water · family: the missing yardstick
where the sludge goes, nobody counts
43% of the world uses pits and septic tanks, but data on whether the sludge is ever treated covers only 1% of the global population — the faecal-sludge accounting gap
Problem statement
Globally, about as many people rely on on-site sanitation (pit latrines and septic tanks) as on sewers, and on-site systems are growing faster than sewers even in cities. The Sustainable Development Goal indicator that matters — "safely managed" sanitation — asks whether the excreta from those pits and tanks is either safely disposed of in place or emptied, transported and actually treated. For sewered populations most countries have data on wastewater treatment; for on-site sanitation very few do. The JMP reports that data on excreta "emptied and treated off-site" were available for just 1% of the global population (JMP Table 3; on-site sanitation was used by 43% of the global population in 2020), from only seven countries, all high-income. The failure is structural: households cannot say what happened to their sludge after the truck (or the man with a bucket) left; emptying is often informal and unrecorded; and where treatment-plant records exist they count trucks or cubic metres from undefined catchments, which are difficult to align with the population-based numbers that surveys produce. The amounts collected and delivered to treatment plants may be known at the scale of individual municipalities, but the JMP says that information is typically not aggregated nationally: outside those seven countries, there was no national-level figure for how much emptied faecal sludge is treated.
Why this matters
Without that number, unsafe emptying and dumping — the JMP calls it "a growing risk to public health and the environment" — is invisible in national statistics, so investment in emptying services and treatment capacity is hard to size or justify, and progress toward SDG 6.2 cannot be fully tracked for the 43% of humanity on on-site systems, which dominate in the areas that are home to "the world's poorest people." The same missing data hampers engineering: Eawag's Sandec group wrote in 2018 that "no reliable method exists to estimate total accumulated quantities and qualities" of faecal sludge, and its project page says in-place amounts are difficult to estimate because sludge "is often stored underground in containments, records frequently do not exist, and construction is not standardized." In the past, the page says, people estimated quantities and qualities from literature values from other contexts or from educated guesses by users and emptying service providers, and those estimates "were not adequate to capture the desired accuracy and range of variability." The 2018 article adds that engineers try to make reasonable estimates when designing solutions, typically without adequate resources or time, and that the result is treatment plants that are immediately at capacity (it gives Lubigi in Kampala as its example) or well under- or over-capacity. In the JMP's absence-of-data rules, countries where sewers dominate are simply assumed to treat sludge as well as wastewater, and where on-site dominates no estimate is made unless the country has its own sludge-treatment data — meaning the countries with the most on-site sanitation are the ones most likely to have no figure for sludge emptied and treated off-site.
What’s been tried and why it hasn’t worked
The JMP, with Gates Foundation support, ran SMOSS data-collection pilots in six countries (including Bangladesh n=3,149, Zambia n≈23,000, Serbia n=1,560, plus Indonesia, Ecuador and Kenya) from 2020, adding questions to national surveys and running dedicated household surveys with sanitary inspections. Household questions worked for containment and for whether a pit had ever been emptied, but hit a wall downstream: "household survey respondents can't reliably indicate if fecal sludge is treated once it leaves the property; additional information is needed from administrative sources." Administrative data proved the weak link — "few [countries] maintain centralized records of desludging services that would allow calculation of how much of the removed excreta is actually delivered for treatment," and when records exist "it is often in different units (i.e. number of trucks, volume or mass of waste) or from undefined catchment areas that make it difficult to align it with household survey data or determine the equivalent population served." The volume disposed to treatment "is not a direct function of the number of users but also depends on the containment size and emptying practices (partial or full emptying, adding water, etc.)." Where informal or private emptiers operate alongside regulated ones, "the data may only cover a portion of the emptying activities." Integrating questions into national surveys was constrained by "the limited number of questions that could be included, the low capacity of enumerators regarding technical sanitation aspects" or difficulty training them directly, and the risk of questions being cut. Separately, the report notes that finding the relevant policies and regulations can be difficult because on-site sanitation services may be delivered or regulated by different departments than off-site wastewater. Sandec's Q&Q method — predicting sludge quantities from spatially available demographic, environmental and technical data — has produced city-scale estimates (accumulation of 270 L/cap·year for pit latrines and 280 L/cap·year for septic tanks in Kampala; a later seven-country study pooled 1,206 data points to fit a relationship between volatile and total solids), but one of its co-authors, in a reply relayed on the SuSanA forum in 2018, called the method "still a baby" and encouraged context-specific adaptation.
What would unlock progress
The missing piece is a reconciliation layer that turns what is actually recorded — truck trips, plant intake volumes, emptier logbooks, GPS traces — into population-equivalent flows that survey data can be compared with: a "faecal-flow accounting" method analogous to the mass-balance and material-flow accounting used in solid-waste and energy statistics, or to the way remittance flows are estimated from partial formal records. Cheap instrumentation of the emptying step (a truck-mounted volume/GPS logger, or a mobile-money-linked emptying receipt) would make the informal chain legible without a census; predictive Q&Q models could fill the gaps between measurements. On the governance side, the JMP notes there are "no global assumptions regarding safety of different types of emptying or transport" — countries must define them — so a defensible, reusable classification scheme would itself be a contribution.
