Scholarly article on topic 'Harvest to harvest: Recovering nutrients with New Sanitation systems for reuse in Urban Agriculture'

Harvest to harvest: Recovering nutrients with New Sanitation systems for reuse in Urban Agriculture Academic research paper on "Environmental engineering"

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Abstract of research paper on Environmental engineering, author of scientific article — Rosanne C. Wielemaker, Jan Weijma, Grietje Zeeman

Abstract To maintain the city as a viable concept for human dwelling in the long term, a circular metabolism needs to be adopted that relies on recovering, reusing and recycling resources, in which output (‘waste’) from one metabolic urban conversion equals input for another. Urban Agriculture (UA) and source-separation-based New Sanitation (NS) are gaining momentum as measures for improved urban resource management. UA aims to localize food provisioning while NS aims to reorganize wastewater and organic waste management to recover valuable and crucial resources. The objective of this paper is to assess the match between the supply by NS systems and the demand from UA for nitrogen, phosphorus and organic matter, in terms of quantity and quality, to foster a circular metabolism. The research is contextualized in the city of Rotterdam. The methodology used is based on the Urban Harvest Approach (UHA), developed previously for the urban water cycle. Novel to this research is adapting the UHA to nitrogen, phosphorus and organic matter loads for two practiced UA typologies (ground-based and rooftop) and four NS concepts for the treatment of domestic urine, feces and organic kitchen waste. Results show that demand for nutrients and organic matter from UA can be minimized by 65–85% and a self-sufficiency of 100% for phosphorus can be achieved, while partial self-sufficiency for nitrogen and organic matter. This research reveals that integration of NS and UA maximizes urban self-sufficiency.

Academic research paper on topic "Harvest to harvest: Recovering nutrients with New Sanitation systems for reuse in Urban Agriculture"

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ELSEVIER

Contents lists available at ScienceDirect

Resources, Conservation and Recycling

journal homepage: www.elsevier.com/locate/resconrec

Full length article

Harvest to harvest: Recovering nutrients with New Sanitation systems for reuse in Urban Agriculture

Rosanne C. Wielemakera*, Jan Weijmaa b, Grietje Zeemanab

a Sub-department of Environmental Technology, Wageningen University & Research, PO Box 17, 6700AA Wageningen, The Netherlands b Lettinga Associates Foundation (LeAF), Wageningen, The Netherlands

ARTICLE INFO

Article history:

Received 25 February 2016

Received in revised form 5 September 2016

Accepted 8 September 2016

Available online xxx

Keywords: Urban Agriculture New Sanitation Urban metabolism Urban Harvest Approach Nutrients Organic matter

ABSTRACT

To maintain the city as a viable concept for human dwelling in the long term, a circular metabolism needs to be adopted that relies on recovering, reusing and recycling resources, in which output ('waste') from one metabolic urban conversion equals input for another. Urban Agriculture (UA) and source-separation-based New Sanitation (NS) are gaining momentum as measures for improved urban resource management. UA aims to localize food provisioning while NS aims to reorganize wastewater and organic waste management to recover valuable and crucial resources. The objective of this paper is to assess the match between the supply by NS systems and the demand from UA for nitrogen, phosphorus and organic matter, in terms of quantity and quality, to foster a circular metabolism. The research is contextualized in the city of Rotterdam. The methodology used is based on the Urban Harvest Approach (UHA), developed previously for the urban water cycle. Novel to this research is adapting the UHA to nitrogen, phosphorus and organic matter loads for two practiced UA typologies (ground-based and rooftop) and four NS concepts for the treatment of domestic urine, feces and organic kitchen waste. Results show that demand for nutrients and organic matter from UAcan be minimized by 65-85% and a self-sufficiency of 100% for phosphorus can be achieved, while partial self-sufficiency for nitrogen and organic matter. This research reveals that integration of NS and UA maximizes urban self-sufficiency.

© 2016 Published by Elsevier B.V.

1. Introduction

Cities depend on regional and global hinterlands for the supply of water, energy, nutrients and materials and for the disposal of wastes (Agudelo-Vera et al., 2012b; Brunner, 2007; Hodson et al., 2012; Kennedy et al., 2007), deeming cities hotspots for resource conversion. This conversion presently follows a linear metabolism from high quality resource inputs and low quality waste outputs (Fig. 1a). Few resources are currently recovered for reuse. This linear metabolism leads to two major challenges: first, cities' high rate of consumption puts stress on resource availability (e.g. phosphorus, fossil fuels), and second, the disposal of vast amounts of waste causes pollution (e.g. water and resource contamination, biodiversity loss, deforestation, and pollution in air, water and land). Cities currently import large quantities of food not only from their hinterlands, but also from locations across the globe. At the same time, they produce low or even negative value waste loads containing disposed and excreted nutrients. These are often mixed and col-

lected via large-scale engineered infrastructures that endorse this linear tendency and make it difficult to effectively recover resources (Balkema et al., 2002; Hodson et al., 2012). With more than half of the world's population currently residing in cities, this linear tendency is further intensified (United Nations, 2014).

