Opportunities for Building-Scale Urine Diversion and Challenges for

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Opportunities for Building-Scale Urine Diversion and Challenges for Implementation Published as part of the Accounts of Chemical Research special issue “Water for Two Worlds: Urban and Rural Communities”. Treavor H. Boyer* and Daniella Saetta

Acc. Chem. Res. Downloaded from pubs.acs.org by IDAHO STATE UNIV on 03/25/19. For personal use only.

School of Sustainable Engineering and the Built Environment (SSEBE), Arizona State University, PO Box 873005, Tempe, Arizona 85287-3005, United States CONSPECTUS: Urine diversion (i.e., urine source separation) has been proposed as a more sustainable solution for water conversation, nutrient removal and recovery, and pharmaceutical sequestration. As wastewater regulations become more stringent, wastewater treatment plants reach capacity, and water resources become more strained, the benefits of urine diversion become more appealing. By using nonwater urinals and urine-diverting toilets, urine diversion systems seek to collect undiluted human urine for nutrient recovery and pharmaceutical sequestration. Urine is a unique, nutrient-rich waste stream that constitutes an overall low volume of waste entering a wastewater treatment plant. If urine is separated at the building-scale, various technologies can be used to recover nutrients and sequester pharmaceuticals at their most concentrated location. However, the implementation of urine diversion requires a paradigm shift from conventional comingling of wastewater and centralized treatment to source separation and decentralized treatment. This Account proposes a vision for building-scale implementation of urine diversion with the goal of clarifying the opportunities and challenges in this context. The main components of urine, i.e., nitrogen, phosphorus, potassium, and pharmaceuticals, are major drivers for technology development and system implementation. Stepping back, the benefits from water conservation and effects on wastewater treatment are an extension of the system boundary that can impact the sustainability of adjacent systems. However, major challenges have been identified in the literature as hurdles for widespread implementation of urine diversion. Challenges include the comparison of recovering nutrients at the wastewater plant versus at the source, the collection and storage of urine, the ability to recover nutrients and sequester pharmaceuticals, and the overall environmental and economic impacts of urine diversion systems. While these challenges exist, studies have been conducted to address some of the underlying research questions. As more research is conducted, the vision of a seamless urine diversion system with building-wide plumbing and storage comes closer to reality. As such, the application of urine diversion systems will benefit from technology development and research to fill gaps that have been identified. It is important to classify urine diversion systems as a process and not a product. This has implications for the way these systems are evaluated, as their impact on peripheral systems can be of benefit to different stakeholders. In the same light, new research areas, such as cyber-physical systems, reverse logistics, and sustainability transitions, can be applied to urine diversion as approaches for ensuring a robust process for widespread implementation. However, established technologies should be constantly reassessed and enhanced by newer techniques. For example, membrane distillation, eutectic freeze concentration, and solar evaporation should be considered for nutrient recovery and volume reduction because they offer benefits over conventional technologies. Finally, the human behavior component of urine diversion cannot be ignored, as negative user acceptance and improper maintenance of these systems can have a detrimental impact on their future implementation.

1. INTRODUCTION

recovered and used as fertilizer. In addition to resource recovery, urine diversion can make potable water production and wastewater treatment (hereafter the urban water system) more efficient by eliminating potable water requirements for flushing and conveying urine and thereby decreasing the

Urine diversion is the separate collection and treatment of human urine apart from sanitary wastewater.1−3 Urine diversion represents a paradigm shift in wastewater management where one of the key waste inputs to sanitary wastewater, i.e., human urine, is viewed as a valuable resource instead of as a pollutant. The resources in human urine include nitrogen (N), phosphorus (P), and potassium (K), which can be © XXXX American Chemical Society

Received: December 1, 2018

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DOI: 10.1021/acs.accounts.8b00614 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Experimental data and equilibrium isotherms showing phosphate sorption to HAIX-Fe resin for (a) fresh urine (urine-F), (b) hydrolyzed urine (urine-H-1), (c) hydrolyzed urine (urine-H-2), (d) anaerobic digester supernatant (ADS-1), (e) anaerobic digester supernatant (ADS-2), (f) greywater (GW), and (g) secondary wastewater effluent (WW). Data are the mean value of triplicate samples with error bars showing one standard deviation. Reproduced with permission from ref 11. Copyright 2013 Elsevier.

