Introduction

Access to effective and sustainable water, sanitation, and hygiene (WASH) services is of paramount importance for public health and environmental protection. Despite significant progress in delivering WASH services worldwide in the last two decades, a significant portion of the Pacific population still lacks adequate WASH services1, with climate-driven hazards compounding risks for vulnerable populations2,3.

The Joint Monitoring Programme (JMP), led by the World Health Organization (WHO) and the United Nations Children’s Fund (UNICEF), is responsible for coordinating assessments of the current condition of WASH services across countries and states globally (available at washdata.org). The Pacific (Oceania) Region faces challenges in meeting the Sustainable Development Goals (SDGs) regarding drinking water and sanitation, with the slowest rate of improvement compared to any other region globally, and sanitation coverages considerably trailing behind those of drinking water1.

Within the Pacific region’s 21 countries, the Melanesia region, comprising Fiji Islands, New Caledonia, Papua New Guinea, Solomon Islands, and Vanuatu, has approximately 90.0% of the total Pacific population4. It is estimated between 25.0 and 45.0% of the total Melanesian urban population resides in informal or underserved settings, a figure equivalent to the entire population of Micronesia and Polynesia combined5.

Informal urban settlements are characterised by either 1) non-compliance with regulations and legal procedures, 2) lack of access to basic services and infrastructure, and 3) uncertainty in land tenure2,3,4,5. A surge in migration from rural to urban centres in Melanesian countries, driven by aspirations for an enhanced quality of life, education, income, and security against natural hazards4,6, combined with increasing urbanisation in those countries (ranging from 1.3 to 4.2% in 20227), lack of affordable and appropriate housing8, and escalating living costs in urban areas8 can explain the increasing proportion of the Pacific population currently residing in informal urban settings.

In the Pacific context, informal settlements have been characterised as “rural villages in the city” 9, where some of the norms, values and attitudes of rural life-ways are transposed into urban contexts, with the centrality of kin relationships being one of those. Although family ties often determine where migrants choose to settle in urban areas10, over time settlements have become increasingly ethnically diverse11. In other regions of the world, ethnic diversity in urban settlements has been shown to affect the preference for sanitation services12,13,14,15, such as the use of shared systems. Meeting these preferences is crucial for their adoption, long-term effectiveness, and sustainability12,13,14,15. Additionally to that, Melanesian countries are especially susceptible to severe impacts from hazards driven by climate-change16, meaning it is critical that sanitation services in informal urban settlements are able to withstand the environmental hazards they are frequently exposed to17.

The JMP WASH monitoring system provides important insights into sanitation access and rate of progress. However, it does not distinguish between formal and informal urban settings. To meet SDG 6 goals of leaving no one behind, understanding sanitation access in marginalised informal settlements is essential. While earlier research has offered preliminary insights into water and sanitation services in informal urban settings in Melanesia5, monitoring data regarding in place sanitation systems, residents’ preferences for sanitation systems, and exposures to, and impacts from, climate-driven hazards in informal urban settlements across Melanesia remains notably scarce18. Given the significant proportion of people residing in informal urban settings in the Pacific, it is imperative to gain a deeper understanding of existing sanitation systems, as well as the challenges and opportunities in ensuring adequate WASH services in those areas. This understanding is crucial for forward planning and adaptation to climate change, with the goal of meeting SDG 6 within the Pacific Region.

In this context, this research aimed to characterise in place sanitation systems, residents’ perceptions and preferences, and exposures to, and impacts from, climate-driven hazards, in sanitation systems in urban informal settlements in Melanesia. A comprehensive household survey, including visual verification of sanitation systems, was conducted in a sample of urban informal settlements across two countries in Melanesia. To the best of the authors’ knowledge, this research provides novel evidence by targeting the scarcely documented urban informal settlements of Melanesia, and, by combining information beyond in-place sanitation systems, to include residents’ preferences, and the impacts of climate-driven hazards. This integration offers valuable insights that add further depth to existing monitoring efforts and can help accelerating the SDG goal of universal access to sanitation services, particularly for marginalised and vulnerable populations in the slowest progressing region globally.

