Researchers must address regulatory regimes to scale up adoption of urine diversion systems in the U.S.

Urine diversion (UD) is a system-of-systems that involves source separation of waste to maximize recovery of valuable nutrients, including phosphorus. Recent research shows how UD systems offer valuable ecological benefits and can aid in water conservation efforts, and public perception studies suggest that UD systems are generally viewed positively by end-users and the general public. Nevertheless, adoption and implementation of this promising sustainability solution remains limited in many countries, including the United States (U.S.). In this perspective, we argue that in order to scale up adoption in the U.S., UD researchers and innovators must do more to address regulatory barriers. We draw on insights from political science research on ‘regulatory regimes’ to introduce the array of regulations that apply to UD systems, with a focus on commercial and institutional buildings. We examine regulatory regimes all along the UD system-of-systems, beginning at the point of collection and ending at the point of beneficial reuse. We then propose next steps to address current regulatory challenges that impact adoption, with an emphasis on the importance of stakeholder coordination. Throughout, we argue that law and regulation plays a critical role in shaping adoption of UD technologies because: (1) different regulatory regimes will be important at different points in the system-of-systems, (2) there may be multiple regulatory regimes that apply to a single subsystem, and (3) it is important to consider that legal and regulatory definitions of a technology may not match scientific understanding.


Overview of urine diversion systems and sustainability
Human urine is a rich source of nutrients, including phosphorus (P).By diverting urine and treating it separately from conventional wastewater, more of those nutrients can be recovered and put to beneficial reuse [1,2].Urine diversion (UD) has the potential to contribute to much-needed circular economies in wastewater by reducing P losses to waterways that may otherwise contribute to harmful algal blooms and eutrophication [3][4][5].UD can be particularly useful in countries like the U.S. because of its potential to improve the efficiency of existing centralized municipal wastewater treatment by reducing inputs of nutrients that must be removed or recovered [6][7][8].
Crucially, UD is not a single technology or product, but instead should be understood as a system composed of interconnected subsystems, or a 'system-of-systems' as described by Boyer and Saetta [1] (figure 1).First, urine must be collected through separate piping in a restroom through e.g. a waterless urinal or urine-diverting toilet.Once collected, urine must be treated to remove potential health hazards and other contaminants.As part of treatment, urine may also be stored for a period of time.After the urine is treated, it may be further processed into a finished product (e.g.struvite, ammonium nitrate) and transported from its storage location to the point of beneficial reuse [1,9].Schematic representation of UD system-of-systems (yellow boxes) with regulatory regimes overlaid in 'layers' (gray boxes).Boxes with dotted lines indicate voluntary or quasi-voluntary regimes.This figure assumes that all private/international/transnational standards are voluntary to some degree.
Studies that involve surveys of end-users and the general public reveal that potential users view UD and source separation technologies in a generally favorable light, especially in comparison to other wastewater recovery and reuse technologies [10][11][12].These studies suggest that users may be more likely to tolerate additional technical issues if they felt involved with the decision-making process and were given information about the purpose of the system [11].Users expressed less enthusiasm for systems if they were required to pay for them [12].A multinational survey suggests that the general public may be less averse to the use of human urine as fertilizer than researchers expect [13].However, most, but not all, of these studies are limited by a small sample size and a focus on specific populations (e.g.university students) who may be more likely to view UD systems favorably.
Despite the aforementioned benefits and generally positive public perception of UD, adoption and implementation remains limited outside of a few communities or pilot projects [14][15][16].Pilot projects tend to be located in small towns or eco-villages where community members are more likely to be bought in to the environmental benefits of UD systems [15,17,18].The most common technical barriers encountered during pilot projects include odors and clogging, which are more likely to occur if systems are not adequately maintained [2,19,20].Additionally, retrofitting existing buildings to accommodate multiple waste streams can be technically complex and costly for building owners [21].
Another important barrier that has affected scale-up of UD systems is the array of legal and institutional frameworks, or 'regulatory regimes' [22] that apply to each subsystem.Some UD pilot projects had to overcome legal barriers in order to install the technology [23] or to transport diverted urine [16].More generally, recent surveys show that stakeholders view regulation as a key barrier to adoption of P recovery technologies, including urine diversion [24,25].Each subsystem in the UD system encompasses different jurisdictions of regulatory oversight, including the built environment, water quality, transportation, and agriculture [1].The relevant regulatory regimes span multiple levels of governance, from state and local bodies to federal agencies to international, quasi-private organizations [26].
This perspective focuses on the UD system-of-systems and applicable regulatory regimes for commercial and institutional (CI) buildings in the United States.We focus on CI buildings in the U.S. because of the higher potential for urine recovery from CI buildings relative to residences [27].We also suspect that addressing regulatory barriers in CI buildings could create a regulatory 'ratchet to the top,' in which improvements in one influential jurisdiction or sector can lead to improvements on a broader scale [28].Nevertheless, we recognize that the specifics of the laws and regulations that affect CI buildings in the U.S. are not broadly applicable to all UD systems, even within the U.S. In particular, because of the way the U.S. federal system delegates selected powers to states, there can be (and often are) state-by-state variations in interpretation and enforcement of federal laws [29].
In this perspective, we introduce the array of regulatory regimes that affect UD systems in CI buildings, beginning at the point of collection (e.g. in the restroom of a CI building) and ending at the point of beneficial reuse (e.g. as fertilizer) (figure 1).We then propose next steps to address current regulatory challenges in UD systems with a focus on scaling up adoption.Throughout, we argue that law and regulation plays a critical role in shaping adoption of UD technologies because: (1) different regulatory regimes will be important at different points in the system-of-systems, (2) there may be multiple regulatory regimes that apply to a single subsystem, and (3) it is important to consider that legal and regulatory definitions of a technology may not match scientific understanding.