volume of wastewater generated and altering its composition.4,5 For instance, diverting urine from wastewater can decrease N and P concentrations in wastewater effluent. There are, however, challenges to implementing urine diversion in homes and buildings. To appreciate the opportunities and challenges for urine diversion, it is necessary to place urine diversion into the proper context for wastewater management. Modern urban water systems have evolved to protect public health and the environment whereby waste streams such as greywater and black water are comingled and conveyed as sanitary wastewater to centralized wastewater treatment plants. Many wastewater treatment plants practice secondary treatment (e.g., activated sludge) whereby the focus is on reduction of biochemical oxygen demand (BOD), nutrients, and pathogens. Nutrient recovery is not the focus of secondary wastewater treatment. In addition, most wastewater treatment plants are not designed to remove trace organic contaminants, such as pharmaceuticals and hormones, and instead discharge the contaminants in wastewater effluent. Hence, although modern wastewater treatment plants have contributed to protecting public health and environmental quality, the lack of nutrient recovery and trace organic contaminants removal present a growing concern for urban water systems. This concern is magnified when one considers that many wastewater treatment plants in the United States and elsewhere are reaching the end of their design life and need to be replacedeither with similar processes or with radical changes to increase their sustainability.6 The first paper on urine diversion was published in 1996 and advocated the separate collection of anthropogenic nutrient solution (i.e., urine) for the benefit of wastewater treatment.7

That is, diverting a substantial fraction of urine from wastewater would decrease the N to carbon (C) ratio of wastewater and allow N removal to be achieved through BOD oxidation and assimilation rather than nitrification/denitrification. Since then, research on urine diversion has expanded to consider the potential for urine and urine-derived products to substitute for commercial fertilizer,8 studied the installation and operation of urine diversion systems in homes and buildings,2 investigated various microbiological and physicalchemical processes for urine treatment,9 and surveyed user attitudes toward urine diversion.10 The approach to urine diversion taken by the first author, as this Account highlights, has focused on ion exchange due to its ability to remove a wide range of constituents from urine and function at different scales. Research on ion-exchange treatment of urine and its insights has led to other technologies and approaches, and ultimately a vision for urine diversion at the building-scale has materialized. This Account explores the opportunities for urine diversion at the building-scale and the challenges for implementation. The opportunities for urine diversion originate from its composition as a nutrient solution, the fact that most wastewater systems use potable water to convey urine and other waste streams to a centralized treatment facility, and that most wastewater treatment plants practice nutrient removal and not nutrient recovery. Another major opportunity for urine diversion owing to its composition is that pharmaceuticals are excreted in urine. Urine provides a small volume and concentrated waste stream to remove or destroy pharmaceuticals, and prevent them from ever entering the wastewater treatment plant and ultimately receiving waters. Finally, there B


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Ca5(PO4)3OH,16,17 which is advantageous for P recovery but problematic during urine collection. Nonwater urinals have been used since the 1990s in existing building plumbing as a way to conserve water.18 Nonwater urinals and urine-diverting toilets are key components in a urine diversion system to collect undiluted urine.2 For both water conservation and urine diversion, there is published and anecdotal evidence of numerous maintenance challenges.1,18,19 Foremost, unwanted struvite precipitation can occur in urinal traps and drain lines. This leads to clogging and malodors, and eventually major maintenance such as fixture replacement. Several approaches have been proposed for the problem including a removable cartridge trap, increased education on proper maintenance requirements for users, and increased education on the proper installation of fixtures for plumbers. Initial work by Boyer et al. sought to remove calcium and magnesium from fresh urine via ion-exchange using a cation exchange resin cartridge placed inside the nonwater urinal trap.20 Although the approach was able to remove calcium and magnesium as hypothesized, the cartridge design parameters limited the extent of removal due to a short contact time, bed shape, and bed size. As an alternative to urea hydrolysis inhibition described below, a urea hydrolysis reactor can be placed near or in the urinal/toilet to accelerate urea hydrolysis and thereby enable phosphate precipitation and ammonia recovery in a controlled manner.21 Ray et al., Saetta and Boyer, and others have pursued the topic of urea hydrolysis inhibition by chemical addition at the point of collection.22−26 Inhibition of urea hydrolysis (i.e., urease enzyme) in soils is well established;13,15 however, the results are not transferable to urine due to high concentrations of chloride, phosphate, and bicarbonate. Ray et al. showed that chemicals commonly used for urease inhibition, such as metals, were not effective in the urine matrix and instead acid was effective by changing the chemical environment for urease.23 Shifting the pH of urine, either by acid or base addition, outside the effective range for urease was shown to be an effective approach to inhibiting urea hydrolysis. Saetta and Boyer extended the work by Ray et al. by applying acetic acid directly into nonwater urinals and showed acid addition could inhibit urea hydrolysis and its adverse impacts (see Figure 2).22 The challenge then becomes how to implement acid addition or other forms of pretreatment at the urinal or toilet. Saetta and Boyer have implemented a cyber-physical system to deliver acetic acid by responding to real-time urine chemistry and variable user urinations, which was able to keep 90% of phosphate in solution and hydrolyzed less than 1% of the available urea-nitrogen.27 Others have shown that base addition and raising the pH is also effective.24−26 For example, Flanagan and Randall proposed adding solid calcium hydroxide to small urine storage tanks at each urinal to inhibit urea hydrolysis by increasing the pH above 12.25 Simha et al. has shown the use of anion exchange resins as a source for hydroxide ions to increase the pH before dehydration for volume reduction and nutrient recovery.24 Inhibiting urea hydrolysis at the point of collection can serve an additional purpose as well. Urea has been mostly neglected in terms of N recovery from urine, although urea is an effective fertilizer and has other industrial uses. The main reason for neglecting urea recovery is the necessity to inhibit the urea hydrolysis reaction such that urea remains in its unaltered form. As a result, N can be recovered from fresh urine as urea as opposed to ammonia in hydrolyzed urine. Owing to urea’s

