Understanding barriers and facilitators to the adoption of sanitation technologies is crucial to the diffusion of these innovations. Urine-diverting dry toilets (UDDTs) have been implemented throughout regions that are lacking improved sanitation. This review applied the Diffusion of Innovation theory to 18 research articles that collected end-user/potential end-user feedback on UDDT pilot projects in low- and lower-middle-income countries to better understand barriers and facilitators to adoption. Results show the main barriers to adoption were beliefs incompatible with UDDT by-product reuse, a lack of supporting infrastructure, high capital costs, and ineffective promotional communication strategies. The benefits promoted by the diffusion agency were often misaligned with the desires and interests of the potential adopters. Ensuring supporting infrastructure was in place and tailoring the promotional communications to the adopters’ perspectives facilitated UDDT adoption. The main facilitator was the perceived relative advantages of the UDDT over existing sanitation options, including fertilizer production. This indicates that safe treatment and reuse processes need to be implemented. Urine diversion was rarely mentioned as a barrier. These findings can help communities and organizations working to increase sanitation technology adoption in low-income, rural areas by better understanding common barriers and facilitators to adoption.

  • • Collection and reuse of human waste as fertilizer is the main facilitator to adoption but may be incompatible with existing beliefs.

  • • Providing ongoing training on safe emptying or creating an emptying/reuse service is crucial to successful UDDT implementation.

  • • Ensuring sufficient ash or dry material facilitates dry sanitation adoption.

  • • Propagators of UDDTs should focus on household scale economic, water, user experience, and health benefits.

  • • Urine diversion was not a barrier to technology adoption.

Only 54% of the global population has access to safely managed sanitation facilities as of 2020 (U.N. 2021). Sanitation coverage values are skewed towards urban areas (Hutton & Chase 2016), indicating that rural areas have less access. The number of rural sanitation users is likely even lower, given that access does not necessarily equate to use (Kanda et al. 2021). This illustrates a large gap between the current state of sanitation and the Sustainable Development Goal of access to and sustainable management of sanitation for all by 2030 (U.N. 2021).

The sanitation reality is far from the objective of resource recovery systems that can benefit communities. Eighty percent of global wastewater is untreated, meaning treatment alone is occurring only in a minority of cases (U.N. 2020). Blackett et al. (2014) found that only two of 12 studied urban sanitation systems in low- and lower-middle-income countries showed any capacity for fecal sludge reuse or recycling. One group of solutions that have been designed to address sanitation and resource recovery needs are EcoSan (Ecological Sanitation) toilet systems. These sanitation solutions take an ecological approach and include resource recovery and reuse (Langergraber & Muellegger 2005). One subcategory of these toilets is the urine-diverting dry toilet (UDDT).

Urine diversion toilets are sanitation solutions that collect or distribute urine separately from the fecal matter at the point-of-use. One type of urine-diverting toilet is the UDDT (Figure 1). UDDTs are unique among urine diversion toilets because they are designed to avoid any liquid entering the fecal chamber. The systems may include anal wiping or anal washing with water diversion away from the fecal chamber. UDDTs have been introduced in several low- and lower-middle-income countries. Designs vary and are continually evolving to overcome barriers to acceptance (Hu et al. 2016).
Figure 1

Diagram of a UDDT structure with fecal matter being emptied for potential reuse (Tilley et al. 2008).

Figure 1

Diagram of a UDDT structure with fecal matter being emptied for potential reuse (Tilley et al. 2008).

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From the vantage point of implementing agencies, here called propagators, the UDDT design has several advantages compared to traditional pit latrines (Figure 2). UDDTs are typically built above ground, making them less susceptible to flooding during the rainy season or in areas with high water tables (Borges Pedro et al. 2020). Manual emptying can be done more easily and without pit entry. An unexcavated design also removes the risk of pit collapse and separating the urine and fecal matter reduces malodors. The ability to reuse treated feces and urine from the UDDT as fertilizer improves nutrient recovery and adds economic value (Roma et al. 2012).
Figure 2

Pit latrines are often unlined and unemptied allowing waste infiltration into groundwater (OpenWASH 2016).

Figure 2

Pit latrines are often unlined and unemptied allowing waste infiltration into groundwater (OpenWASH 2016).

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Despite these advantages, UDDT projects have not always been successful in promoting widespread adoption. To remove barriers to adoption, the adopters’ mindsets, the technology, or both must be altered (Lienert & Larsen 2006). Larsen et al. (2013) noted that research should focus on facilitators and barriers to the adoption of urine-diverting toilet technologies so engineers can design needed improvements, as the collection and evaluation of feedback on sanitation technology was limited in the past. It is important to understand sanitation solutions in the complex set of contextual and motivation factors (Novotný et al. 2018).

To better understand the perspectives of communities engaging with UDDT as a potential sanitation solution, Diffusion of Innovation theory was utilized. Diffusion of Innovation theory focuses on how innovation is disseminated through a social system and either adopted or rejected (Rogers 1995). This framework incorporates aspects of both how the potential adopter of the innovation (i.e., technology) engages with the product and the various strategies by which they are engaged by the propagator (i.e., company, public service) promoting the innovation (Brown 1981; Rogers 1995). Numerous exploratory, pilot, and full-scale implementations of UDDT have occurred across the Majority World. The literature discussing feedback from end-users in these projects is included in this review.

