Abstract

Water-saving agricultural practices can reduce negative environmental impacts in water-scarce regions all over the world. This study deals with an innovation that combines hydroponic crop production and municipal wastewater reuse for irrigation purposes. The research question was what impacts such hydroponic water reuse systems have on product confidence, economic viability, groundwater recharge, biodiversity and landscape quality. It should also be clarified under which conditions and with which measures these systems can be sustainable. To answer these questions, a number of generic hydroponic water reuse systems were modeled and assessed using a Bayesian Belief Network that included both numerical values and expert knowledge. The hydroponic water reuse systems with the most positive overall impacts are small-scale food production systems (tomatoes) equipped with lighting and heating whose products are marked with a quality label or with a label for regional products. The systems are located in a former industrial area. In addition, a wetland system and landscape integration are implemented as landscaping measures. Hydroponic systems can be operated economically viable, their products have a high level of product confidence and their ecological impacts can be positive. No tradeoffs have to be accepted between economic, social and ecological goals.

HIGHLIGHTS

  • The study shows that hydroponic water reuse systems can be operated economically viable, that their products have a high level of product confidence and that their ecological effects can be positive if appropriate landscaping measures are taken.

  • Landscaping as well as acceptance measures should be carried out to accompany hydroponic crop production to improve its social and ecological impacts.

INTRODUCTION

Agriculture accounts for approximately. 70% of withdrawals of global freshwater resources and is thus by far the largest consumer of water compared with other sectors. In arid and semi-arid areas, the withdrawals can even be as high as 85% (Chmielewski 2011). About 40% of the agricultural food produced is irrigated (Chmielewski 2011). Agriculture is dependent on irrigation not only in arid and semi-arid zones. Even in intensively cultivated agricultural areas in Germany, agriculture is confronted with problems of water scarcity. In total, 451,800 ha (or 1.3%) of agricultural land in Germany are irrigated (Statistisches Bundesamt 2017). In the German strategy for adaptation to climate change, it was found that regional water use conflicts could arise with regard to surface and groundwater during dry periods (BMU 2008). Therefore, new water-saving cultivation methods and additional water resources are of interest in Germany in order to avoid conflicts of use, but also for the protection of groundwater. Intensification of agriculture can reduce land use and therefore reduce negative environmental impacts (Ellis et al. 2013).

One option for the water-efficient cultivation of crops is hydroponic greenhouse production. In hydroponics, plants are grown without soil. Instead, mineral nutrient solutions dissolved in water are used (dos Santos et al. 2013). Crops are cultivated in such a way that only their roots are hanging in the nutrient solution. The roots can be physically supported by an inert medium such as mineral wool, volcanic stones or other substrates. One possibility of operation is that the nutrient solution continuously flows past the roots (continuous-flow solution culture). This particularly facilitates the adjustment of the nutrient concentration (Rockel 1997). The nutrients used in hydroponic systems can come from various sources, e.g. fish excrement, chemical fertilizers or nutrient solutions (Jones 2004). Crops that are usually grown hydroponically include tomatoes and lettuce, but also ornamental plants. The main advantage of hydroponics is the significantly lower water consumption compared with conventional agriculture (Zhang et al. 2018). In particular, water loss through evapotranspiration is low in a closed greenhouse. This enables the cultivation of food even in extremely dry areas.

One possible resource for feeding hydroponic systems is the reuse of wastewater. In water reuse, wastewater is treated with technical means, so that it can be reused for other purposes (e.g. agricultural irrigation). The use of recycled water instead of fresh water for corresponding purposes can be a water-efficient measure. Water reuse can thus be part of sustainable water management. This can reduce water scarcity and alleviate pressure on groundwater and other water resources (Andersson et al. 2016). In addition, not only can water be saved when reusing wastewater, but the nutrients contained in the wastewater can also be used to fertilize the plants.