Methods

A household survey was conducted between September and November 2023, during the Spring season, covering 393 households in nine underserved urban areas or informal urban settlements in Suva, Fiji, and Port Vila, Vanuatu.

Locations

Survey sites (Fig. 1) were selected aiming to represent the diversity of natural environmental (e.g., type of terrain, slope, proximity to water resources and coastline), built environment (e.g., land tenure, land ownership), and human (e.g., ethnic groups) characteristics within the urban areas of the Fijian and Vanuatu capital cities. Formal and planned urban areas were not covered in this study. Information about natural and built environment, and human characteristics were acquired through desktop assessments and site visits before location selection (Table 1).

Fig. 1
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Location of surveyed urban settlements in Suva (Fiji) and Port Vila (Vanuatu).

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Table 1 Characteristics of surveyed urban settlements in Fiji and Vanuatu
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Suva, the capital city of Fiji, is located on the southeastern coast of the island of Viti Levu (Fig. 1). Fiji is predominantly an urban nation, with around 58.0% the population living in urban areas, mostly in Suva7. With a population of over 100,000 people, Suva is not only the largest city in Fiji but also its political, economic, and cultural hub. Known for its tropical climate characterized by warm temperatures and high humidity, Suva’s diverse population reflects Fiji’s multiculturalism, with indigenous Fijians (iTaukei), Indo-Fijians, and other ethnic groups. The city faces challenges such as the management of customary land, ensuring access to clean water resources, and addressing issues related to informal settlements19. It is estimated that at least 20.0% of the urban population in Suva lives in informal settlements across customary, state, and freehold land20,21.

In Suva, four informal settlements were surveyed: Muslim League, Namadi, Ragg Avenue, and Veidogo. The topography of settlements varies across all settlements (Table 1). The availability of local water resources also differs among settlements, with Muslim League, Namadi, and Ragg Avenue having surface streams within their boundaries, while Veidogo being closer to the sea. Muslim League and Veidogo are mostly on private freehold land, while Namadi is on state land, and Ragg Avenue on customary land.

Port Vila is located on the southern coast of the island of Efate (Fig. 1), serving as the capital and largest city of Vanuatu. With a population exceeding 50,000 residents, Port Vila is a key centre for commerce, administration, and tourism in Vanuatu. Its tropical climate is characterized by warm temperatures throughout the year and a tropical cyclone season during summer months22. Port Vila’s population reflects Vanuatu’s cultural diversity, encompassing indigenous Ni-Vanuatu, French, British, and Chinese influences. Increased migration to Port Vila23 results in challenges such as ensuring access to water and sanitation services, and managing informal settlements24. Most of the population growth is on the urban and peri-urban outskirts of Port Vila, with estimations that over 50.0% of the urban population lives in either customary or informal settings23.

In Port Vila, five settlements were surveyed: Blacksands, La Smet, Le Lagoon, Man Ples, and Ohlen Mataso. The settlements Blacksands and Le Lagoon are situated in flat terrains, while La Smet and Man Ples are in steep hilly areas. Local water resources include surface streams and the sea, with a common practice of rainwater harvesting across all settlements. Land ownership ranged from private freehold to customary land (Table 1).

Household survey questions and data collection

The results reported in this study are a subset of a larger study covering both water and sanitation services in informal settlements. The complete survey encompassed 43 primary questions, supplemented by follow-up inquiries contingent upon respondent answers, totalling up to 125 questions per respondent. Its objectives were threefold: 1) to understand, quantify, and map the water and sanitation service delivery models within underserved settlements; 2) to explore and record residents’ technological preferences, familiarity, and challenges in operating, maintaining, and repairing systems; and 3) to discern susceptibilities to hazards intensified by climate change, along with residents’ perceptions concerning climate change’s impact on water and sanitation systems.