Collection
UD involves separation of liquid waste streams from solid wastes at the source, i.e. the toilet or urinal.However, the collection subsystem includes not just the toilet or urinal, but the built environment that enables source separation, including the piping and building design.Architects design buildings according to a set of public and private standards, including building codes that are adopted at the state or local level.However, most localities in the U.S. do not develop their own codes.Instead, local officials adopt generic 'model codes' which are developed by one of two competing 'standards-developing organizations' (SDOs): the Uniform Plumbing Code (UPC), developed by the International Association of Plumbing and Mechanical Officials (IAPMO), and the International Plumbing Code (IPC), developed by the International Code Council (ICC) [30,31].Both IAPMO and ICC are accredited by the American National Standards Institute (ANSI) to develop standards through a voluntary consensus process [32].Voluntary consensus standards are widely used in highly technical fields around the world [26,33,34].Federal agencies are not usually involved in these processes but do provide input on areas such as energy efficiency [35] and disaster resilience [36].Waterless urinals are approved for use in most plumbing codes, but urine-diverting flush toilets may require a variance for existing plumbing codes.Supplements to plumbing codes like We STAND provide guidance on pipe material used to convey urine through buildings [37].
Individual architects, designers, or planners may decide to go beyond the requirements of building codes to pursue additional certifications that reflect best practices.These 'third-party' certifications typically involve audits to ensure ongoing compliance [38,39].LEED is the most well-known certification for the built environment [40].Some firms and organizations also develop their own internal guidelines for new construction [41,42].

Storage
The storage subsystem can include both the plumbing system that diverted urine travels through as well as any storage containers or other receptacles to contain diverted urine.Diverted urine is generally stored for a period of time before treatment, in part because storage is one of the cheapest and most straightforward approaches to ensure inactivation of pathogens [9].Plumbing codes provide basic guidance on storing urine onsite but additional regulations related to worker safety may apply, depending on how workers interact with stored urine.The Occupational Safety and Health Administration (OSHA) sets basic standards for safety and sanitation in water closets, which include provisions regarding 'receptacle(s) for solid or liquid waste [43].' State and local environmental or public health agencies may also be involved, depending on their mandate and authority.

Treatment
The treatment subsystem includes a variety of approaches, processes and methods to convert raw urine into a usable product.When designing treatment processes, it is important to consider how treatment systems fit into existing regulatory regimes [44].While the EPA sets environmental rules at the federal (national) level, states have primary authority for enforcement according to principles of 'cooperative federalism' [29].As a result, interpretation and enforcement of environmental laws can vary from state to state.This is especially important when considering the legal definition of treated and untreated urine: because the regulations are written so broadly, urine could technically fit into the EPA's legal definition of 'domestic sewage' under the agency's biosolids rule [45].While drawing on existing rules for biosolids may allow UD system designers to take advantage of an established regulatory pathway, this approach also undercuts a key argument in favor of UD systems, which is that UD can generate a higher quality product than traditional wastewater treatment [16].Thus, it may be necessary to advocate for changes to the way state and federal agencies define recovered nutrients from waste, or to more clearly distinguish between diverted urine and other kinds of domestic sewage.A more effective approach may be to model UD regulations on rules for greywater reuse and rainwater harvesting.Another consideration for this subsystem is the process itself; if treatment processes involve hazardous chemicals, federal and state laws and regulations governing chemical safety in the workplace may apply [46].

