Potential of Nature-Based Nutrient Cycles
Nutrient Recovery Efficiencies
Table 2 below presents the nutrients discharged per person as well as by the 1.92 million population of Vienna  to the urban sewers via raw wastewater, as well as additionally via compostable household waste (municipal organic waste—MOW), every year. Table 2 also presents the nutrients that are contained in the recovered TW effluent and therefore could be applied to agricultural fields as fertigation water, as well as the nutrients contained in the biogas digestate. These values are based on 50% liquid-solid separation (LSS) efficiency. Finally, Table 2 presents the conversion efficiencies from N, P, and K input to nutrients recovered in nutrient solutions, i.e. contained in fertigation water and digestate fertilizer applicable to fields.
With the majority of nutrients contained in raw wastewater and TW effluent (in contrast to MOW, solids in wastewater, and recovered digestate), the removal rates of TW strongly determine the amount of nutrients that could be recovered. The removal rates for N (35%) and P (20%) used in the present study are based on Dotro et al. . However, nutrient removal rates reported in recent literature vary a lot. For example, Tanner et al.  report 8.9 to 49.0% TN removal and 39.0 to >55.0% TP removal. Morari and Giardini  report even 91.0% TN removal and 72.5% TP removal in a pilot-scale vertical subsurface flow TW. Tables S1A and S1B (see supplementary material) provide an overview of nutrient removal efficiencies of TW treating municipal wastewater found in literature. The overview includes TW at scales from 1 to 6,080m2 in different countries worldwide.
The studies presented in Tables S1A and S1B (see supplementary material) focus on subsurface flow (SSF) TW. The authors consider SSF TW to be well suited for densely settled areas due to their lower area requirement, minimum odor development, and provision of a barrier between the wastewater and the surroundings through the subsurface flow, i.e. mitigating contact of humans or animals with wastewater, in contrast to free water surface (FWS) wetland systems.
The mentioned calculated values for nutrient conversion from MOW to recovered digestate correspond well to the nutrient content of digestate from co-digestion of MOW and municipal sewage sludge analyzed by Sogn et al. . This suggests that the method applied in the present study is suitable to quantify recoverable nutrients from MOW and solids separated from municipal sewage processed by anaerobic digestion.
While nutrients are preserved to a greater degree in biogas digesters, the recovery of nutrients by a TW brings the added benefits of (urban) greening as well as the supply of fertigation water, which can easily be applied to fields especially for vegetable production, which is commonly irrigated even in temperate climates such as in Austria. Digestate may need to be hygienized before application to food crops. There are several options to hygienize the digestate, such as sludge composting, a sludge treatment wetland or other TW designs depending on the solids content, or dry fermentation. Another option is to simply extend the retention time in the digester so that no additional hygienization step should be necessary if the digestion process works properly.
In the present study, it was assumed that the population of Vienna spends 100% of the day (24 h) within the system, in this case the city, and therefore, the total amount of wastewater produced by them enters the calculation. When quantifying the urban nutrient budget for a city or another urban scale (e.g., district, quarter, building), the number of people and the share of the day spent within the boundaries of the city could be further differentiated. For example, Jönsson et al.  suggest dividing the 24-h day into 60% active (awake) time out of a day spent at work and 40% spent at home. This could be multiplied by the people residing in the area of scope, and those commuting in and out of the city (or district, quarter, or building).
The conversion efficiencies above refer to the process from raw wastewater and MOW to recovered nutrient solutions. But looking at the full cycle, the efficiency of the recovery and reuse system is impacted by the actual utilization by crops and the valorization as food. As mentioned in “Materials and Methods,” if the N:P:K ratio of the nutrient solution added to soil as fertigation water or fertilizer corresponds to that of the specific crop nutrient demand and crops take up their net nutrient requirements from the soil, the nutrient balance in the soil is assumed to remain the same, and 100% of added nutrients are used. However, if the N:P:K ratio of the recovered nutrient solution does not correspond to the N:P:K ratio of specific crop net nutrient demand and NPK are added to fully cover the requirements of each N, P, and K, there will be excess application of the nutrients that are available to a larger proportion. This results in losses (unutilized nutrients) and lower conversion efficiencies from raw “waste” input to the food products.
Linking Recoverable NPK to Crop Nutrient Demand
To supply the entire vegetable production within Vienna, per year, total 212.04 t N, 26.4 t P, and 199.6 t K must be added as fertilizer (net nutrient demand). The wastewater and MOW produced by 77,250 people, metabolized by the described system, could supply total 212.05 t N, 39.9 t P, and 59.0 t K as recovered fertigation water (TW effluent) and liquid fertilizer (biogas digestate) per year. This would cover the N demand and exceed the amount of P needed for the whole actual vegetable production in Vienna.