This literature review applies the Diffusion of Innovation theory to UDDTs in low- and lower-middle-income countries (per 2020 World Bank definitions) to evaluate the current state of social and cultural acceptance and elucidate focal improvement areas for future implementation. This approach is unique in that only information collected from surveyed or interviewed adopters/potential adopters is considered. The objective is to understand barriers and facilitators to the adoption of UDDTs from the adopters’/potential adopters’ perspective. The results can inform communities, diffusion agencies, and engineers of areas for program and design adaptation.

This literature review targeted information on adopter/potential adopter feedback on UDDTs. The research databases searched were Scopus, PubMed, and Web of Science. Search terms were used to identify UDDTs, specifically, as a discussed technology and to select for articles describing an intervention or the solicitation of adopter/potential adopter (end-user) feedback. Only articles written in English were included. Searches were conducted on January 29, 2021.

Search terms

  • • Terms selecting for technology:

  • ‘dry toilet’ OR UDDT OR ‘urine diver* dry toilet’ OR ‘urine diver* latrine’ OR ‘urine diver* toilet’ OR ‘UD toilet’ OR ‘UD latrine’ OR ‘dry latrine’

AND

  • • Terms selecting for intervention and end-user opinions solicited:

  • adoption OR adopter OR uptake OR operation OR maintenance OR behaviour OR behavior OR ‘decision making’ OR demand OR attitude OR norm OR improvement OR intervention OR program OR marketing OR promotion OR ‘health education’ OR ‘cultural values’ OR beliefs OR ‘perceived risk’ OR preferences OR perception OR ‘cultur* accept*’ OR ‘feedback’ OR ‘social* accept*’ OR ‘survey’ OR ‘barrier*’ OR ‘facilitator*’ OR ‘adopt*’ OR ‘interview*’ OR ‘opinion’ OR ‘community meet*’ OR ‘iteration’ OR ‘pilot’ OR ‘taboo’ OR ‘religious belie*’ OR ‘ostraci*’ OR ‘tradition* belie*’

Selection process

The selection process began with all articles resulting from the search terms. Scopus, PubMed, and Web of Science returned 474, 167, and 51 items, respectively. The selection process is described in Figure 3 and resulted in 18 relevant articles.
Figure 3

Method of article selection, indicating inclusion criteria and number of articles at each step.

Figure 3

Method of article selection, indicating inclusion criteria and number of articles at each step.

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Coding process

To analyze the adoption of UDDTs, Diffusion of Innovation theory, as articulated in the work of Rogers (1995) and Brown (1981), was used. The two stakeholders considered in this framework are the adopter/potential adopter, synonymous with end-user in this instance, and the propagators or diffusion agents who aim to promote adoption. Rogers (1995) focuses on the demand-side of innovation diffusion. This is articulated through five aspects of how the technology or innovation is perceived by the adopter/potential adopter (Table 1). Brown (1981), in the market and infrastructure perspective, focuses on the supply-side of Diffusion of Innovation. This includes the actions and approaches taken by the propagator to make the innovation available to adopters (Table 1). For each aspect, relevant data were coded as either a barrier or facilitator to adoption.

Table 1

Framework used to code text based on the Diffusion of Innovation theory

Demand-side (Rogers 1995) 
Trialability The extent to which the innovation can be tested or experimented with before a commitment to adopt is made 
Observability The extent to which the innovation provides tangible results and can be seen by others 
Relative advantage The degree to which an innovation is seen as better than the idea, program, or product it replaces 
Compatibility How consistent the innovation is with the values of the potential adopters 
Complexity How difficult the innovation is to understand and/or use 
Supply-side (Brown 1981) 
Physical proximity to diffusion agency The innovation is propagated or diffused in a nearby location (market, store, via extension agent, etc.) 
Financial resources/price The price set by the propagator and the ability of potential adopters to afford the innovation 
Supporting infrastructure Any infrastructure (energy, water, roads, delivery/collection services) needed for the innovation to be feasible are accessible to all potential adopters. Supporting infrastructure was interpreted to include physical infrastructure, supply chains, supply access, associated services, supporting systems and the UDDT design itself. 
Promotional communication strategy The propagator or diffusion agency's strategy for informing potential adopters about the innovation 
Demand-side (Rogers 1995) 
Trialability The extent to which the innovation can be tested or experimented with before a commitment to adopt is made 
Observability The extent to which the innovation provides tangible results and can be seen by others 
Relative advantage The degree to which an innovation is seen as better than the idea, program, or product it replaces 
Compatibility How consistent the innovation is with the values of the potential adopters 
Complexity How difficult the innovation is to understand and/or use 
Supply-side (Brown 1981) 
Physical proximity to diffusion agency The innovation is propagated or diffused in a nearby location (market, store, via extension agent, etc.) 
Financial resources/price The price set by the propagator and the ability of potential adopters to afford the innovation 
Supporting infrastructure Any infrastructure (energy, water, roads, delivery/collection services) needed for the innovation to be feasible are accessible to all potential adopters. Supporting infrastructure was interpreted to include physical infrastructure, supply chains, supply access, associated services, supporting systems and the UDDT design itself. 
Promotional communication strategy The propagator or diffusion agency's strategy for informing potential adopters about the innovation 

