Faecal sludge simulants to aid the development of desludging technologies

Abstract

This paper presents a review of currently available data from the literature on the undrained shear strength, bulk density, stickiness and debris content of faecal sludge. Those data have been used to develop two different simulants that replicate the full range of shear strengths and densities reported for faecal sludge. Comprehensive specifications are also presented for the debris or solid waste found in latrines to more closely replicate the challenge of pumping faecal sludge. Finally, a design guide has been produced to capture these results and support quantitative performance testing of desludging pumps. The simulants have already been used as part of the Bill & Melinda Gates Foundation's Faecal Sludge Omni-Ingestor project and by Water for People's SaniHub in developing improved desludging pumps. The wider use of these simulants could accelerate the development of pit emptying technologies and help standardize the quantitative evaluation of their performance.

Re-evaluating the strength of pit-latrine faecal sludge from dynamic cone penetrometer test data

“…It should be noted that these values, at around 6 kPa, are substantially greater than the sludge strengths reported in previous work, yet only in 21 of the 439 pits tested did the penetrometer sink to the base of the pit with no blows. This indicates that in any pit latrine for which dynamic cone penetrometer testing has been carried out and a blow-count reported, the sludge strength must exceed 6 kPa and hence be greater than the upper limit of 2 kPa previously proposed by Radford et al (2015).…”

“…Despite this, faecal sludge with strengths exceeding the 'portable' penetrometer's maximum reading was found in 7 of the 30 pits. While 87% of the tested pits contained some 'weak sludge of comparable strength to that previously reported' , 60% of them were also found to contain sludge which was stronger than 400 Pa. Radford et al (2015) collected existing data on the shear strength and bulk density of faecal sludge. The highest undrained shear strength directly measured was 2 kPa, but rheological data which was extrapolated from weaker samples in Woolley et al (2014a, b) suggested that the maximum shear strength of faecal sludge could be 10 kPa, for sludge with total solids contents of up to 40%.…”

“…Shear strength has since been used as a more appropriate measure of sludge state than viscosity, as consolidated faecal sludge does not readily flow, rather it behaves more like a solid than a fluid (Radford and Sugden, 2014). The undrained shear strength of faecal sludge dictates whether the pit latrine material will begin to flow at a given suction (Radford et al, 2015).…”

Development of trash exclusion for mechanized pit latrine emptying

“…In this study, we used 0% bentonite to simulate septic tank wastewater, 3-7% bentonite to simulate thin sludge found in wet pit latrines, and 9-12% bentonite to simulate thick fecal sludge in wet and dry pit latrines. The model pit latrine also contained a standard trash formulation 20 and contained items such as but not limited to plastic bags, condoms, magazines, and sanitary napkins (Table 1).…”

Field testing of a prototype mechanical dry toilet flush

“…While the disposal of solid waste into pit latrines and other sanitation systems is detrimental to transport and treatment of the faecal sludge, and is therefore discouraged, it frequently happens nonetheless. The reasons for this vary, but the lack of alternative solid waste disposal options and the stigma surrounding menstrual hygiene are two common examples (Radford et al, 2015; Tembo et al, 2017). A flush should not necessarily accommodate this behaviour, but it is likely that menstrual absorbents are disposed regardless of user instruction.…”