A comparative analysis of composite and grab sampling methods for fecal sludge characterization: a case study from Pilani, India

Abstract

Fecal sludge (FS) is biohazardous waste from on-site sanitation (OSS) containers like septic tanks and pit latrines, potentially harming the environment if discharged untreated. The design of the FS treatment system depends on its characteristic properties. Earlier and already existing characterization studies have shown that FS age, OSS type, water inclusion, and usage of additives significantly impact FS characteristics. There are various sampling methods to collect and characterize the sample. However, no study has compared the sampling methods of FS, which may potentially impact characterization. This study compares composite and grab sampling methods by analyzing 15 samples of each collected from the same FS discharge during a vacuum truck emptying vehicle in Pilani, a town in Rajasthan, India. The characterization of FS samples from OSS revealed variations between the two sampling methods, even though the samples were obtained from the same FS discharge. In composite sampling, total solids (TS) varied from 14.9 to 90 g/l (mean: 42.3 g/l, median: 33.4 g/l), and chemical oxygen demand (COD) varied from 16 g/l to 122.7 g/l (mean: 54.7 g/l, median: 42.7 g/l). While in grab sampling, TS varied from 12.1 to 91.5 g/l (mean: 36.2 g/l, median: 25.6 g/l), and COD varied from 8.7 g/l to 114.7 g/l (mean: 43.9 g/l, median: 29.3 g/l). A paired Wilcoxon signed-rank test shows that sampling methods significantly affect the TS (p = 0.041) and COD (p = 0.018) of FS samples.