Although studies have already been carried out on hydroponic systems in which reused wastewater is used, no large-scale implementation has yet been carried out. The use of treated wastewater as a nutrient solution has so far been little investigated (Magwaza et al. 2020). In the HypoWave project, a new concept was tested, which deals with the connection of treated municipal wastewater and plant production (Bliedung et al. 2019). The key question was to what extent municipal wastewater has to be treated in order to be able to use it in hydroponic systems with as little additional nutrient input as possible. A pilot plant with a modular set-up of different wastewater treatment processes was implemented in Wolfsburg Hattorf, Germany, on the site of the municipal wastewater treatment plant (WWTP) (Bliedung et al. 2019). In addition to the option of using the biologically treated (activated sludge including nitrification and denitrification) WWTP runoff, a number of alternative treatment processes have been tested. These processes include an anaerobic expanded granular sludge bed reactor and an aerobic sequencing batch reactor for nitrification. In this way, a large part of the nitrate remains in the treated water. In addition, a biological activated carbon biofilter for the removal of trace substances and, if necessary, ozonation to eliminate pathogens was used. Depending on the process combination, water of different quality could be produced for the hydroponic system, which among other things resulted in different nutrient levels.

The hydroponic system of the pilot plant consisted of several parallel rows of pipes in which lettuce plants were cultivated. The pipes were fed with different water qualities. In addition, lettuce was grown in one pipe in a conventional Hoagland solution with a 50% concentration for comparison. The hydroponic system was designed as a mixture of nutrient film technique and deep water culture.

In addition to the above-mentioned advantages of hydroponic water reuse systems in terms of water and nutrient efficiency, a number of impacts are also unclear, such as questions about the social acceptance of the produced crops, as well as ecological consequences and economic viability. Accordingly, the key research questions are: (1) What are the social, economic and ecological impacts of selected hydroponic water reuse systems? (2) Under which conditions are hydroponic water reuse system socially, economically and ecologically sustainable? To answer these questions, a number of generic hydroponic water reuse systems were modeled and assessed using a software-based modeling process that can make use of both numerical values and expert knowledge.

METHODS AND MATERIALS

General methodology and state of the art

The assessment of social, ecological and economic impacts of hydroponic water reuse systems was carried out using Bayesian Belief Networks (BBNs). BBNs are statistical multivariate models based on the theorem of Thomas Bayes (1740) for calculating conditional probabilities. BBNs combine a qualitative, graphical representation of a system with a quantitative, probabilistic evaluation of the interactions between the variables of such a system (Castelletti & Soncini-Sessa 2007; Barton et al. 2012). BBNs are particularly suitable for depicting complex systems in a comprehensible way (Molina et al. 2013), can process qualitative and quantitative data (Chen & Pollino 2012) and allow the inclusion of stakeholder and expert knowledge (Bergmann et al. 2010). BBNs consist of a directed acyclical graph containing variables and nodes. These are connected by directional arrows that represent cause–effect relationships between the nodes. Conditional probability tables (CPTs) assigned to each node reflect the conditional probabilities for the values of a node as a function of the values of the parent nodes (Cain 2001). A disadvantage of conventional BBN is that feedbacks cannot be mapped. Furthermore, spatial and temporal dynamics are difficult to represent (Uusitalo 2007).

BBNs are used as versatile tools for the investigation of interactions between variables of a system and as decision support systems for the development of management strategies (Kuikka et al. 1999; Cain 2001; Castelletti & Soncini-Sessa 2007; Jensen & Nielsen 2007; Pagano et al. 2013; Reichert et al. 2015), in the fields of water resource management and ecology (Cain 2001; Bromley 2005; Marcot et al. 2006; Castelletti & Soncini-Sessa 2007; Pollino et al. 2007; Aguilera et al. 2011) and on environmental impact assessment and risk assessment (Voie et al. 2010; Liu et al. 2012; Tighe et al. 2013; Money et al. 2014; Brandmayr et al. 2015).

The structure of the BBN (i.e. system variables and their interrelations) was pre-developed together with external experts and project partners. For this purpose, a total of six expert interviews with researchers from different disciplines (e.g. civil and environmental engineering, plant science, ecology and landscape architecture) were carried out. Five social, economic and ecological impact variables were jointly chosen, namely product confidence, economic viability, groundwater recharge, biodiversity and landscape quality. The data required to model the BBN were collected through qualitative or semi-quantitative expert surveys in the case of product confidence, biodiversity and landscape quality (Table 1). Other impacts, namely economic viability and groundwater recharge, were calculated or assessed based on literature values and assumptions (Table 1). The relations among system variables (nodes) were expressed in the form of a CPT in the case of probabilistic values and in the form of a function table in the case of deterministic values (Supplementary Material, Appendix A). The BBN was finally evaluated using the software Netica. The BBN eventually shows which social, ecological and economic impacts the experts attributed to various hydroponic water reuse systems and selected measures.

