Influence of Salt and Cr(VI) Shock Loadings on Oxygen Utilization and COD Removal in SBR

The effects of salt (NaCl) and chromium (VI) shock loadings on oxygen use and chemical oxygen demand (COD) removal efficiency were investigated by using two laboratory-scale sequencing batch reactors (SBR). The feed water used in this investigation was synthetic sewage with a ratio of 100/5.4/1.1 as COD/N/P. Reactors were operated at a 0.34–0.52 F/M ratio, 2000–2500 mgL⁻¹ mixed liquor suspended solids (MLSS) concentration and 10 days sludge retention times (SRT). All results were compared with those from an identical reactor operating at normal conditions. At concentrations of 20 gL⁻¹ NaCl, 5 mgL⁻¹ Cr(VI), and 10 mgL⁻¹ Cr(VI) shock loadings were applied to the shock-loaded reactor. Temperature, dissolved oxygen (DO), and pH in the reactors were measured at hourly intervals during the cycle period; however, the COD, MLSS, and sludge volume index (SVI) were daily. For the salt shock loading, COD removal efficiency decreased and the system could return to steady-state conditions within 3 days. For 5 mgL⁻¹ Cr(VI) shock loading, COD removal efficiency was affected and the system tended to recover itself within 3 days. However, for 10 mgL⁻¹ Cr(VI) shock loading, COD removal efficiency decreased dramatically, and it required more time to return to steady-state conditions, and the system could recover itself within 5 days. Although the differences in MLSS values were about 10% for the Cr(VI) shock loadings, there were no apparent effects observed on the biomass concentration and sludge-settling properties for the salt shock loading.