The Potential of the Co-Recycling of Secondary Biodegradable Household Resources Including Wild Plants to Close Nutrient and Carbon Cycles in Agriculture in Germany

The aim of this study is to evaluate the potential for conserving natural resources (fossil resources, mineral fertilizer, fertile soil and biodiversity) with alternative circular concepts in order to contribute to the achievement of global sustainability goals. This study examines the potential contribution of substituting conventional products for three alternative circular economy concepts. This includes the household resources black water, kitchen and green waste for the production of design fertilizer, plant charcoal, biopolymers (concept 1) and biogas (concept 2), as well as the combination of household kitchen waste with wild plants for the production of biogas (concept 3). For evaluation, literature values were combined with analyzed parameters of input streams and biogas tests. The production and consumption values determined all relate to the functional unit of a person and year in Germany. Concept 1 has the highest potential for substitution in terms of the amount of recycled products. Co-recycling of organic household waste can account for 20% of NPK (nitrogen, phosphorus, potassium) mineral fertilizer, 19% of plastic consumption and 11% as a soil improving measure in soils in agriculture that are at risk of degradation. Concept 2 has the potential to contribute 12% of the final energy consumption in private households, which is an alternative solution regarding energy recovery due to the extensive practical experience. The joint recycling generates 141 kWh without, and 174 kWh with, fermentable green waste. If 75%, by weight, of fresh wild plants are added to the kitchen waste in concept 3, a wild plant area of 5 m2 is required, which could replace 41% of the biogas corn area, which is concept 3. This mix generates 193 kWh with the potential to reach 78% of corn energy production. The share of wild plants in kitchen waste of 50 or 25% by weight has the potential to achieve 115 or 104% of the corn energy yield, which is a promising concept for rural areas regarding energy recovery from an ecological point of view. The results show a considerable contribution potential of household resources in alternative cycle concepts to increase resource efficiency, and indirectly to diversify the agricultural landscape.


Problem Status and Relevance
The consumption of resources associated with global economic performance has been increasing for years and has already exceeded the natural growth limits, which has a significant negative impact on people and the environment. To take an example, mineral fertilizer causes high energy demand, which is one of the main environment impacts of crop cultivation [1]. In addition, the substitution of phosphorus as a finite resource is also an important issue due to global demand, growing world population, geopolitical dependency and security of food supply [2][3][4]. In this context, the United Nations have formulated Figure 1. Concept 1 of a rural urban nutrient partnership (RUN), as an example for short circular economy. The focus is on the products design fertilizer, plant charcoal and biopolymers [10]. Further-more, biogas is considered (concept 2 and 3) and the resource of wild plants (concept 3) which cannot be found in this figure.

Objectives
The primary aim of this study is to evaluate the theoretical potential to conserve natural resources by using alternative circular concepts in order to contribute to the achievement of global sustainability goals. To this end, the following research question is being investigated: Can alternative concepts close material cycles through the joint use of secondary biodegradable household resources in agriculture or have significant effects on resource efficiency? Figure 1. Concept 1 of a rural urban nutrient partnership (RUN), as an example for short circular economy. The focus is on the products design fertilizer, plant charcoal and biopolymers [10]. Furthermore, biogas is considered (concept 2 and 3) and the resource of wild plants (concept 3) which cannot be found in this figure.

Objectives
The primary aim of this study is to evaluate the theoretical potential to conserve natural resources by using alternative circular concepts in order to contribute to the achievement of global sustainability goals. To this end, the following research question is being investigated: Can alternative concepts close material cycles through the joint use of secondary biodegradable household resources in agriculture or have significant effects on resource efficiency?
There is a lack of investigation of the joint exploitation of household resources for recovering nutrients and carbon [7,11]. In addition, basic research on closing loops is an important aspect to strengthen the focus away from the linear towards the circular economy, among other things to leapfrog outdated concepts with new solutions [3,7,9]. This shows the need to evaluate the theoretical potentials of these waste streams in alternative concepts in order to create a better basis for a further step towards alternative circular concepts.