The complete survey comprised six sections: 1) participant and household attributes (age, gender, education, location, number of residents); 2) water systems (main water source, drinking water source, accessibility convenience, operation and maintenance requirements and roles, preferences); 3) hazards concerning water systems (exposure history, types of hazards, incident response, impact level); 4) sanitation systems (toilet types, accessibility convenience, operation and maintenance requirements and roles, preferences); 5) hazards concerning sanitation systems (exposure history, types of hazards, incident response, impact level); and 6) perceptions of climate change (concerns about climate change impacts on water and sanitation, perceptions of mitigation strategies). Questions were either single or multiple answers, open answers, and visual infrastructure recordings. For this study, we focus solely on the questions regarding sanitation systems and household attributes according to its delimited scope.

Data collection occurred in October and November 2023, conducted by a team of six local research assistants. Ensuring gender parity, team members were proficient in relevant local languages besides English (Bislama, iTaukei, Hindi), and survey questions were translated into these languages prior to conducting fieldwork. Field researchers underwent comprehensive training in research ethics and water and sanitation systems.

The survey (Supplementary Table 1) was administered via SurveyCTO® utilising tablets connected to the internet. Prior consent for participation was obtained, with surveys conducted on the participants premises. Upon survey completion, participants were further solicited for consent to inspect toilets and containment units. Visual recordings (photographs) were made accordingly, and assessments regarding sanitation system types were conducted comparing participants’ responses and observed systems.

The number of households surveyed per settlement was determined to encompass 20.0 to 35.0% of all estimated buildings (B) within the settlement, determined utilising Open Street Map (OSM) data. In Suva, building densities within the surveyed urban settlements ranged from approximately 2000 B/km² in Ragg Avenue to 3700 B/km² in Namadi, while in Port Vila, building densities were generally lower, with the highest density observed in Ohlen Mataso at 3300 B/km² and the lowest in Blacksands at 370 B/km² (Table 1). Household selection followed a cluster random sampling method aimed at spatially covering the entire settlement area while avoiding neighbouring houses. Prior to the fieldwork, each settlement area was divided into zones based primarily on environmental features (e.g. low lying, furthers from the road, hillside, coastline). After completing a household survey within a zone, enumerators skipped 3 to 5 neighbouring houses before conducting another survey within the same zone. After a representative number of households surveys in a settlement zone was completed, enumerators moved to the next settlement zone until the entire settlement area was covered. Surveyed households were contingent on the presence of residents and their willingness to participate.

Classifying sanitation systems involved identifying and visually verifying facilities in the surveyed households. Each facility was assessed based on the design and functionality of the toilet and containment and/or treatment unit (if applicable), and the reported emptying and disposal of waste practices. Categories included both dry and water-based systems, where the latter encompasses both flush and pour flush systems. The categories were: dry pit latrine without slab, dry pit latrine with slab, ventilated improved pit latrine, composting toilets, water-based toilets without containment, water-based pit latrines without slabs, water-based pit latrines with slabs, water-based toilets to septic tanks, and water-based toilets connected to sewers. For more information about the key infrastructure, processes, strengths, limitations and differences among these sanitation systems, the reader is referred to other literature25,26. The options for actions taken in response to full sanitation containment units were categorised as: emptying by a household member, emptying by a private contractor, closing of full unit with the construction of a new containment unit, not happened yet, or respondent unaware.

Climate-driven hazards encompassed cyclones, heavy rain, sea level inundation, flooding, drought, and contamination (either to humans or perceived environmental contamination). The level of damage from exposures to hazards were categorized as minor, moderate, or heavy. Minor damage was classified as cases that did not interrupt access and operation of sanitation systems, moderate damage as limited access, and heavy damage rendered facilities inaccessible. Instances where hazards had not occurred were also noted.

Data analysis

Data were processed and analysed using R 4.3.127. Pre-processing included removal of outliers, categorisation of open questions, and consolidation of sanitation system in place. The consolidation of sanitation system in place was done by comparing the reported type of sanitation system by the resident, the observed system by the trained enumerator, and photography records by the rest of co-authors. The reported results refer to the processed data with verified sanitation system. In average, it was observed a 10.0% (3.5-37.5% across settlements) disagreement between the reported and observed sanitation system in place across all surveyed locations.