This research focused on articles where adopter/potential adopter feedback was explicitly solicited and reported. As a result, information gathered from adopters/potential adopters was coded into the Diffusion of Innovation framework. Efforts were made to omit content that was reported from the view of the article authors, who were not target adopters/potential adopters. Propagator opinions were also not coded, when possible. Therefore, the demand and supply sections of the coding were, as best as possible, from the perspective of the adopter. For each aspect, relevant data were coded as either a barrier or facilitator (Table 2). Coding was done using Nvivo 12 software.

Table 2

Examples of text assigned to a facilitator and barrier code within the demand and supply sides of the framework

Code assignmentExample text coded (source)
Demand → Complexity → Barrier ‘The technology was seen as complex’ (Seleman & Bhat 2016
Demand → Complexity → Facilitator ‘Indicated that the new toilets were easier to install (compared to digging a pit)’ (Nyoka et al. 2017
Supply → Financial Resources/Price → Barrier ‘After they left [NGO officials], some people showed interest in EcoSan latrines but no one built it because they do not have money to buy cement.’ (Kumwenda et al. 2016
Supply → Financial Resources/Price → Facilitator ‘Whereas it was seen as a financial benefit by some owners as water is costly: ‘With our ecological latrine we don't waste water anymore […]. We monthly pay water to a private company, and it is expensive you know.’ (Eelderink et al. 2017
Code assignmentExample text coded (source)
Demand → Complexity → Barrier ‘The technology was seen as complex’ (Seleman & Bhat 2016
Demand → Complexity → Facilitator ‘Indicated that the new toilets were easier to install (compared to digging a pit)’ (Nyoka et al. 2017
Supply → Financial Resources/Price → Barrier ‘After they left [NGO officials], some people showed interest in EcoSan latrines but no one built it because they do not have money to buy cement.’ (Kumwenda et al. 2016
Supply → Financial Resources/Price → Facilitator ‘Whereas it was seen as a financial benefit by some owners as water is costly: ‘With our ecological latrine we don't waste water anymore […]. We monthly pay water to a private company, and it is expensive you know.’ (Eelderink et al. 2017

Inter-coder consensus building

The described coding framework was provided to a rural sociologist who researches and teaches in the Diffusion of Innovation space. They coded three of the 18 articles. The primary coder, first author on this article, coded the same articles and then noted areas of agreement between the two coders. The two coders then met to discuss areas of disagreement and interpretations of the coding labels. The agreement was reached on all coding for the three articles and the primary coder used this informed understanding to code the remainder of the articles.

Consolidating themes

Text coded as barrier or facilitator within each of the nine subcategories (Table 1) was reviewed. Specific aspects acting as barriers or facilitators were listed and the number of articles in which the theme recurred was noted. Barriers or facilitators mentioned in three or greater articles are listed in the Results section.

Limitations

This article provides useful information for those interested in understanding UDDT adoption, but several limitations should be noted. The authors chose to limit the research findings to only those published in peer-reviewed journals or conference proceedings and have specific methods for end-user/adopter feedback. This removed several reports or articles discussing UDDT that did not meet these two criteria. This was done to enable a focus on end-user/adopter perspectives but likely resulted in the exclusion of some relevant resources. Three articles were coded by two individuals, with the remainder having only a single coder.

Rural areas of low- and lower-middle-income countries were partially selected to control for people's previous experience of owning indoor, plumbed toilets, which would conceivably impact their reaction to UDDT. This is an imperfect assumption. The inclusion of low-income and lower-middle-income countries covers an enormous swath of the human experience and although similarities may be found, the fact that each community and individual’s response to UDDT is informed by unique opinions and experiences should not be ignored by practitioners.

Eighteen articles were selected in total (Appendix A, Supplementary material). One article each focused on Malawi, Palestine, the Philippines, Rwanda, and Tanzania. Two articles each focused on Bolivia, Burkina Faso, Morocco, Nepal, and Uganda. Three articles focused on Kenya.

All selected articles used a methodology section to describe the data collection processes utilized. The most common approaches were surveys/questionnaires and interviews. Many articles used additional methods including focus group discussions (FGDs), mass gatherings, photo-elicitations, transect walks, ethnography, observation, and/or accessing secondary data. Only two articles mentioned approaches to selecting male/female respondents specifically in the interview methodologies and a third indicated a gender analytical framework was applied but gave no further detail.

Text coded to the Diffusion of Innovation framework resulted in more instances of coding to the demand than the supply side (Figure 4). Aspects of relative advantage, compatibility, and complexity played important roles in adopter decisions. Propagators’ approaches to pricing, supporting infrastructure, and communication strategies also impacted UDDT diffusion.
Figure 4

Proportion of selected articles with coding as facilitator (F) or barrier (B) within the supply and demand sides of the Diffusion of Innovation framework. Shading light to dark represents increased number of articles coded to category.

