Systematic and stratified characterization of fecal sludge from septic tanks of bungalow type buildings

Abstract

On-site sanitation systems have proven to be widely acceptable in developing societies due to their ease of construction compared to sewer systems. However, management of the effluent sludge from the system has remained an environmental challenge. Proper understanding of the widely varying physico-chemical characteristics of sewage sludge will aid in the proper management of the system and disposal of septic tank sludge after desludging. Hence, this study was aimed at investigating the physical characteristics of septic tank systems, accumulation rate and the physico-chemical characteristics of septic tank sludge in bungalow-type buildings. The methodology for the study involved physical inspection and measurement of systematically and purposively selected septic tanks, measurement of sludge accumulation rates, and collection of samples from various layers (top liquid layer, bottom liquid layer, top sludge layer, and bottom sludge layer) of the systems for laboratory physico-chemical tests. It was observed that more than 60% of the sampled septic tanks have a volume of 10–15 m³, more than 35% of the sampled septic tanks had between five and seven users, and also, 80% of the sampled septic tanks was located 1–3 m from the building line. An average accumulation rate of 0.2015 m³/capita/year was recorded. A decrease in the accumulation rate of fecal sludge over time was observed with an R² = 0.8429. Phosphate exhibited the highest concentration among observed minerals, ranging from 346.46 mg/L at the top of liquid layer to 680.50 mg/L at the bottom of sludge layer, while nitrite had the lowest concentration, ranging from 0.1535 mg/L at the top of sludge to 0.3185 mg/L at the top of liquid. The other observed parameters showed varying concentrations in the different fractions. Significant variations were noted in moisture content, pH, and phosphate concentrations among different fractions due to p values lower than 0.05, other parameters showed no significant difference with p values ranging from 0.108 to 0.857. Significant correlation values were observed for some of the physico-chemical parameters such as nitrate (NO₃⁻) and TVS (− 0.944), nitrate (NO₃⁻) and TFS (0.758), temperature and nitrate (0.820), pH and temperature (0.751), E. coli and moisture content (0.833), E. coli and COD (− 0.790), and E. coli and phosphate (0.848). These findings provide critical insights for fecal sludge management, guiding treatment method selection, process optimization, and efficiency improvement.