Search strategy

We searched eight journal databases in October 2020, and 39 international organization websites, between January 2020 and July 2021. The search was completed primarily in English, and in Spanish for the LILACS database. There was no restriction on publication date. The list of databases and organization’s websites searched, with the date of search and the keywords and categories used, are in S1 Appendix. For several databases and websites, including Google Scholar and UNHCR website, the number of titles we could access was restricted to the first 1,000 results. For reference screening, we reviewed the references of documents that met final inclusion criteria.

Screening process and eligibility criteria

For databases, title results were exported into Zotero and screened. For websites where downloading was not possible, title screening was done directly online, and included documents downloaded for abstract screening. Search results after title screening were exported into Zotero, and duplicates removed. References were initially screened by title, then abstract, and finally full text, based on inclusion and exclusion criteria.

Initially, documents were included if they: assessed FST implementations during the steps of containment, emptying, transport, treatment, and reuse/disposal; including at least output-level results; and, were implemented in a humanitarian setting, as defined by “important losses and damage that were inflicted upon communities and individuals, possibly including loss of life and livelihood assets, and that left the affected communities unable to function normally without outside assistance” [7]. Humanitarian settings included disasters triggered by natural or technological hazards, conflicts, fragile states and protracted crises, and communicable disease outbreaks that occurred in a low- or middle-income country.

Initially, documents were excluded if they: 1) were not related to fecal sludge management as described in the eligibility criteria; 2) were related to healthcare sanitation; 3) were not implemented in a humanitarian context as described in the eligibility criteria; 4) did not include output level data from primary data collection on an intervention; 5) were a duplicate; and/or, 6) were not accessible. When criteria could not be deduced from the title or the abstract, the document was included in the next stage of review. The reason for not including a document during the full document review is recorded in S1 Checklist.

As 266 documents were included after full-text review, we then added additional eligibility criteria on intervention and setting. Studies and reports were included into the review if they were about FST implementation in camps in humanitarian settings.

Abstracts, full text, and additional review screenings were completed independently by two authors. At each stage, discrepancies were discussed between the authors and resolved. The list of documents assessed with the additional criteria, and reason for exclusion is recorded in S2 Table.

Review update

In April 2023, we updated the systematic review. We searched Pubmed with date constraints from October 10, 2020 to April 24, 2023 and Google Scholar with dates constraints from 2020–2023. The list of databases searched, with the date and keywords used, is in S1 Appendix. Inclusion and exclusion criteria were the same as previously used. The list of documents assessed is recorded in S2 Table.

Data extraction process

The following parameters were extracted from included documents: title, publication year, country, crisis phase, FST category, study type, type of treatment, barrier or facilitator, barrier or facilitator type, description of barrier or facilitator, and page number. Before data collection, we established options for data extraction categories (Table 1). An intervention was defined as a unique treatment system implemented in one setting; please note one intervention could have been described in several documents. A facilitator was defined as a characteristic supporting FST implementation, and a barrier hindering FST implementation. The dataset is available in S1 Table.

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Table 1. Options set for four categories for data collection.

https://doi.org/10.1371/journal.pwat.0000289.t001

Data was extracted in Microsoft Excel (Redmond, WA, USA) independently by two authors, and discrepancies discussed and resolved.

Quality appraisal

We used the Bond Evidence checklist [13] to rate and categorize interventions into four quality categories: weak evidence (up to 37%), minimum standard (up to 62%), good standard (up to 81%), and gold standard (up to 100%). Interventions were graded independently by two authors, and discrepancies discussed and resolved.

Data synthesis

After cleaning the data, including remove duplicate interventions, we developed facilitators and barriers categories. We grouped treatments into treatment modules, defined as one treatment technology (e.g. constructed wetlands). A treatment system was defined as one or multiple treatment modules (e.g. lime treatment associated with constructed wetlands and incineration). We analyzed results by country, crisis phase, treatment module, and quality assessment. We synthesized results by category, grouped similar barriers and facilitators, calculated their percentage of occurrence, and provided descriptive examples in the text. We summarized overall results into the top 10 mentioned barriers or facilitators, and recommendations for implementation guidance and future research.