Figure 4

Proportion of selected articles with coding as facilitator (F) or barrier (B) within the supply and demand sides of the Diffusion of Innovation framework. Shading light to dark represents increased number of articles coded to category.

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Tables 36 list facilitators and barriers recurring in three or more articles. Categories not included indicate no common themes presented in three or more articles were noted within that category. For example, no specific complexity facilitator was noted in three or more articles, resulting in the complexity heading not being present in Table 3. Major components were related to the reuse of human waste as fertilizer, household scale benefits, concerns around capital cost, dry material acquisition, and a variety of relative advantages.

Table 3

Demand-side facilitators reported in >3 articles and their frequency

Relative advantageArticles
Benefit of using human urine and/or excrement as fertilizer 
Improved health 
No/reduction of bad smells 
Improved agricultural yields and/or crop quality 
Economic savings 
Water savings 
Reduction in insects/flies 
Cleaner toilets and surroundings 
No/reduced contamination of water sources or environment 
Safety and/or convenience due to proximity to house 
Improved dignity and/or seen as modern 
CompatibilityArticles
Feel that reuse of by-products is safe 
TrialabilityArticles
Ability to trial UDDT fertilizer on land 
Relative advantageArticles
Benefit of using human urine and/or excrement as fertilizer 
Improved health 
No/reduction of bad smells 
Improved agricultural yields and/or crop quality 
Economic savings 
Water savings 
Reduction in insects/flies 
Cleaner toilets and surroundings 
No/reduced contamination of water sources or environment 
Safety and/or convenience due to proximity to house 
Improved dignity and/or seen as modern 
CompatibilityArticles
Feel that reuse of by-products is safe 
TrialabilityArticles
Ability to trial UDDT fertilizer on land 
Table 4

Demand-side barriers reported in >3 articles and their frequency

Relative advantageArticles
People who do not have own use for by-products do not see advantage 
CompatibilityArticles
Handling excreta seen as unacceptable 
Reuse of excreta on food crops unacceptable 
Anticipated risk of disease from handling sludge 
Difficulty or inability to do anal cleansing and maintain UDDT design 
Incompatibility with religion or religious beliefs (separate from anal cleansing) 
ComplexityArticles
Seen as complex and labor intensive 
Tenants, guests, children, or new users do not understand how to use 
ObservabilityArticles
Opinions, judgement, and misperceptions by neighbors 
Relative advantageArticles
People who do not have own use for by-products do not see advantage 
CompatibilityArticles
Handling excreta seen as unacceptable 
Reuse of excreta on food crops unacceptable 
Anticipated risk of disease from handling sludge 
Difficulty or inability to do anal cleansing and maintain UDDT design 
Incompatibility with religion or religious beliefs (separate from anal cleansing) 
ComplexityArticles
Seen as complex and labor intensive 
Tenants, guests, children, or new users do not understand how to use 
ObservabilityArticles
Opinions, judgement, and misperceptions by neighbors 
Table 5

Supply-side facilitators reported in >3 articles and their frequency

Communication strategyArticles
Sensitizing, workshops, and awareness days were beneficial in improving adoption 
Users having the opportunity to see technologies in use 
Promoting reuse of waste as fertilizer or to improve soil conditions 
Financial resources or priceArticles
Maintenance costs decreased or low 
Communication strategyArticles
Sensitizing, workshops, and awareness days were beneficial in improving adoption 
Users having the opportunity to see technologies in use 
Promoting reuse of waste as fertilizer or to improve soil conditions 
Financial resources or priceArticles
Maintenance costs decreased or low 
Table 6

Supply-side barriers reported in >3 articles and their frequency

Supporting infrastructureArticles
Issues with accessing, refilling, and keeping dry wood ash 
Lack of interest in emptying and/or reuse of by-products 
Lacking a third-party to handle emptying and sale of by-products 
Communication strategyArticles
Lack of training on how to empty and/or monitoring visits to follow-up 
Financial resources or priceArticles
High capital costs 
Construction reliant on subsidies or UDDT being free 
Supporting infrastructureArticles
Issues with accessing, refilling, and keeping dry wood ash 
Lack of interest in emptying and/or reuse of by-products 
Lacking a third-party to handle emptying and sale of by-products 
Communication strategyArticles
Lack of training on how to empty and/or monitoring visits to follow-up 
Financial resources or priceArticles
High capital costs 
Construction reliant on subsidies or UDDT being free 

In this section, the most commonly occurring barriers and facilitators are explored further in the context of literature on sanitation solution acceptance. Each section is followed by a list of considerations for facilitating the future adoption of UDDT in low- and lower-middle-income countries. A section on urine diversion notes that this defining feature of UDDT was not frequently mentioned as either a direct barrier or facilitator. The facilitators of adoption were heavily weighted toward relative advantage. The barriers to adoption centered around compatibility, supporting infrastructure, and financial resources/price. Human excreta reuse is discussed as a common facilitator and barrier, with the safety of human excreta reuse expanded upon as an associated barrier. The promotion of household scale benefits is contextualized because the relative advantage facilitators focused on these benefits. Strategies and approaches to capital cost are considered, as the high capital cost was the most mentioned barrier to acceptance of UDDT. Lack of dry materials was a frequently noted supporting infrastructure barrier and context and potential solutions are described.

