The influence of lower temperature, influent fluctuations and long retention time on the performance of an upflow mode laboratory-scale septic tank

Abstract

The influence of temperature in a range of 15–5°C, fluctuations in soluble chemical oxygen demand (sCOD) and suspended solids (SS) of the influent and also long hydraulic retention times (HRT) of 36 or 50 d on the performance of a laboratory septic tank were studied. A laboratory-scale septic tank with a volume of 20 L, started up at 15°C was fed with domestic wastewater from two settlements with sCOD up to 450 and up to 2,000 mg/L, respectively. The efficiency of the septic tank was assessed based on pollutant removal and biogas production. A stepwise decrease in temperature by 5°C in the range of 15–5°C halved the daily emission of biogas. Removal efficiencies for sCOD and SS were in the range of 74–86 and 86–88%, respectively. An increase in sCOD of influent from 450 to 4,000 mg/L resulted in a decreased performance of the septic tank (to ~25% less). The highest biogas emission was observed at 10°C and at retention time (36 d), owing to increased consumption of CO₂ and CH₄ by autotrophic microorganisms over the longer retention time (50 d). At the 15°C, biogas emission was lower than in case of lower temperatures applied. In the range of 15–5°C, operational temperature did not correlate significantly (p > 0.05) with the removal efficiency of sCOD or SS, assuming as a consequence of the long HRT.

Keywords:

• Anaerobic wastewater treatment
• Domestic wastewater
• Low temperature
• On-site system
• UASB-septic tank
• Biogas

Acknowledgements

Support for this study was provided by the target-financed project of the Estonian Ministry of Education and Research No. IUT20-16. Laura Daija was supported by a DoRa Grant from the Archimedes Foundation. We thank Anne Paaver for her help with the analysis of water samples.