Transportation
The transportation subsystem includes handling and transportation of urine and/or end product, typically after treatment but before end use.Today, much more clarity is needed on the logistics of this subsystem for UD systems to be scaled up.Especially as UD systems scale up, it is likely that some or all aspects of UD treatment, storage, and application may be conducted offsite (i.e.transported to a quasi-centralized treatment location).In the U.S., anytime commodities are transported in commerce, truck drivers ('motor carriers') must adhere to the relevant state and federal regulations governing highway safety, licensing, and certification (table 1).These rules may differ slightly depending on whether the transportation occurs within a state or crosses state lines.UD system operators could learn from current practices and procedures that are used in the portable sanitation industry [47].

Product
After the urine is treated, the next step will be to convert the nutrient-rich urine into a product for beneficial reuse, or a fertilizer.The product subsystem includes production processes, marketing, and any required testing or inspection of finished products; distribution and logistics are included in the transportation subsystem.The urea present in human urine could also be separated and concentrated to a level that would be useful in the chemical industry for products like diesel exhaust fluid, where private testing and certification would be required for purity.For fertilizers, if the finished product undergoes nitrification to produce ammonium nitrate, additional regulations related to fire safety and handling of explosive materials would apply (not depicted in figure 1).Once converted into a fertilizer product, the treated urine would then be subject to a different set of regulatory regimes that govern quality, safety, and uses of fertilizer.Fertilizer inspection and labeling is governed by individual states rather than the U.S. Department of Agriculture (USDA) [48].

Beneficial reuse
The beneficial reuse subsystem includes any uses of finished products that involve recycling or reuse of P from diverted urine.While it is more likely that diverted urine from CI buildings will be used to supplement or replace existing fertilizer used for landscaping, on a global scale, it will be important to ensure that UD systems produce fertilizers that could be applied to food crops.Thus, UD system designers would do well to keep in mind the World Health Organization (WHO)'s guidelines governing the reuse of wastewater, greywater, and excreta, which includes guidance on pathogen reduction to ensure fertilizers do not compromise food safety [49].In the U.S., regulations related to fertilizer or biosolids may affect the ability of farmers or end-users to apply the urine as fertilizer.States, municipalities or watersheds may recommend 'Best Management Practices' (BMPs), which farmers and other end-users can adopt on a voluntary basis [50].EPA does not provide limits for application of chemical fertilizer, but does regulate land application of biosolids from wastewater treatment plants [44,45].As noted above, if treated urine is defined as sewage, then users can employ an established regulatory pathway for land application of biosolids.However, defining urine as a biosolid also prohibits the use of diverted urine in certified organic farming, because 'sewage sludge' is explicitly excluded from the list of possible nutrient management options for crops adhering to the USDA Organic standard [51].The Organic Materials Review Institute, which is a private certification scheme for organic inputs, does not even include an entry for human urine in its database of materials [52].

Stakeholder coordination to advance adoption of UD systems
To advance adoption of UD systems in buildings, researchers may benefit from coordinating with other organizations and stakeholders [53], especially given that UD represents a system-of-systems.Revising the model codes is an essential first step which can then trickle down to state and local codes [54].However, even if model codes are updated to permit UD systems, private standards, including LEED, may continue to impact adoption decisions for UD systems in CI buildings.Thus, it may be helpful to work directly with firms and commercial developers to ensure that any certifications or internal standards are aligned with basic prerequisites for UD systems.Another approach might be to advocate for changes in LEED or other certifications to award more points for water conservation and UD systems.Lastly, U.S. researchers should consider working with the U.S. Environmental Protection Agency and state environmental agencies to create a clearer regulatory pathway for reuse of diverted urine.

Conclusion
UD systems afford numerous environmental benefits and are generally viewed positively by end-users.Nevertheless, adoption of UD systems in developed countries like the U.S. remains low.As the previous sections have shown, UD involves a system-of-systems rather than a single technological intervention, and each subsystem is governed by multiple regulatory regimes.Even if other social and economic concerns are addressed, it is unlikely that urine diversion can be scaled up beyond the pilot or trial stage if innovators do not attend to these regulatory regimes.By coordinating with other stakeholders, including federal agencies, SDOs, and third-party certifiers, researchers can help ensure that implementing UD systems in CI buildings is actively incentivized through the language of codes, standards, and regulations.

Figure 1 .
Figure 1.Schematic representation of UD system-of-systems (yellow boxes) with regulatory regimes overlaid in 'layers' (gray boxes).Boxes with dotted lines indicate voluntary or quasi-voluntary regimes.This figure assumes

Table 1 .
Key regulatory regimes reflected in figure1.