As hotspots of resource conversion, however, cities also present an excellent opportunity to adopt a high-impact circular metabolism, in which output ('waste') from one process equals input ('resource') for another. As opposed to the current linear urban metabolism, a circular urban metabolism aims to recover and reuse (recycle) resources within or between urban functions to reduce both the external input of virgin resources and the output of waste (Agudelo-Vera et al., 2012b) (Fig. 1b). To move towards a circular urban metabolism, resource input-output flows of urban functions need to be identified, described and matched in terms of quantity and quality. New Sanitation and Urban Agriculture are currently gaining global interest individually as measures to improve urban resource management (Degaardt, 2003; Metson and Bennett, 2015; Mougeot, 2006; Vernay et al., 2010). Linking these two urban functions could lead to mutual benefit in terms of resource cycling, especially for fertilizers.

* Corresponding author. E-mail address: rosanne.wielemaker@wur.nl (R.C. Wielemaker).

http://dx.doi.org/10.1016/j.resconrec.2016.09.015 0921-3449/© 2016 Published by Elsevier B.V.

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Fig. 1. a) A linear metabolism of inputs and outputs. b) A circular metabolism reuses, recycles and recovers resources from urban waste streams, reducing resource inputs and outputs.

1.1. Urban Agriculture

Urban Agriculture (UA) is the local production of food within (peri-)urban areas, which in addition fosters education, employment, community building and/or closing organic resource cycles (Mougeot, 2000; Smit et al., 2001). UA involves intensive cultivation/breeding methods that yield a diverse selection of flora and fauna, and integrates it with the local urban economic, social and ecological systems; thus, UA assimilates a plurality of activities, locations, scales, purposes and engagement. Exemplary of this variety, UA can include low-tech and high-tech production systems, such as community gardens, rooftop farming, indoor controlled environment agriculture, and animal husbandry.

1.2. New Sanitation

Sanitation is the promotion of hygiene via the management and treatment of wastes, and includes both the physical and organizational structure (Brikke and Bredero, 2003; Mihelcic et al., 2011). New Sanitation (NS) is a new paradigm for the collection, transport, treatment, and recovery of solid waste and wastewater (e.g. urine deviated vacuum toilets, anaerobic digesters, struvite (Mg(NH3)PO4) precipitation) with the aim to recover resources (i.e. water, nutrients, organic matter), increase efficiency, reduce energy costs, and/or offer solutions to waste management (Kujawa-Roeleveld and Zeeman, 2006; Lens et al., 2001; Maurer et al., 2012; Zeeman, 2012). NS systems minimize transport and are therefore locally-oriented systems (source, recovery and reuse are in close proximity) and the technical design serves this aim. The design varies with the local context but often includes source separation of waste and wastewater streams, collecting organic kitchen waste, black water (urine and feces), grey water (shower/bath, sink, laundry, dish washer) and/or urine separately. Depending on the types of streams separated and the local context, NS concepts can be configured for treatment and recovery to achieve reuse or discharge parameters. The respective recovery and removal efficiencies of the sanitation technologies determine the quantity of nutrients that can be harvested and the quality of the product for human and environmental hygiene.

1.3. Linking Urban Agriculture and New Sanitation

Re-establishing a partnership between agriculture and sanitation is not a new phenomenon. Various studies have looked at the possible cycling between sanitation and crop production including: wastewater reuse/irrigation for crop production (Beuchler et al., 2006; Smit and Nasr, 1992; Strauss, 2001), treatment, recovery and reuse of fertilizers from wastewater (Jenkins, 2005; Lens et al.,

2001; Mihelcic et al., 2011; Tervahauta et al., 2013; Tidáker et al., 2006), reuse of urine (Maurer et al., 2003, 2006), bioavailability of recovered products to crops (Jonsson et al., 2004; Oenema et al., 2012), guidelines on urine and feces reuse in agriculture to ensure safe handling (Heinonen-Tanski and van Wijk-Sijbesma, 2005; Jonsson et al., 2004), risks of micro-pollutants, pathogens and heavy metals (Heinonen-Tanski and van Wijk-Sijbesma, 2005; Tervahauta, 2014; Winker et al., 2009), policymaking for resource recovery (van der Hoek et al., 2016) and the link between UA and sanitation systems as an economic and food security measure in developing countries (Cofie et al., 2013; Kone, 2010; Streiffeler, 2001).

The feasibility, however, to match input and output flows between UA and NS systems at the urban scale is not known. To start, data on the quantity and quality of the input demands from UA systems is lacking, as UA is very diverse in practice and for the most part unregulated (Belevi and Baumgartner, 2003; Martellozzo et al., 2014). This diversity results in varied fertilization practices and therefore requires that UA typologies be clearly defined to identify respective input and output flows. Second, although data on the quantity and quality of the products produced by NS systems has, and continues to be, researched, the extent of their reuse potential in UA is uncertain (e.g. plant availability, nutrient ratios, pathogen and micro-pollutant contamination) (Lens et al., 2001; Tervahauta et al., 2013; Zeeman and Kujawa-Roeleveld, 2011).