Scope and Functional Unit
The resources black water, kitchen and green waste and wild plants, and the products potential for biogas, magnesium-ammonium-phosphate (MAP) and complete (NPK) fertilizer, polyhydroxyalkanoates (PHA) for biopolymers and plant charcoal were taken into account for this investigation. All values were transferred to a functional unit of production and consumption per inhabitant and year in Germany (visible as p·a), with these values relating to selected areas of resource consumption, in accordance with the planned scope of the RUN project (representative in concept 1) from Figure 1 (Table 1), which relates to the use in the surrounding agriculture. The potential use of plant charcoal is related to the improvement of soil fertility in arable land in Germany that is at risk of degradation [12]. This reference was chosen because there is no classic substitution product. For concepts 2 and 3, the final energy consumption of private households was used as a parameter in order to fit into the framework of the functional unit of this study.

Scenarios and System Boundaries
The system was modeled from the provision of resources to the potential product gate. For an overview, a flow chart with system boundaries of the different concepts of consideration is illustrated in Figure 2. It shows the used resources of separated and joint exploitation with the abbreviations and resulting products in closed systems. Concept 1 showed the approach of RUN, and concept 2 considered a modified version of it, in order to consider energy generation as an alternative. Concept 3 was integrated to further strengthen ecosystem-based circularity and the direct use in rural areas.

Feedstock Characterization
Kitchen waste can vary widely in its composition, which is why a design kitchen waste was created based on the literature [17][18][19]. The composition is shown in Figure 3, which was collected, created and shredded for all investigations. The shredded green waste was made available by the nearby Wilhelma Zoo, which is responsible for maintaining the landscape. The green waste was sorted manually in woody and easy fermentable parts. Kitchen and green waste were stored until use in the freezer at −18 • C. Black water was taken from vacuum toilets of the Institute of Sanitary Engineering, Water Quality and Solid Waste Management at the University of Stuttgart, which require 1 liter per flush. It was homogenized with the collection and stored in the refrigerator at 4 • C until it was used. Combinations of black water, kitchen and green waste were mixed in quantities that arise on average in households in Germany, with 38:1:0.5 by weight. Wild plants were provided by a farmer from Kißlegg in the Ravensburg district, who cultivates wild plants with the Sustainability 2022, 14, 5277 4 of 18 mixture BG70 from Saatenzeller for the production of biogas. For mixtures with kitchen waste, fresh wild plants with wt-% of 75%, 50% and 25% were taken into account.

Scenarios and System Boundaries
The system was modeled from the provision of resources to the potential product gate. For an overview, a flow chart with system boundaries of the different concepts of consideration is illustrated in Figure 2. It shows the used resources of separated and joint exploitation with the abbreviations and resulting products in closed systems. Concept 1 showed the approach of RUN, and concept 2 considered a modified version of it, in order to consider energy generation as an alternative. Concept 3 was integrated to further strengthen ecosystem-based circularity and the direct use in rural areas.

Feedstock Characterization
Kitchen waste can vary widely in its composition, which is why a design kitchen waste was created based on the literature [17][18][19]. The composition is shown in Figure 3, which was collected, created and shredded for all investigations. The shredded green waste was made available by the nearby Wilhelma Zoo, which is responsible for maintaining the landscape. The green waste was sorted manually in woody and easy fermentable parts. Kitchen and green waste were stored until use in the freezer at −18 °C. Black

Input Parameters and Fermentation Tests
The data were generated and analyzed from September 2019 to December 2020. To calculate the substitution potentials, various parameters were analyzed at the laboratory level. Measured input parameters of the considered resources included the following: dry matter (DM), water content (WC) at 105 °C, volatile solids (VS) at 550 °C, potential of hydrogen (pH) measured with the pH meter, total phosphorus (TP) [20], total nitrogen (TN) [21], potassium (K) and magnesium (Mg) [22] and chemical oxygen demand (COD) [23]. Furthermore, heavy metal contents were analyzed [23]. The analyzes follow the DIN (German standard) which corresponds to the ISO (international standard). Parameters of four different substrates were analyzed, obtaining up to 3 replicates each. The arithmetic mean and the standard deviation were determined.
The biogas potentials of co-fermentation were identified experimentally with the help of fermentation tests. For the fermentation tests, measurements of biogas volume, carbon dioxide (CO2), methane (CH4), temperature and pressure were conducted continuously. The experimental procedure (including the preparatory steps) of the fermentation tests followed the guidelines of the Association of German Engineers (VDI 4630) [24]. The biogas and -methane potential were measured with the yieldmaster system (from the BlueSens gas sensor GmbH; Herten; Germany), which records the biogas volume, temperature, pressure and CH4and CO2-gas formation potential. These data were documented with the BACVis (from the BlueSens gas sensor GmbH) program. In addition, these measurements were reviewed with MilliGascounters (from the Dr.-Ing. RITTER Apparatebau GmbH & Co. KG; Bochum; Germany), which record the gas yield volume mechanically. These values were read five times a week for the same period for at least twenty-five days at an average temperature of 35 °C and recorded manually. For comparability, the time-limited gas yield from day 21 (GB21) was used for all illustrations, which also forms the basis for calculating the substitution potentials [25]. A total of four different substrates and five mixtures were examined in fermentation tests, obtaining up to three replicates each. The arithmetic mean, and the minimum and maximum values were determined.