Performance

The most commonly identified performance barrier (61%) was not meeting effluent discharge standards, for some or all parameters. For example, in a treatment system using a settling tank, ABR, and a gravel filter, the final BOD₅ effluent values were still 40 times above the national discharge standards, although the treatment efficiency reached 70% [38]. Another identified barrier was when further treatment was needed for the solid and/or the liquid part of the sludge (31%). For example, in a treatment system including an anaerobic digester and a horizontal filter, treatment of sludge accumulating in the anaerobic chamber and the filter wasn’t planned for, so the accumulated sludge was periodically desludged and disposed of on the nearby land [38]. Lastly, systems with low reduction values were also identified as a barrier (10%). For example, a system with hydrated lime treatment, drying beds, ponds, and incineration achieved only 27% BOD₅ reduction [38].

Systems with high reduction efficiencies were the most commonly identified performance facilitator (48%). For example, a system with a dumping station, hopper-bottom tank, ABR, constructed wetlands, maturation pond, infiltration basin, drying beds, infiltration trenches, and incinerator reached a treatment efficiency of 96% for TSS, 85% for COD, and 3 log reduction for E. coli [31]. Another facilitator was when systems had effluent values within standards (30%), especially for pathogens. For example, a system including lime stabilization and geobags had no E. coli recorded in the liquid supernatant [42]. Another facilitator was when effluents could be used as fertilizer in agriculture or energy, with sometimes better quality than commercial products, and having planned discharged routes that were functioning (18%). For example, the briquettes produced through a solar-thermal process burned for longer than normal charcoal and produced less smoke [30].

Operation

A need for strong operational supervision, with frequent labor or specific technical skills was the most commonly identified operation barrier (39%). For example, a biogas system needed strict supervision to add the adequate mixture every day in the reactor to ensure the proper ratio to maximize gas production [16]. Additionally, difficulties in system operation were reported as a barrier (23%), such as the sensitivity of biological treatment to hard-to-control factors [8]. For example, for a solar-thermal system, the briquette recipe had to be refined each day due to variation in sludge quality and quantity [30]. Another barrier was when inputs (e.g. water, fuel, lime) were needed for operation (20%). For example, for a septic tank system, a constant supply of water was necessary to flush the pans and ensure sewage was diluted and bacteriological action could proceed, which was critical in a context with water scarcity [28]. Lastly, monitoring was a barrier (12%) because of the additional tasks it required, high staff turnover, decentralized units to reach, and lack of access to adequate testing facilities. For example, significant human resources were necessary to monitor a lime stabilization system through pH checks in every barrel treated [42].

The most common operation facilitator was when systems were easy to operate and maintain (66%), requiring basic technical skills from locally-available labor that could be easily trained on with limited interventions from staff. For example, a system with settling tanks, an upflow filter, constructed wetlands, soak pits, and solids burial pits run by gravity needed limited intervention from the operator [8]. A system with ABR, aerobic treatment, settling tank, glass bead filter, disinfection, anaerobic digestion, and lime treatment required basic mechanical and electric skills for daily operation and maintenance [46]. Additionally, not being dependent on energy source was a facilitator (9%). For example, the infrastructure for a system using thermophilic composting was not dependent on an energy source, which was critical in a context where power supplies were unreliable [56]. It was also a facilitator when processes were flexible (8%), for example when there were multiple geobags to allow time for solids to dry out completely before burial [8].

Technical

The most commonly identified technical barrier was when systems were over or under-designed, or an inappropriate type of material was used (52%). This led to issues such as plants not surviving because of the high sludge concentration [16], or necessary retention time not maintained because of frequent emptying due to high volumes of sludge [32]. For example, the filter material from a system with upflow filtration, constructed wetlands, and solids burial pits clogged because of excessive solid sludge levels at the inlet [58], and in a system with unplanted drying beds, the local rice bags used were too compact to allow large-scale rapid infiltration [40]. Additionally, the need for technical expertise or supervision for design or construction was a barrier (17%). For example, a biogas system needed highly skilled masons for construction [16]. Another barrier was construction quality (10%), with brick layer collapsing [25], or pipes leaking [48]. For example, in a system with a dumping station, a hopper-bottom tank, an ABR, constructed wetland, a maturation pond, an infiltration basin, drying beds, infiltration trenches, and an incinerator, the pipes got clogged with solid sludge, and the brick walls cracked due to poor brick and mortar quality [31]. Lastly, it was a barrier when scaling-up the system was difficult (7%), especially when constructed with concrete. For example, in a system with lime treatment, settling tanks, solids burial pits, and infiltration trenches, structures were made of concrete and were less simple to expand than excavated lagoons [8].