Urine diversion

Issues regarding urine diversion at point-of-use were only identified in two articles (Kumwenda et al. 2016; Nyoka et al. 2017). This indicates that the urine diversion aspect of the UDDT is not a prominent barrier to adoption and current designs meet community needs in this regard. Difficulty in successfully diverting urine for people who had undergone female genital mutilation was noted as a concern in one article (Nyoka et al. 2017). Urine diversion systems could be blocked by soil, ash, or children's feces, causing urine to back up. This caused some people to avoid urinating in the diverter. The issue of clogged pipes indicates that the act of utilizing the diverter is not in itself a barrier to acceptance, but that lack of training and proper use leads to malfunctions. Research into reducing urine diversion clogging due to struvite precipitation from urine is focused on removable cartridge traps and responsive dosing of acids or bases to move the pH outside the functional range of urease (Boyer & Saetta 2019). Training individuals on the potential for UDDT urine pipe clogging is recommended (Kilbride et al. 2013).

Training and explanations would also address the issue noted in four articles that tenants, guests, children, or new users often do not understand how to use the UDDT. The reluctance to have visitors use the toilet because of concerns around incorrect use was echoed in a pilot of an updated urine-diverting toilet design in a peri-urban area (Sutherland et al. 2021). This concern can be addressed through ongoing training. One article in this review noted that having instructive posters within the UDDT assisted in communicating proper use (Eelderink et al. 2017).

Alternative solutions may be needed for young children (under 6 years old). Communities often allow open defecation by young children with the expectation that adults will move the feces into the UDDT. An alternative solution is a non-separating children's toilet made of a 5-gallon bucket that is placed inside the superstructure (Kilbride et al. 2013).

Human excreta reuse

Despite excluding articles focused solely on acceptance of UDDT by-product reuse, human excreta reuse still appeared as a main facilitator and barrier to UDDT adoption. The most noted relative advantage facilitator was the reuse of human waste as fertilizer, mentioned in nine articles. At the same time, five articles noted a main compatibility barrier that handling human waste was seen as unacceptable. This contradiction can be seen elsewhere in the literature. The agricultural value of human excreta compost was expressed as a main benefit of Arborloo adoption in rural Ethiopia, noting that the seedlings in the Arborloo grew faster and were healthier than those planted elsewhere (Fry et al. 2015). Contrary findings in another Ethiopian study found that composting toilets were left open for pigs and dogs to spread the waste because the concentrated waste was seen as dangerous and undesirable (Rheinlander et al. 2010). In a study of latrine adoption, Juran et al. (2019) found that both adopter and non-adopters used hygiene, purity, and ‘naturalness’ as reasons for their contradictory latrine adoption decisions. Differences in individual and community acceptance align with the theory of variation in norms between and within communities (Rogers 1995). Therefore, community attitudes towards handling and reuse of human waste must be considered ahead of UDDT implementation.

Dry material availability concerns

Dry material, specifically ash, availability was noted as a UDDT maintenance issue in five articles. Conversely, two articles did mention the benefit of being able to use ash, a by-product of the existing daily process of cooking, in the UDDT (Abarghaz et al. 2013; Taouraout et al. 2018). The issue of sourcing and storing dry material was noted by several UDDT owners in a study in Panama (Mehl et al. 2011).

A variety of dry materials have been reported as being used in UDDT. Surveyed end-users in Panama noted using sawdust, wood ash, sawdust/wood ash combination, dry grasses or leaves, dry dirt, sand, and rice or coffee husks (Mehl et al. 2011). Traditional urine diversion toilet users in Northern Vietnam utilized sawdust, ash, and dry soil (Harada et al. 2010). Sugar cane husks are commonly used in UDDTs in Haiti (Berendes et al. 2015).

Studies have also been conducted on dry material combinations that have not been described in practice. Oyster shells and oyster shell/ash combinations were tested at the lab-scale (Magri et al. 2013). McKinley et al. (2012) recommended combining ash or lime with an organic matter, such as sawdust or rice husks, to improve composting and reduce odors. In a study of odor control of fecal sludge from public latrines in Ghana, Senanu et al. (2021) tested locally available waste materials as potential additives, including ash of coconut husks and cocoa husks, biochar of rice husks, bamboo, and sawdust, and powder of rice husks, sawdust, moringa leaves, and neem seeds. They found that sawdust biochar had the best performance in H2S and NH3 suppression and was economically feasible, with rice husk biomass as a less effective but functional alternative (Senanu et al. 2021).

