*3.2. Strategies and Production Potentials of Integrated Plant Growth into Building* 3.2.1. Strategies to Integrate Plant Growth into Buildings

Our feasibility study considered four scenarios for house-internal gardens (plant production) and compared them regarding productivity, nutrient utilization, energy demand, required skills and social inclusion. For scenario 1 and 2, crop is produced in open roof-top gardens, while protected cultivation in greenhouses or plant factories, also called vertical farming [51], is assumed for scenario 3 and 4. Thus, the scenarios increase from low level to high-level control from scenario 1 to scenario 4 (Figure 8). Plant factories allow to cultivate crops in multiple layers with a high productivity and uniformity [52]. These systems are completely isolated from the exterior climate with the control of light, temperature, relative humidity and CO2 concentration [51]. Especially by controlling the light quality, the yield and the nutritional value of lettuce can be increased [53]. However, with increased control from scenario 1 to scenario 4, energy consumption increases through the supply of mechanical heat in greenhouses [54] and electric light in plant factories [52,55].

**Figure 8.** Relative assessment of the cultivation scenarios. Scenario 1: roof-top raised-bed, community residents; Scenario 2: roof-top vertical hydroponic system, community residents; Scenario 3: roof-top greenhouse, professional management; Scenario 4: basement vertical farm, professional management.

In scenario 1, plants are cultivated in raised-beds on the roof-top and maintained by community residents. In this scenario, the residents have a high ecological "feelgood factor". Maintenance requires only low skills, and the solid fraction of the composted digestate could be utilized, too. While food production is less efficient, a high crop diversity can be attained. With scenario 2, the production efficiency (per m2) can be increased. Here, the residents cultivate the plants in vertical hydroponic tiers. Vertical tiers are more space-efficient, while through optimal nutrient availability, hydroponic cultivation systems allow a faster and higher crop production [56]. For example, Li et al. (2018) [57] realized in hydroponic systems about twice as much shoot fresh weight of two lettuce cultivars than for the cultivation in culture substrate. However, hydroponic systems require more experience and regular control of the composition of the nutrient solution [58].

Better suited are scenario 3 and 4 because of the controlled or semi-controlled environment as in plant factories or greenhouses. In addition, increased dynamics of the crop water and nutrient demand like in scenario 2 create unfavorable situations with the necessity to discard parts of the nutrient solution [59].

In scenario 3, the crop is cultivated in a roof-top greenhouse using hydroponics. This scenario enables very high year-round production, but with a higher demand for heating and lighting (see Table 1). The energy consumption is, in particular between autumn and spring, very high. Furthermore, a certain dynamic in water and nutrient demand, as well as in yield throughout the year, is still present. In scenario 4, this dynamic is more or less completely eliminated. Here, the plant factory is installed in the basement or rooms without natural light and the climate is fully controlled [60]. Due to the cultivation of crops in layers, even less space than for the roof-top greenhouse is needed. However, since there is no natural light, high amounts of artificial light for plant growth must be provided, and the exhaust heat needs to be removed. Thus, scenario 4 is, despite its higher productivity, a better product quality as well as a stable and constant harvest, the most energy demanding. In addition, it requires maintenance and resources all year round. As the community provides a year-round output in biowaste, continuous use of it can be best achieved with scenario 4. Thus, a decision must be made between the installation of a buffer system for seasonal provision of nutrient-rich irrigation water or a high energy demand. Next to that, non-technical issues also need to be taken into consideration, e.g., the roof-top floor is highly desired by residents and the most expensive floor in the building. In contrast, skyscrapers contain inside rooms without daylight, which must not be used for apartments or offices, at least in Germany.

#### 3.2.2. Production Potentials and Energy Demand of Simulated Scenarios

To evaluate the potential of in-house food production, crop cultivation in protected environments with semi-closed and closed systems, i.e., greenhouses in scenario 3 and plant factories in scenario 4, a model-based simulator tuned to the respective cases was developed. Scenarios 1 and 2 were neglected since their outcome varies, among other factors, highly on the residents' skills and motivation, which complicates simulations regarding pure bio-technological scenarios significantly. As one of the most common crops in plant factories and greenhouses, lettuce was chosen as a model crop because it is relatively easy to handle during cultivation [61], and it is suitable to produce lettuce even with wastewater [62].