are opportunities for urine diversion that accrue at the system level including water conservation and reduced electricity use for potable water production and wastewater treatment. The challenges for implementing urine diversion can be summarized in the following questions, which also serve as section topics in this Account. Is P recovery from source-separated waste streams more efficient than from combined wastewater? How can the function of nonwater urinals and urine-diverting toilets be improved? Can nutrients and pharmaceuticals in urine be separated and recovered? What are the environmental, economic, and social impacts of urine diversion? After exploring these questions, a vision for urine-diversion at the building-scale is described. Finally, this Account concludes with key points on the opportunities and challenges for urine diversion, and the next steps required to advance the implementation of urine diversion.

2. IS P RECOVERY FROM SOURCE-SEPARATED WASTE STREAMS MORE EFFICIENT THAN FROM COMBINED WASTEWATER? O’Neal and Boyer investigated the effectiveness of nutrient recovery from source-separated waste streams by comparing phosphate adsorption to hybrid anion exchange (HAIX) resin in fresh urine, hydrolyzed urine, anaerobic digester supernatant, greywater, and biologically treated wastewater effluent, including mixtures of urine and greywater diluted with tap water.11 The waste steams were selected to span greater than 2 orders of magnitude in phosphate concentration (1.8−668 mg P/L), and capture both source-separated (e.g., urine) and combined (e.g., wastewater effluent) waste streams. The maximum loading of phosphate on HAIX resin was fresh urine > hydrolyzed urine > anaerobic digester supernatant ≈ greywater > wastewater effluent (see Figure 1), and when considering the initial phosphate loading of the waste streams, the P recovery potential was fresh urine > hydrolyzed urine > greywater > wastewater effluent > anaerobic digester supernatant.11 Hence, phosphate adsorption to HAIX resin in concentrated phosphate solution such as urine was a more efficient approach (i.e., greater phosphate loading per unit of resin) to P recovery than phosphate adsorption to HAIX resin in dilute phosphate solution such as wastewater effluent. Kocaturk and Beler Baykal showed similar results for ammonium adsorption to clinoptilolite with higher loading in undiluted urine than diluted urine.12 Collecting and treating undiluted urine also has benefits for nutrient removal where incorporating urine diversion can produce wastewater effluent with lower N and P concentrations (and lower energy requirements for wastewater treatment) than without urine diversion.4 3. HOW CAN THE FUNCTION OF NONWATER URINALS AND URINE-DIVERTING TOILETS BE IMPROVED? Freshly excreted urine contains urea, inorganic salts (e.g., phosphate, potassium), endogenous metabolites, and possibly pharmaceuticals. Fresh urine ages due to the hydrolysis of urea by urease-active bacteria,13−15 where 1 mol of urea is converted into 2 mol of ammonia, 1 mol of bicarbonate, and 1 mol of hydroxide. The increase in pH and presence of ammonia create supersaturated conditions for precipitation of st ru vite, MgNH 4 P O 4 · 6 H 2 O, a nd h y dr ox y apa ti te , C