Visual inspections and statistical tests (i.e., chi-square test, multinomial regression, odds ratio) were performed using R base packages, ggplot228, and nnet29. All tests were set at significance level of p < 0.05, and reported tests were significant at this level, otherwise they are reported as non-significant. Where pertinent (e.g., small sample size), categories of sanitation systems were clustered according to the use of water in the sanitation system (e.g., dry pits encompassing dry pit latrines without slabs, dry pit latrines with slabs, ventilated improved pit latrines and composting toilets).

Multinomial regressions were calculated using the multinom() function from the nnet R package29. Candidate potential independent variables were screened using chi-square tests regarding the outcome variable. The final regression variables were selected based on the significance outcome of Wald’s test, ensuring at least one significant relationship for each category related to the outcome variable. Odds ratios were derived from regression coefficients. Two key outcomes were derived from this analysis: 1) residents’ preference for sanitation systems different from the one in-place, categorized by the type of sanitation system in place, and 2) the reported level of damage due to a climate-driven hazard per sanitation system in place.

Ethics

Ethical approval for this research was obtained from Griffith University (Griffith University Humans Research Ethics Committee, Approval Number 2023/210), The University of the South Pacific (USP) for research in Fiji and Vanuatu (USP Research Ethics Committee, Approval Number CR-09), and the Vanuatu National Cultural Council (Vanuatu Cultural Centre, Approval Number not applicable). The ethical protocols outlined in these approvals were diligently complied with, including strict adherence to participant consent procedures. Only adults over 18 years of age who have provided verbal and voluntary consent after being informed of the study’s purposes and protocol participated in this study.

Results and Discussion

Sanitation systems in place

The sanitation systems observed across the settlements in Suva (Fiji) and Port Vila (Vanuatu) varied in terms of toilet type (Table 2) and practices for waste collection and final disposal (Table 3).

Table 2 Distribution of sanitation systems in surveyed settlements in Suva (Fiji) and Port Vila (Vanuatu)
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Table 3 Reported actions for full sanitation containment unit by sanitation system in place in Suva (Fiji) and Port Vila (Vanuatu)
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In Suva settlements, most households rely on water-based pit latrines with a slab (71.3%), followed by water-based septic tanks (20.7%) and water-based to sewers (6.2%) (Table 2). Water-based latrines without slab were not observed. Namadi was the settlement in Suva with the lowest proportion of water-based pit latrine with slab (4.1%) and the highest connection to sewers (17.6%). Households connected to sewers were mostly located in outskirts of the settlements, with no sewerage going through the settlement. Reports of shared toilets were limited in Suva, with only 5.6% of respondents stating that their household did not own a toilet. Shared toilets were primarily shared with family members living in neighbouring houses instead with non-relatives. Around 10.7% of surveyed households in Suva did not meet standards for basic sanitation according to UNICEF and WHO definition due to shared toilets among households. No clear relationships between the sanitation system in place and land or household tenure were observed.

Most households in Suva (56.2%) reported that their containment unit never required emptying. Most respondents reported residing in the same household over 10 years, with the median living period of 15 years and cases of the same containment unit being used for over 55 years without the need of emptying it. For the few cases that the containment unit required emptying (< 38.0% of all surveyed households in Suva), the most common reported procedure was to seal the filled unit and build a new one (34.2%), for both water-based pit latrines (44.1% of reported water-based pit latrines) and septic tanks systems (13.5% of reported cases of septic tanks) (Table 3). Very few respondents in Suva (< 4.0%) reported any emptying of the containment unit by either a household member or a private contractor. The few respondents who did empty their system reported that it was disposed in a nearby water stream or the sea. No households reported removal and safe management of sludge. In cases of system failure, almost all respondents (93.5–100% across settlements) did not know who to report to or seek assistance from to repair the sanitation system.