Based on the N content in the liquid fraction from the biowaste digestate, it was calculated that the production of 7.6 t fresh mass of lettuce per year would be theoretically possible. A challenge for implementing closed hydroponic systems in the ICU concept is the open question after what time period the fertilizer has to be replaced due to the accumulation of salts (mainly natrium and other deleterious substances. However, in comparison to open systems (the drained nutrient solution is discarded), closed re-circulating hydroponic systems reduce water and fertilizer by about 30% and 50%, respectively [63,64].

Using the ICU concept, uncertainties in the dynamic of crop water demand in scenario 3 (through seasonal fluctuations) could result in an unwanted discharge of nutrient solution. This can partly be solved by increasing the area of the cultivation system. Therefore, the area for the greenhouse in scenario 3 was with 70 m<sup>2</sup> partly oversized. Due to that, all available N will be taken up by the crop. This dynamic is not relevant in scenario 4 and thus a perfect sizing of the plant factory is possible in this case. Accordingly, a total annual

yield of 6.4 t or 7.3 t fresh lettuce (i.e., 91 kg m−<sup>2</sup> or 242 kg m<sup>−</sup>2; Table 1) was predicted for the greenhouse (scenario 3) and the plant factory (scenario 4), respectively. These values correspond to the yields reported in other studies [36,65]. In these scenarios, a weekly harvest of roughly 760 or 1377 lettuce heads in a 70 m<sup>2</sup> greenhouse or in a plant factory can be expected. Since 1 kg of lettuce contains about 150 calories and, assuming a person requires about 2000 calories a day without burden, this would nourish about one people for one year. However, while human nutrition is not covered solely by lettuce, in a real implementation of innovative urban agriculture practitioners a broad mix of vegetables and herbs with various nutritious values should be considered in follow-up studies [66] (Table 2).

**Crop Potential Fresh Mass [kg] Calories [kcal/100 g]** Broccoli 3050 35 \* Bush bean 5490 25 \* Brussel sprouts 2771 43 \*

> Lettuce 7625 11 \*\* Spinach 3431 23 \*

**Table 2.** Potential biomass production per year for different vegetable crops based on the total available N amount in the liquid effluent and respective calories.

\* Footspring magazine—Kalorientabelle [67]. \*\* wikifit—Kalorientabelle, Gemüse, Salat [68].

On the downside: The roof-top greenhouse in scenario 3 (m<sup>2</sup> ground) requires an annual amount of ~143 GJ (39.7 MWh) energy for heating and 25 GJ (7.1 MWh) electrical power for light. While the energy demand is highest during autumn and winter, the yield is highest in summer. Thus, the efficiency of energy use strongly drops in the cold season. Using a cultivation period of March to October requires only about 44% of the energy for heat and 10% for light. The latter, however, would require an additional storage solution for crops during that period. Furthermore, power consumption for greenhouse lighting depends on many parameters, e.g., geographical location, greenhouse cover light transmission, light source, crop, set points. Comparable results for power demand in greenhouses as in scenario 3 were reported earlier [37,69–71].

The energy demands for the plant factory case in scenario 4 are even higher. This corresponds to an annual electrical power demand of 201 GJ (56.1 MWh) per year. As each plant factory is unique and large differences exist between implementations, e.g., the numbers of layers, layer size, empty spacing (room use efficiency), light source, light set point, etc., the power consumption needs to be compared taking into account the net production area and the net installed and used light capacity. Based on our assumptions, the results obtained are in the same range as reported earlier [72].

The lettuce crop produced in scenario 4 using a full climate-controlled plant factory consumes 95% of the fertilizer of the ICU system when a (nearly) closed system is attained. About 36 m3 water would be additionally needed if the evaporation water is not reused.

As both scenario 3 and 4 have a high energy demand, these solutions are not the best concerning the CO2 footprint. However, to make controlled hydroponic systems more sustainable, regenerative energy could be used more intensively in this culture system, as it is already done for some urban farms [66]. Moreover, due to further advantages, like reduced water consumption and space requirements in comparison to a field cultivation, less transport and utilization of non-used spaces in the city, reliable and stable resource demand and crop yield, higher quality and food safety, there is a great potential for controlled and semi-controlled crop production in an urban scenario. Additionally, local food production is highly advantageous. In particular, it represents an alternative for highly populated mega cities lacking space or for regions lacking agricultural areas.