Input Parameters and Fermentation Tests
The data were generated and analyzed from September 2019 to December 2020. To calculate the substitution potentials, various parameters were analyzed at the laboratory level. Measured input parameters of the considered resources included the following: dry matter (DM), water content (WC) at 105 • C, volatile solids (VS) at 550 • C, potential of hydrogen (pH) measured with the pH meter, total phosphorus (TP) [20], total nitrogen (TN) [21], potassium (K) and magnesium (Mg) [22] and chemical oxygen demand (COD) [23]. Furthermore, heavy metal contents were analyzed [23]. The analyzes follow the DIN (German standard) which corresponds to the ISO (international standard). Parameters of four different substrates were analyzed, obtaining up to 3 replicates each. The arithmetic mean and the standard deviation were determined.
The biogas potentials of co-fermentation were identified experimentally with the help of fermentation tests. For the fermentation tests, measurements of biogas volume, carbon dioxide (CO 2 ), methane (CH 4 ), temperature and pressure were conducted continuously. The experimental procedure (including the preparatory steps) of the fermentation tests followed the guidelines of the Association of German Engineers (VDI 4630) [24]. The biogas and -methane potential were measured with the yieldmaster system (from the BlueSens gas sensor GmbH; Herten; Germany), which records the biogas volume, temperature, pressure and CH 4 -and CO 2 -gas formation potential. These data were documented with the BACVis (from the BlueSens gas sensor GmbH) program. In addition, these measurements were reviewed with MilliGascounters (from the Dr.-Ing. RITTER Apparatebau GmbH & Co. KG; Bochum; Germany), which record the gas yield volume mechanically. These values were read five times a week for the same period for at least twenty-five days at an average temperature of 35 • C and recorded manually. For comparability, the time-limited gas yield from day 21 (GB21) was used for all illustrations, which also forms the basis for calculating the substitution potentials [25]. A total of four different substrates and five mixtures were examined in fermentation tests, obtaining up to three replicates each. The arithmetic mean, and the minimum and maximum values were determined.

Substitution Potential
The substitution potentials were calculated by combining literature values and our own analyses by transferring consumption and production to the functional unit calculated with 83.1 Mio inhabitants in Germany [13][14][15][16]. The biogas potential estimation was based on the fermentation tests; meanwhile, the biogas yield for green waste was determined through using the carbon oxygen demand (COD) [26]. The methane yield was used to calculate the energy potential in kWh·(p·a) −1 . The potential for recycling phosphorus as magnesium-ammonium-phosphate (MAP) was derived from total phosphorus (TP) and the mass potential in combination with the estimate of 50% recovery potential [27]. The potential for nitrogen and magnesium was calculated stoichiometrically from the amount of phosphorus recovered [28]. The estimation for biopolymers orientated on literature calculations and our own investigated COD values. Black water was calculated with 11% acidification and 0.63 g PHA from COD with a usability of 50% [29,30]. The PHA for kitchen and green waste was calculated to be 0.05 g polyhydroxyalkanoates (PHA) per g COD [31]. For mixtures, the values of the substrates were added up. Waste streams vary in their components resulting in different ratios of COD, BOD (biolog-ical oxygen demand) and also VFA (volatile fatty acids), which is important as the COD can only be partially utilized for PHA production. The COD was used for the calculations because the values used take this aspect into account and are based on the same input substrates [29][30][31][32][33]. In concept 1, after acidification, a solid filter cake was created that was separated from the liquid permeate. A separation of the filter cake with a dry matter (DM) content of 50% was presumed, which needed to be dried to a DM content of 85% [34]. From this, 30% plant charcoal from the DM of the filter cake was estimated [35,36]. The value of 10 tones (t) per hectare was used for the application of plant charcoal as a soil improver [37]. For the substitution potential of corn acreage, the energy potential of corn was replaced by that of wild plants and mixtures, which in turn was projected onto the current biogas corn area in Germany of 0.99 Mio hectares [38] For scaling the values on the functional unit, kitchen waste was accounted from the full amount accruing per person and year. From this amount (as 75%, 50% and 25%), wild plants were added on with 25%, 50% and 75%. The substitution of corn was calculated with a cultivation area rounded to 100,000 ha which resulted in 12 m 2 ·(p·a) −1 [39].