In Port Vila settlements, most toilets were water-based toilets connected to a septic tank (54.4%), though proportions of dry pit latrines without a slab were more common (14.9%) than in Suva (Table 2). It is important to note that centralised sewerage systems are not available in Port Vila, and that water-based latrines without slab were not observed. The proportion of households relying on dry toilets in Port Vila (29.8%) was considerably higher than in Suva (1.1%). Half of dry toilets in Port Vila informal settlements had no slab and 68.2% were shared with other households. Dry toilets were present in all surveyed settlements in Port Vila except in La Smet where all sanitation systems were water-based. The highest proportion of dry toilets (48.9%) within a single settlement, from which 57.8% were dry pit latrines without slab, was observed in Blacksands (Table 2). No clear relationships between the sanitation system and the land or household tenure were observed.

Reports of shared toilets were more common in Port Vila, with 23.7% of surveyed households not having their own toilet, and 63.7% sharing it with other households, either with family members (49.5%) and/or non-relatives (30.0%). Around 47.4% of surveyed households in Port Vila did not meet basic sanitation standards according to UNICEF and WHO definition (i.e., as either being a shared toilet or not having a slab or platform).

Septic tank systems in Port Vila settlements are most often emptied by a private contractor (53.8%), with a smaller percentage reporting septic tank emptying by a household member (10.3%) (Table 3). Around 31.6% of septic tank systems have not yet required emptying, with the median living period of 23 years and cases of the same containment unit being used for 60 years without the need of emptying. Regarding water-based pit latrines, most respondents either did not know what is done when the pit gets full (37.5%), or it had not happened yet (40.6%). In contrast, most of the dry pit latrines were reported to have reached capacity and been emptied, with the most common practice (71.9%) being shutting down the full containment unit and constructing a new one. For the sanitation systems emptied by a private contractor, 67.1% of respondents reported that the waste was disposed at the Bouffa septage treatment plant, while the remainder were not sure of where waste is disposed. In cases where a resident emptied the containment unit, the sludge was either disposed in a nearby pit, water stream, unused area, or a second containment unit used in alternation. Overall, 23.7% of respondents in Port Vila reported that sludge is collected and treated off-site. In cases of system failure, most respondents either did not know who to report or seek assistance from (59.9% in average across settlements). For the remainder who knew who to seek assistance from, 76.4% stated that a family member from the household would repair the sanitation system.

Residents’ preferences for sanitation systems

Residents preferred sanitation systems displayed consistent patterns within countries (Table 4) and correlated with the current sanitation system in place (Tables 5, 6).

Table 4 Residents’ preferred sanitation systems in Suva (Fiji) and Port Vila (Vanuatu)
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Table 5 Resident’s preferred sanitation systems in Suva (Fiji) and Port Vila (Vanuatu) per current sanitation system in place
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Table 6 Multinomial regression of preferring a different sanitation system than the one in place per type of system in place
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In Suva settlements, most respondents would prefer sewer systems (49.4–62.7%), followed by septic tanks (27.5–34.9%) (Table 4). The odds ratio of a resident preferring a sewer system instead of a water-based pit latrine with slab or a septic tank is of 83.3 and 14.1, respectively (Table 6). A small proportion (9.8–14.5%) of respondents reported preferring water-based systems connected to pits. Most respondents in Suva (76.9%) would prefer to change the current sanitation system in place at their household, with a significant odds ratio of approximately 10 (Table 6), except for respondents already using water-based systems connected to sewers (Table 5). No respondents stated preferring dry pits systems, and preference for sanitation systems did not show significant correlations with gender, education, or age. Testing for differences between each informal settlement, including different land tenure, was not possible due to relatively small and variable sample sizes.

In Port Vila settlements, no respondents reported a preference for sewerage systems, with the majority (82.8%) of respondents preferring septic tanks (Table 4). The odds ratio of a resident preferring water-based systems connected to a septic tank instead of dry or water-based pit latrines is of 90.1 and 38.5, respectively. In the Blacksands settlement, a relatively higher proportion of residents (21.7%) prefer dry pits, all of which were VIP latrines. Even though some respondents indicated preference for other sanitation systems than the current sanitation system in place (Table 5), most respondents in Port Vila (57.7%) preferred to maintain the current sanitation system they use instead of changing to an alternative one, with an odds ratio of 43 (Table 6).