Table 1

Methods used to assess specific impacts of hydroponic water reuse systems

Impact dimension Social impacts Economic impacts Ecological impacts 
Impacts Product confidence Economic viability Groundwater recharge Biodiversity Landscape quality 
Method Expert survey Calculation Qualitative estimation Expert survey 
Type of parameter Probabilistic Deterministic Deterministic Probabilistic 
Impact dimension Social impacts Economic impacts Ecological impacts 
Impacts Product confidence Economic viability Groundwater recharge Biodiversity Landscape quality 
Method Expert survey Calculation Qualitative estimation Expert survey 
Type of parameter Probabilistic Deterministic Deterministic Probabilistic 

Calculation of the economic viability

The economic viability was determined for a total of 16 different generic hydroponic water reuse systems. These systems differed in terms of product (food vs. non-food crops), size (small-scale vs. large-scale), production process (with or without lighting and heating) and the water regime applied (open vs. closed mode of operation). Furthermore, landscaping measures (i.e. wetland systems and landscape integration) were considered in the cost calculation. Tomatoes were selected as food crops and Chrysanthemum as non-food crop or ornamental plants. The size of the hydroponic systems corresponded either to the water quantity of 1,000 (small-scale) or 10,000 (large-scale) population equivalents. The greenhouses were either equipped with lighting and heating for improved growing conditions or not. Finally, an open and a closed water regime were distinguished. In the first case, the water flows continuously through the hydroponic system which makes it nutrient-efficient (e.g. regarding micro- and macronutrients). In the second case, the water is circulated in the system which makes it water-efficient. Fertilizer, however, has to be added regularly. The water is circulated until a certain salinity threshold (e.g. 8 mmol NaCl) is reached. It can then be treated by reverse osmosis for further reuse.

Based on these preconditions, investments and operating costs (e.g. labor costs, energy costs, overhead and additional water treatment) of the 16 systems were determined. A depreciation period of 10 years has been applied. Capital costs have been calculated on the basis of an interest rate of 3%. Literature values for expenses and revenues were taken from the KWIN model (WUR 2018). Additional costs for the water reuse treatment steps and the landscaping measures were added. Finally, the economic viability was measured based on the annual rate of return which represents the profit expressed as the proportion of the investments and costs.

The market price for tomatoes was set at 1.86 EUR with lighting and heating and 1.74 EUR for tomatoes without lighting and heating as suggested by the KWIN model for European conditions (WUR 2018). The market price for Chrysanthemum Santini with lighting and heating was adjusted due to recent market fluctuations from 0.38 EUR to 0.28 EUR per stem. For Chrysanthemum Santini without lighting and heating, the price remained as suggested by the model at 0.17 EUR per stem. Wages were set at 12.00 EUR per hour for workers, 18.75 EUR for trained staff and 20.00 EUR for operators. For the landscaping measures, a land value (lease) of 800 EUR per ha and year was applied according to a related case study (Mohr et al. 2019). Additional costs for construction and operation of the landscaping measures, however, were not considered due to the diversity of possible options (e.g. wetland systems, hedges and trees).

Expert surveys and estimations of social and ecological impacts

A total of 12 experts from research and practice were surveyed using questionnaires regarding product confidence, biodiversity and landscape quality, covering different disciplines and sectors. Even if the number of 12 experts cannot be representative, their positions and experience mean that they are multipliers who can provide a well-founded appraisal of the impacts surveyed.

Consumer acceptance of the food and non-food products made using the hydroponic water reuse system was considered an important factor. The decision of food and flower retailers to add products to their range depends heavily on how they assess consumer acceptance. Therefore, the product confidence of corresponding retailers was chosen as an indicator of consumer acceptance. Additionally, measures to increase product confidence were taken into account. In addition to taking no action, these measures comprise products with a quality label (e.g. products are harmless to health, sustainable or without genetic engineering), products that are labeled as regionally produced as well as branded products (e.g. Bonduelle and Ardo).