One issue to consider when selecting or promoting dry materials is the treatment process being employed. Fecal matter can be treated through either composting, which relies on increased temperatures to kill pathogens, or desiccation, which relies on low moisture content to inactivate pathogens. Composting will require carbon-rich additives, such as sawdust or rice husks, while desiccation will mainly rely on alkaline additives, such as ash or lime. Both the type and quantity of dry material added will impact the treatment efficacy. Mehl et al. (2011) found that household users of composting toilets in Panama did not add sufficient dry material for treatment via desiccation only, but most had added enough dry material to reach moisture contents supportive of composting. Alternatively, adding too much dry material, except for lime, was seen to preserve pathogens in a pilot-scale study (Magri et al. 2013). Therefore, understanding and communicating the effective dose of dry material(s) to be added after each defecation is important to safe UDDT use. A design of a urine diversion toilet with an ash dispenser was introduced in Kenya to reduce direct handling of dry materials (Mayaka et al. 2018). Although such a design does not resolve sourcing issues, this could be used to match the ash content required for treatment to the amount dispensed per use. Understanding which dry materials can be interchanged and identifying source streams ahead of implementation can address sourcing issues by providing various streams without sacrificing treatment outcomes. This dependency on access to, or delivery of, raw materials illustrates infrastructure dependence as discussed by Brown (1981). The market and infrastructure lens indicate that innovations need supporting systems to be in place to facilitate diffusion (Brown 1981). The ability of the local infrastructure to provide sufficient dry material is often overlooked in UDDT promotion and should be considered in future projects.

Safety and processing considerations for emptying and reuse

An anticipated risk of disease when handling waste products and a lack of training on how to empty UDDTs were each noted as barriers in three articles. If human waste treatment and reuse in agriculture is agreed upon as an objective of the UDDT implementation, understanding the safety of recommended processes is critical. Communities with a willingness to reuse the waste may not utilize effective safety protocols without ongoing training. Jensen et al. (2008) found that farmers in Vietnam did not express safety concerns when using latrine waste as fertilizer, despite using it prior to the recommended 6-month composting period. In line with recommendations by Heppleston (2009), this review asserts that knowing the conditions and timelines that lead to safe emptying and reuse is required on the part of the propagator. This information must also be communicated and understood by those interacting with the waste, whether that be UDDT adopters or a third-party service provider. UDDT adoption is thus infrastructure constrained, per Brown (1981), because the innovation cannot spread unless training programs or collection services are implemented.

Treatment of fecal matter from UDDT for pathogens is typically approached through means of desiccation, pH, temperature (composting), ammonia addition, or a combination of these treatments. Additionally, Austin & Cloete (2008) found soil to be an effective additive for removing pathogens, which the authors attributed mainly to competition by the soil microbiome. Different means of treatment can have more or less impact on different pathogen types such as bacteria, bacteriophages, and helminths (Magri et al. 2013).

Ascaris eggs are considered the most persistent fecal indicator and are often used to determine treatment efficacy. A study of Ascaris egg survival using various percentage lime additions found 99% inactivation of Ascaris eggs within 4 months (Jensen et al. 2009). Free ammonia, released at high pH, was thought to play an important role in egg deactivation, indicating that the same results would not apply to urine diversion systems because the ammonia content would be limited through urine diversion (Jensen et al. 2009). Several studies have since explored the use of urea or urine as an additive to fecal sludge from UDDT to improve pathogen inactivation, particularly of Ascaris eggs.

The use of free ammonia, added in the form of urine, to inactivate Ascaris lumbricoides eggs in a fecal matter/ash mixture from UDDTs found that a 2:1 ratio was most effective (Trimmer et al. 2016). The addition of ash to the mixture beyond that used by the UDDT users did not significantly impact inactivation (Trimmer et al. 2016). The treatment testing was done outside the chamber and success was based on a 2-log decrease during the secondary treatment. Off-site composting of UDDT fecal chamber contents found that high temperatures due to composting were the main cause of a decrease in Ascaris egg viability, but that treatment may have been aided by the addition of urine to the compost bin potentially causing ammonia inactivation (Berendes et al. 2015). In secondary treatment of dewatered fecal sludge, increased sawdust to sludge ratio was seen to decrease Escherichia coli, fecal coliform, Salmonella, and helminth egg presence in dried biosolids, indicating sawdust addition aids pathogen removal, but does not accomplish complete inactivation (Mensah et al. 2013).

Out-of-chamber or off-site treatment systems still necessitate interaction with the waste before complete treatment. Magri et al. (2013) tested potential additives to create acceptable pathogen reduction within the fecal chambers. Urea was added directly to an oyster shell/ash mixture and layered with human waste to replicate a UDDT fecal chamber. Treatments combining desiccation and urea treatment (free ammonia) were most effective against Ascaris eggs. In a study of feces from a UDDT system, Ascaris eggs inactivation was associated with moisture contents (<3%), but was not impacted by treatment with high pH additives (Endale et al. 2012). Temperature, total ammonia and moisture content may be more important to helminth inactivation than high pH levels.

Ongoing research on ammonia inactivation of helminths provides interesting considerations for UDDTs when balancing odor reduction and treatment efficacy.

Lime was seen to inactivate fecal coliforms, whereas reduction of moisture content to 3% did not; the opposite of impacts on Ascaris eggs (Endale et al. 2012). This indicates that various means of treatment may be required to inactive bacterial and helminth pathogens within UDDT fecal chamber contents. A 50:50 mixture of oyster shells:ash adjusted to 0.5% urea concentration was found to be the most effective additive when studying bacteria, bacteriophage and helminth egg viability in UDDT fecal chamber conditions (Magri et al. 2013). It is likely that designers will need to consider inactivation solutions for helminth egg and other pathogens separately. Design and use protocols should be developed to support multiple treatment practices.