Dry Matter Content
The quantity and quality amount of waste accruing per person and year in Germany is listed in Table 2, including combinations with Table 1. Dry matter, water content and volatile solids (Table 2(a)) are confirmed by other studies within their range [26,40]. The dry matter for green waste in the literature is given in between 38 and 68% [26,40], which corresponds with the results of the total amount of green waste. Woody parts were separated from easy fermentable parts, as our samples contained a lot of fresh grass in combination with woody substrates, which explains the low (16%) and high (85%) dry matter content. Hereby, the low dry matter values are in the range of fresh grass cuttings [41,42]. Table 2. Quality (analyzed values) and quantity (calculated per functional unit) of the considered waste streams. Shown are mass balances of (a) various parameters for potential product calculations (b) and heavy metal contents to identify possible disruptive properties in comparison to the limits of the Biowaste Ordinance (noted as limit) (c).

Carbon Oxygen Demand and Nutrients
The literature also shows a wide range of values of the carbon oxygen demand (COD) and the contents of nutrients of these various waste streams examined. Nevertheless, the investigated values of Table 2(b) can be found confirmed within or close to these values [18,26,40,43,45,[49][50][51][52]. A higher nitrogen content in green waste is very likely to result from the separation of woody parts from the easy degradable substrates [26,45]. The highest potential for nutrient recovery can be seen in black water, while the COD has a comparable value for all three waste streams. This indicates an advantageous yield of different products when used together. The difference through the addition of easy fermentable green waste to black water and kitchen waste is small, due to the low amount of fresh matter per household. Nevertheless, an increase in the total organic carbon is visible, which can be derived from the COD content.

Heavy Metals
Compared to the heavy metal contents based on our own analyses, there are clear differences for lead (Pb), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni) and zinc (Zn) compared to the literature values (Table 2(c)). According to the literature, kitchen waste has much higher heavy metal contents, whereby it is noticeable that only zinc and copper have comparable values [26,53]. It can be assumed that this is due to the fact that kitchen waste was designed and not collected in the classic way via waste bins, which excludes, for example, contamination such as foreign matter (such as batteries and metal cutlery) [54]. There could be similar reasons for the differences in the values in green waste, which are also higher in the literature for Pb, Cr and Ni than in these investigations; meanwhile, comparable values were identified for Cd, Cu and Zn [26]. With regard to black water, the values in the literature are lower, which can be attributed to the natural inhomogeneity and fluctuation [40,43]. This is also suitable with findings which show higher heavy metal contents for black water per person [51,55]. Mercury (Hg) was not investigated further here because preliminary research has shown that it is not a relevant variable in this case. Overall, the heavy metal contents in the combinations of black water, kitchen and green waste are below the limit values of the Biowaste Ordinance of Germany [25,56].

