Cadmium-effect on performance and symbiotic relationship of microalgal-bacterial granules

So far, the microalgal-bacterial granular sludge process has attracted growing interest as an emerging wastewater treatment technology. Cadmium ion (Cd²⁺) commonly found in wastewater is toxic to microorganisms, thus its effect on microalgal-bacterial granules was investigated in this study. Results showed that Cd²⁺ at the concentration above 1 mg/L could compromise the performances of microalgal-bacterial granules. The removal efficiency of chemical oxygen demand decreased from about 70% in the control to 42.2% and 25.0% after 30-day operation at the respective Cd²⁺ concentrations of 5 and 10 mg/L, while the ammonia-nitrogen removal also declined from 70.4% to 30.5% with the increase of the Cd²⁺ concentration from 1 to 10 mg/L, indicating that nitrifying bacteria were susceptive to the presence of Cd²⁺. It was further revealed that Cd²⁺ could stimulate the production of extracellular polymeric substances, e.g. 190.19 ± 7.04 mg/g VSS in the presence of 10 mg/L of Cd²⁺ versus 100.26 ± 3.82 mg/g VSS in the control after 10-day operation. More importantly, about 84.1%–94.8% of Cd²⁺ was found to bind to the extracellular proteins in microalgal-bacterial granules at the Cd²⁺ concentrations studied. In addition, Chlorococcum and Cyanobacteria in microalgal-bacteria granules were withered in the presence of 10 mg/L of Cd²⁺, suggesting uncoupled symbiosis between microalgae and bacteria induced by Cd²⁺. Consequently, this study showed that Cd²⁺ could negatively impact on the microbial structures and metabolisms of microalgal-bacterial granular sludge, leading to a compromised process performance in terms of organic and nitrogen removal.