Unsafe return of human excreta to the environment: A literature review

Sanitation facilities are often assumed to create a barrier between humans and excreta; however, this may not be the case if excreta is insufficiently contained or improperly disposed of. Thus, we conducted a literature review of peer-reviewed studies and grey literature to identify pathways of human excreta leaking unsafely into the environment. The fate of collected fecal sludge and the extent of wastewater treatment was not well documented. Future sanitation research should specifically document the location of fecal disposal to aid in characterizing public health risks.

As part of a project funded by Bill and Melinda Gates Foundation, the Water Institute undertook a review of peer-reviewed and grey literature to examine pathways of unsafe return of human excreta to the environment. The review was conducted to inform a proof of concept to estimate the fraction of human excreta unsafely returned to the environment.

 The review has a wide scope encompassing diverse technologies and all stages of the sanitation delivery chain and was solicited to add to the discussion on global sanitation monitoring and the agenda for the Sustainable Development Goals (SDGs). The Water Institute believes the review has value to the WaSH sector and has made it publicly available for comment in order to incorporate questions and comments from WaSH professionals and practitioners.  We encourage you to download the document and provide feedback via e-mail to ashley.williams@unc.edu.

The full review may be downloaded here.

Executive Summary

This literature review was funded by Bill & Melinda Gates Foundation as part of a proof of concept project titled “Estimates for the Unsafe Return of Human Excreta to the Environment”. The aim was to compile evidence on the pathways and extent of unsafe return of human excreta to the environment throughout the sanitation delivery chain. This document was submitted to Bill & Melinda Gates Foundation on June 4, 2015. The authors invite readers to provide feedback, suggestions, and questions that may be addressed and incorporated in a future version of the review.

Within monitoring and evaluation, sanitation facilities are often assumed to be safe if, by design, they create a barrier between humans and human excreta. However, human excreta may be released into the environment if the waste is not sufficiently contained on-site, if the waste is “leaked” into the environment through improper disposal or transport, or if the waste is insufficiently treated. Human waste contains pathogens that are harmful to health; thus leakage of human excreta into the household, community, and greater environment is a public health concern.

This review investigated leakage of excreta in the containment, disposal, transport, treatment, and disposal stages of the sanitation delivery chain for the following technologies: pit latrines, septic systems, and sewerage. The review focused on “leakage” of fecal sludge, liquid waste stream, wastewater, and pathogens contained in excreta.

The review findings indicate that while there are few studies on “leakage” of latrines and septic systems, studies report that many latrines and septic tank are not emptied and are sources of groundwater contamination. Unlined latrines, damaged facilities, and pits serving as septic tanks do not provide effective containment and can cause microbial contamination of the household premises. Findings from a few studies indicate that latrines are widely affected by storms, heavy rainfall, and floods; while no studies reported on effects of weather events on septic systems, vulnerability to floods and extreme weather events may also be expected. In addition, findings indicated the additional hazard of households resorting to open defecation when their latrines became damaged or unusable.

While several studies and reviews cited latrines and septic systems as potential or likely causes of groundwater contamination events, this was not universal across included studies; some studies reported no contamination effects from nearby facilities. The range of findings emphasize that the impact of latrines and septic systems on groundwater quality is dependent on soil type, distance between groundwater and pit or drain field, and hydrological conditions. Additionally, seasonal effects on well contamination in areas with a high density of latrines or septic systems were reported in several studies.

Household latrine and septic tank emptying behavior is not well understood or characterized in the literature. Included studies on household emptying behavior for latrines and septic tanks commonly reported a large proportion of respondents had never emptied their on-site sanitation facility or did not know the last time they had emptied it. This appeared to vary across study settings; some studies cited a high percentage of respondents routinely emptied their on-site sanitation facility. However, emptying was self-reported in all studies and subject to recall bias.

Methods for pit latrine emptying also varied by study setting; mechanical emptying was more prevalent in some regions, whereas manual emptying, burying pits, pit diversion, and mechanical emptying were more commonly used in other study settings. “Flooding” latrines appears to be a common practice in certain areas, yet was cited in few papers. Literature findings indicate that availability of diverse emptying options was associated with routine emptying. Several studies cited that while private and public companies may provide emptying services, they were often not sufficient to meet regional demand. Only two studies in peri-urban and rural areas were found, therefore the behaviors practiced in these settings when pits fill up are relatively undocumented.

Very little literature was found on certain topics related to on-site sanitation facilities. Septic systems are used in many urban and rural settings of developing countries, yet few studies on septic system performance and maintenance were retrieved from developing countries. Within a small number of studies, septic system maintenance was found to be infrequent. Older septic systems are prone to failure and common in the US, but little data was found on system age and performance.

Critical gaps identified in the literature included the fate of collected fecal sludge, and the extent of primary, secondary, and tertiary treatment. While some studies reported volumes of fecal sludge collected, treated, and properly disposed in certain cities, there were no estimates or studies found for many regions. Having more reliable estimates from collection through disposal would better illustrate regional gaps and opportunities within the sanitation delivery chain. Similarly, there are global estimates for wastewater that is treated, but the effectiveness and level of treatment is unknown. The results from the reviewed studies show, even with advanced treatment processes, some wastewater effluent still contains high levels of pathogens.

In order to understand the increased hazard to public health through the unsafe return of human excreta, it is necessary to determine where excreta is “leaking” back into the environment. From the literature, it is unclear what fraction of sludge is being disposed of, untreated, into surface water through practices such as flooding or discharging sludge into drains which may lead to wastewater treatment plants or directly into surface water through storm water drains. Since the rate of pathogen die-off varies in soil and water, future research on fecal sludge management behavior should report more specifically the location of disposal in order to better characterize the associated public health risks.


Unsafe return of human excreta to the environment: A literature review. A. R. Williams, A. Overbo. 2015. The Water Institute at UNC, Chapel Hill, NC, USA. https://waterinstitute.unc.edu/wp-content/uploads/sites/3640/2015/07/BMGF_UnsafeReturn_LitReview_UNC_16June15.pdf.