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4. CAN NUTRIENTS AND PHARMACEUTICALS IN URINE BE SEPARATED AND RECOVERED? Human urine contains N, P, and K, which are the same macronutrients required for healthy plant growth and used in large quantities as commercial fertilizer. For example, humans excrete 7−11 g N/p·d, 0.6−1 g P/p·d, and 2.2−3.3 g K/p·d in urine.31 The potential for nutrient recovery from urine can be substantial when nutrient generation rates from human urine are scaled to multiple buildings in a city. However, human urine can also contain pharmaceuticals. Because the main driver for urine diversion is often nutrient recovery, the fate of pharmaceuticals is an important consideration in producing recovered nutrient products. Initial attempts to remove pharmaceuticals from urine investigated membrane separation and ozonation.32,33 Membrane separation was effective at rejecting pharmaceuticals but rejected nutrients as well; hence, it was counter to the goal of recovering a contaminant-free nutrient product. Research on ozone and advanced oxidation processes have shown limited effectiveness in oxidizing pharmaceuticals due to inhibition by the urine matrix.34 In reviewing pharmaceuticals excreted in urine and considering their ecotoxicity potential,35,36 pharmaceuticals from the analgesic medicine class, including acetaminophen (paracetamol) and nonsteroidal anti-inflammatory drugs (NSAIDs), are a high priority for urine diversion and treatment. Many pharmaceuticals in the analgesics class contain acid functional groups that give the pharmaceutical a net negative charge in urine. Hence, ion-exchange (using anion exchange resin, AER) was investigated as a novel approach for pharmaceutical removal from urine. Landry and Boyer showed that polystyrene AER exhibited greater removal of diclofenac than polyacrylic AER due to the presence of both electrostatic and van der Waals (VDW) forces of attraction, and that interactions were similar in fresh and hydrolyzed urine (see Figure 3).37 Considering several analgesics (mostly NSAIDs) with varying physical-chemical properties, polystyrene AER had a higher selectivity for more hydrophobic pharmaceuticals, such as diclofenac and naproxen, than more hydrophilic pharmaceuticals, such as ibuprofen and paracetamol (see Figure 4).38 Together, the results show the importance of considering both pharmaceutical characteristics and resin properties in understanding the capacity and selectivity of removal by ion exchange. For instance, polystyrene AER is more selective for pharmaceuticals than polyacrylic AER because of VDW interactions, and VDW interactions make polystyrene AER more selective for pharmaceuticals than phosphate as described below. Biochar was investigated as an alternative to AER based on the premise that the diverse physical-chemical properties of biochar could adsorb a wider range of pharmaceuticals than AER, and because biochar is a lower cost adsorbent than AER. Solanki and Boyer followed guidelines by the California Department of Public Health regulations pertaining to recycled water to select pharmaceuticals representing various functional group categories.39 In addition, biochars from different source materials were investigated. The results showed that bambooand southern yellow pine-derived biochars exhibited higher removal of all pharmaceuticals than coconut shell- and northern hardwood-derived biochars. Considering the diverse characteristics of the pharmaceuticals investigated, biochar adsorbed pharmaceuticals with varying degrees of aromaticity and functional groups. In addition, because pharmaceutical

Figure 2. Phosphate concentrations in synthetic urine for the (a) urea hydrolysis, (b) urea hydrolysis inhibition (acetic acid), and (c) urea hydrolysis inhibition (citric acid) synthetic urine experiments. Measurements were taken every 30 min. Measurements for each urinal are shown along with the average concentration for the three urinals. Phosphate concentration at t = 0 was the concentration in the initial synthetic urine. Reproduced with permission from ref 22. Copyright 2017 American Chemical Society.

small size and neutral character, Ray et al. has explored different combinations of membrane processes to separate and concentrate urea.28 The results are promising whereby a reverse osmosis or forward osmosis membrane can be used to separate urea from other constituents in urine and membrane distillation can be used to concentrate urea beyond its initial concentration in undiluted urine. Eutectic freeze crystallization is another process that has the potential to separate urea from salts in urine and produce a concentrated urea solution.29 The final urea product could be a concentrated liquid or taken to complete dryness, where there are novel approaches for stabilizing solid urea.30 D


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adsorption to biochar is dominated by VDW interactions, biochar can adsorb charged and uncharged pharmaceuticals. For example, biochar adsorbed ibuprofen and paracetamol whereas AER was not effective for these same pharmaceuticals. Further work by Solanki and Boyer has confirmed the dominance of VDW forces in governing pharmaceutical sorption to biochar.40 Because pharmaceuticals are biologically active compounds, the extent of removal should consider factors such as ecotoxicity potential and not just mass removal. Escher et al. explored this for different combinations of physical-chemical treatment processes and bioassays.41 One process not considered, however, was adsorption (i.e., ion exchange). Landry and Boyer compared the mass removal of NSAIDs by ion-exchange with the reduction in COX-1 inhibition.42 Figure 5 shows an example of the results where the mass removal (i.e., normalized effluent concentration, C/C0) follows a similar breakthrough behavior for all NSAIDs whereas the percent inhibition varies by NSAID, and as such, illustrates the tradeoffs in design between mass removal and reducing ecotoxicity potential that must be considered for a real urine diversion system. The previous discussion illustrates that removal of pharmaceuticals in urine is possible through adsorption (i.e., ion exchange), and likely realistic considering different size applications of ion exchange and granular activated carbon in drinking water treatment. A key consideration for urine treatment is the scale at which adsorption is applied, e.g., at individual urinals and toilets or urine collected from buildings.

