂ equivalents) was emitted in the production of German imported goods in 2015 ^[49]. Offshoring environmental damages were also criticised concerning Europe’s Green Deal ^[50], but, currently, the EC 2021 proposals for making the EU’s policies fit for reducing net greenhouse gas emissions by at least 55% by 2030 ^[51] include a carbon border adjustment mechanism ^[52].

Comparably, local food production increases local resource use and thus impairs the ‘local ecological footprint’ while simultaneously reducing the footprint of distant production and possibly the overall ecological footprint.

The same applies to the water sector. A significant proportion of the tomatoes consumed in Berlin are produced on the Spanish Almeria peninsula around the town, El Ejido. In this region, the rapid development of greenhouse horticulture since the 1950s has dramatically affected the availability of groundwater resources ^[53], which causes aquifer overexploitation. In addition, water quality deterioration occurs due to an increase in water salinity in aquifers as a result of marine intrusion processes and unsustainable aquifer management ^[43]. However, the share of water needed under these troublesome circumstances to cultivate tomato for export to Germany does not appear in the German water consumption statistics.

This study examines the boundary conditions for a production-location shift from other countries to Berlin based on year-round production. Compared to fish and lettuce, tomatoes have the quantitatively highest share of food production in the upscaled AP scenario (cf. Table 3), which is why tomato production is used to illustrate the dependencies of a production shift to Berlin as visualised in Figure 3.

Figure 3. Impact of simplified causal relations on the FWE nexus concerning year-round tomato production and aquaponic setup as boundary conditions for production-location shift; FWE ranking was conducted according to the main dependencies of the sectors; swim lanes are explained in Table 5.

Figure 3 contains subdivisions such as the so-called swim lanes which are explained in more detail in Table 5. The swim lane ‘local AP’ comprises urban and peri-urban AP, as they are both within the system boundaries of the circular city (CC). Baganz et al. ^[54] noted the potential for integrating AP into the CC through resource streams such as greywater, plant leftovers, and sewage; the diagram element ‘heat coupling’ in Figure 3 is related to this.

Table 5. Causal relations of significant tomato production-location variables.

Production Variables	Selected Causal Relationships
Demand Berlin	In 2018/2019, 9.3 kg/PC fresh tomatoes and 17.9 kg/PC processed tomatoes were consumed in Germany (DE); we assumed the same for Berlin.
Supply for Berlin	In 2019, the production shares on fresh tomatoes for Germany (DE) include the following: The Netherlands (NL) 48.1%, Spain (ES) 23.5%, DE 11.5%, and Italy (IT) 2.3% ^[55]. We assume that these values also apply to Berlin. The share of tomatoes produced in Berlin is not known. The footprint evaluation of processed tomato products is not the subject of this study; nevertheless, an LCA of packaged tomato puree exists in the literature ^[56]. All deliveries result in an import of embodied CO₂. It should be noted that China is the globally most significant producer of tomatoes—some tomato products are distributed in the EU under an Italian label ^[57].
FWE nexus	The FWE ranking in Figure 4 indicates the main dependencies of the sectors: the climate crisis (CO₂, energy) is the greatest global challenge. If it is not solved, the global water balance will face significant problems and water scarcity will increase. Water, in turn, is the basis for all forms of food production.
Local aquaponic	Concerning urban AP, increasing building integration will reduce land consumption, which is required to achieve zero net land take by 2050 ^[58]. On the other hand, increasing competition for urban space will decrease urban AP applications. Peri-urban AP results in the conflict of objectives that, on the one hand, mitigates competition for use in the city but, on the other hand, is usually built as a standalone facility that results in increased land consumption. The high standard of the Dutch (NL) greenhouse production is the energy-related benchmark concerning greenhouse production in Berlin. Heat coupling and/or low-energy greenhouse are required for production in Berlin to have a lower impact on the energy sector than production in the Netherlands. Increasing local AP will induce the following: decrease imports, reduce embodied CO₂, mitigate water scarcity in Almeria, and increase local food production. Due to the double use of water by AP, the overall water consumption will decrease (WCO in Table 5) but local water demand will increase.

In terms of global environmental impacts and only these are considered in this study; relocation of production only makes sense if it reduces these impacts.