The product confidence was determined by using standardized expert surveys. In particular, grocery retailers from organic supermarkets and farmers' markets as well as florists were interviewed. The interviewees were asked to rate the product confidence on a 5-level Likert scale from 1 to 5, with 1 meaning that the respondent would include the product in their range without hesitation and 5 that the respondent would under no circumstances do so. Two separate questionnaires for two separate groups of interviewees were used for vegetables (tomatoes) and ornamental plants (Chrysanthemum). One question, for example, was: ‘Would you include foods (e.g. tomatoes) produced with the described hydroponic system in the assortment of your market if they are labelled as regional products?’ The responses were converted to probabilities by first calculating the mean and then performing a linear interpolation where a mean of 1 corresponded to a product confidence of 100% and a mean of 5 to a product confidence of 0%.

The implementation of hydroponic greenhouses can have adverse ecological impacts, for instance increased land consumption or impervious surfaces. Landscaping measures to mitigate these effects can be the creation of wetland systems around the greenhouses and the integration of greenhouses into the landscape by using hedges, trees or extensive farming elements such as short rotation forestry. Both measures can also be combined and might have positive impacts on biodiversity and landscape quality.

Similar to the case of product confidence, the impacts of landscaping measures (i.e. wetland systems and landscape integration) on biodiversity and landscape quality were determined by means of standardized expert surveys. In particular, experts from science and practice (e.g. ecologists and landscape architects) were interviewed. The interviewees were asked to rate the likelihood of positive impacts of the landscaping measures on biodiversity and landscape quality on a scale from 0 to 100% where 100% represents the strongest positive effect. Two questionnaires were presented to the interviewees – one for the impacts on biodiversity and one for the impacts on landscape quality. It was assumed that wetland systems would expand the area of the hydroponic systems (including the greenhouses) by 20%. Measures for landscape integration would expand the area by 40% and both measures combined by 60%. These values were derived from the proposal to keep more than 50% of the area free from intensive use in order to preserve biodiversity in the long term (Ellis 2019). Furthermore, the questionnaires also took into account whether the hydroponic systems were implemented in former industrial or agricultural areas and whether they were small-scale (approximately 3–6 ha or 1,000 population equivalents) or large-scale systems (approximately 27–58 ha or 10,000 population equivalents). One question, for example, was: ‘What is the probability that greenhouses built on a former agricultural area with a size of 100 ha and a continuous green area with wetland systems of an additional 20 ha have a positive and/or negative impact on biodiversity?’

Impacts on the local groundwater recharge were qualitatively estimated on the basis of land consumption and impervious surface of the hydroponic systems as well as the capacity of the considered measures to support rainwater infiltration. Five qualitative levels of groundwater recharge were distinguished, namely highly reduced, reduced, neutral, improved and highly improved. Different rates of groundwater recharge were assessed on the basis of two different initial situations, namely former agricultural areas (pervious) and former industrial areas (impervious). The implementation of various landscaping measures (i.e. wetland systems and landscape integration) was then considered for these areas. The area used for landscaping measures is assumed to be permeable.

RESULTS AND DISCUSSION

Structure of the BBN

The expert discussions resulted in a BBN that consists of five system variables describing properties of hydroponic water reuse systems, two measures relating to product confidence and landscaping as well as the pre-defined five social, economic and ecological impact variables. The system variables each have two instances, namely food production systems (tomatoes) vs. systems for the production of ornamental plants (Chrysanthemum), small-scale (1,000 population equivalents) vs. large-scale systems (10,000 population equivalents), systems with lighting and heating vs. systems without lighting and heating, systems with a closed water regime (water-optimized) vs. systems with an open one (nutrient-optimized), and finally systems implemented in a former industrial area vs. systems in a former agricultural area. Furthermore, the BBN includes two measures which represent possible interventions to avoid or mitigate undesirable impacts. The measures are aimed at social and ecological impacts and consist of the four above-mentioned instances each.

The expert discussions also revealed how the system variables, measures and impacts (i.e. product confidence, economic viability, groundwater recharge, biodiversity and landscape quality) are interrelated which eventually leads to the BBN (Figure 1 and Supplementary Material, Appendix C). A total of 512 combinations (25 × 42) of the system variables and measures are possible. Figures with examples of the model's behavior are attached in Supplementary Material, Appendix B.