1.4. Scope of research and research objectives

The scope of this research focuses on the recovery of nitrogen (N), phosphorus (P) and organic matter (OM) from domestic wastewater and organic kitchen waste to determine the extent to which these resources can cover the demand from UA, in Rotterdam, the Netherlands (population 620,000) (Gemeente Rotterdam, 2013). The reason for this focus is threefold. First is the global concern regarding resource depletion and environmental pollution due to current consumption and disposal trends of nutrients, N and P, and OM (Carter, 2002; Cordell and White, 2011; Galloway et al., 2004). Second is the increased regional interest in the Netherlands for the professionalization of UA and the recovery of resources from waste streams (Green Deal Stadsgerichte Landbouw, 2013). Third is Rotterdam's interest in improving local resource management and implementing UA (Cityportal Rotterdam, 2014; Gemeente Rotterdam, 2012). In fact, Rotterdam currently houses a few leading UA initiatives in the Netherlands, including: Uit Je Eigen Stad, Rotterdamse Munt, Rotterzwam, and De DakAkker.

The objective of this study is to model combined UA and NS systems to evaluate the degree to which N, P and OM input-output flows can be matched and quantify the degree of self-sufficiency.

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Fig. 2. Schematic of the management strategies of the UHA adapted to show nutrient flows between UA and NS (nutrient losses are not shown).

This will be done in three steps: a) select and characterize relevant UA typologies and quantify the demand of nutrients and organic matter for each selected typology, b) select the NS technologies (proven at lab and pilot scale) most appropriate for the recovery nutrients from residual waste streams and quantify the harvested nutrients and organic matter, c) quantify the extent to which the demand for nutrients and organic matter from UA can be met by recovered nutrients and organic matter from the selected NS systems.

2. Methodology

2.1. Methodological framework: urban harvest approach

The methodology used in this research is an adaptation of the Urban Harvest Approach (UHA) developed at the Sub-department of Environmental Technology (ETE) at Wageningen University & Research (Agudelo-Vera et al., 2012a; Leusbrock et al., 2015). It has been most extensively applied to the urban water cycle to improve urban resource management towards self-sufficiency starting with a baseline assessment and applying three management strategies: demand minimization, output minimization (by resource cascading, recycling and recovery), and multi-sourcing (harvesting local primary and secondary resources) (Agudelo-Vera et al., 2012b). Multi-sourcing will not be included in this research as there are few renewable sources of N, P and OM (e.g. nitrogen fixing cover crops).

These strategies are shown in Fig. 2 as applied in this research. The designed systems are evaluated using the two indices developed by Agudelo-Vera et al. (2012a,b), including: Demand Minimization Index (DMI) and Self-Sufficiency Index (SSI).

2.1.1. Strategy 0) baseline demand

The baseline assessment describes the existing situation, including demand inventory and current technologies. Here the baseline identifies the quantity and type of nutrient input demand for UA, and the output of nutrient flows from domestic sanitation waste flows.

The baseline assessment was conducted for two selected UA typologies: ground-based Urban Agriculture (ground-based UA) and rooftop Urban Agriculture (rooftop UA). These were selected because both typologies can be found in Rotterdam, which served as reference case studies for this research. Ground-based UA grows edible plants at ground level in soil (e.g. commercial or community farms, permaculture farms and forest gardening). Rooftop UA involves cultivating crops on the rooftops of urban buildings, usually flat roofs that are most suited to carry additional weight (between 60 and 150kg/m2). This typology can cultivate plants in soil or in a soil-like substrate.

The nutrient baseline demand was calculated for each typology (kg/ha) from interviews with individual urban farmers and the respective records they had on the practiced fertilization regime. This demand was compared to fertilizer regulations for conventional agriculture in the Netherlands, and values for equilibrium fertilization (plant uptake). The conventional norms and the equilibrium fertilization values were averaged from 22 different types of horticultural crops,1 to reflect the diversity of crops grown at the UA typologies (Fink et al., 1999; Rijksoverheid, 2014a,b). Equilibrium fertilization reflects the nutrients a plant takes up, or the nutrients contained in the total harvested fresh matter (harvest residues and

1 Dwarf bean, broccoli, Brussel sprouts, carrot, cauliflower, celery root, Chinese cabbage, cucumber, fennel, iceberg lettuce, kale, kohlrabi, leek, lettuce, onion, radic-chio, radish, red beet, red cabbage, savoy cabbage, spinach, white cabbage.

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Table 1

Mean compositions of urine, feces, black water and organic kitchen waste calculated based on European data as reported in literature, including respective standard deviations (Daigger, 2009; Friedler et al., 2013; Kujawa-Roeleveld and Zeeman, 2006; Magid et al., 2006; Tervahauta et al., 2013).