Exposure and impacts of climate-driven hazards on sanitation systems

Climate-driven hazards impacting sanitation systems were more frequently reported in Port Vila than in Suva, both in number of occurrences and level of damage (Table 7), with an association of level of damage with the current sanitation system in place (Table 8) and its reliance on water (Table 9).

Table 7 Hazards and level of damage affecting sanitation systems in surveyed urban settlements in Suva (Fiji) and Port Vila (Vanuatu)
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Table 8 Number of reports of damage to sanitation systems due to climate-driven hazards per current sanitation system in place and level of damage, in Port Vila (Vanuatu)
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Table 9 Multinomial regression of occurring damage due to a climate-driven hazard per sanitation system in place in Port Vila (Vanuatu)
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Overall, most respondents in both countries (77.4% of all respondents) reported never having their sanitation system impacted by a climate-driven hazard (Table 7). In Port Vila, higher reports of damage to at least one climate-driven hazards (36.3% of all respondents in Port Vila) was observed compared to settlements in Suva, which were limited to only 6.2% of respondents. In Suva, only 11 reports of damage, mostly resulting in minor damage (91.0%) were made, encompassing cyclones, heavy rain, flooding, and sea level inundation. Interestingly, 5 out of 11 cases reported being affected by more than one hazard (Table 7).

Among the reported cases of climate-driven hazards affecting sanitation systems in Port Vila, cyclones were the most common hazard (83.3% of all reported cases in Port Vila) affecting sanitation systems in all settlements (Table 7). Exposure to more than one hazard was common, with one in three cases reporting exposure to two or more climate-driven hazards. The proportion of moderate (25.6%) and major (23.1%) damage to sanitation systems was higher in Port Vila than in Suva (9.0 and 0.0%, respectively). Despite the higher report of damage to sanitation systems in Port Vila, reports of contamination, both to the environment and to residents, were very limited (only two cases). La Smet, Blacksands, and Man Ples were the settlements where more than 20.0% of residents reported having their sanitation system affected by at least one climate-driven hazard.

The level of damage, particularly moderate and heavy damage, was higher in dry sanitation systems (mostly comprised by dry pit latrines without slab) than in water-based systems with slab connected to either pits or septic tanks (Table 8). The odds ratio of moderate or heavy damage occurring on dry pit latrines compared to septic tank systems is 4.2 and 8.5 (Table 9). There were no significant differences in odds of damage level between septic tank systems and water-based pit latrines with slab, nor significant correlations of damage level with education and number of residents in the household.

Sanitation coverage and service levels: implications for SDG monitoring

A widespread availability of water for sanitation was observed across the surveyed urban informal settlements, which was accompanied by relative improvements in superstructure (i.e., presence of slabs and platforms in water-based systems compared to dry systems), indicating potential improvements in sanitation services in terms of user hygiene and safety. However, the study findings also revealed that there is still a high reliance on unimproved sanitation, with considerable rates of toilet sharing among households, and challenges in sludge treatment and final disposal incurring potential risks for public and environmental health.

This study reveals significant discrepancies in sanitation coverage compared to JMP country-level data for urban populations, which estimates that only 6.0% in Vanuatu and almost no one in Fiji’s urban populations rely on unimproved sanitation services. In contrast, our findings indicate that in Fiji informal settlements around 10.7% of households share toilets, and almost all households have inadequate faecal sludge disposal, relying therefore on unimproved sanitation. In Vanuatu, our findings indicate that nearly half of the surveyed households do not meet basic sanitation standards due to inadequate infrastructure, toilet sharing and inappropriate sludge disposal. These observations indicate that informal urban settlements are significantly underserved, a discrepancy masked by underreporting of such areas within the urban region and by the lack of differentiation between formal and informal urban areas in the national surveys used in JMP assessments.

