3.5.3. In the Framework of the Paris Agreements, Government Promote a CO2-Society

In the Paris Agreement, 195 states including the European Union agreed to limit global warming below 2 ◦C [81]. Therefore, netto CO2 emission has to be reduced to zero by 2040. In addition to the targets for the energy and building sector, new goals and measures are also being set for agriculture. In Germany, 8% (72 million tons of CO2-eq) of the greenhouse gas emission came from agriculture in 2014. Despite the fact that of most agricultural greenhouse gas emissions are caused by natural physiological processes, the ability to reduce them is limited. The largest source with about 25 million tons of CO2-eq is the use of N fertilizers. The use of organic nutrients considered in the ICU project, in particular the soiling®-products, are one important step towards minimization of the N use. The main goal until 2030 is to significantly reduce the emissions of mineralic fertilizers in agriculture. One option is to use financing instruments under the Common Agricultural Policy. Another option is to increase the percentage of land used for organic farming by circular economy approaches [82].

3.5.4. ICU Implementations must Fulfil High Legal Standards Favoring Large Projects or Tiny Ones for Personal Need

The implementation of ICU projects requires observing the laws of the state. In the following, the most critical regulations for implementation ICU concepts in Germany are exemplarily addressed. One important factor is to fulfil the guidelines for building security (BauGB §29–§38, in particular admissibility of projects §34, BauNVO). Additionally, urban development plans and urban planning law must be complied with. Biogas production using AD for biowaste and black water is critical since biogas is flammable and requires sufficient ventilation. Therefore, the storage of large biogas volumes should be avoided. The produced biogas should be consumed immediately, upgraded to natural gas, fed in the gas grid, or outsourced from the buildings [83]. Nevertheless, fire prevention (§§ 3 and 14 MBO) and explosion control (DGUV Regel 113-001, [84]) must be considered. For the removal of black water digestate and the solid fraction of the biowaste fermentation, the laws for sludge disposal have to be considered (AbfKlärV, [85]). Utilization of the biowaste as fertilizer requires compliance with the German laws for biowaste (Bioabfallverordnung

(BioAbfV)) and fertilizer ordinance (DüMV, [28]) as well as the EU regulations for fertilizer ordinance (EU-FPR). In particular, base materials must be allowed (DüMV, Supplementary 2, Table S7, [28]). The fertilizer has to be listed in a positive list (DüMV, Supplementary 1, Table S1, [28]) or equals any allowed fertilizer type. Furthermore, emission limits (DüMV, supplementary 2, Table S1 [28]) and minimum hygiene requirements (§ 5 DüMV, [28]) have to be fulfilled. Exception exist in the case the biowaste and the produced fertilizer are only used for personal needs. However, it is questionable if biowaste utilization in a cooperative of more than 100 residents accounts as a personal need. Due to the high legal requirements, implementation of ICU concepts seems only manageable for large projects or tiny implementations for personal needs (Supplementary File 3). Another question is liability, which is difficult to address in general and usually depends on the specific case. Therefore, no general recommendation can be given here, except to address this issue in a contract between the stakeholders (Supplementary file 6).

#### 3.5.5. Communication and Participation Are Important for the Acceptance of Residents

Implementation of ICU projects require the participation of the residents. Residents have to separate the biowaste accurately, agree to install vacuum toilets and use the urban gardens either as gardeners or as consumers. In general, there is a high acceptance in Germany to waste separation [86], vacuum toilets [87], and urban gardening [79]. However, it is always useful to integrate all stakeholders as early as possible to successfully implement projects [88] and, in addition, to guide their participation by teaching material.

Furthermore, it is recommended to communicate potential risks [89]. For example, ICU operation has the risk of microbial contamination of the food collected. Pre-treatment of biomass at 70 ◦C can ensure the inactivation of harmful microorganisms. Another issue is the produced biogas, which is explosive. However, when immediately consumed, the risk is reduced to the level of a conventual gas heater.

An important cultural aspect is the utilization of black water as fertilizer. Theoretically, animal dung or manure usage and spreading it on fields are quite similar to the use of black water digestate for hydroponics. However, this is neither allowed nor accepted [90].

#### *3.6. Strategies to Extend the ICU Concept*

The ultimate goal of the ICU project is to close energy and material flow cycles in an urbane building. Additional components could increase yields and productivity and allow for a more robust operation.