Figure 1

BBN of hydroponic water reuse systems consisting of system variables (blue), measures (green) and impacts (red). Please refer to the online version of this paper to see this figure in color: http://dx.doi.org/10.2166/wrd.2020.026.

Figure 1

BBN of hydroponic water reuse systems consisting of system variables (blue), measures (green) and impacts (red). Please refer to the online version of this paper to see this figure in color: http://dx.doi.org/10.2166/wrd.2020.026.

Selected hydroponic water reuse systems and their impacts

The hydroponic water reuse system with the most positive overall impacts is a small-scale food production system (3–6 ha, tomatoes) equipped with lighting and heating whose products are marked with a quality label (Table 2). The system is located in a former industrial area, and both above-mentioned landscaping measures (wetland system and landscape integration) are implemented. With regard to the investigated impacts, it is irrelevant whether it is a system in which the water is circulated (closed system) or through which the water flows only once (open system). Such a system would achieve a very high product confidence (90%), a high economic viability (6% rate of return), a greatly improved groundwater recharge, a very positive impact on biodiversity (77%) and a very positive effect on the landscape quality (84%). A slight variation in this system in which the tomatoes are marked with a label for regional products would achieve a slightly lower product confidence (70%). Apart from that, the system achieves the same very positive results. Small-scale food production systems without lighting and heating, however, have a very poor economic viability (−12% rate of return).

Table 2

Hydroponic water reuse systems with most positive impacts

SystemsMeasuresProduct confidenceEconomic viabilityGroundwater rechargeBiodiversityLandscape quality
Food production (tomatoes); small-scale; lighting and heating; open or closed system; former industrial area Quality label; wetland system, landscape integration 90% High Greatly improved 77% 84% 
Food production (tomatoes); small-scale; lighting and heating; open or closed system; former industrial area Labeled as regional product; wetland system, landscape integration 70% High Greatly improved 77% 84% 
Food production (tomatoes); large-scale; no lighting and heating; open or closed system; former industrial area Quality label; wetland system, landscape integration 90% Positive Greatly improved 76% 80% 
Food production (tomatoes); large-scale; no lighting and heating; open or closed system; former industrial area Labeled as regional product; wetland system, landscape integration 70% Positive Greatly improved 76% 80% 
SystemsMeasuresProduct confidenceEconomic viabilityGroundwater rechargeBiodiversityLandscape quality
Food production (tomatoes); small-scale; lighting and heating; open or closed system; former industrial area Quality label; wetland system, landscape integration 90% High Greatly improved 77% 84% 
Food production (tomatoes); small-scale; lighting and heating; open or closed system; former industrial area Labeled as regional product; wetland system, landscape integration 70% High Greatly improved 77% 84% 
Food production (tomatoes); large-scale; no lighting and heating; open or closed system; former industrial area Quality label; wetland system, landscape integration 90% Positive Greatly improved 76% 80% 
Food production (tomatoes); large-scale; no lighting and heating; open or closed system; former industrial area Labeled as regional product; wetland system, landscape integration 70% Positive Greatly improved 76% 80% 

Large-scale food production systems (tomatoes with quality label) in former industrial areas also perform fairly well but only without lighting and heating (Table 2). In this case, the economic viability is just positive (Table 2). Apart from this, the large-scale system scores slightly worse than the small-scale system with regard to the criteria biodiversity (76 instead of 77%) and landscape quality (80 instead of 84%) (Table 2). Large-scale food production systems with lighting and heating, however, do not appear to be economically viable (rate of return −5%, Table 5).

As in the case of small-scale systems, product confidence drops from 90 to 70% if the tomatoes are labeled as regionally produced. Food systems without consumer acceptance measures (regardless of whether they are large- or small-scale) lead to a loss of product confidence (56 instead of 90 or 70%). Branded products even lead to a product confidence of only 42%.

In the case of food production systems in former agricultural areas, the positive ecological effects are not so pronounced. Assuming that both landscaping measures are carried out, the groundwater recharge is neutral in small-scale systems with lighting and heating, the biodiversity is 55 instead of 77% and landscape quality 67 instead of 84%. For large-scale systems without lighting and heating, groundwater recharge is also neutral, the biodiversity is 53 instead of 76% and landscape quality 53 instead of 80%.