Parameter unit Urine s.d. Feces s.d. Kitchen waste s.d. Total

Volume L/p/d 1.3 0.12 0.13 0.06 0.2 - 1.63

COD g/p/d 12.5 1.91 47.9 12.23 59 - 119.4

TN g/p/d 10.2 1.10 1.4 0.38 1.4 0.52 13

TP g/p/d 1.1 0.34 0.5 0.05 0.2 0.06 1.8

COD = chemical oxygen demand, TN = total nitrogen, TP = total phosphorus.

marketable yield) assuming an optimal yield per hectare (Fink et al., 1999). A further distinction was made between total N and P and available N and P. Available N and P values take into account availability of organically-bound nutrients (slow release) as advised by Dutch fertilization regulations. According to set coefficients ('werk-ingscoëfficient') only a percentage of the N in organic fertilizers counts toward the regulatory norms. For instance, the N coefficient is 10% for compost and 30-60% for manure, depending on liquid or solid composition (Rijksoverheid, 2014b). Total P counts towards the norm with the exception of compost, for which only 50% counts The baseline assessment for the supply of nutrients first includes an overview of the current waste and wastewater treatment in Rotterdam. Second, the baseline supply from domestic sanitation was calculated per waste stream by using mean compositions (Table 1) of urine, feces, black water and organic kitchen waste generation per person as recorded in literature (Daigger, 2009; Friedler et al., 2013; Kujawa-Roeleveld and Zeeman, 2006; Magid et al., 2006; Tervahauta et al., 2013).

2.1.2. Strategy 1) demand minimization

The Demand Minimization Index (DMI) describes the change in demand in reference to the baseline demand. Baseline demand (Do) reflects the current resource demand (status quo) from UA and the minimized demand (D) describes the demand adjusted to reflect equilibrium fertilization values. A DMI of 0 indicates that no demand minimization has taken place. The DMI is calculated using Eq.(1).

Baseline demand (Do) -Minimized demand (D) Baseline demand (Do )

Demand minimization reduces the demand for nutrients via the implementation of new technologies or via changes in human behavior. For N, P and OM, a change in farming technologies or fertilizer regimes can reduce the initial demand. For this research, the minimized demand was based on equilibrium fertilization. The equilibrium values were used to assume an ideal scenario (zero waste) in which the fertilization regime reflects the amount of nutrients that crops take up, and not more. The baseline demand was used when these values were below the equilibrium values. The ratio of slow release vs. quick release fertilizer for the minimized demand was assumed to be the same as for the baseline demand. OM was minimized to reflect the suggested compost load per hectare in literature of 15,000 kg of compost, with a maximum of 3,000kg0M/ha, or the baseline demand if below 3000kg0M/ha (Goed boeren in kleinschalig landschap, 2011).

2.1.3. Strategy 2) output minimization

This strategy minimizes outputs via three strategies: cascading (direct use of outputs for a purpose with lower quality demand), recycling (the reuse of a resource flow after a quality upgrade, which generally costs energy) and/or recovery (the extraction of valuable resources from waste streams) from the outputs. Cascading will not be used because primary and/or secondary treatment

of human excreta is needed to secure the removal of pathogens (Jönsson et al., 2004).

For the recovery of nutrients, urine, feces and organic kitchen waste are the most promising streams since they have the highest loads of N, P, and OM (de Haan and van Geel, 2013). Feces and organic kitchen waste contain most organic matter, suitable for making compost and soil conditioners, while urine contains the largest fraction of N and P. Therefore, urine, feces, black water (BW) and organic kitchen waste (KW) were considered for recovery, whereas greywater (GW) was not considered.

Four NS concepts (Fig. 3) were selected based on systems demonstrated on lab and pilot scale.The sanitation system installed in Sneek, the Netherlands for source-separated BW, was used as a starting point for Concept 1, and variations upon that system were configured for Concepts 2-4, further separating urine, feces, and/or organic kitchen waste with respective treatment systems (Tervahauta et al., 2013; Waterschoon, 2011). Concept 1 includes source-separation of BW combined with KW (via a grinder). The BW and KW are both treated anaerobically in an UASB (up-flow anaerobic sludge blanket) reactor, followed by an OLAND (oxygen limited anaerobic nitrification denitrification) reactor and a struvite precipitation reactor. Concept 2 includes the same treatment steps as Concept 1, although with separate collection of KW for composting (Dekker et al., 2010; Eklind and Kirchmann, 2000; Fricke and Vogtmann, 1994; Hargreaves et al., 2008). Concept 3 is similar to Concept 1 with the exception ofurine, which is collected separately and stored (Jönsson et al., 1998, 2004; Maurer et al., 2006). Concept 4 separates KW for composting and urine for either (a) storage or (b) struvite precipitation. Feces are not considered in Concept 4 for recovery of nutrients. Treatment systems for GW and for byproduct effluents from the technologies were not further quantified, and are therefore not shown in Fig. 3.

In Concepts 3 and 4, urine is separated at source via a urine-diverting toilet using 0.2L of water per flush. In Concept 3 and 4a urine is stored and in Concept 4b urine undergoes struvite precipitation. In Concept 3, struvite precipitation was not considered for the separated urine because the treatment stream of the feces and KW already includes a struvite precipitation step.