The most common technical facilitator was when systems were easy to scale-up (50%), by adding modules in parallel when there was sufficient space. For example, in a system with lime treatment, geobags, and gravel and sand filters, the number of barrels, geobags, and infiltration beds could be increased to raise the system capacity [29]. Proper design and using appropriate materials were also facilitators (18%). For example, a solar-thermal system had two chambers to allow adequate time for proper maturation of fecal sludge [22]. Lastly, a facilitator was when systems were easy to design and/or construct (18%), with low technical expertise needed. For example, lime treatment was identified as a simple, straightforward, robust technology that could be designed and constructed in the local context, as construction did not require prior knowledge and experience [47].

Economic

High capital and/or operational costs were the most common economic barrier (62%), such as building infrastructure [8] or sourcing lime [37]. For example, the worms cost US$210/kg in 2015 in a system using vermicomposting [35], and a system with lagoon lime treatment and dewatering beds had a whole life cost of more than US$200,000 [8]. Additionally, an identified barrier was when local authorities could not cover the costs and external funding was necessary (22%). For example, in a system with lime stabilization, the government agreed to manage the site even though they lacked funds to support it financially; when external funding ended, the government was no longer able to maintain and operate the site [49]. In another system with thermophilic composting, compost sales could only cover ~70% of labor inputs, excluding infrastructure investments, general site maintenance, or transport and collection of the wastes, so external funding was necessary [56].

Low capital and operational costs, due to low salaries, inexpensive locally-available materials, and/ or low power consumption were the most commonly identified economic facilitator (81%). For example, a system with an ABR, aerobic treatment, a settling tank, a glass bead filter, disinfection, anaerobic digestion, and lime treatment had an operational cost of US$5/m³ treated, that could be reduced using renewable energy [46]. A system with a settling tank, ABR, unplanted and planted gravel filters, and a polishing pond had a capital cost of less than US$500/m³ treated [8]. Another facilitator was when outputs helped cover operational costs, with reduced desludging [22], or generating revenues by selling compost or briquettes [50] (14%). For example, in a system with a feeding tank and a biogas, biogas use saved US$95 in cylinder gas costs [27]. In a system with feeding tank, ABR, anaerobic filter, and planted drying beds, around US$200/year was saved due to using treated wastewater for farmland [27].

Environmental

Heavy rains were the most common environmental barrier (45%), because they could halt construction work [25], induce landslides [32], or increase sludge drying time [16]. For example, the time to dry sludge in drying beds increased from three to five weeks during the rainy season [31]. Another barrier was floods or earthquakes damaging the systems (27%). For example, a system with the effluent at a low level and an ABR below the ground level was not able to discharge when the surrounding area flooded [8]. Lastly, an identified barrier was a high water table (18%) because it could overflow the systems [26] or decrease the soil infiltration capacity [28]. For example, because of the additional flow going through the treatment system during the rainy season, infiltration basins were not able to infiltrate the wastewater which overflowed to the neighboring field [31].

The most commonly identified environmental facilitator was when systems were working well in certain climates, like the ponds in dry seasons [16], or were resistant to environmental issues, like the constructed wetlands for earthquakes [8] (54%). For example, a UDDT system was set up in a warm and arid climate, which resulted in a warm, very dry and moderately alkaline environment in the UDDT vaults, inhospitable for many microorganisms [15]. Adaptations were also facilitators when done to resist environmental issues (35%), like roofs to cover drying beds from the rain [44], or constructing the system above the ground to prevent flooding [8]. For example, locally made polytunnels (bamboo and see through plastic) were covering unplanted drying beds during the rain and removed during a sunny day or before a cyclone [40]. Lastly, systems with additional positive environmental impacts were identified as facilitators (11%), like biogas production reducing firewood use [21]. For example, briquettes produced through solar-thermal process created lower smoke and soot than charcoal [20].

Spatial

The most commonly identified spatial barrier was lack of space to construct the system or additional modules for scale up, and systems with high land occupation like constructed wetlands (58%). Available space constrained the systems that could be implemented [37]. For example, the amount of suitable land required for thermophilic composting operation (0.6 hectares for 10,000 people) was a limitation [56]. Another barrier was hilly terrains (39%), which led to access issues [8], and increasing costs and time for construction and operation [29]. For example, multiple systems in Cox’s Bazar, Bangladesh were accessible only by foot, not to motorized and non-motorized transport [47].