Figure 3. Effect of anion exchange resin properties on equilibrium (24 h) removal of diclofenac in (a) fresh urine (pH 6) and (b) ureolyzed urine (pH 9). Initial diclofenac concentration 0.2 mmol/L. Reproduced with permission from ref 37. Copyright 2013 Elsevier.

Figure 4. Experimental equilibrium data and isotherm models determined by nonlinear regression of (a) diclofenac (DCF) (C0 = 2.96 × 10−3 mmol L−1), (b) ibuprofen (IBP) (C0 = 3.65 × 10−3 mmol L−1), (c) ketoprofen (KTP) (C0 = 7.80 × 10−3 mmol L−1), and (d) naproxen (NPX) (C0 = 7.51 × 10−3 mmol L−1) using Dowex 22 AER. Figure (d) *Naproxen illustrates the plotted experimental isotherms excluding the lowest resin dose of 0.16 mL L−1) (i.e., excluding the data point with the highest Ce and corresponding nonlinear isotherm models (Freundlich, Langmuir, Dubinin−Astakhov (D−A), and Dubinin−Radushkevich (D−R)). Reproduced with permission from ref 38. Copyright 2015 Elsevier. E


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Figure 5. Column breakthrough curves for (a) diclofenac (C0 = 0.55 μmol/L), (b) ketoprofen (C0 = 1.3 μmol/L), (c) naproxen (C0 = 3.0 μmol/ L), and (d) O-desmethylnaproxen (C0 = 1.4 μmol/L) removal by Dowex 22 AER in synthetic ureolyzed urine. The treated effluent is shown as the normalized effluent concentration (C/C0) (solid line) predicted by the HSDM or transformed to % COX-1 inhibition (dashed line) using the individual compound dose−response curves. Reproduced with permission from ref 42. Copyright 2017 American Chemical Society.

hydrolyzed urine, and the option to recover P in different mineral forms other than struvite. Following phosphate removal by ion exchange, ammonium and potassium can be removed from urine using cation exchanger such as clinoptilolite.51−53 Hence, a three-stage adsorption process can be used to remove pharmaceuticals and recover phosphate and ammonium/potassium. As an alternative to adsorption, Jagtap and Boyer investigated an integrated, multiprocess approach to total nutrient recovery from urine.54 The premise was to combine processes that have been well studied individually (struvite precipitation, air stripping and acid absorption, evaporation) but have been overlooked in terms of their interactions. For example, base addition for ammonia air stripping can decrease the purity of the final potassium product depending on the chemical used. Given real hydrolyzed urine as the input, the result was 91% P recovery as struvite, 99% N recovery as ammonium sulfate, and 80% K recovery as potash. In addition to maximizing nutrient recovery, a multiprocess approach emphasizes the integration and interaction of processes. For example, ammonia air stripping with acid absorption is enhanced by elevated pH and/or temperature. pH is commonly increased by addition of NaOH, which is suitable for improving N recovery; however, the addition of sodium from NaOH decreases the purity of the recovered K product, potash. Regardless of the approach to nutrient recovery, pharmaceutical removal by adsorption can be applied as a pretreatment without affecting the subsequent nutrient recovery.

