Investigation of faecal sludge accumulation rates in Septic tanks and lined pit latrines of low income densely populated area of Kampala
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The rapid expansion of populations in low-and middle-income areas of sub-Saharan African (SSA) cities without a concomitant increase in the provision of services such as sanitation endangers public health and the environment. Moreover, cities in SSA are largely served by on-site sanitation containments, which have to be frequently emptied to continue rendering the service. Consequently, there is a need to understand the faecal sludge accumulation rates to devise appropriate management interventions. To do this, there is a need to have reliable estimates for the quantities and qualities (Q&Q) of faecal sludge (FS) that accumulates in these systems at the community to city-wide scales. There are ongoing attempts to provide more reliable estimates of Q&Q, but these lack more studies and data to improve their applicability in a wide spectrum of settings, considering the highly variable nature of FS. There is therefore a need to add value to the global ongoing attempts to develop reliable, empirical, field-based approaches for the estimation of Q&Q of FS at scales relevant to the design of management and treatment solutions. Hence the objectives of this study were to: 1) determine the in-situ volumes of FS in the selected onsite sanitation containments; 2) evaluate the increments in FS inside the containments, and hence their filling rates; 3) assess the spatial relationship between demographic, environmental, and technical (DET) variables of FS in the containments; and 4) determine the new design parameters for onsite sanitation containments. The study was undertaken in Kawempe division, Kampala, covering both dry and rainy seasons. The Q&Q measurements in septic tanks and lined pit latrines were performed in 22 containments using a volaser measuring device, and samples were taken out using a locally built pit sampler for analysis in the laboratory. Besides, a questionnaire was administered to the users and/or owners of these containments to determine the DET variables. The Volaser consists of a distance laser measuring unit, a tripod stand, and a collapsible probe to measure total depth as well as a smartphone app to operate the device. This device was used to measure the depth of containment from the surface of the FS to the top of the containment. It was also used to determine the volume of the containment and its shape. The sampler typically consisted of a sample container and was used to collect representative samples of FS in triplicates from the septic tanks and lined pit latrines for laboratory analysis of pH, TS, VS, EC, and COD. The in-situ volume of accumulated FS in containments was 5.6 . The average containment size in the study area was 8.14 ± 4.4 m3 filling at a median rate of 8.64±39.68 L per day. A correlation of 0.58 (R2 = 0.92) was obtained for VS and TS. For TS and COD, a correlation of 0.62 (R2 = 0.53) was observed. The estimated median faecal sludge accumulation was 295.05 L/cap.year. The results from this study contribute to enhancing Correlating data collection and analysis enhances the ability to plan and model effectively, relying on dependable estimates of Q&Q. Consequently, conducting comparable future studies can aid in streamlining future characterization studies, reducing the need for numerous laboratory analyses.