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

The influence of temperature in a range of 15-5 degrees 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 50d on the performance of a laboratory septic tank were studied. A laboratory-scale septic tank with a volume of 20L, started up at 15 degrees C was fed with domestic wastewater from two settlements with sCOD up to 450 and up to 2,000mg/L, respectively. The efficiency of the septic tank was assessed based on pollutant removal and biogas production. A stepwise decrease in temperature by 5 degrees C in the range of 15-5 degrees 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 4,000mg/L resulted in a decreased performance of the septic tank (to similar to 25% less). The highest biogas emission was observed at 10 degrees C and at retention time (36d), owing to increased consumption of CO2 and CH4 by autotrophic microorganisms over the longer retention time (50d). At the 15 degrees C, biogas emission was lower than in case of lower temperatures applied. In the range of 15-5 degrees 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.