One possible approach is pharmaceutical removal by adsorption followed by nutrient recovery by adsorption. Although the main emphasis on P recovery from urine has been struvite precipitation,43 phosphate adsorption is another mechanism that has been investigated in domestic and industrial wastewater but overlooked in urine.44,45 Sendrowski and Boyer were the first to investigate phosphate adsorption in urine using HAIX resin,46 which incorporates iron oxide nanoparticles into a polystyrene AER.47−49 This gives the resin dual functionality of ion-exchange (electrostatic attraction to strong-base functional groups) and ligand exchange (innersphere complexation with iron oxide). Sendrowski and Boyer showed high removal of phosphate using HAIX resin in both synthetic fresh urine and synthetic hydrolyzed urine, which is an important result because struvite precipitation is only possible in hydrolyzed urine.46 Because the HAIX resin is polystyrene, it adsorbed diclofenac so a pharmaceutical removal step is necessary before phosphate adsorption.46 In contrast, polystyrene AER intended for pharmaceutical adsorption showed minimal uptake of phosphate because of the lack of inner-sphere complexation.37 One caveat to phosphate adsorption is that it is a removal process and not necessarily a recovery process. However, regeneration of the phosphate-loaded HAIX resin can be used to recover P in different forms by precipitation. O’Neal and Boyer investigated such an approach where struvite precipitation in hydrolyzed urine was compared with adsorption−regeneration−precipitation in hydrolyzed urine and fresh urine.50 Specifically, HAIX resin was loaded with phosphate by treating fresh urine or hydrolyzed urine, and then regenerated to create a concentrated phosphate solution. Either struvite or potassium struvite, KMgPO4·6H2O, was precipitated in the waste regeneration solution. Although direct precipitation of struvite in hydrolyzed urine is simple and effective, it is restricted to hydrolyzed urine as the input and struvite as the output. In contrast, the advantages of adsorption−regeneration−precipitation are the ability to remove P from either fresh urine or

5. WHAT ARE THE ENVIRONMENTAL, ECONOMIC, AND SOCIAL IMPACTS OF URINE DIVERSION? The foundation for urine diversion is collecting undiluted urine using nonwater urinals and urine-diverting toilets. Because toilet and urinal flushing accounts for 30% or more of water use in buildings,55 implementing urine diversion can have F


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Figure 6. Necessary components and operational options for building-scale implementation of urine diversion system. Urine diversion benefits include nutrient recovery and water conservation, while challenges include collection and user acceptance.

in wastewater. From a life-cycle perspective, Ishii and Boyer showed that decreasing the N in wastewater could result in environmental benefits due to corresponding reduction in electricity requirements during wastewater treatment.56 Following potable water saving and nutrient recovery, the third system-level benefit of urine diversion is pharmaceutical sequestration and thereby reducing the ecotoxicity of wastewater effluent. The discussion above highlighted research efforts on removing pharmaceuticals from urine mainly by adsorption. Assuming that a significant fraction of urine is diverted and the pharmaceuticals are effectively removed, there is the potential for urine diversion to serve as a form of pretreatment to obviate the need for advanced treatment at the centralized wastewater plant. Landry and Boyer explored this idea using an LCA model of a university community.57 They showed that urine diversion could reduce the ecotoxicity of the wastewater effluent relative to secondary wastewater treatment.

significant reductions in water use (by installing no-flow and low-flow fixtures) that translate to reductions in potable water produced and wastewater generated. Producing less potable water results in decreased electricity and chemical requirements for drinking water treatment. Likewise, producing less wastewater results in decreased electricity requirements for wastewater treatment. Ishii and Boyer conducted a life cycle assessment (LCA) on hypothetical scenarios of urine diversion in dorms at a university and confirmed that water conservation was one of the major environmental benefits to urine diversion at the system level.56 Landry and Boyer expanded the hypothetical LCA scenarios of urine diversion across the entire university and showed water conservation was one of the main benefits of implementing urine diversion.57 Other LCA studies on urine diversion have explored the logistics of N recovery.58 The main motivation for urine diversion is typically nutrient recovery, with the goal of producing a urine-derived fertilizer product that has economic value. Although struvite precipitation in urine has received the most attention, it is challenging for struvite precipitation to be cost-effective because P has a lower concentration in urine than N or K, and the chemical requirements (i.e., magnesium) are high. Setting aside the economics of struvite precipitation, recovering P from urine does have environmental benefits in terms of providing fertilizer offsets within the system boundary. Considering N and P recovery is often more profitable than P alone; however, the technology for N recovery is important. Although counterintuitive, struvite precipitation can be used for N recovery with the addition of magnesium and P. Ishii and Boyer showed that N and P recovery by struvite precipitation was profitable compared with the status quo approach of no nutrient recovery; however, the high chemical requirements made for more harmful environmental impacts than the status quo.56 As such, the LCA approach was able to show trade-offs between economic and environmental costs/benefits and identify hot spots in the system where more attention is needed, e.g., N recovery technology. The LCA on N recovery further explores trade-offs between treatment chemicals, transportation, and other logistics to find an optimal solution.58 Finally, nutrient recovery from urine also has the benefit of reducing the nutrient concentration, particularly N,