Systems with low land occupation like lime treatment was the most common spatial facilitator (75%). For example, a system with upflow filters, constructed wetlands and solids burial pits required a 700 ft² area for a maximum influent of 5 m³/day [41]. Another identified facilitator was systems that could be implemented in multiple topographies and laid out to reduce occupied space, like geobags or rectangular shaped tanks [8] (17%). For example, small biogas plants (2 to 4 m³ for two to four latrine blocks) were flexible and preventing destroying existing shelters or WASH facilities for their implementation [16].

Social/cultural

The most commonly identified social and cultural barrier was when the population had difficulty accepting the treatment solution (45%), sometimes leading to stopping the system [57]. Communication and education were identified as important to overcome this issue [56]. For example, questions on using biogas for cooking were raised by users regarding its compatibility with the Muslim faith [16]. Another barrier was when how people used the latrines influenced how the system functioned (35%), like using flushing water in UDDT [22] or chemicals with vermicomposting [61]. For example, people threw waste material (plastic, piece of bricks, etc.) into the organic waste chamber and the slurry pit of a biogas system [25]. Lastly, systems with bad odors were a barrier (20%). For example, ponds resulted in olfactory nuisance for the surrounding population [16].

The most common social and cultural facilitator was user acceptance and ownership (37%), when users had benefits from the treatment outputs, like using the biogas or briquettes [20], or if the treatment system was far from the settlements [33]. For example, households benefiting from biogas for cooking had strong ownership of the system and were willing to be involved in the maintenance [16]. Another facilitator was when systems were already in line with people’s habits, or were improving their life (26%). For example, briquettes produced through solar-thermal process could be used with current cooking infrastructures and required minimum behavioral change [20]. User acceptance was more investigated and discussed in the included documents for on-site systems, like for the UDDT or the biogas [35].

Temporal

The most commonly identified temporal barrier was when systems had long start up time (64%), due to long construction or commissioning time. For example, in a system with constructed wetland, sand filter, chlorination and an infiltration pit, six months were necessary for the system to reach targeted reductions for BOD, pathogens and nutrients [8].

Rapid implementation, due to short set up or commissioning time, was the most common temporal facilitator (76%), thanks to prefabricated tanks or simple technicities like lime treatment and geobags [8]. For example, simple excavated lined lagoons for lime treatment were quick to construct and decommission [8].

Safety

Systems putting people in contact with dangerous substances were the most commonly identified safety barrier, like lime from lime treatment [42] (44%) or smoke from an incinerator affecting the workers [31]. Contact with pathogens from the sludge, through flies or through the plant’s operation [8], was another barrier (32%). For example, many flies bred on maturation ponds [31]. Lastly, open pits and possible theft of materials were also an identified barrier (24%). For example, to avoid accidents, ponds were fenced and families were instructed to keep their children away [16].

Systems preventing sludge contact by having it enclosed up to the discharge routes, were the most commonly identified safety facilitator (67%). For example, an upflow filter was contained in closed plastic tanks which limited physical contamination and vectors proliferation [8]. Another facilitator was adaptations limiting sludge contact and vectors spreading (22%). For example, a mosquito net covered the containers of a lime treatment system [60].

Supply

The most common supply barrier was material or spare parts difficult to source locally (63%), imported from abroad, or with long shipping time. For example, the fuse from a locally-bought control module for solar-thermal process was not locally or in-country available, and had to be shipped from Europe [30]. The components poor quality and durability was another identified barrier (19%). For example, the tank panels from an aerobic system needed regular replacement because of the repetitive stress of the mixing action [54].

Locally available materials for construction and operation were the most commonly identified supply facilitator (92%). For example, hydrated lime, gravel and sand were identified as common building materials that can be purchased locally and are readily available [47].

Institutional

Local authorities not involved in the management of the system due to a lack of funds or required skills was the most common institutional barrier (41%). For example, a local government had little interest in managing a lime stabilization system, because they could not foresee a sizable benefit from it [49]. Another barrier was administrative requirements hindering the systems implementation, such as complex authorization processes [19] (35%). For example, for a system with lime treatment, geobags, and gravel and sand filters, the government did not allow permanent infrastructure, so the infrastructure needed to be able to be decommissioned and the use of bricks or cement was not accepted [29].

The only identified institutional facilitator was local stakeholders supporting the system implementation, by paying salaries or identifying the land (100%). For example, for a system with a feeding tank, biogas, a stabilization tank, ABR, an anaerobic filter, and planted drying beds, local stakeholders identified the land to establish the plant, and the municipality provided the operator’s salary [24].