Biogas Potential of Household Organic Resources
In order to compare the biogas yield and in orientation to the gas formation rate (GB21), a limited time period of 21 days is shown in Figure 4. The biogas yield for black water is 501, for kitchen waste 600, for mixture one 634 and for mixture two 505 standard liters (l N ) per kg volatile solids (VS). The methane accounts are 69, 67, 68 and 63%, respectively (Figure 4a,b). These values were compared with estimates obtained through the methane calculation with carbon oxygen demand (COD) and the values found in the literature for black water and kitchen waste [8,40,57]. In terms of methane content, the results show the highest biogas potential per kg VS in black water that is fermented with kitchen waste in the proportion 38:1 with 430, followed by pure kitchen waste with 403 and pure black water with 347 that is mono-fermented. The lowest potential of l N methane per kg VS lies in the co-fermentation of black water with kitchen and green waste with 317 ( Figure 4b). In order to stick to the functional unit, the results were converted to standard biogas yield per person and year. This shows that black water, which is fermented together with kitchen and green waste, has the highest potential due to the substrate quantities with 27,854 l N biogas and 17,490 l N methane (CH 4 ) per person and year. This is closely followed by the co-fermentation of black water and kitchen waste with 20,795 l N biogas and 14,112 l N CH 4 per person and year. Kitchen waste with 10,129 l N biogas and 6816 l N CH 4 , and black water with 8764 l N biogas and 6065 l N CH 4 per person and year have a comparatively low and similar biogas potential.

Biogas Potential of Wild Plants and Kitchen Waste
Wild plants were mono-and co-fermented with kitchen waste in three different ratios by weight of fresh matter (FM) (Figure 4c

Biogas Potential of Wild Plants and Kitchen Waste
Wild plants were mono-and co-fermented with kitchen waste in three different ratios by weight of fresh matter (FM) (Figure 4c,d). The highest methane (CH 4 ) yield per kg of volatile solids (VS) was observed for the mixture with 25 wt-% wild plants (M3), which had 660 l N biogas·(kg VS) −1 and 418 l N CH 4 ·(kg VS) −1 . This makes 119% of the corn energy yield, accounted with 350 l N CH 4 ·(kg VS) −1 [42]. This was followed by the mixture with 50% wild plants with 544 l N biogas·(kg VS) −1 and 341 l N CH 4 ·(kg VS) −1 . The potentials of the mixture with 75% wild plants were 464 l N biogas·(kg VS) −1 and 263 l N CH 4 ·(kg VS) −1 . Mono-fermented wild plants had the lowest potential of 320 l N biogas·(kg VS) −1 and 190 l N CH 4 ·(kg oVS) −1 . This strengthens the importance of the optimal combination of co-substrates. The results were converted into kilowatt hours (kWh) per kg of fresh matter (FM). These showed the highest yield for the mixture with 50 wt-% wild plants. The biogas yield was 115 l N ·(kg FM) −1 and 1,15 kWh·(kg FM) −1 . This potential is higher compared to the corn yield [39].

Synergistic Effects on Methane Yield of Black Water, Kitchen Waste and Wild Plants
When considering the methane yield per kg volatile solids (VS), synergistic effects are recognizable based on the mean values. It was 23.3% for black water with kitchen waste, 8.1% for kitchen waste with 75% wild plants, 15% for kitchen waste with 50% wild plants and 19.5% for kitchen waste with 25% wild plants. However, taking into account the maximum and minimum values, this could not be confirmed significantly. According to other studies, the synergistic effect can be greatly enhanced by an optimal combination of proportions [53,[58][59][60][61][62][63][64]. Examples of this are that the co-fermentation could increase the amount of biogas per VS between 25 and 29% [58,59].

Synergistic Effects on Methane Yield of Black Water, Kitchen Waste and Wild Plants
When considering the methane yield per kg volatile solids (VS), synergistic effects are recognizable based on the mean values. It was 23.3% for black water with kitchen waste, 8.1% for kitchen waste with 75% wild plants, 15% for kitchen waste with 50% wild plants and 19.5% for kitchen waste with 25% wild plants. However, taking into account the maximum and minimum values, this could not be confirmed significantly. According to other studies, the synergistic effect can be greatly enhanced by an optimal combination of proportions [53,[58][59][60][61][62][63][64]. Examples of this are that the co-fermentation could increase the amount of biogas per VS between 25 and 29% [58,59].
By using wild plants with these amounts in WM1, WM2 and WM3, the current biogas corn acreage could be reduced by 41%, 14% and 5%, respectively, in order to achieve the same amount of energy.
Moreover, a joint production of fertilizer and energy is worthwhile as the combination increases the energy yield for black water (57% for M1; 66% for M2) as for kitchen waste (52% for M1; 61% for M2).