Around one-third of vegetables consumed in Vienna is produced within the city boundaries (Stadtentwicklung Wien, n.d.). Therefore, if the vegetable consumption patterns by product groups are similar to production, 231,750 people could supply all the N and P needed to produce all vegetables consumed in Vienna. Nutrients that could be recovered if the entire population of Vienna would be connected to described nature-based treatment trains amount to 25 times the N, 37 times the P, and 7 times the K needed to sufficiently supply the net nutrient requirements of the approximated production of all vegetables consumed by Vienna’s population. The significant excess nutrients recovered from wastewater and MOW could supply the production of other food groups (fruit, grain) and feed and industrial crops to fully close the resource loop within the city and between the city and its agricultural hinterland.
Figure 1 illustrates the nutrient flows from consumers through the treatment steps to agricultural production and via food back to the consumers. The flows represent N (beige arrows), P (blue arrows), and K (orange arrows) that is emitted via wastewater and MOW by the 1.9 million population of Vienna. Non-edible food parts are not included in the diagram, because they are considered to be left on the field and become a source of nutrients to the crops, or they travel back to the consumer as part of marketable vegetables and then become kitchen waste, re-entering the recovery and reuse system.
Figure 1 highlights that, while municipal organic waste is widely processed to compost and even biogas, municipal wastewater (the flow from consumers to liquid solid separation) is a much more significant untapped resource. Further, only a small fraction of the recovered nutrients are needed to cover the peri-urban Viennese vegetable production, which, as mentioned, covers one-third of the entire vegetable consumption in the city. The excess nutrients could be recovered and returned to other agricultural production within and beyond the city’s borders. Even if nutrients from the entire (non-industrial) urban wastewater and MOW are recovered and returned to food production, there is a smaller flow returning to consumers than is emitted by consumers. This gap could be filled by recovering nutrients from landscaping waste and livestock manure, which are already widely processed by composting and anaerobic digestion. Figure 1 also showcases that K is contained in the liquid wastewater fraction and recovered fully via the TW.
Figure 2 below showcases that the N that could be recovered from municipal wastewater and MOW produced by 77,250 people could cover the net N-fertilizer demand of the entire vegetable production of Vienna; recoverable P even exceeds the demand, while K must be added from another source. Consistent with previous research, availability of K is relatively low in human excreta (e.g., ), and consistently in overall treated municipal wastewater (e.g., ). K could be supplemented with sustainable and cheap K sources such as biomass ash or livestock manure. K in ash is soluble and can be extracted by washing ash. Even though livestock manure adds not only K, but also N and P, the share of K content is so high that the addition of livestock manure easily fills (and can exceed) the K gap [1, 6]. Livestock manure could be co-digested with solids and landscaping waste.
Figure 2 also displays the shares of nutrients supplied by TW effluent and digestate, considering a 50% LSS (i.e., 50% of solids are retained, the rest join the liquid fraction). As shown, the TW effluent supplies most of the N (75%), P (58%), and K (70%). Compared to 70% LSS, these ratios hardly change, with the TW effluent supplying 71% N, 70% K, and 49% P, i.e., biogas digestate supplies just over half of recovered P (51%) with 70% LSS.
Adaptation of Nutrient Sources to Product Groups to Optimize the Recovery and Reuse Efficiency
As mentioned above, the plant macronutrients N, P, and K are allocated to the three vegetable product groups in proportion to their actual share of total vegetable production in Vienna, namely 49% fruity vegetables (product group 1), 44% cabbage, leafy, and pole vegetables (product group 2), and 7% pulses, root, and bulb vegetables (product group 3) (Stadt Wien, n.d.). This distribution could be adapted to specific crop production of any defined community, city, region, or country.
N:P ratio of nutrient demand for Viennese vegetable production is 8:1, which is the same as the N:P ratio of the TW effluent, if LSS is 70%. Therefore, for the described combination of crops, the TW effluent is a suitable nutrient source. The N:P ratio of recovered nutrient solutions (TW effluent and digestate combined) is 5:1 and the digestate has an N:P ratio of 3:1, due to higher relative P-levels. The N:P ratios change with a lower LSS. In the case of 50% LSS, the N:P ratio in the TW effluent sinks to 1:7, which still corresponds more closely to the N:P ratio of crop nutrient demand of the vegetables produced in Vienna than the combination of TW effluent and digestate or digestate alone. This suggests that the net N and P fertilizer requirements of the vegetable crop mix produced in Vienna are best met by using the TW effluent, or by combining TW effluent and digestate as nutrient sources.