In fact, JMP data reveals a declining trend in basic sanitation coverage in the region18. As this is occurring alongside the expansion of urbanisation and informal urban settlements in Melanesia4,7, there is an increasingly urgent need to understand sanitation in urban informal settlements. This study confirms the need to disaggregate sanitation monitoring data from informal and formal urban populations to fully understand urban sanitation access situations and needs.

SDG6 target on safely managed sanitation: O&M and public health risks

The SDG 6 target is to achieve universal access to safely managed sanitation by 2030. This requires an understanding of existing sludge management practices. While optimal desludging intervals for septic tanks vary depending on the specific system, previous research, mostly from temperate regions, suggests that for most organic and hydraulic loading rates, intervals typically range from 3 to 5 years, aiming to prevent sludge accumulation exceeding 50.0% of the unit’s volume, thus minimising the carryover of solids and organic matter into the effluent30. The low rates of septic tank emptying observed in this study may indicate system malfunction, attributed to factors such as leakages (lack of watertightness), inadequate knowledge of system operation by residents, or suboptimal design for effective particle sedimentation and anaerobic digestion, which leads to sludge accumulation and the eventual need for tank emptying31,32. However, more detailed facilities inspections than those performed in this study are required to verify such a hypothesis.

Sludge accumulation in septic tanks is also dependent on temperature33 but data from tropical regions are notably scarce limiting a direct comparison of the findings of this study with literature benchmarks. Future efforts to improve on-site sanitation should include research to understand sludge accumulation and optimal desludging intervals in tropical regions, and raise awareness of these requirements with householders, who ultimately bear responsibility and decision-making power on operation and maintenance.

This study observed a considerable portion of individuals lacking awareness of the distinctions between various sanitation technologies, ranging from 3.5% to 37.5% of households. Education, coupled with a sense of ownership and pride in such systems, plays a crucial role in ensuring the long-term success of decentralised solutions, as householders are primarily responsible for system maintenance and operation, as observed in this study and in the literature elsewhere33,34. Improper on-site sanitation practices have potential to significantly heighten public health risks, particularly within Melanesia where an average of 17.0% of the urban population rely on self-supply sources for drinking water, such as groundwater and rainwater35, which may become contaminated with faecal pathogens due to unsafe sanitation. This situation is compounded by the limited monitoring of groundwater quality in these regions (e.g., SOPAC36). Consequently, poor on-site sanitation has been identified as a major threat to groundwater contamination in significant urban centres in Vanuatu37 and Papua New Guinea38.

Preferences for sanitation systems and challenges for climate-resilient sanitation

The widespread preference for water-based sanitation systems, particularly on-site water-based solutions that eventually require desludging and treatment, illustrates the importance of ensuring a comprehensive service chain for the delivery of resilient sanitation systems. This includes provisions for emptying and faecal sludge treatment to minimise the risk of diffuse pollution, which could compromise public and environmental health, particularly after flooding events (e.g., Phanuwan, et al.39), which are expected to increase with climate change.

Successful and enduring sanitation services must extend beyond infrastructure for water supply and wastewater treatment. They should also encompass integrated urban planning40, strategic governance and policy harmonisation41, community social dynamics42, a cohesive and systems-based service chain perspective, incorporating public43 and market-based sanitation approaches44, trust in the organisations providing these services45 as well as fostering a sense of ownership and appreciation within the community to support the payment and maintenance of such services46. While many studies have been performed in Africa, Asia and South America to understand optimised approaches to improved sanitation in informal urban settlements (see systematic literature review by Annamalai et al.47), a very limited number of studies (e.g., Brown, et al.48) focusing on Melanesian countries have been reported and should be the focus of future research.