To stress the full potential of the ICU concept, further extrapolations were performed. The complete fertilizer produced from the biowaste (Ntot in liquid and solid fraction neglecting possible N-losses during further processing like composting and/or immobilization

processes) allows for the cultivation of about 19.5 t lettuce. Interestingly, the 720 L of black water produced by the residents already contain 183.95 kg NH4, which could be converted to 170.15 kg NO3. This amount per year could theoretically allow us to produce 1.1 t/m<sup>2</sup> lettuce in a roof-top greenhouse, and clearly demonstrates the potential of reusing nutrients from the digestate of biowaste and black water to produce food and nourish urban populations.

#### *3.3. Implementation of a ICU Concept for a Building with 100 Residents Saves up to 6468 kg CO2-eq*

Implementation of the ICU project for a building with 100 residents reduced CO2 emission due to reduced transport of 11 tons biowaste by 693 kg CO2. The reuse of NH4 as fertilizer saved 2363 kg CO2 compared to the new synthesis by the Haber–Bosch process. Usage of the produced 25,855 m<sup>3</sup> biogas for heating saved 3412 kg CO2 and is comparable to the CO2 fingerprint for heating reported in literature [73]. In total, the implementation of the ICU concept can save 6468 kg CO2-eq. Based on a CO2 emission price of 25€ per ton CO2, the CO2 saving value is currently 161.7€ (BMU, 2021 [74]) (supplementary Figure S9). Additionally, our concept has the potential to save further CO2. For example, in the case of black water usage, less CO2 is emitted during wastewater treatment [75]. However, further studies are required to accurately incorporate then the CO2 emission for black water tubes and the wastewater treatment plant.

#### *3.4. ICU Concept Becomes Economically Feasible in Large Buildings and with Growing Food Prices*

Necessary for the implementation of the ICU concept is its profitability. Although the numbers for a specific implementation will differ, projection of the costs and benefits enables identifying scaling effects and targets for further optimization. To estimate the economic feasibility, yearly costs and yields of the ICU concept were estimated. Operation of an AD for biogenic waste (Figure 9, Biowaste Anaerobic Digester) costs 9535–9948 € and yields 1615–1727 € annually. In contrast, operation of an AD for blackwater utilization costs 8594–8758 € and yields 2917–3606 € annually. In consequence, the in-house use of biowaste and black water is not profitable for small buildings (Salerno et al., 2017). For large buildings, however, personal costs remain more or less the same and investment costs for larger fermenters rise only slightly. Therefore, implementation is economically feasible for large buildings or agglomeration of several buildings, favoring the ICU implementation in large cities or at the district level.

Generally, black water utilization is economically more promising than biowaste usage under the premise that the saved cost for the wastewater removal compensates for the black water system's cost. Combined usage of black water and biowaste would create the synergy that personal costs for the daily lookup and the co-generation unit can be shared.

The annual cost for a 70 m<sup>2</sup> greenhouse on the roof-top was 69,000 € (scenario 3), and in the basement 70,000 € (scenario 4). The annual benefit of both solutions is 11,418–14,737 €, based on a lettuce price of 0.87 €. Extrapolation showed that the system is profitable with lettuce prices of 1.80 €. This rise is possible when the agricultural space vanishes further, and the population grows. For example, in Singapore, the lettuce price is already 1.00–2.50 €. In contrast to the AD, the hydroponics' economic efficiency grows only slightly with larger systems since it scales quite well.

Whether the hydroponic should be integrated on the roof-top, in the basement, or in rooms without light depends heavily on the price. The top floor's rental price is roughly 30% more expensive. Therefore, it could bring more profit to use the top floor as a penthouse. In a prestigious skyscraper with around 100 m2 of living space, a six-digit amount in major German cities is possible.


**Figure 9.** Summary of cost and yields. Yields are considered from the point of the residents using Table S3. = scenario 3, S4 = scenario 4. 1. pers. communication AAT Abwasser- und Abfalltechnik GmbH, Konrad-Doppelmayr-Str. 17, 6960 Wolfurt, Austria. 2. Regulations for the expansion of renewable energies (Renewable Energies Act—EEG 2021) §43 Fermentation of biowaste (2017). 3. pers. Communication Mr. Huber, GEFOMA GmbH, Germany. 4. pers. Communication Mr. Grantham, Heliospectra AB, Sweden [76]. 5. Waste transport Magdeburg [77]. 6. Jassen Kunststoffzentrum GmbH, Germany [12]. 7. Average price of 4 Discounter 0.87 €.