Models and low-resource testing procedures are being developed to improve the ability of individuals to monitor the safety of composted human waste. The ability to model E. coli and helminths based on total solids testing could help increase knowledge and trust in fecal sludge safety (Kalulu et al. 2021). A predictive model for safety was developed and found that ammonia, temperature, moisture content, and pH impact different pathogens to different extents and sometimes in contradictory ways (Oishi et al. 2021). Until these models and the associated tests become more robust and accessible, UDDT diffusion will be impacted by limited infrastructure to support safety monitoring of treated fecal matter.

Promoting household scale benefits

Nine relative advantages unrelated to fertilizer reuse were mentioned as facilitators in three to five articles each. These relative advantages described benefits to the household or individuals. Focusing the promotional communication strategy on relevant household scale benefits will improve the value proposition for UDDTs. Keraita et al. (2013) found that latrine adopters in rural Ghana focused more on benefits of safety, privacy, convenience, and dignity than those associated with health. These findings were mirrored in a study of latrine acceptance in rural Niger where adopters again did not see health as the main benefit, but instead noted privacy and proximity (Diallo et al. 2007). Convenience, hygiene, and safety/security were similarly the most noted relative advantages in case studies of ecological sanitation across seven countries (Heppleston 2009). This review did find that improved health was a commonly mentioned relative advantage (five articles) but echoed the idea that household benefits such as convenience, dignity, economic and water savings, and a better user experience were important (Table 3). These findings suggests that propagators should utilize these items in communications, rather than focusing on community-scale benefits. This aligns with the concept of heterophily, where differences in socioeconomic status, education and lived experience between the propagator and adopter can lead to difficulty in diffusing information (Rogers 1995). The propagator may view overall public health and environmental pollution reduction as benefits, but these are not aligned with the adopters’ perspective.

The reduced odors associated with UDDTs compared to traditional pit latrines was noted in five articles as a relative advantage and should be key in UDDT promotional communications. Rheinlander et al. (2010) found that adopters/potential adopters of latrines noted that totally enclosed toilets were seen as potentially dangerous disease agents because of the dirty smell within the latrine. The idea that unpleasant smells could cause disease and that the strength of the smell indicated the severity of illness was noted in studies in both Ethiopia (Alemu et al. 2017) and Vietnam (Jensen et al. 2008).

Fear of smell may partially explain why Harvey (2011) found that 80–90% of latrines designed by community members in Zambia had toilet/orifice covers, but less than 45% had roofs. Two articles in this review noted missing or leaking roofs as barriers to UDDT adoption. The resulting issues were an uncomfortably hot toilet seat on sunny days and difficulty maintaining dry ash for the UDDT, respectively (Kumwenda et al. 2016; Nyoka et al. 2017). It may be worth discussing the benefits and drawbacks of UDDT roofs from the perspective of each adopter/potential adopter to balance desires for reduced malodor and increased comfort. The feasibility of excluding water from dry materials and fecal chambers through bin covers and toilet lids should also be considered.

Improved modernity/dignity was noted in three articles as a facilitator of UDDT adoption. Perceptions of the UDDT as a modern sanitation system will be partially dependent on adopters/potential adopters’ previous sanitation experiences. In sub-urban Northern Vietnam, it was found that, compared to flush toilets, traditional urine diversion toilets were associated with older households (Harada et al. 2010). One article in this review described the differing opinions of UDDT end-users in a refugee camp coming from situations of open defecation and improved sanitation, respectively. Those not having previously used latrines tended to have a higher regard for the UDDT than those who had experience with improved sanitation. This highlights the need to consider existing sanitation structures and experiences when beginning a UDDT project.

Discussing costs

High capital costs were mentioned in six different articles as a barrier to adoption. This concern is common in research on sanitation adoption (Rheinlander et al. 2010; Keraita et al. 2013; Alemu et al. 2017). Higher prices, within a relative range, associated with a product (i.e., sanitation) are known to result in lower adoption rates (Brown 1981). Lowering prices may align with the objectives of propagators such as government or non-governmental agencies (Brown 1981) and this can be expressed in the use of subsidies. Subsidies have not produced long-term, sustainable latrine use (Sesan et al. 2018) and three articles in this review noted reliance on subsidies as a barrier to UDDT adoption. Only one article in this review mentioned subsidies as a reason for increased UDDT adoption (Uddin et al. 2014).

Cost–benefit analysis can be determined with communities by having conversations about the anticipated capital and maintenance costs of UDDTs and alternative latrines. Perceptions of UDDT maintenance costs as low (three articles) facilitate adoption. This is mainly because pit systems often require new pits be dug each time the pit fills. Ongoing structural maintenance creates an opportunity for latrine abandonment (Fry et al. 2015), indicating that the permanent structure of UDDT may be beneficial for long-term use. Savings gained through the reuse of by-products defraying fertilizer costs, increasing yields and/or offsetting energy costs should also be included in the conversation.