The Potential for Mineral Phosphorus Fertilizer Substitution
The highest potential for nutrient recovery is visible in black water (Table 3). This potential is only marginally higher in the mixtures, but has slightly more magnesium present, mainly due to the kitchen waste, whereby no statement can be made about the availability. This is an important consideration because if there is not enough magnesium, it will need to be added for the correct stoichiometric ratio for magnesium-ammoniumphosphate (MAP) recovery [28]. Black water alone has insufficient magnesium, as 169% of the inflow would be necessary for the recovery of P. Very high concentrations of nitrogen are available in all waste streams, but in particular in black water, which is also confirmed by other studies [40,50]. The amount shows that the waste streams are important for nutrient recycling, in particular because studies proved that MAP is a safe and efficient fertilizer from black water [2]. The fertilizer recovery potential (Figure 7) is mainly provided by the wastewater flow of black water with 13% of the P-recovery and 15% of the total NPK-recovery. This can be increased slightly through combinations (M1: 15% and 18%; M2: 16% and 20%), but the differences are minimal. Kitchen waste is negligible when it comes to the substitution potential of fertilizers with 1% and 3%, as is green waste fermentable with 1% for P-recovery and 2% for total NPK-recovery. With the addition of kitchen waste, the substitution potential for fertilizer increases by 9% with M1 and 14% with M2. This potential increases with regard to the total recovery of NPK fertilizer to 17% for M1 and 25% for M2. The values of the phosphorus recovery potential are at the lower end of the values given in the literature [18,40,43,50,52]. In addition, the high fertilizer consumption leads to a low substitution potential. Phosphorus has a high recovery potential, especially in black water, as it is for the total recovery of NPK.

The Potential for Plastic Substitution
For the waste steams black water: 8% and kitchen waste: 11%, a relatively high potential for the substitution of plastics consumption in agriculture can be seen, but especially for mixtures with 18% in M1 and 19% in M2 based on consumption and production per person and year in Germany (Figure 7). The potential of the fermentable green waste is negligible with 1%. Table 3 shows that this is also an effect of the amount produced and also proves the negligible difference between the mixtures and the lowest potential of the green waste. The joint production of phosphorus and biopolymers is also worthwhile, as biopolymer production increases for black water (58% for M1 and M2) and for kitchen waste (43% for M1 and M2). The addition of green waste is negligible.

The Potential for Substitution of Soil Improving Measures
For green waste, plant charcoal came into consideration and its combination with the mechanical separation of the mixture of black water and kitchen waste from the liquid part. It was calculated that the amount of green waste per person and year could cover 5%, and the mixture of all household organic resources (M3) 11%, of the economically used arable land threatened by degradation in Germany in order to regenerate soil fertility, create carbon sinks and to replace other soil improving measures. Due to the small proportion and the wide range of possible uses of plant charcoal, this clearly shows that cascading use, for example, with prior use in livestock farming and end use as a soil improver, could be the most sustainable solution [37].

The Potential of Alternative Circular Concepts
All three concepts can contribute to increased resource efficiency, with concept 1 having the greatest potential in terms of quantity (Figures 5-7). Concept 2 is the most mature and widespread in practice and therefore requires the least amount of effort to apply. These practical experiences could also be positively transferred to the implementation of concept 3. Concept 3 integrates the aspect of biodiversity, which makes it the most promising from an ecological point of view ( Figure 6). However, even if this concept improves environmental friendliness, it will not be economical without support and suitable framework conditions. In general, the significant influence of alternative circulatory systems on resource efficiency and environmental friendliness can be confirmed by other studies [7,9].. This is especially true for the receipt of various products [9]. However, the potential is mainly seen in new settlements, growing cities and regions without wastewater and

The Potential for Plastic Substitution
For the waste steams black water: 8% and kitchen waste: 11%, a relatively high potential for the substitution of plastics consumption in agriculture can be seen, but especially for mixtures with 18% in M1 and 19% in M2 based on consumption and production per person and year in Germany (Figure 7). The potential of the fermentable green waste is negligible with 1%. Table 3 shows that this is also an effect of the amount produced and also proves the negligible difference between the mixtures and the lowest potential of the green waste. The joint production of phosphorus and biopolymers is also worthwhile, as biopolymer production increases for black water (58% for M1 and M2) and for kitchen waste (43% for M1 and M2). The addition of green waste is negligible.