3. Current Insights

3.1. Food: Demand-Side Impact of Dietary Shifts

AP based food production meets the EU and global circular economy trends and creates possibilities for green entrepreneurship development ^[59]. The causal relationships shown so far are only a small part of the aquaponics-related impact structure. Two causalities shall be highlighted: (1) the impact of a ‘human’ pescatarian diet on GHG emissions and (2) the mitigation of phosphorus depletion by recycling the element by AP.

Agriculture is the primary driver of land system change, e.g., through tropical deforestation ^[60]. The food system also impacts biodiversity loss ^[61]—related to the biosphere integrity planetary boundary—and while domestic livestock currently has an estimated biomass of 100 Mt C, all wild mammals globally account for only about 7 Mt C ^[62]. Food systems are currently threatening human health and environmental sustainability ^[6] and environmental impacts can be reduced on the supply as well as on the demand side ^[63]. At the EU level, rapid changes in our habits and behaviour are requested ^[64] in order to reduce the environmental and climate footprint of the EU food systems ^[65]. The negative impact of meat consumption on the environment is well known ^[66]. Fish represents an alternative: global aquaculture has a rather modest share of approximately 0.49% of anthropogenic GHG emissions in 2017 ^[67] than terrestrial livestock farming (approximately 15%). Other alternatives include insects, which can be used both as fish feed ^[68] and as human feed, e.g., dried yellow mealworms ^[69]. A GHG emission tax on food products can support dietary shifts but must be introduced globally or trade restrictions must be considered to be fully efficient ^[70]. Concomitantly, environmentally harmful subsidies should be avoided ^[1]. The IPCC ^[71] investigated the role of dietary preferences and the demand-side GHG mitigation potential of different diets by 2050. A pescatarian diet consisting of seafood could save about 4.0 GtCO₂-eq a⁻¹, whereas a vegan diet without animal source food has a doubled effect of about 7.9 GtCO₂-eq a⁻¹ ^[71]. The GHG savings potential of a pescatarian diet with a high share of freshwater fish would be between these two values. None of these scenarios will fully unfold, but one crucial aspect of the food environment and the desirability of food is inter alia embossed by socio-cultural aspects ^[72], which can be changed.

However, changes in the diet affect not only CO₂ emissions but also many other components of the food system. Modern food production is entirely dependent on the non-renewable resource phosphorous (P) ^[73]^[74]: Biogeochemical flows—mainly nitrogen and phosphorous fluxes—are seen as a planetary boundary ^[75] and agriculture is a major driver exceeding it ^[8]. The use of phosphate causes the phosphorous dilemma: while mineral fertiliser facilitated the intensification of plant production ^[76]^[77], it has results in an enormous P-input into the biosphere ^[16] with P as a dominant driver of eutrophication with all its adverse effects ^[16]. Without P recycling, food security will inevitably be violated in the long run ^[78], preventing us from ‘living well, within the limits of our planet’ ^[79]. P recycling is also becoming increasingly crucial concerning circular cities and urban farming. In aquaculture and aquaponics, the treatment and recycling of potential P-sources are also of interest. After fish feeding, a considerable fraction of dietary P is not retained in fish but excreted and dissolved P is strongly adsorbed onto particles ^[80]. In RAS, solid waste from faeces and uneaten feed pellets represents a substantial reservoir of nutrients, especially P, and needs to be captured ^[81], e.g., by using drum filters or passive sedimenters. Therefore, efforts are focused on increasing nutrient retention in fish or using sludge as a nutrient sink in RAS ^[82]. Another possibility to increase the effectiveness of aquaponics in terms of P is the substitution of fishmeal and fish oil with other ingredients (algae and poultry meal). Such fish diets reduce the footprint for carnivorous finfish production ^[45], also regarding P ^[83].

3.2. Water: Trans-Aquaponics

Concerning human food production, AP impacts the circular economy in a positive manner ^[84]^[85]. Following the circular economy concept, the CC consists of loops formed by NBS, which are defined as concepts derived from nature and focused on resource recovery ^[29]. AP is itself an NBS and the wastewater generated in the RAS, instead of being treated, can be provided as nutrient water for, e.g., a vertical green system (VGS) ^[54]. Systems based on the aquaponic principle that extend crop production from hydroponic to soil-based methods are referred to as trans-aquaponics ^[48]. Such a trans-aquaponic solution emerges when the two NBS units, in this case aquaculture and VGS, are coupled to tackle circularity challenges in cities ^[86].