One proposed solution to capital costs concerns is community-led total sanitation (CLTS). CLTS focuses on motivating communities to improve sanitation systems without external funding by drawing attention to sanitation issues to create a felt need (Sesan et al. 2018). CLTS still requires resources from the propagator in the form of sensitizations, trainings and monitoring and evaluation, but hardware purchasing is not a part of the process (Harvey 2011). CLTS may also be paired with training local masons/technicians in latrine construction/maintenance and marketing these services via mass media (Briceno & Chase 2015).

When working with communities practicing CLTS, Alemu et al. (2017) found that sensitization did lead to individuals thinking latrines were healthier, but did not translate into open defecation free resolutions. It should be noted that CLTS can have negative impacts on households less able to afford sanitation improvements by creating or exaggerating stigmas (Sesan et al. 2018). Previous exposure to hardware subsidies has been seen to reduce the effectiveness of CLTS (Harvey 2011), and this should be considered before beginning a CLTS project within a community.

One of the benefits of CLTS is that adopters/potential adopters or local artisans are encouraged to design and build their own sanitation systems, with technical support as needed (Harvey 2011). This can lead to distinct solutions fitted to community desires and needs. Yet, this must be balanced with the technical components of UDDT, especially those impacting treatment efficacy. Working with local contractors to meet treatment goals, while understanding local material availability and preferences, could lead to higher UDDT adoption by overcoming price and infrastructure barriers (Brown 1981) and promoting perceptions of dignity, modernity and structural integrity shown to facilitate adoption.

This review focuses on understanding the opinions of adopters/potential adopters of UDDT from rural settings across low- and lower-middle-income countries to facilitate adoption and identify areas for consideration and improvement. Using the Diffusion of Innovation theory to frame direct feedback from adopters/potential adopters highlights barriers and facilitators to adoption as perceived in communities. The results illustrate that infrastructure, price, and compatibility are main barriers. The former two aspects relate to the supply-side of Diffusion of Innovation theory and point to areas for propagators to improve the likelihood of adoption. Compatibility, along with the main facilitator of relative advantage, express the adopters’ perception of and demand for the innovation. Knowing the specific issues and benefits as perceived by the adopter can assist in identifying areas where UDDTs may diffuse quickly. Specific considerations for facilitating adoption are summarized here in relation to each stage of a UDDT project.

Community and objective (self-)identification:

  • • Agricultural communities that are open to UDDT by-product reuse should be targeted.

  • • Communities where existing unimproved latrine systems have been used may be good candidates for UDDT sanitation solutions.

  • • Determine the desired end-use of fecal chamber materials early to determine appropriate additives.

  • • Determine the effective dose of additives and calculate amount required. Mehl et al. (2011) provides a brief example.

  • • Discuss prevalence of female genital mutilation within the community (understanding cultural appropriateness of discussing this topic is a prerequisite).

Pre-project discussions:

  • • Communicate the means of treatment with adopters to overcome safety concerns.

  • • If using a secondary treatment outside of the chamber, understand health concerns and necessary precautions.

  • • If UDDT reuse unacceptable for reasons beyond fear of safety, consider utilizing a third-party service-based model or alternative uses such as biochar (Koetlisi & Muchaonyerwa 2019), briquette (Ward et al. 2014), or fuel (Sharma et al. 2020) production, thermal treatment (Krueger et al. 2021) or incineration for ash production (Niwagaba et al. 2006).

  • • Promote household scale benefits of economics and water savings, reduced flies and odors, and improved individual/family health.

  • • Be open to variation in design if they do not compromise safety and treatment efficacy.

  • • Identify all waste materials that can be used as additives and if any processing capacity exists (i.e., to make biochar).

  • • Discuss maintenance costs, by-product value. and capital costs when considering UDDT.

  • • Communicate the role of dry material(s) in the treatment process of the UDDT.

  • • Plan for ongoing trainings of new residents or community members.

Design:

  • • Understand and design for effective in-chamber treatment through selection of dry material or additional additives and required ratios.

  • • Consider designs with automated dispensers to reduce variation and likelihood of pathogen persistence.

  • • Consider designs that limit variation in amount of additive used.

  • • Incorporate alternative designs or locations for use by small children (under 6 years old).

  • • Utilize dry material dispenser above fecal chamber to reduce risk of dry materials being added to urine diverter.

  • • Improve design to reduce likelihood that backed-up urine will flow into fecal chamber.

  • • Design ways to reduce clogging potential of urine diversion pipe.

Findings indicate that safe reuse of by-products as fertilizer for agricultural communities should be understood and promoted. Household scale benefits should be the focus of communication. Pre-emptively determining a sufficient supply of dry material or additives will reduce barriers to adoption, as will having proactive discussions around cost benefit analysis of the UDDT. Ensuring the proposed treatment process results in sufficient pathogen reduction within the UDDT by-products should be explored early in any project. By discussing these results in the broader sanitation context, identifying focus areas, and noting potential solutions, this review points UDDT propagators and adopters to important considerations for future implementation and research.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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