The effect of cadmium on a semi-self-sustaining microalgal-bacterial granular sludge process for wastewater treatment

Abstract

Microalgal-bacterial granular sludge (MBGS) is an innovative and sustainable technology for wastewater treatment. This research investigated the effect of Cd2+ on the performance and characteristics of such semi-self-sustaining MBGS reactors (with additional intermittent aeration). After 30 days of exposure to 0, 2, and 10 mg·l−1 Cd2+, the three reactors consistently exhibited robust removal efficiencies, achieving 89 % removal for chemical oxygen demand (COD) and 99 % removal for NH4+-N. However, the MBGS in control exhibited better performance in T-N removal compared to MBGS exposed to 10 mg·l−1 Cd2+ (especially during the light phase), indicating a potential impact of Cd2+ on the N-assimilation process of microalgae. The high concentration of Cd2+ also negatively affected the MBGS structure and increased the production of extracellular polymeric substances. Further investigations of the dissolved oxygen profile were conducted under phototrophic conditions, the maximum oxygen production rate contributed to microalgae in the control group was 20.9 % and 46.4 % higher than that of groups with 2 and 10 mg·l−1 Cd2+. The results show that Cd2+ mainly impacts the microalgae in the outer layer of the structure, and additional aeration helps compensate for the decrease in oxygen production caused by Cd2+ in MBGS, thereby making the system more stable in removing COD and NH4+-N.