The Potential for Substitution of Soil Improving Measures
For green waste, plant charcoal came into consideration and its combination with the mechanical separation of the mixture of black water and kitchen waste from the liquid part. It was calculated that the amount of green waste per person and year could cover 5%, and the mixture of all household organic resources (M3) 11%, of the economically used arable land threatened by degradation in Germany in order to regenerate soil fertility, create carbon sinks and to replace other soil improving measures. Due to the small proportion and the wide range of possible uses of plant charcoal, this clearly shows that cascading use, for example, with prior use in livestock farming and end use as a soil improver, could be the most sustainable solution [37].

The Potential of Alternative Circular Concepts
All three concepts can contribute to increased resource efficiency, with concept 1 having the greatest potential in terms of quantity (Figures 5-7). Concept 2 is the most mature and widespread in practice and therefore requires the least amount of effort to apply. These practical experiences could also be positively transferred to the implementation of concept 3. Concept 3 integrates the aspect of biodiversity, which makes it the most promising from an ecological point of view ( Figure 6). However, even if this concept improves environmental friendliness, it will not be economical without support and suitable framework conditions. In general, the significant influence of alternative circulatory systems on resource efficiency and environmental friendliness can be confirmed by other studies [7,9]. This is especially true for the receipt of various products [9]. However, the potential is mainly seen in new settlements, growing cities and regions without wastewater and organic waste manage-ment, but not in regions that already have existing solutions with a low impact on the environment [7,60,66].

Conclusions and Outlook
In view of the results, the considerable potential for increasing resource efficiency and the share of renewable energies, alternative circular systems with joint use of resources make sense in order to achieve the Sustainable Development Goals. In addition, the results of the substitution potential show that consumption and production are very unbalanced. The focus on the reuse of secondary resources should be increased in order to minimize this gap and move closer to a sustainable model such as organic farming.
With regard to the concepts considered in this study, concept 1 shows the highest quantitative substitution potential for recycling, including the design fertilizer with 4.3 kg per person and year (p·a), biopolymers with 2.5 kg·(p·a) −1 and biochar with 21 kg·(p·a) −1 co-produced from the mixture of organic household resources, taking into account black water 38 kg·(p·a) −1 , kitchen 1 kg·(p·a) −1 and green waste 0.5 kg·(p·a) −1 . The biogas production of concept 2 has the potential to produce 174 kWh·(p·a) −1 from the same mix of resources. This is an alternative for energy recovery because it is a mature and widely used technology. This practical experience could be used for concept 3, which shows the potential of co-fermentation of kitchen waste (75 wt-%) with wild plants (25 wt-%) with 4 kWh per volatile solids in biogas plants. When using 100% kitchen waste and adding 75 wt-% of wild plants, 193 kWh·(p·a) −1 could be generated. Due to the same amount of energy, 41% of the currently 0.99 Mio hectares of biogas maize cultivation area could be replaced by wild plants. Concept 3 is a promising concept for rural areas regarding energy recovery from an ecological point of view due to the primary use of secondary organic resources and the additional diversification of the agricultural landscapes.
Overall, the results showed that waste streams have the potential to subsidize a noteworthy proportion of conventional products, which underlines the importance of promoting alternative, sustainable and environmentally friendly recycling concepts and the need to think in terms of alternative concepts. This applies in particular to new settlements and areas without a sewage and waste treatment system, but also to regions with increased capacity requirements and non-environmentally friendly concepts.

Further Research
This study is an excerpt and cannot stand on its own. Other aspects must also be taken into account for implementation, such as socio-economic ones. Therefore, this study should be supported by life cycle assessments, for example. Countries of the global south should be integrated, which could skip our outdated concepts for alternative, more ecosystemfriendly circular concepts and address several goals of the UN Sustainable Development Goals. In addition, pilot projects involving stakeholders are very important to scale the results and improve the applicability of the new concepts.