Entry points for student teams
A team could build and test, with one city's data (the JMP report mentions a review of "SFDs in Kenya which focus on urban areas" — shit-flow diagrams, an abbreviation the report does not spell out — and Sandec's project page lists a Kampala study that recorded 5,653 emptying events from GPS units installed in trucks), a reconciliation model that converts truck-load and plant-intake data into "population served" with explicit uncertainty, then compare it against household-survey emptying rates to expose the informal share. A hardware team could prototype a low-cost emptier logger (volume, timestamp, GPS, disposal-site geofence) and pilot it with a vacuum-truck operator. A data/GIS team could reproduce Sandec's SPA-DET approach for a new city without a lab or field site: the project page lists the sharing of raw data for 845 samples taken in four cities (Kampala, Hanoi, Sircilla, Lusaka), and the abstract of the Hanoi–Kampala modelling study on that page says the collected data and developed scripts were made available for replication. The page does not say where the samples are deposited; its footer links to Eawag's research-data portal (https://opendata.eawag.ch/). Skills: environmental engineering, survey statistics, GIS, embedded systems, public-sector data systems.
Genes
Structural cousins: stuck the same way, in other fields
Sources
"Monitoring safely managed on-site sanitation (SMOSS): Synthesis of lessons from phase 1 pilots and recommendations for phase 2 pilots," WHO/UNICEF Joint Monitoring Programme for Water Supply, Sanitation and Hygiene (JMP), December 2021, accessed 2026-08-17; supporting: Eawag/Sandec, "Faecal sludge quantities and qualities (Q&Q)," accessed 2026-08-17 go to source 1 ↗ go to source 2 ↗
Working notes
Notes made while writing and checking this brief, shown as written. They use working shorthand.
Primary source is the JMP's own SMOSS synthesis report (tier 1 — the SDG custodian agency documenting its monitoring gap and pilot results); the 1%/seven-country figure is the JMP's Table 3 citing WHO/UNICEF 2021 — verified at intake as 1% of the global population (not of the on-site-sanitation population; an earlier draft of this brief misstated the denominator) and should be re-checked against the latest JMP progress report, which may have improved coverage. Sandec Q&Q figures are from the Eawag project page and the SuSanA forum thread on Strande et al. (2018); the 1,206-data-point relationship and per-capita accumulation rates should be verified against the journal article. `failure:ignored-context` (data/information sub-pattern) chosen: monitoring instruments (household surveys, plant records) were designed around sewered or household-level facts and do not capture the emptying/transport link — not `failure:not-attempted` (SMOSS pilots are serious attempts) and not `failure:theoretical-gap`. `stakeholders:multi-institution` passes the three-criteria test: national statistical offices, water/sanitation regulators (often different departments for on-site vs sewered), utilities/treatment operators, and private or informal emptiers each hold a non-substitutable piece of the flow; the report explicitly names the departmental split as a barrier. `constraint:coordination` was considered and rejected: the binding constraint is that the data does not exist in reconcilable form, not that willing actors cannot coordinate. `temporal:static` rather than `worsening`: on-site sanitation is growing faster than sewers, but the measurement barrier itself is unchanged (growing salience, not growing difficulty). Related collection briefs: `water-pit-emptying-demand-supply-gap` (household demand for safe emptying) and `humanitarian-faecal-sludge-emergency-treatment` (camp treatment); this brief is the distinct national/city-scale accounting problem.
Source type: Agency gap analysis (SDG custodian documenting its own monitoring failure).
Verified at intake 2026-08-17: gate (net) + adversarial source check + contested-tag second coding.