2.1.4. Sanitation technologies, removal efficiencies and harvested products

The collection system for each concept depends on the separated waste streams. In Concepts 1 and 2 vacuum toilets are used with 1L of flush water. In Concepts 3 and 4 urine-diverting vacuum toilets are used for collection with 0.2L of water per flush. In urine-diverting toilets, it is assumed that the urine separation efficiency is 75%, whereas 25% joins the feces stream (Larsen and Lienert, 2007; Tervahauta et al., 2013). With regards to KW, it is assumed that 100% of the KW per household is collected via a kitchen grinder in Concepts 1 and 3, where KW is digested together with feces streams. In Concepts 2 and 4 KW is collected separately and composted.

De Graaff et al. (2010) studied the fate of nutrients and organic matter in the anaerobic treatment of black water using a UASB reactor with a short HRT at 25 °C. Data for recovery and removal efficiencies from de Graaf et al. was used here for further calcula-

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Fig. 3. New Sanitation concepts (adapted fromTervahautaet al., 2013). Arrows indicate nutrient flows. (For clarity of the figures, nutrient losses are not indicated, see Section 2.1.4).

tions. The COD in the UASB reactor undergoes anaerobic biological decomposition reaching a methanization level of 54%, 10L CH4/p/d can be produced from black water. Of the remaining COD, 19% is found in the sludge and 27% remains in the effluent stream of the reactor (De Graaff et al., 2010). The sludge from the UASB is thermally hygienized to deactivate pathogens (Capizzi-Banas et al., 2004). The OM of the sludge is calculated using a fixed COD to OM

ratio of 1.4 (Zeeman and Gerbens, 2002). The available N from the UASB sludge is assumed to be the same percentage as what is available from sewage sludge identified by the Dutch fertilizer policies ("Mestbeleid: werkingscoefficient voor stikstof"). The available P is assumed to be 50%, similar to compost, a comparable stabilized organic sludge. The removal efficiencies for the UASB, OLAND and Struvite reactors used in Concepts 1,2, and 3 are provided inTable 2.

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Table 2

Removal efficiencies for Concept 1, 2 and 3.

Removal Efficiencies (%)

Parameter UASB OLAND Struvite reactor

COD 73a 53b -

BOD5 73a 53b -

TN lb 73b 16-c

TP 33b - 96b

a (deGraaff, 2010).

b (Tervahauta et al., 2013; Wilsenach et al., 2007). c Calculated per concept based on the molar ratio of N:P of 1:1.

Kirchmann, 2000). The vegetable, fruit, and yard waste (VFY) produced is 0.338 kg/p/d. The composition of the VFY can be calculated using the percentages of dry matter (DM) (40.6%) and organic matter (65.3% of DM) (van Haeff, 2012). The total available N and P from the compost is calculated using the "werkingscoefficient" identified by the Dutch fertilizer policies (10% ofN is available and 50% of P).

Increased self-sufficiency is achieved by reusing output as an input, (partially) covering the input demand. The Self-Sufficiency Index (SSI) was used as a measure for the extent to which the recovered nutrients from NS systems fulfill the demand from UA. The SSI is defined by: the resources reused (Rr) against the minimized demand (D). The SSI is calculated using Eq. (2).

Losses occur in the UASB (OM is methanized), in the OLAND reactor (release NO2-, NO3- and N2) and in the effluent of the struvite reactor (84% ofN of the influent). Precipitation of struvite (magnesium ammonium phosphate) from UASB effluent (Concept 1, 2, and 3) and from urine (Concept 4) conveys two nutrients, N and P, in solid form at a molar ratio of 1:1 (Maurer et al., 2006). However, urine contains ammonium and phosphate in a ratio of 20:1, meaning that only about 3% of the nitrogen can be recovered as struvite (Maurer et al., 2006). The rest of the nitrogen remains in the effluent.

Urine is assumed to be collected via a well-sealed collection system and storage tank to prevent loss as gaseous NH3 (Jonsson et al., 2004; Maurer et al., 2003). The nitrogen loss during collection is 0.02 kg NH3/yr for 1000 inhabitants, which is considered negligible. Urine storage recovers the largest amount of N from wastewater compared to the other treatment steps. It is assumed that urine is stored for >6 months for hygienization and conserves 100% of the nutrients that are present in the fresh urine. During storage the urea hydrolyzes, increasing the pH and ammonium concentration, and precipitating struvite and calcium phosphate. The amount of struvite and calcium phosphate precipitated, both slow release fertilizers, is small and depends on the storage time. These are therefore not considered in further calculations and stored urine is assumed to be a quick release fertilizer. The stored urine is rich in N and P, and also contains some OM. In this research, the OM found in stored urine is ineffective because it degrades quickly (~73%) in the first year (Kuntke, 2013), and therefore we do not take it into account in the OM balance.

