Algal-bacterial granular sludge for simultaneous denitrification and yttrium removal

This study investigated the performance and stability of algal-bacterial granular sludge (ABGS) for the treatment of inorganic wastewater containing ammonia and Y(III), which aimed to shed light on efficient treatment of yttrium-rich rare earth mine wastewater. By seeding heterotrophic granules and dosing exogenous algae, cationic polyacrylamide and sodium acetate in a photo-sequencing batch reactor, the algal and granular symbiosis system was successfully established on Day 50, and mature ABGS was harvested on Day 165. When the concentration of Y(III) was no more than 4 mg/L, the system showed good performance for simultaneous removal of nitrogen and Y(III). However, further increasing the Y(III) concentration to 30 mg/L in Days 144–210 gradually inhibited the bacterial activity and killed most of the functional algae, resulting in significant decrease of the activity of bacteria and chlorophyll a content after Days 135 and 180, as well as the oxygen yield rate of the algae and ammonia oxidation rate after Days 140 and 95. The removal efficiency of total inorganic nitrogen maintained mostly above 90% in Days 46–153, and gradually decreased to approximately 50% in Days 154–210. The Y(III) removal efficiency remained above 98% in Days 96–125, and gradually decreased to approximately 60% in Days 126–210. Most of the algal genera disappeared on Day 210. Conversely, the increase of the relative abundance of Nitrosomonas and Thauera was observed. The nitrogen removal pathways included denitrification and algal assimilation. However, the contribution rate of algae gradually decreased from 22% to 4% due to the accumulation of Y. [Display omitted] •A method for rapid construction of algal and granular symbiosis system was proposed.•Stability and performance of algal-bacterial granules for nitrogen and Y(III) removal were investigated.•Contribution rate of bacteria and algae for nitrogen removal was calculated.