Hydroponic water reuse systems for the production of ornamental plants (in our case: Chrysanthemum) have a negative or very negative economic viability in all calculated scenarios (Table 5). However, these systems can still achieve positive social and ecological impacts. Ornamental plants (Chrysanthemum) show high product confidence if they are labeled with a quality label (100%) or as regionally produced (100%) and even if neither of these two consumer acceptance measures have been carried out (92%). The potential for very good ecological effects only exists if the system is implemented in a former industrial area and not on an agricultural one. Conclusions regarding the advantages and disadvantages of systems with an open or closed water regime cannot be drawn either for food production systems or for systems for the production of ornamental plants.

Specific impacts and their interdependencies

Social impacts

The highest product confidence is found in ornamental plants (Chrysanthemum) with (100%) and without (92%) consumer acceptance measures (quality label and/or labeled as regionally produced) as well as in food products (tomatoes) with quality labels (90%) followed by ones that are labeled as regionally produced (70%). In general, food retailers were positive about potential products from hydroponic water reuse systems. They said that they could imagine including the products in their range. The best measure to improve the product confidence was a quality label which can be compared with labels that certify sustainable production or GMO-free labels. Labeling as a regionally produced product also improved product confidence. Branded food products even had a lower product confidence (42%) than products without any consumer acceptance measure (56%). A sensitivity analysis of the factors influencing product confidence shows that the influence of acceptance measures is significantly greater than that of the product type.

Florists did not mention concerns with selling ornamental flowers produced with hydroponic water reuse systems. The use of labels (quality label or labeling as a regional product) was preferred over no label. However, it was also argued that labels are not required in case of ornamental plants since customers would not be concerned. They were said to prefer quality and price over other factors. In some cases, it was said that costumers would ask where the flowers were made. This might indicate an interest in regional products and their social or environmental impacts.

The results indicate that the production of flowers from hydroponic water reuse systems would be well accepted. Flowers labeled as regionally produced could even influence the consumer's purchase decision in a positive way if they are interested in the social and environmental sustainability of the production. Apart from this, the attitude of grocery retailers towards food crops from hydroponic systems was predominantly positive. Nevertheless, quality labels or labeling as a regional product might be required to increase product confidence. Big supermarket chains could not be reached by means of this study. Further research is necessary to understand how the big players will position themselves towards the circular economy and future developments in food production. To our knowledge, there are no other detailed studies on product confidence concerning products from hydroponic systems with water reuse.

Ecological impacts

Implementing both landscaping measures (i.e. wetland system and landscape integration) leads to the most positive impacts in terms of biodiversity and landscape quality. This is irrespective of whether the hydroponic system was set up in a former industrial or agricultural area or whether it is a small-scale or large-scale system (Table 3). Undertaking no landscaping measures in a former agricultural area leads to the most negative effects in this regard.

Table 3

Impacts of landscaping measures on biodiversity and landscape quality

SizeMeasuresBiodiversity (%)Landscape quality (%)
Former agricultural area: 
Large-scale (27–58 ha) No measures 
Wetland system 21 28 
Landscape integration 37 52 
Both measures 53 53 
Small-scale (3–6 ha) No measures 
Wetland system 34 35 
Landscape integration 41 58 
Both measures 55 67 
Former industrial area: 
Large-scale (27–58 ha) No measures 23 53 
Wetland system 47 63 
Landscape integration 62 70 
Both measures 76 80 
Small-scale (3–6 ha) No measures 26 57 
Wetland system 48 66 
Landscape integration 58 76 
Both measures 77 84 
SizeMeasuresBiodiversity (%)Landscape quality (%)
Former agricultural area: 
Large-scale (27–58 ha) No measures 
Wetland system 21 28 
Landscape integration 37 52 
Both measures 53 53 
Small-scale (3–6 ha) No measures 
Wetland system 34 35 
Landscape integration 41 58 
Both measures 55 67 
Former industrial area: 
Large-scale (27–58 ha) No measures 23 53 
Wetland system 47 63 
Landscape integration 62 70 
Both measures 76 80 
Small-scale (3–6 ha) No measures 26 57 
Wetland system 48 66 
Landscape integration 58 76 
Both measures 77 84 

Hydroponic systems in former agricultural areas need at least landscape integration measures to achieve positive impacts on landscape quality. In terms of biodiversity, both landscaping measures are necessary to achieve positive impacts. In contrast, hydroponic systems in former industrial areas have positive effects on landscape quality even without any landscaping measures. Regarding biodiversity, at least measures of landscape integration are necessary to reach positive impacts.