Composting in Concept 2 and 4 is achieved in an open static pile composting system that allows for the regulation of temperature, humidity and pH by forcing air through the compost (Gomez, 1998). Source separated KW is nitrogen-rich (N ratio of 13:1) accounting for substantial gaseous N losses, (55%) (Eklind and

SSI = Resource reused (Rr) ^ Minimized demand (D)

3. Results

3.1. Baseline demand and demand minimization

3.1.1. Baseline demand

The baseline demands for both ground-based UA and rooftop UA reflect the fertilizer regime followed by urban farms of respective typologies in Rotterdam. For ground-based UA, this fertilization regime included the use of both slow release (15%) and quick release (85%) fertilizers distributed in a compost mixture, chicken manure, and an organic liquid fertilizer. The baseline demand for rooftop UA is based on the fertilization regime of a rooftop farm that uses a growing substrate low in organic matter, to decrease its weight, to adhere to the 180kg/m2 capacity of the roof. Therefore no compost is added for fertilization. Only slow release granulates (100% slow release) and no quick release (0%) fertilizers are used.

Fig. 4 compares the baseline demand with the norms and regulations for N and P use in conventional agriculture in the Netherlands and with equilibrium fertilization values. This figure shows that the baseline demand for N for both ground-based UA and rooftop UA lies well below the equilibrium fertilization value. For both UA typologies, the baseline demand for P, however, exceeds the conventional norms, meaning that over-fertilization of P is occurring. For ground-based UA, the baseline demand for P exceeds the conventional norms by a factor three and the equilibrium fertilization values by a factor seven. The amount of P over-fertilization that occurs in both typologies is wasteful and demands attention considering that P is a finite resource.

Fig. 4. Comparison of nutrient demand from ground-based and rooftop UAto conventional norms and equilibrium fertilization. Where "conventional norms" are the average N and P use norms and regulations (clay and sandy soils) in the Netherlands (Rijksoverheid, 2014a, 2014b), and "equilibrium fertilization" reflects the nutrients that crops take up, averaged for 22 vegetable crops (Fink et al., 1999).

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Table 3

Annual baseline nutrient demand and minimized demand for ground-based UA and rooftop UA.

units Available Nd Available Pc Organic matterd

Ground-based UA

Baseline Demand3 (Do) kg/ha 109 96 7861

Minimized Demand (D) kg/ha 109 14 2685

DMI % 0 85 66

Rooftop UA

Baseline Demandb (Do) kg/ha 113 41 1743

Minimized Demand (D) kg/ha 113 14 1743

DMI % 0 65 0

a Table on fertilizer advice (Van lerssel, 2013).

b Technische Fiche ECO-MIX 1 (DCM Nederland BV, 2014) and Organische Gedroogde Koemest (Humuforte, 2014).

c Nutrient values for N and P are usually expressed by weight of N and P2O5. P is 44% of the P2O5 value. N is expressed as elemental N. Both N and P are calculated using the "werkingscoefficient" for compost and animal manure. Available N is defined as 10% in compost and 55% from chicken manure. Available P is 50% in compost with a maximum of 3.5 g P2O5/kg dry matter of compost. d OM=32% of dry matter, Samenstelling en werking van organische meststoffen (de Haan and van Geel, 2013).

3.1.2. Minimized demand

The baseline demand was minimized (Table 3) to reflect a maximum value equivalent to that of equilibrium fertilization. For ground-based UA the N demand does not need to be minimized (DMI= 0%), while the demand for P and OM is minimized, with respective DMI values (Eq. (1)) of 85% and 66%. For rooftop UAthe DMI for N and OM is 0%, while the DMI for P is 65%.

3.2. Baseline supply from waste and wastewater

Rotterdam, with an area of 319.35 km2, has a population of approximately 620,000 people (Gemeente Rotterdam, 2013). The city produces a total of 76,000 tons of household organic solid waste; however, most of this organic solid waste is collected together with municipal solid waste and incinerated for the generation of energy. A small fraction, 1% of household VFY waste, is collected separately at source, composted and sold via a third party to the agricultural sector. The city's wastewater is treated at wastewater treatment plants by the Waterschap Hollandse Delta and Hoogheemraadschap Schielanden en Krimpenerwaard. Using Table 1 the loads of the nutrients can be calculated for the whole population of Rotterdam. Total household BW and KW generated daily represent a load of 1356 kg P and 316,850 kgN and 88,764 kg OM per day.

3.3. Output minimization

The demand for N, P and OM from each UA typology was compared with the supply generated by each NS concept. In total ten combinations were evaluated for the degree of self-sufficiency achieved using the self-sufficiency index (SSI) (Eq. (2)). The combinations aim at a SSI of 100% for P (as the most critical nutrient in terms of global scarcity and EU policies), both slow release and quick release; this determines the number of people (waste producers) needed per NS concept per ha of UA to provide that self-sufficiency, as well as the respective reuse of the harvested N and OM. Fig. 5 (ground-based UA) and Fig. 6 (rooftop UA) show the mass flows of the harvested N, P and OM per concept and the respective self-sufficiency achieved for each for 1 ha of UA. The deficits of resources, which would need to be imported or added to the UA system, and the excess nutrients harvested, resources which can be exported or traded outside of the system, are also shown, as well as the number of people needed per concept to achieve the indicated SSI.

