Methods for estimating quantities and qualities ( Q & Q ) of faecal sludge : fi eld evaluation in Sircilla , India

Estimates of accumulated quantities and qualities (Q&Q) of faecal sludge are essential for developing city-wide management plans. However, standardized approaches are lacking, and examples in scientific literature make use of diverse methodologies and parameters, making their comparability and transferability difficult. This study field-tested an approach for estimating Q&Q in Sircilla, India, and compared three methods for measuring accumulated sludge: (1) faecal sludge accumulation rate from in situ measurement with a core sampler; (2) faecal sludge accumulation rate with volume emptied by desludging truck; and (3) sludge blanket accumulation rate in situ with a core sampler. Measurements were taken at households and commercial establishments, samples were analysed for characteristics, and demographic, environmental, and technical data were collected with a questionnaire. The median total solids (TS), volatile solids (VS), and chemical oxygen demand (COD) concentrations for all containments were 26.8, 17.8, and 32.0 g/L, respectively. The median faecal sludge accumulation rate estimated with the core sampler and truck were 53 and 96 L/cap·year, respectively. The median sludge blanket accumulation rate was 17 L/cap·year. Continued data collection in this fashion will lead to a better understanding of what is accumulating in onsite containments at regional levels.

Although the most difficult to reasonably estimate, rates of faecal sludge accumulation are essential because it represents the total (latent) amount of faecal sludge that needs to be managed (Strande et al. ). However, there is no standard for methods used to determine accumulation rates reported in the literature. Examples include measuring in situ volume using a sludge sampler and tape measure (Lugali et al. ) or a laser-based measuring device (Still &   For definitions of faecal sludge used in this paper, refer to Velkushanova et al. (). Quantity of faecal sludge was defined as in situ rate of accumulation for septic tanks (commonly referred to as 'septage') and pit latrines. This was calculated both for the total amount of faecal sludge in containment, and on the sludge fraction that was settled out (referred to here as a sludge blanket). As shown in Figure 1 In addition, to validate measurements with the core sampler, the total height of faecal sludge was measured six times  over a period of 8 months for 28 elliptical septic tanks, 5 of which were emptied at the beginning of the study. Sludge blanket was also measured at these time points, to understand how sludge blanket accumulated over shorter time intervals.

Analytical methods
For qualities (characteristics), chemical oxygen demand

Qualities (characteristics)
Summary statistics for TS, VS, and COD are presented in  Other parameters with significant differences for TS, VS, and COD include containment type (concentrations were higher in pit latrines than septic tanks), containment shape (elliptical septic tanks had lower concentrations than circular pit latrines and rectangular septic tanks), and containment age (concentrations increased with age up to 20 years). Differences were not statistically significant for the number of users (no difference between categories), locality (no difference between slum and non-slum), and income level (no difference between low, middle, and high incomes).
As shown in Figure   Accumulation rates were also evaluated for differences between the categories of collected questionnaire data. All three types of accumulation rates were significantly higher for septic tanks as compared with pit latrines (FSAR-C (p ¼ 1.32 × 10 À6 ), FSAR-T (p ¼ 1.07 × 10 À4 ), and SBAR (p ¼ 3.92 × 10 À4 ); Figure 4). The relationship between the parameters used in the accumulation rate calculation (time, users, and sludge volume) was analysed to understand their influence on the results. Users and sludge volume were higher in septic tanks, though the time since last desludging was not significantly different when comparing septic tanks and pit latrines. Between types of septic tanks, elliptical septic tanks had higher accumulation rates than rectangular septic tanks for all three methods (    Figure 4 | Boxplots for faecal sludge accumulation rate measured using the core sampler (FSAR-C), the faecal sludge accumulation rate measured using truck volumes (FSAR-T), and the sludge blanket accumulation rate measured using the core sampler (SBAR-C) showing the difference between pit latrines and septic tanks, and the spread of the data. The diamonds represent mean values. Non-overlapping notches represent significant differences between categories.
To calculate accumulation rates, questionnaire data for 'time since last emptied' and 'number of users' is used. A sensitivity analysis was conducted to evaluate the impact of these assumptions. The maximum variability in the median was ±27 L/cap·year for FSAR-C, ±40 L/cap·year for FSAR-T, and ±4 L/cap·year for SBAR (calculation provided in Supplementary Table S1). To improve future estimates for accumulation rates, the concept of population equivalents for each onsite containment could be incorpor- proportion. This is in contrast to pit latrines, which conven- To evaluate the consistency of core sampler measurements, and how sludge blanket accumulates over shorter time periods, 28 of the elliptical septic tanks were measured six times over 8 months. Over this period of time, the height of the total faecal sludge in each of the tanks remained constant, other than the first measurement for the 5 that were emptied at the start of the study (Supplementary Material, Figure S4). However, no clear trend was observed for the height of the sludge blanket, in contrast to expectations (Supplementary Material, Figure S3)

CONCLUSIONS
This study compared three different methods for estimating accumulation rates that were each carried out on pit latrines and two types of septic tanks, and observed a wide range of values depending on the method. In previous studies, it was often not clear or fully reported how rates were estimated, or whether SBAR or FSAR was being measured. The variability of the results from the same systems when using the different methods in this study illustrates the importance of accurately defining and reporting terminology and methodology. The discourse surrounding containment type is also highly variable, and moving forward, providing more detailed containment classifications such as lined, partially lined, or unlined, together with overflow or no overflow will make results more comparable.
The differences in accumulation rates in this paper also Most of the trends that were observed for qualities and quantities with demographic, environmental, and technical data are the same as in other regions of the world (e.g. Uganda, Indonesia). However, context-specific data is still necessary for the design of management solutions. Hopefully in the future, as the relation between trends in different regions is better understood, it will help us to understand more fundamental processes that govern faecal sludge accumulation in order to design optimal management practices.