6. VISION FOR BUILDING-SCALE IMPLEMENTATION The vision for urine diversion described in this Account resides within the boundaries of multistory commercial or institutional (CI) buildings (see Figure 6). In this context, urine diversion exists as a system not obviously different to building occupants from current wastewater management in CI buildings. Nonwater urinals and urine-diverting toilets would be used in place of traditional water-flushing fixtures. Apart from the difference in the fixtures, the system would blend in to current infrastructure norms to reduce the amount of buy-in necessary from occupants. A complementary approach proposed by Randall and Naidoo and Simha et al. is to collect at treat the urine in or near the toilet.24,59 This approach would eliminate the need for separate urine piping; however, it would introduce new challenges of physically moving the stored urine. It is important to minimize behavioral changes required by the users, as the survey conducted by Ishii and Boyer showed lower acceptance for urine diversion systems if users were asked to dispose of soiled toilet paper in a nearby trash can.60 The building plumbing would be unique to each wastewater flow, with the urine plumbing directing urine to a lower-level urine storage location. The next step in the process is the consideration of where treatment occurs, specifically whether it would be implemented in a decentralized or centralized G


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manner. The LCA conducted by Landry and Boyer explored the trade-offs of centralized versus decentralized treatment of urine for ecotoxicity reduction.57 The LCA results showed that decentralized, building-level treatment and vacuum truck delivery to centralized treatment had a similarly lower environmental impact than vacuum sewer conveyance to centralized treatment. However, all three urine diversion options had a lower environmental impact than conventional wastewater treatment. As explained above, the maintenance of urine diversion systems, especially at collection, is important for the function of the entire system. The technologies applied in this context must be robust, relatively easy to use and operate, and low risk for building maintenance staff. Saetta and Boyer have shown that cyber-physical systems can be used to reduce workload on maintenance staff by predicting treatment using computer models and a low-risk treatment solution, i.e. dilute acetic acid.27 Connecting technologies to cyber components has been identified as an area of opportunity for water and wastewater systems.61 Urine diversion systems should incorporate novel ideas from the cyber-physical system literature to ensure the effective application and longevity of these systems in the CI building context.62

The authors declare no competing financial interest. Biographies Treavor H. Boyer received his Ph.D. at the University of North Carolina at Chapel Hill in 2008. He is an associate professor in the School of Sustainable Engineering and the Built Environment and program chair of Environmental Engineering at Arizona State University. His research and teaching interests include water quality and physical-chemical processes. Daniella Saetta received her B.S. and M.S. at the University of Florida in 2014 and 2016, respectively. Currently, she is a Ph.D. student in Environmental Engineering at Arizona State University. Her research focuses on building-scale implementation of urine diversion systems.

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ACKNOWLEDGMENTS This publication was made possible by the following grants and awards to T.H.B.: NSF CAREER Grant No. CBET-1150790, U.S. EPA Grants RD835569, SU835326, and SU835719, ASU Fulton Schools of Engineering start-up funding, and ASU initiative Future H2O. T.H.B. would like to acknowledge the many undergraduate and graduate students who have contributed to the research discussed in this Account.

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7. CONCLUSION AND PERSPECTIVE This Account reviewed our progress on identifying opportunities for urine diversion at the building-scale and challenges to implementation. There are real opportunities for urine diversion in terms of potable water conservation, nutrient recovery for use as fertilizer, and pharmaceutical sequestration to reduce the ecotoxicity of wastewater effluent. In studying urine diversion at the building-scale, it is important to recognize that urine division is a collection of processes (i.e., a system) and not a product. Moreover, the benefits of urine diversion are derived from extension of the system boundary to include drinking water production, wastewater treatment, fertilizer production, and ecotoxicity reduction. The most practical and beneficial implementation of urine diversion is likely as a distributed system that includes decentralized treatment and centralized treatment with vacuum truck collection. Finally, the implementation must consider user opinions and human behavior. The next steps in advancing the implementation of urine diversion should focus on new technologies and approaches to fill gaps. Acid and base addition to inhibit urea hydrolysis, membrane distillation, solar evaporation, and eutectic freeze concentration are a few examples of technologies that align with the benefits of urine division but have research gaps. Cyber-physical systems and reverse logistics are examples of approaches that could assist with the implementation of urine diversion in buildings but require further research. Finally, researchers and others interested in advancing the implementation of urine diversion should study how other technologies have evolved and specifically how sustainability transitions occur.63,64

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 1-480-965-7447. E-mail: [email protected]. ORCID