The evaluation of impacts on biodiversity and landscape quality can only provide first indications for decisions about landscaping measures. Activities to improve biodiversity need to be focused on a larger spatial scale than the rather narrow system boundaries, especially with a view to mobile organisms such as birds (Söderström et al. 2001). From a legal point of view, nature conservation legislation in Germany, for instance, prohibits a deterioration of land to avoid negative effects on nature and landscape (‘Verschlechterungsverbot’). According to the German Federal Nature Conservation Act, interventions which cannot be avoided need to be compensated. The landscaping measures considered here could play a role in connection with these compensations.

Hydroponic systems implemented in former industrial areas always show an improved or greatly improved groundwater recharge if landscaping measures are taken (Table 4). Systems implemented in former agricultural areas have at best a neutral effect on groundwater recharge if both landscaping measures are carried out. Without any measures or with only one of the considered measures, groundwater recharge is highly reduced or at least reduced.

Table 4

Impacts of landscaping measures on groundwater recharge

MeasuresGroundwater recharge
Former agricultural area: 
No measures Highly reduced 
Wetland system Reduced 
Landscape integration 
Both measures Neutral 
Former industrial area: 
No measures Neutral 
Wetland system Improved 
Landscape integration 
Both measures Highly improved 
MeasuresGroundwater recharge
Former agricultural area: 
No measures Highly reduced 
Wetland system Reduced 
Landscape integration 
Both measures Neutral 
Former industrial area: 
No measures Neutral 
Wetland system Improved 
Landscape integration 
Both measures Highly improved 

For the assessment, it was assumed that former industrial areas are impervious. Therefore, there would be no further reduction of groundwater recharge in the case of the construction of a greenhouse. The surfaces of the landscaping measures mentioned above are regarded as pervious. Apart from surface infiltration, various construction measures can be taken to improve groundwater recharge such as infiltration basins or percolation trenches.

A sensitivity analysis of the influencing factors on all three ecological impact variables shows that the influence of landscaping measures and the former type of land use is significantly greater than that of the size of the greenhouses.

To our knowledge, there are no other detailed studies on the effects of hydroponic systems with water reuse on biodiversity, landscape quality or groundwater recharge. Studies dealing with the environmental impacts of hydroponic systems deal either with their general land requirements (Moore et al. 2020), water pollution (Tewolde et al. 2016), energy demand (Barbosa et al. 2015; Chen et al. 2020) or greenhouse gas emissions, respectively, their carbon footprint (Martinez-Mate et al. 2018; Vinci & Rapa 2019).

Economic impacts

As mentioned above, good economic viability (6% rate of return) is associated with small-scale food production systems with lighting and heating (Table 5). Large-scale food production systems without lighting and heating are at least barely economically viable. Other studies confirm that hydroponic systems can be economically viable under various conditions (Waldhauer & Soethoudt 2015; Becraft 2017; de Franca Xavier et al. 2018; Souza et al. 2019). Systems for ornamental plant production or small-scale food production systems without lighting and heating, however, always have a poor economic viability. A sensitivity analysis of the factors influencing the economic viability shows that the influence of the product type, the size of the greenhouses as well as lighting and heating is greater than that of the water regime and landscaping measures.