The N:P:K ratio required to meet Viennese vegetable production is 8:1:8, while the N:P:K ratio of TW effluent is 8:1:2, for 70% LSS. This reflects the shortage of K noted above. A look at the specific nutrient distributions to the solid and liquid fraction of municipal wastewater reveals that there is no K in the solid fraction. The K in recovered digestate comes from the MOW co-digested with the solid wastewater fraction. If digestate is to be used as fertilizer, the solid wastewater fraction should be co-digested with MOW to gain K, or liquids and solids co-digested, which however eliminates the benefits of functional greening and ecosystem services that TW can provide.
This knowledge and present model can inform the selection of sources to match the crop nutrient demand (considering N:P or N:P:K ratios), and thereby also to maximize the use efficiency of recoverable nutrients.
Figure 3 below shows the area that could be sufficiently supplied with nutrients from fertigation water and digestate, if either only product group 1 (fruity vegetables), group 2 (cabbage, leafy, and pole vegetables), or group 3 (pulses, root, and bulb vegetables) are produced. The variety of areas across the product groups and nutrients reflects the different crop nutrient requirements of NPK per m2 as well as NPK availability in recovered fertigation water and digestate fertilizer. For example, average product group 1 vegetables demand higher K input per field area than product groups 2 and 3. Therefore, K limits the field area that could be fertigated so that crops obtain sufficient NPK. Conversely, the area of fields cultivating group 3 products that could be fertigated with available K is much higher than for groups 1 and 2. Groups 2 and 3 vegetables require similar P per area. For the scenario that only fruity vegetables are produced and K is supplemented, N represents the limiting value and an area of 1,140 ha could be sufficiently supplied with recovered nutrients. If only pulses, root, or bulb vegetables are produced, with a selection of group 3 vegetables representing the average nutrient demand, and K is supplemented, then again N represents the limiting value and 1,933 ha could be supplied.
Meanwhile, K could relatively easily be added by adding organic waste-based pot-ash, which is generally considered sustainable and cost-efficient. If N:P:K ratios in recovered nutrient supply sources and specific crop field demand are considered, crop selection and/or dimensioning of the system could be adapted to N:P:K ratios in the fertigation water (TW effluent), or fertilizer (digestate) in order to maximize the production achieved from applying only recovered nutrients, and/or to maximize the replacement of synthetic fertilizer. The calculations for this study were conducted with average net nutrient requirements of different crop species within each product group (1–3). When applying this to a specific field case, the specific crop nutrient requirements could be considered and secondary nutrient input sources tapped adequately.
As mentioned, matching the N:P:K ratio of recovered macronutrient solutions with the N:P:K ratios of crop net nutrient requirements would minimize the losses and increase the conversion and utilization efficiencies. The optimization is beyond the scope of this paper, but in practice, this could be done by comparing the net nutrient requirements of specific cultivated crops with the recoverable nutrient sources and adapt the mix of nutrient sources (household wastewater, MOW, green clippings, livestock wastewater, other wastewaters), sustainable and waste-based specific nutrient supplements, treatment technologies and specific designs, or even selected crop mix. How much discrepancy between NPK ratios results in lower quantity or quality of production should be looked at in further detail.
From the yearly 89,702,400 m3 of municipal wastewater produced by the total population of Vienna, a volume of 57,028,263 m3 of fertigation water could be recovered as TW effluent, if accessing it during 8 months per year. This includes water recovered from the municipal wastewater and precipitation captured by the TW. For greenhouse cultivation, the average annual precipitation must be supplemented, which results in an average annual irrigation water demand of 880 L/m2. Considering the recommended irrigation of outdoor-cultivated field vegetables and mentioned compensation of precipitation in greenhouse-cultivated vegetables (see “Materials and Methods” above), the total vegetable production in Vienna requires 8,787,527 m3 irrigation water per year, cultivated on an area of 1,235 ha. This is equivalent to recovering the wastewater produced by 151,600 people. The secondary fertigation water that could be produced by treating and reusing the wastewater generated by the total population of Vienna could cover the irrigation demand of 15,643 ha of vegetable field area, if the shares of production in greenhouses and open fields are maintained. This is equivalent to the 13-fold actual field area of vegetable production within the boundaries of Vienna.
