Over-Produced Extracellular Polymeric Substances and Activated Antioxidant Enzymes Attribute to Resistance of Pb(II) for Algal–Bacterial Granular Sludge in Municipal Wastewater Treatment

Algal–bacterial granular sludge technology is a new type of wastewater treatment and resource regeneration process, which has received widespread attention due to its excellent nitrogen and phosphorus removal performance, and energy-saving and emission reduction effects. Although algal–bacterial granular sludge technology has achieved an ideal nutrient removal ability, some pol-lutants in wastewater might affect the symbiotic relationship between algae and bacteria. This study investigated the impact of coexisting Pb(II) on the symbiosis of algal–bacterial granular sludge. It was found that 2.5–10.0 mg/L of Pb(II) exposure increased the relative abundance of Pro-teobacteria. In addition, more protein in extracellular polymeric substances (EPS-PN) was secreted at 2.5 mg/L of Pb(II) exposure while EPS-PN content reduced at a rate of 5.0–10.0 mg/L of Pb(II). Under different concentrations of Pb(II), the damage degree of algal–bacterial granular sludge was exacerbated, evidenced by increased malondialdehyde (MDA) content. To cope with these adverse circumstances, the antioxidant enzyme activity of both super-oxide dismutase (SOD) and peroxidase dismutase (CAT) was boosted. With the help of these adaptive strategies, the symbiosis of algal–bacterial granular sludge was stable. Moreover, the performance of algal–bacterial granular sludge in treating COD, ammonia-N and phosphate-P was kept at above 95%. This study approved that a Pb(II) concentration less than 10.0 mg/L had little effect on the performance of algal–bacterial granular sludge in wastewater treatment. It is hoped that this study can provide useful information for an improved engineering feasibility of algal–bacterial granular sludge process.