Table 5

Annual rate of return and size of hydroponic water reuse systems

Food production (tomatoes) Lighting and heatingOrnamental plant production (Chrysanthemum) Lighting and heating
withwithoutwithwithout
Large-scale Open system 27 ha 27 ha 33 ha 33 ha 
−5% 1% −10% −4% 
Closed system 58 ha 58 ha 51 ha 51 ha 
−5% 0.5% −14% −8% 
Small-scale Open system 3 ha 3 ha 3 ha 3 ha 
6% −12% −17% −12% 
Closed system 6 ha 6 ha 5 ha 5 ha 
6% −12% −19% −13% 
Food production (tomatoes) Lighting and heatingOrnamental plant production (Chrysanthemum) Lighting and heating
withwithoutwithwithout
Large-scale Open system 27 ha 27 ha 33 ha 33 ha 
−5% 1% −10% −4% 
Closed system 58 ha 58 ha 51 ha 51 ha 
−5% 0.5% −14% −8% 
Small-scale Open system 3 ha 3 ha 3 ha 3 ha 
6% −12% −17% −12% 
Closed system 6 ha 6 ha 5 ha 5 ha 
6% −12% −19% −13% 

Additional water treatment costs are lowest for systems producing ornamental plants. Corresponding costs range between 0.2 and 0.5% of the production costs in systems with an open water regime and between 2.0 and 4.6% in closed ones. In food production systems, they range between 2.0 and 5.5% in open systems and between 2.4 and 6.5% in closed ones.

Market prices of the products and energy costs have a big influence on the profitability. The calculations presented in Table 5 are based on rather conservative assumptions. A further sensitivity analysis was made with different assumptions considering energy supply and market prices. If prices and energy production are used as initially suggested by the model, six scenarios are highly profitable (15–35%), further four scenarios are positive, two scenarios are negative, and four scenarios are very negative (−6 to −13%). Furthermore, a combined heat and power plant for the production of heat and electricity would result in a surplus and therefore an income from sales of electricity. The market prices for Chrysanthemum (with lighting and heating) were assumed to be 0.38 EUR according to the model.

Costs for disposal of residues from water treatment were not considered in the calculations. In systems with closed water regimes, reverse osmosis is suggested to recycle the greenhouse runoff. It would be possible to irrigate a short rotation plantation in the landscape park which is much less sensitive to salinity than crops in hydroponic systems. In this way, water treatment costs and the question of disposal of residue would not arise.

CONCLUSIONS

The results of the study show that hydroponic water reuse systems can be operated in an economically viability way, that their products have a high level of product confidence and that their ecological effects can be positive if appropriate landscaping measures are undertaken. This applies in particular to the small-scale food production system described above. The results mean that no tradeoffs have to be accepted between economic goals on the one hand, as well as social and ecological goals on the other. In order to achieve this, however, the results suggest that landscaping as well as acceptance measures should be carried out to accompany hydroponic crop production. Land use through the construction of greenhouses should be compensated in particular by measures such as wetland systems and landscape integration in order to avoid or at least mitigate undesirable effects on the environment.

The BBN method allowed a rough, but nevertheless comprehensive and quick overview of the interdependencies of generic hydroponic systems. On the one hand, the modeling of generic systems has the advantage that the results can offer a general orientation and can be transferred to a specific case under certain conditions. On the other hand, the circumstances of a specific case can be such that the effects of the modeled systems might not apply. From a business perspective, however, such a model can serve as a decision support for companies (e.g. horticultural companies) or investors for a pre-feasibility study. The advantage of using a method such as the BBN is that quantitative assessments (e.g. cost–benefit analysis) are expanded to integrate qualitative expert knowledge. This means that impacts that are difficult to assess and to quantify can be included, which are often neglected in purely quantitative assessments.

Further investigations could address questions that remained open. One of these aspects is to clarify how an adequate quality management for hydroponic crop production including water reuse should be. In connection with this, the possibilities for the certification of the products with regard to consumer acceptance could be examined in more detail. Another aspect would be to include further assessment criteria such as measuring the carbon footprint in order to get a more complete picture of the impacts of hydroponic systems. A life cycle analysis could, in particular, provide insights into energy and material consumption for water reuse and hydroponic crop production in greenhouses. Unfortunately, this was not possible within the scope of the study. Nevertheless, the findings of this work may contribute to a successful take-off of the discussed innovation and the associated transformation processes in agriculture and water management.

ACKNOWLEDGEMENTS

The authors thank all project partners and participants who took part in the expert interviews and the survey. The research project ‘HypoWave – New Pathways Towards Wastewater Re-Use in Agriculture’ was funded by the German Federal Ministry of Education and Research (grant no. 02WAV1402).

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

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Supplementary data