Study on the effect of total dissolved solids (TDS) on the performance of an SBR for COD and nutrients removal

In this study, the effect of total dissolved solids (TDS) on the performance of a sequencing batch reactor (SBR) system to treat synthetic wastewater with microbial inoculum was evaluated. The SBR was operated continuously for eight days on a 6-h cycle with anaerobic/anoxic/aerobic phases in each cycle after entering the steady state, and the influent TDS was tested at five levels, i.e., 750, 1500, 3000, 4500, and 6000 mg L⁻¹. The results showed that only two TDS levels (750 and 1500 mg L⁻¹) could achieve good COD removal efficiencies (94.8 and 92.2%, respectively). For TDS levels equal to, or greater than, 3000 mg L⁻¹, a 20% reduction in COD removal efficiency resulted. Different from COD, removal of NH₄⁺–N appeared not to be affected by the TDS content, and a removal efficiency of higher than 97% was obtained, regardless of the TDS content. However, only the lowest two TDS levels achieved high phosphate removals (>99%), and the removal efficiency dropped to 57.8 and 45.9%, respectively, for TDS levels of 3000 and 4500 mg L⁻¹. More interestingly, a phosphate release, instead of uptake, was observed at the TDS level of 6000 mg L⁻¹. It may be concluded that for effective phosphate removal, the TDS level in the liquid should be controlled under 1500 mg L⁻¹, and higher liquid TDS levels were detrimental to the aerobes and could disrupt the aerobic metabolism, leading to the failure of the SBR treatment system. A tendency that raising TDS content would adversely affect the aerobic oxygen uptake rate was observed, which could also result in SBR upset. A power regression with an R of 0.9844 was established between the influent TDS concentration and the TDS removal efficiency, which may be used to estimate the SBR performance in TDS removal based on the influent TDS content.