Reconciliation 2026-10-01: Fact repair after reading the whole write-up against every source it names. Three sources were fetched on 2026-10-01. The JMP report was read as full text (93 pages, searched and the relevant passages read) from https://washdata.org/sites/default/files/2022-05/jmp-2021-smoss-synthesis-report.pdf. The Eawag/Sandec project page, including the publication abstracts it lists, was read in full from https://www.eawag.ch/en/department/sandec/projects/ewm/qaq/. The SuSanA forum thread named in the notes above was found and read at https://forum.susana.org/forum/categories/214-vault-content-research-and-faecal-sludge-characteristics/22656-new-article-methods-to-reliably-estimate-faecal-sludge-quantities-and-qualities-by-eawag. The journal article by Strande et al. (2018) was not opened by the first reader, who read its abstract on the Eawag page; the second reader read its full text (19-page PDF, text extracted and searched) from the Internet Archive copy of the Eawag repository file. (1) The Source line gave the report's title as "Synthesis report of phase 1 pilots and recommendations for phase 2 pilots" and its date as 2021/2022. The cover reads "Synthesis of lessons from phase 1 pilots and recommendations for phase 2 pilots" and "December 2021 - FINAL"; the Source line now follows the cover. (2) A quotation was altered: the text had Sandec saying there has been "no reliable method to estimate total accumulated quantities and qualities." The abstract on the Eawag page says "no reliable method exists to estimate total accumulated quantities and qualities"; the quotation now matches and is dated to 2018, the year of the paper, and the reason given (underground storage, missing records, non-standard construction) is attached to the difficulty of estimating in-place amounts, as on the page. (3) A quotation was altered by one word: the JMP report says the volume disposed to treatment "is not a direct function of the number of users but also depends on" containment size and emptying practices; the text had "depend." The quotation now matches. (4) The text said treatment plants "have been designed from literature values borrowed from other cities." The Eawag page says that in the past people estimated quantities and qualities from literature values from other contexts or educated guesses from users and emptying service providers; it does not mention plant design there, and neither does the full text of Strande et al. (2018). The sentence now says what the page says, including that those estimates "were not adequate to capture the desired accuracy and range of variability," and no longer presents the past practice as a consequence of the storage problem; the second reader added the article's own account of the consequence, that engineers estimate when designing solutions without adequate resources or time and the result is plants immediately at capacity (Lubigi in Kampala is its example) or well under- or over-capacity. (5) The text said "Nobody — not the country, not the city, not the plant — can produce a defensible number." The JMP report says the information may be available at the scale of individual municipalities but is typically not aggregated nationally, and that only seven countries had national-level data on off-site treatment of sludge from emptied pits and tanks; the sentence now says that. In the same paragraph, "cannot be reconciled" became "difficult to align" (the report's words), and the sentence on sewered populations now gives the report's statement that most countries have data on wastewater treatment while very few have data on on-site excreta management. (6) The text listed the departmental split between on-site and sewered services as one of the constraints on adding questions to national surveys. The report raises it in a different place, as a reason the relevant policies and regulations can be hard to identify; the text now keeps the two apart, and adds the report's third survey constraint (the risk of questions being cut). (7) The text attached "1,206 data points" to the Kampala accumulation figures and to the prediction method. On the Eawag page the 1,206 data points belong to a later seven-country study (Andriessen et al. 2023) that fitted a relationship between volatile solids and total solids; the Kampala figures of 270 and 280 L/cap·year come from Strande et al. (2018), where they were confirmed in the full text; the article does not contain the 1,206 figure. The sentence now separates them, and "promising" was dropped because neither source uses that judgment. (8) "Its authors describe it as 'still a baby'": the phrase is in the forum thread, in a reply from one co-author (Miriam Englund) relayed by a moderator on 15 August 2018, which goes on to "encourage context specific adaption." The sentence now attributes it that way. (9) The absence-of-data rule was said to leave the most on-site countries with "no safely-managed figure at all." The report says no estimates are made for off-site sludge treatment in those countries unless they have data; the sentence now says that. "The on-site half of humanity" became the report's 43%, and "cannot be sized or justified" and "cripples" were softened, since the report says only that such data inform policy, implementation and research. (10) The entry point said "several African and Asian cities have published shit-flow diagrams and plant intake records." Neither source says this. The JMP report mentions a review of "SFDs in Kenya which focus on urban areas" (it does not spell the abbreviation out), and the Eawag page lists a Kampala study with 5,653 emptying events recorded from GPS units installed in trucks; the sentence now names those. The GIS entry point now says what the page offers: it lists the sharing of raw data for 845 samples from four cities, and the abstract of the Hanoi–Kampala study says its data and scripts were made available for replication. The page does not say where the samples are deposited; the link to Eawag's research-data portal is in the page footer (the portal opened on 2026-10-01; the individual datasets were not downloaded). Confirmed in the sources and left as written: on-site sanitation used by 43% of the global population in 2020 and growing faster than sewers, even in urban areas; sewer and on-site use approximately equal; data on excreta emptied and treated off-site for 1% of the global population, seven countries, all high income (Table 3); six pilot countries from 2020 with Gates Foundation funding; the sample sizes for Bangladesh (3,149), Serbia (1,560, which includes rural schools and healthcare facilities) and Zambia (23,000, households and institutions); every other JMP quotation; the JMP assumption for countries where sewers are more common. Could not be checked: the two "related collection briefs" comparisons in the notes above, which are about this collection and not about the sources. The sentences under "What Would Unlock Progress" on flow accounting, truck loggers and mobile-money receipts are the write-up's own proposals and are not claims about the sources. No title changed. Genome tags untouched. A second reader went over these repairs against the sources on the same day, opened the full text of Strande et al. (2018) through the Internet Archive copy of the Eawag repository file, kept all of the repairs, tightened the wording of three (the 2018 dating, the abbreviation SFD, and where the shared data are), and added to (4) the article's sentence on plant sizing. A further second-reader pass the same day re-fetched all four sources, confirmed every remaining quotation verbatim, and changed nothing else.