Treavor H. Boyer: 0000-0003-0818-5604 Daniella Saetta: 0000-0003-3107-1154 H


Article

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(34) Zhang, R.; Sun, P.; Boyer, T. H.; Zhao, L.; Huang, C.-H. Degradation of Pharmaceuticals and Metabolite in Synthetic Human Urine by UV, UV/H2O2, and UV/PDS. Environ. Sci. Technol. 2015, 49, 3056−3066. (35) Lienert, J.; Buerki, T.; Escher, B. I. Reducing micropollutants with source control: substance flow analysis of 212 pharmaceuticals in faeces and urine. Water Sci. Technol. 2007, 56, 87−96. (36) Lienert, J.; Gudel, K.; Escher, B. I. Screening method for ecotoxicological hazard assessment of 42 pharmaceuticals considering human metabolism and excretory routes. Environ. Sci. Technol. 2007, 41, 4471−4478. (37) Landry, K. A.; Boyer, T. H. Diclofenac removal in urine using strong-base anion exchange polymer resins. Water Res. 2013, 47, 6432−6444. (38) Landry, K. A.; Sun, P.; Huang, C.-H.; Boyer, T. H. Ionexchange selectivity of diclofenac, ibuprofen, ketoprofen, and naproxen in ureolyzed human urine. Water Res. 2015, 68, 510−521. (39) Solanki, A.; Boyer, T. H. Pharmaceutical removal in synthetic human urine using biochar. Environmental Science: Water Research & Technology 2017, 3, 553−565. (40) Solanki, A.; Boyer, T. H. Physical-chemical interactions between pharmaceuticals and biochar in synthetic and real urine. Chemosphere 2019, 218, 818−826. (41) Escher, B. I.; Pronk, W.; Suter, M. J. F.; Maurer, M. Monitoring the removal efficiency of pharmaceuticals and hormones in different treatment processes of source-separated urine with bioassays. Environ. Sci. Technol. 2006, 40 (16), 5095−5101. (42) Landry, K. A.; Boyer, T. H. Fixed Bed Modeling of Nonsteroidal Anti-Inflammatory Drug Removal by Ion-Exchange in Synthetic Urine: Mass Removal or Toxicity Reduction? Environ. Sci. Technol. 2017, 51, 10072−10080. (43) Etter, B.; Tilley, E.; Khadka, R.; Udert, K. M. Low-cost struvite production using source-separated urine in Nepal. Water Res. 2011, 45, 852−862. (44) Blaney, L. M.; Cinar, S.; SenGupta, A. K. Hybrid anion exchanger for trace phosphate removal from water and wastewater. Water Res. 2007, 41, 1603−1613. (45) Sengupta, S.; Pandit, A. Selective removal of phosphorus from wastewater combined with its recovery as a solid-phase fertilizer. Water Res. 2011, 45, 3318−3330. (46) Sendrowski, A.; Boyer, T. H. Phosphate removal from urine using hybrid anion exchange resin. Desalination 2013, 322, 104−112. (47) DeMarco, M. J.; SenGupta, A. K.; Greenleaf, J. E. Arsenic removal using a polymeric/inorganic hybrid sorbent. Water Res. 2003, 37, 164−176. (48) Cumbal, L.; Sengupta, A. K. Arsenic removal using polymersupported hydrated iron(III) oxide nanoparticles: Role of Donnan membrane effect. Environ. Sci. Technol. 2005, 39, 6508−6515. (49) Puttamraju, P.; SenGupta, A. K. Evidence of Tunable On−Off Sorption Behaviors of Metal Oxide Nanoparticles: Role of Ion Exchanger Support. Ind. Eng. Chem. Res. 2006, 45, 7737−7742. (50) O’Neal, J. A.; Boyer, T. H. Phosphorus recovery from urine and anaerobic digester filtrate: comparison of adsorption-precipitation with direct precipitation. Environmental Science: Water Research & Technology 2015, 1, 481−492. (51) Boyer, T. In Adsorption applications for total nutrient recovery from urine. Abstr. Pap. Am. Chem. Soc., 2015. (52) Boyer, T. H.; Cribbs, K. Effect of Urine Dilution on Ammonium Sorption using Natural Zeolites. Proceedings of the Water Environment Federation 2014, 2014, 7322−7328. (53) Tarpeh, W. A.; Udert, K. M.; Nelson, K. L. Comparing Ion Exchange Adsorbents for Nitrogen Recovery from Source-Separated Urine. Environ. Sci. Technol. 2017, 51, 2373−2381. (54) Jagtap, N.; Boyer, T. H. Integrated, multi-process approach to total nutrient recovery from stored urine. Environmental Science: Water Research & Technology 2018, 4, 1639−1650. (55) USEPA. WaterSense at Work: Best Management Practices for Commercial and Institutional Facilities; Office of Water 2012; EPA 832-F-12-034, 308 pp. I


Article

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