For example, hydroponic modules for the balcony could be added, or food production can be elevated by aquaponic [91] or algae cultivation [92]. For an implementation, strategies to combine agriculture with photovoltaic could also be implemented [93,94]. For hot and dry areas, the ICU concept could be extended to include the water cycle [95]. For example, greywater can be reused [1] or rainwater could be used for adiabatic cooling. In order to achieve a more robust operation, a module for cleaning the biowaste, for example, via conveyor belts, can be added [96]. Additionally, storage capacities for biowaste, biogas, or fertilizers can be added. However, additional stores in the building are expensive and increase the fire load.

#### *3.7. Overall Discussion of the ICU Project*

Overall, we develop a concept for closed urbane biomass circulation and illuminated its feasibility. The final calculation for material flows and potential outcomes for our concept was straightforward. In particular, modeling frameworks such as SIMBA#Biogas for the biogas process and a greenhouse simulator [33,34] for the hydroponic enabled to compare several possibilities. However, selecting the best scenarios was challenging due to the high number of possibilities. Furthermore, besides technical and economic reasons, we recognized that social–cultural reasons such as laws and liability are essential for implementing such concepts.

In consequence of these observations, we can give three main steps to consider for decision-makers aiming to implement biomass circulation. The first step is to define criteria and their weighting (e.g., resource efficiency, economics, CO2-emission, or social–cultural aspects). The second is to select the best technologies for their use case, depending on the climate, culture, resources, and infrastructure. Based on findings 1 and 2, the third step is to compare the different possibilities, as we have done for scenarios 1 to 4.

This will require comprehensive databases that collect the relevant information for biomass circulation (e.g., as available for LCA) [97] as well as models to calculate the best solution for each setting.

One main limitation of our concept is the high investment costs, making it only economically beneficial for large professional implementations. Another one is that our concept could be easily applied to new buildings or districts, but upgrading existing buildings is challenging.

In the future, our concept and evaluations could be improved by more advanced models, more precise parameters, scenarios, or novel technologies. However, even more important is to validate our outcome on a prototype and identify potential challenges during the implementation.

#### **4. Conclusions**

Our study proposes a concept for closed urbane biomass circulation in its entirety and comprehensively analyze its feasibility, which was not done before. Integrating biomass cycles into residential buildings, as proposed by the ICU concept, is technically feasible, reduces CO2 emission, and is of interest to owners of urban buildings and their residents. It is profitable for implementation in large buildings or agglomeration of buildings and in case food prices further increase. However, to achieve this goal, it will require the implementation of prototypes to perfect technical details and to confirm economic and material calculations. Major challenges for the implementation come from legal aspects relate to the biowaste prescription (the German BioAbfV) and the fertilizer prescription (the German DüMV). In sum, the results of this study should bring us one step closer to a reduction in land use and to a sustainable, CO2-neutral society.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/su13179505/s1, Figure S1: file 2,3,5,7,8, Table S1: file 1,4,6.

**Author Contributions:** B.I. and M.I. provided estimates for quantification of the soiling®-process and compared the conversion of NH3 to NO3 with oxygen feedCalculations for in-house hydroponic calculations were made by S.K. and O.K. Legal aspects for the ICU-concept were processed by A.B. and R.B. LCA-analyze was performed by M.W. Simba#Biogas simulations were created by N.M. and I.S. Data evaluation and preparation of the manuscript was conducted by N.M., R.H., S.K. and J.U., U.R., D.B., J.W. and G.S. contributed with valuable advice and by editing the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors acknowledge the financial support by a funding program of the Fachagentur Nachwachsende Rohstoffe e.V. (FNR, FPNR). We additionally thank the GEFOMA GmbH for the estimation of costs of greenhouse construction. J.U. received funding from the EU-program ERDF (European Regional Development Fund) of the German Federal State Saxony Anhalt by Research Center of Dynamic Systems (CDS).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** Jassen Kunststoff GmbH was one of the co-authors of this feasibility study. B.I and M.I. byJassen Kunststoff GmbH have a commercial interest in selling soiling®-modules. Both B.I. and M.I. confirm that they have carried out their evaluation to the best of their knowledge and judgment. All other authors declare that they have no competing interests.