While horizontal space is scarce and under tremendous utilisation pressure for use in densely built urban regions, vertical space—facades and walls—is rarely used apart from billboards and photovoltaic applications. VGS including expensive green walls, modular wall-mounted plant beds, or low-cost and sustainable facade greening including ground-based climbing plants are promoted for several ecosystem services and are simultaneously an aesthetic upgrade of buildings or passive cooling ^[87]^[88]. Food production is even possible on several height zones of the building facades and could include host vine-crops (i.e., climber species) such as kiwifruits or grapes and other suitable crops with artificial cropping adjustments for vegetables such as beans, tomatoes, cucumbers, and peppers; or fruits such as blackberries, blueberries, pears, or apples. Due to the negative climatic water balance, especially in the summer season, irrigation water sources and volumes are a significant factor in determining the sustainability of VGS. In Berlin, VGS would require 240 (north exposure) to 400 L m⁻² (south exposure) of water in summer, of which only 330 L m⁻² can be collected from the roof ^[89]. For the remaining 70 L m⁻², the aquaculture wastewater from a RAS can be used because it does not contain any human faeces or human-active pharmaceuticals. Vertical green could also be integrated into the aquaculture system itself by mostly bringing closed production into open space and allowing the multi functionalities described above. Transporting water through pumps may reduce the sustainability benefits of aquaculture water compared to tap water and fertiliser. The biggest challenge in irrigation is the storage of AP waters in terms of fouling and space demand.

In 2040, the expected water consumption of Berlin amounts to 806 million m³ (cf. Table 6) out of which 103 million m³ are attributable to industry and trade. The reduction in water consumption by 2.0 million m³ (cf. Table 4) corresponds to about 2% of the latter demand. This value could be further increased if water losses due to evapotranspiration were regained and condensed by cooling traps and eventually fed into the aquaculture unit ^[90].

Table 6. Estimated water consumption in Berlin in the year 2040, author’s work based on water supply concept for Berlin ^[91].

Water 2040	(Million m³)
Households	551.0
Industry and trade	102.6
Others	140.6
Environment	11.4
Total	805.6

The implementation process for upscaled aquaponics will take some time and result in higher water demand in the future due to AP water demand. Berlin’s water management faces challenges, e.g., concerning groundwater extraction as shown by a lawsuit filed by the Berlin State Working Group for Nature Conservation to protect peatlands and wetlands ^[92]. In the future, these challenges will increase and new concepts and courses of action will be required ^[93]. AP upscaling mitigates the water problem in the Almeria region but exacerbates it in the Berlin area. As urban agriculture, including aquaponics, claims access to water as a resource, care must be taken and the use of modern semi-closed greenhouses with condensation regaining ^[94] can contribute to care.

3.3. Energy: Low-Energy Greenhouses and Transport Trade-Offs

The comparative-LCA has revealed the energetic disadvantages of greenhouse production in the moderate continental climate of the Berlin area, especially during the winter, compared to the Mediterranean climate in southern Europe. However, in cooler regions, the crop can be cultivated year-round without the need for a summer break or intensive and water consuming cooling as is required in southern Europe. Energy savings from upscaled urban aquaponics are limited when using a standard greenhouse. Winter heating in regions such as Berlin can be supported by excess heat. On the other hand, there are technical solutions for greenhouse crop production for almost all climatic situations ^[95]. Upgrading greenhouses with a package of high-technological equipment such as combined heat and power units, heat pumps, underground seasonal and daytime energy storage systems, and air treatment units as used in the closed greenhouse concept ^[96] can strongly reduce energy consumption ^[94]. In order to achieve optimal energy saving and plant production, smart decision support systems and/or model-based climate control systems for the greenhouse crop production units (or closed units) are needed ^[97].

Another energy-related aspect is that the environmental impact of food transport is often reduced to its CO₂ emissions, which are the so-called food miles, and this accounts for only a tiny part of its environmental impact ^[98]. Here, trade-offs between energy, water, and food transport (FWE nexus) must be considered. For example, an LCA case study of tomatoes originating in Morocco and imported into France reveals that a comprehensive method for assessing freshwater use impacts is lacking for the energy and water trade-off ^[99]. Furthermore, traffic-related non-exhaust particulate matter contributes significantly to the flux of microplastics into the environment ^[100] and tire and brake abrasion particles are transported globally through the atmosphere to distant regions ^[101]. Nota bene: these problems are not addressed by a tax on GHG emissions from transportation.

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