Aggregation/disaggregation of microalgal-bacterial flocs in high-rate oxidation ponds is a response to biotic/abiotic-induced changes in microbial community structure

During wastewater treatment by integrated algal pond systems (IAPS), microalgal-bacterial flocs (MaB-flocs) form naturally but periodically disaggregate, resulting in poor settling, low biomass recovery, and reduced effluent quality. This study investigates biotic/abiotic-induced changes in microbial community structure in high-rate algal oxidation ponds (HRAOP) of an IAPS on MaB-floc formation and stability during sewage treatment. Results show that dominance by Pseudopediastrum, Desmodesmus and Micractinium species in spring and summer and the chytrids, Paraphysoderma sp. in spring and Sanchytrium sp. in summer, occurred coincident with enhanced MaB-floc formation and biomass recovery (≥90%). In winter, poor floc formation and low biomass recovery were associated with dominance by Desmodesmus, Chlorella, and the Chlorella-like genus Micractinium. A principal components analysis (PCA) confirmed that combinations of colonial microalgae and associated parasitic chytrids underpin MaB-floc formation and stability in spring and summer and that unicells dominated in winter. Dominance by Thiothrix sp. coincided with floc disaggregation. Thus, changes in season, composition and abundance of colonial microalgae and associated parasitic fungi appeared to impact MaB-floc formation, whereas species composition of the bacterial population and emergence of Thiothrix coincided with floc instability and disaggregation.