The effects of mercury shock on the performance and microorganisms in biological wastewater treatment process

In this study, the effects of Hg²⁺ shock on the removal efficiencies for organic matters and nutrient, enzyme activities, and microbial community structures in lab-scale sequencing batch reactor (SBR) were investigated. The results showed that the short-term exposure (6 h) to Hg²⁺ at 0.05 mM significantly reduced the removal efficiencies of dissolved organic carbon (DOC), ammonium-nitrogen (NH₄⁺-N) and orthophosphate (PO₄^3--P). At lower concentration (0.01 mM) and prolonged (30 days) exposure condition, NH₄⁺-N removal was severely inhibited, and it was recovered after 15 days of restoration. Higher levels of reactive oxygen species (ROS), catalase (CAT) and superoxide dismutase (SOD) activities were detected in the Hg²⁺ shocked reactor and eventually restored to the control level after 60 days of restoration. No increase in the release of lactate dehydrogenase (LDH) was observed under both short-term and long-term shock conditions, indicating no irreversible damage to the cell membrane. The relative abundance of genus Zoogloea and Paracoccus were decreased after Hg²⁺ shock, which implies these microorganisms may be sensitive to heavy metal exposure.