Both the SSI and the number of persons needed to provide that SSI is relevant for the evaluation of the combined systems. While a high SSI is preferable for the sourcing of local resources, both the type of nutrient demand (slow vs quick release), and the removal

and recovery efficiencies of the NS technologies also determine the potential to implement the NS concepts. The higher the recovery rate, the lower the number of people needed for each concept.

The combinations of ground-based UA with NS (Concepts 3 and 4a) provide a SSI of 100% for both slow and quick release P. Concept 4a, however, requires 10 times as many persons/ha to obtain this SSI, which is a possible barrier for the separate collection of VFY waste in densely (high rise) populated areas of Rotterdam. Concepts 1 and 2 fail to supply the demand for quick release N and P and seem less preferable.

In this research, rooftop UA does not have a demand for quick release fertilizer, and therefore the SSI for both quick release N and P is not applicable. The harvested quick release N and P in Concepts 3 and 4a are considered excess nutrient harvests. For all combinations, except with Concept 4a, the SSI for slow release P was set to 100%, resulting in low SSI values for both slow release N and OM. In Concept 4a setting the SSI for P to 100% would result in a SSI for OM of 263%. To prevent over-fertilization of OM, the SSI for OM was set to 100% instead. The combination of rooftop UA with Concept 4b results in the highest combined SSI for N and P, followed by Concept 2.

4. Discussion

The UHA offers a step-by-step methodology to gain insight into the opportunities that lie in integrating UA and NS, however, its application to N, P and OM input-output flows presented challenges at each step of the methodology.

4.1. Baseline demand

There are very few reliable empirical studies that quantify the demand from UA for nutrient inputs, as well as harvestable yield. In this study, the baseline N, P and OM demand from UA was based on two existing UA sites in Rotterdam. As these likely are not representative for fertilizer regimes of all UA initiatives within the studied typologies, more data is needed on nutrient demand to gain a broader view on the potential to couple UA to NS. For example, whereas rooftop UA in this study did not have a demand for quick release N and P, other rooftop UA initiatives might use quick release fertilizers.

In this research, both fertilization regimes showed strong over-fertilization of P, a consequence of various possible factors including: lack of farmers' education and training on fertilization, the lack of regulations for fertilizer use in UA, the reuse of farm waste (i.e. chicken manure), and fertilizer use based on nitrogen limitation. Considering, however, that conventional agriculture in the Netherlands is heavily regulated in their N and P use to reduce

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Fig. 5. Nitrogen, phosphorus and organic matter mass flows (kg/ha) between New Sanitation concepts and ground-based Urban Agriculture (1 ha) with respective achieved self-sufficiency (%) for organic matter, slow and quick release N, and slow and quick release P. The self-sufficiency for P is set to 100%, determining the number of people needed per concept.

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Fig. 6. Nitrogen, phosphorus and organic matter mass flows (kg/ha) between New Sanitation concepts and rooftop Urban Agriculture (1 ha) with respective achieved self-sufficiency (%) for organic matter, slow and quick release N, and slow and quick release P. The self-sufficiency for P is set to 100%, determining the number of people needed per concept.

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pollution of water resources, and that P is a finite resource of increasing scarcity, UA fertilization regimes should also take measures to prevent over-fertilization. This study demonstrates the need for regulations for UA, especially as UA continues to grow, taking into account the wide range of UA typologies. The fertilization regime also has consequences on the nutrient loads discharged to the urban water cycle, such as the increase of nutrient loads to the sewer system via rooftop UA, especially after heavy rainfall. Therefore, expanding UA across cities has various implications for urban resource cycles and water treatment for which management systems need to developed

4.2. Demand minimization

Minimizing the demand for N, P and OM from UA is achieved by assuming equilibrium fertilization values and avoiding over-fertilization. This is a novel perspective for the application of nutrients in UA, although further research is needed to identify the optimal fertilization regime for each UA typology, considering that nutrients mineralize in the soil and runoff may occur. Especially the monitoring, collecting and sharing of data from UA (pilot) studies are needed in this respect. In addition, technological innovations (i.e. injection fertilization at the plant base as opposed to sprinkler systems) for the administration of fertilizers to minimize the demand were not considered in this research. Such measures, detailed by Schröder et al. (2011), could help farmers administer fertilizers where and when the plant needs them, reduce losses, and thereby minimize the demand.

4.3. Output minimization

The results of applying the output minimization strategies to N, P and OM flows between UA and NS are determinedly context specific; these are dependent on the results of the baseline demand and the demand minimization, specific to the two reference initiatives in Rotterdam, and the specific NS treatment systems selected, with their respective removal and recovery efficiencies. The main challenge in matching the input and output flows was accounting for the difference in N:P:OM ratios. While the demand from UA has one ratio of N:P:OM and a ratio of slow release to quick release fertilizer, the supply from the NS concepts has different ratios of N:P:OM and of slow release to quick release fertilizers. This difference means that 100% self-sufficiency for all three resources, simultaneously, could never be achieved; there would always be a shortage or excess.