The findings from this study further reinforce the notion that the preference for sanitation solutions is influenced by the context of a particular location, including existing infrastructure. This observation was evident in Port Vila, which lacks any existing centralised sewer systems, and where not a single survey respondent expressed a preference for sewers. Moreover, none of the respondents across all surveyed settlements indicated a preference for the implementation of nature-based solutions, such as constructed wetlands and composting toilets. This aligns with the previous argument, as there are very few reports of nature-based solutions being utilised in informal urban settlements in the Pacific region, with only one reported case in Vanuatu and four in Fiji49. The challenges associated with implementing nature-based solutions in urban informal settlements include their relatively higher space requirements50, in addition to the need for appropriate planning and management. This necessitates a collective and integrated approach51 which, while challenging, may result in simultaneous benefits of human well-being, ecosystem services, and increased resilience and biodiversity52.

Monitoring climate resilient sanitation in informal urban settlements

Addressing the challenge of monitoring and promoting climate-resilient sanitation services requires a locally tailored approach. Enhancing both the quantity and quality of data collection through participatory community engagement, capacity-building initiatives, and multi-stakeholder partnerships, is crucial for effective monitoring53. For surveys to assess accessibility, such as is reported by JMP (WHO, UNICEF), it is important to be able to disaggregate urban informal and formal populations. This could be achieved through either georeferencing survey samples and cross referencing with up-to date settlement maps, or, more simply, to include survey questions relating to land tenure.

Given the observation above that sanitation preferences may not always be aligned with the sanitation service usually considered most suitable a local context, monitoring aimed at informing policy and program development needs to include residents’ sanitation preferences12,13,14,15 and also impacts of climate-driven hazards on sanitation services. Monitoring the resilience of existing sanitation to locally relevant hazards cannot rely solely on past hazardous incidents, as future hazards are likely to change as climate change continues to impact these risks. Combining the past resilience of sanitation with projections of future exposures in a locale, based on specified climate scenarios, would provide greater insights to future resilience, though still with some uncertainty. As we demonstrate here, collaborative monitoring benefits understanding of local exposure to hazards and their impacts on sanitation services. Such initiatives can enable research institutions, governmental agencies, international organizations, the private sector, and local communities to collectively enhance routine and event-based data collection and monitoring efforts. Monitoring standards are critical to supporting such collective efforts. For instance, recent efforts to standardise terminology used for on-site sanitation54,55 could address the current use of diverse and often misleading terms.

The observation of single households being affected by multiple hazards illustrates the complexities and challenges involved in delivering climate-resilient WASH services. The findings of this study suggest that water-based sanitation systems have lower odds of experiencing negative impacts compared to dry pit systems, particularly those lacking a slab or platform for the user. However, it is crucial to emphasize that 1) the reported damage by residents’ is likely biased towards a user-centred perspective, focused on the ability to use the toilet interface with limited consideration for the sanitation system fully meeting its purpose of containing and treating human faecal waste to safeguard public and environmental health. This is reinforced by the limited number of reports of perceived environmental contamination on this study; 2) the climate resilience of sanitation systems cannot not be solely attributed to technologies, as emphasized in recent frameworks for climate-resilient WASH56,57; and, 3) the study’s reliance on recall reporting and the unique characteristics of each site highlight the need for further data collection in other Melanesian nations to enhance the understanding of challenges in delivering adequate sanitation services in informal urban settings across the region. Other factors such as environmental settings, community governance and empowerment, institutional support, socio-technical innovation, fit-for-purpose finance, and climate justice framing are key for improving climate resilience in informal settlements58. The intersectionality of factors affecting resilience are also important to consider. For example, we speculate that lower socio-economic households are more likely to rely upon the less robust unimproved dry pit systems (e.g., lack of slab or platform), and live in areas of the settlement that are more exposed to hazards, thereby increasing the likelihood of experiencing adverse impacts to sanitation and public health from climate-linked hazards. Further research is needed to identify the drivers of such clustering, which could include socio-economic and affordability factors associated with more hazard-prone land, and cultural preferences for settlement amongst kin. Integrating field-collected data with satellite imagery and hazard data, including modelling data, could be a useful approach to better understand the interactions between climate risks affecting water and sanitation systems in underserved urban areas, and support the development of tools for integrated urban WASH planning.