The potential irrigable area reduces with greater greenhouse production and increases if the share of open field production is larger, due to compensating precipitation in greenhouse irrigation.
Area Requirements and Integration into the Urban Infrastructure
Table 1 below shows the nutrients that could be recovered from the wastewater and MOW produced by a five-person household, the potential vegetable yield resulting from these secondary nutrients, and the area needed for the TW and the field area that could be supplied with nutrients considering the vegetable product group proportions produced in Vienna. A single household could supply a field area of 938 m2 and would require a TW of approximately 20m2, considering the 4m2 TW area per PE set out in the Austrian standard for TW  (Table 3).
Connecting the 77,250 people, thus treating the amount of wastewater to fully supply Viennese vegetable production with recovered nutrients, an area of 31 ha is needed. To treat the wastewater produced by the whole Viennese population, 768 ha would be needed.
As mentioned above, the specific TW area of 4m2 per PE is set out in the Austrian standard for TW . However, it does not reflect more recent technical advances that enable smaller TW size per PE while fulfilling legal effluent water quality requirements. Thus, the 4m2 are a conservative estimation, and this area could be reduced by implementing available enhanced NBS with design improvements which enable sizing of less than 4m2 per PE. Such design improvements can include, for example, multi-stage systems, using different substrates or intensification through aeration and/or recirculation. TW size could also be reduced if wastewater reuse is the goal instead of treatment for disposal, and therefore nutrients could be preserved instead of removed. Though in this case, firstly, the removal of pollutants that may threaten human health must still be ensured, and secondly, the effluent water quality must always comply with national standards or laws for water reuse (e.g., the Regulation (EU) 2020/741 on minimum requirements for water reuse for agricultural irrigation) or for wastewater treatment for disposal.
TW can also be designed and installed as green façade panels or stacked units, with basins stacked above each other, thus reducing the actual ground-space needed.
Moreover, the vast available unused or underutilized open spaces in cities as well as rooftops could be used for functional urban greening providing not only the increasingly necessary climate services in cities (evaporative cooling, biodiverse habitats), but also treat building wastewater for on-site recovery and reuse for urban farming. Designing green urban infrastructures with wastewater treatment abilities would also reduce the need for synthetic fertilizer and drinking water which existing urban greening, in particular intensive systems, currently relies on.
Finally, the size of the TW could be reduced if the goal is not to remove, but to metabolize and even preserve nutrient content from wastewater in the nutrient solutions recovered as effluents. However, it must be noted that reclaimed water may not contain nutrients only, but could contain a wide range of contaminants as a result of our lifestyle (e.g., pharmaceutical residues, biocides, disinfectants). Therefore, when designing a recovery and reuse system, not only the nutrient mass balance but also health and safety issues must be considered before any such system could be put in place. Several studies have found that TWs can partly remove organic micropollutants, in some cases more effectively than conventional WWTPs [25, 54]. Therefore, TWs have been installed to treat wastewater from pharmaceuticals and cosmetics industry  besides other municipal and industrial effluents. To ensure food safety when irrigating crops with reclaimed water treated by NBS, further research could investigate the design requirements of technologies and the combination within the treatment system to effectively remove organic micropollutants and other potentially harmful substances such as microplastics and heavy metals. Additional treatment steps that do not remove nutrients could be included, such as ozonation.
At the building or household scale in cities, there is limited space to utilize the nutrients, public infrastructure is largely not designed for this, nor can economies of scale be used. However, the ecosystem services of distributed NBS, the efficiency gains of direct cycles, and the supported maintenance through localized ownership could be exploited. At the neighborhood, district, or communal scale, limited economies of scale can be use, and local space for farming and composting may be available. At the urban centralized scale, space for large-scale treatment may be available at the outskirts and distributed treatment units across the city could be implemented. Collection systems are already widely in place, as the management and regulatory frameworks to implement resource recovery and redistribution for reuse with centralized organization. Economies of scale allow high-tech control systems. However, if resources are recovered centrally, potential inefficiencies may be incurred by longer distances across which resources must be transported for treatment and redistribution to the point of reuse.
Instead of fully replacing the centralized, conventional wastewater treatment systems in large cities with NBS, various different types of TW could be integrated into the existing urban built infrastructure, in particular in new buildings or new urban development areas. In particular in peri-urban areas, which are located close to agricultural production, the proposed nature-based resource recovery and reuse system could be implemented to close water and nutrient cycles locally and contribute to a circular economy, a more climate-friendly food system and resilient city.