To address this challenge, a SSI of 100% for P was assumed, which determined the respective SSI for N and OM achieved. Setting N or OM to 100% self-sufficiency would mean over-fertilizing in P per hectare. The ratio of slow release to quick release also influenced the matching of the demand and the supply, especially for ground-based UA. The characterization of the demand was context specific, based on the two reference initiatives in Rotterdam, and could very well be configured differently. This brings to question whether a difference between slow and quick release fertilizer should even be accounted for or that total available (effective in the first year after application) N, P and OM would be a better approach. This again would change the ratios of N:P:OM, as well as the SSI for each.

The reuse of harvested products from wastewater in UA in this research prioritized the cycling of P, a finite and scare resource, over N and OM. However, other criteria and indicators could also be considered for selecting the best combination of NS concept and UA typology, and prioritizing the different harvested products. Criteria could include soil type and health, transport distance, storage requirements, availability of alternatives, costs, etc.

4.4. Self-sufficiency

Combining UA and NS offers the possibility to increase urban self-sufficiency. The city of Rotterdam can fertilize the 2363 ha of available arable land and the 906 ha of rooftop area suitable for UA (available hactares were calculated in a study carried out at the municipality of Rotterdam). With a population of 620,000 people (Gemeente Rotterdam, 2013) and assuming a marketable yield per hectare of 45,000 kg/ha,2 one hectare can supply the daily-recommended vegetable consumption (200 g/p/d) to circa 620 people, or 1010 ha for the entire city. For the recommended consumption of 400 g/p/d of fruits and vegetables, one hectare can supply fruits and vegetables for circa 310 people and 2020 ha for the entire city (Gezondheidsraad, 2015).

5. Conclusion and outlook

The UHA offers a methodology through which to reconsider urban resource flows through three management strategies: demand minimization, output minimization and multi-sourcing. Novel to this research is the application of the UHA on urban nutrient flows, showing preliminary results for future research in the domain of harvesting phosphorus, nitrogen and organic matter from waste for reuse in urban food production. The application of this methodology in different contexts, including low-income countries, could offer new insight on opportunities for nutrient recovery and reuse. The results presented here are context specific and show that partial self-sufficiency can be reached. However, many uncertainties still remain when determining the extent to which UA and NS can be integrated; future research needs to address remaining knowledge gaps of technical, operational and economic feasibility.

Research on safety measures and technical feasibility studies for reuse of harvested products as fertilizers are needed to make sure that reuse does not impose risks to humans and the environment. This especially concerns the presence of heavy metals, micro-pollutants, pharmaceuticals and pathogens in the harvested products, which currently represent a barrier for reuse. In the Netherlands, the use of sewage sludge in agriculture is restricted because of the heavy metal content. Tervahauta et al. (2014), however, show that only Cu and Zn in black water sludge are high compared to Dutch standards and that these metals mainly originate from food intake. Therefore, Tervahauta et al. conclude that sludge from black water should be allowed as a fertilizer, to complete a circular metabolism of metals (2014).

Micro-pollutants, pharmaceuticals, hormones and pathogens found in wastewater continue to be researched to determine the implications of the reuse of recovered products from human waste (Decrey et al., 2011; de Wilt et al., 2016; Escher et al., 2006; Ronteltap et al., 2007; Uysal et al., 2010). Measures for removal of contaminants need to be developed, and/or risk reduction measures need to be implemented through handling and reuse protocols. Since January 2015, the Dutch fertilizer regulations have permitted the use of struvite, falling under the category of 'recovered phosphate', to be used as a fertilizer in the Netherlands, as long as the recovered struvite complies with heavy metal, pathogen and micro-pollutant guidelines (van der Grinten et al., 2015). Reuse of stored urine and sewage sludge as fertilizers are currently not permitted.

The operational feasibility of combined UA and NS systems requires the evaluation of these systems in higher resolution, taking

2 Equal to the national yield for conventional agriculture (based on conventional farming yields in the Netherlands for 'vegetables and melons' for 2013 as reported by FAOSTAT) with a reduction of 20% (organic yield gap) (FAOSTAT, 2013).

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into account spatial and temporal conditions, including, seasonal patterns, storage, and infrastructure capacities. While humans produce waste and wastewater year-round, cultivation and the use of fertilizer only takes place during certain seasons. To account for this temporal disparity, appropriate storage tanks or the export of fertilizers (including to indoor farming and greenhouses) are needed, which also have respective spatial implications, let alone the logistics.

While the Netherlands is interested in closing resource cycles and moving towards a circular metabolism, the marketability of recovered products, especially phosphorus, is limited due to the overabundance of animal manure in the country (van der Grinten et al., 2015). However, for UA, the reuse of struvite and other odorless products within cities could be a promising alternative to animal manure and synthetic fertilizers containing mined P. Finally, the social perception of the reuse of human waste in UA is another barrier that needs to be relieved to secure a future for recovered products.

Acknowledgements

The authors gratefully acknowledge dr.ir. Ingo Leusbrock for his critical review of the manuscript and valuable input, as well as, dr.ir. Frank de Ruijter for his advice on plant fertilizer requirements at the beginning stages of the research.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016Zj.resconrec.2016. 09.015.

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